Toward eradication: the effect of Mycobacterium bovis infection in wildlife on the evolution and future direction of bovine tuberculosis management in New Zealand

New Zealand's bovine tuberculosis (TB) control programme has greatly reduced the burden of tuberculosis on the farming industry, from 11% of mature cattle found with TB at slaughter in 1905 to <0.003% in 2012/13. New Zealand implemented TB control measures in cattle from the mid-twentieth century, and later in farmed deer. Control was based on established methods of tuberculin testing of herds, slaughter of suspect cases, and livestock movement control. Unexplained regional control failures and serious disease outbreaks were eventually linked to wildlife-vectored infection from the introduced Australian brushtail possum (Trichosurus vulpecula), which also triggered a wildlife disease complex involving a range of introduced species. This paper reviews the progressive elucidation of the epidemiology of Mycobacterium bovis in New Zealand's wildlife and farmed livestock, and the parallel development of research-led, multi-faceted TB control strategies required to protect New Zealand's livestock industries from damaging infection levels. The adoption of coordinated national pest management strategies, with increasingly ambitious objectives agreed between government and industry funders, has driven a costly but very successful management regime targeted at controlling TB in the possum maintenance host. This success has led to initiation of a strategy designed to eradicate TB from New Zealand's livestock and wildlife, which is considered a realistic long-term prospect.

Development of integrated surveillance systems for the management of tuberculosis in New Zealand wildlife

Disease surveillance for the management of bovine tuberculosis (TB) in New Zealand has focussed, to a large extent, on the development of tools specific for monitoring Mycobacterium bovis infection in wildlife. Diagnostic techniques have been modified progressively over 30 years of surveillance of TB in wildlife, from initial characterisation of gross TB lesions in a variety of wildlife, through development of sensitive culture techniques to identify viable mycobacteria, to molecular identification of individual M. bovis strains. Of key importance in disease surveillance has been the elucidation of the roles that different wildlife species play in the transmission of infection, specifically defining brushtail possums (Trichosurus vulpecula) as true maintenance hosts compared to those that are predominantly spillover hosts, but which may serve as useful sentinel species to indicate TB persistence. Epidemiological modelling has played a major role in TB surveillance, initially providing the theoretical support for large-scale possum population control and setting targets at which control effort should be deployed to ensure disease eradication. As TB prevalence in livestock and wildlife declined throughout the 2000s, more varied field tools were developed to gather surveillance data from the diminishing possum populations, and to provide information on changing TB prevalence. Accordingly, ever more precise (but disparate) surveillance information began to be integrated into multi-faceted decision-assist models to support TB management decisions, particularly to provide informed parameters at which control effort could be halted, culminating in the Proof of Freedom modelling framework that now allows an area to be declared TB-free within chosen confidence limits. As New Zealand moves from large-scale TB control to regional eradication of disease in the coming years, further integrative models will need to be developed to support management decisions, based on combined field data of possum and TB prevalence, sentinel information, risk assessment in relation to financial benefits, and changing political and environmental needs.

Epidemiology, diagnostics, and management of tuberculosis in domestic cattle and deer in New Zealand in the face of a wildlife reservoir

The control of tuberculosis (TB) in cattle and farmed deer in New Zealand has been greatly influenced by the existence of a wildlife reservoir of Mycobacterium bovis infection, principally the Australian brushtail possum (Trichosurus vulpecula). The reduction in possum numbers in areas with endemic M. bovis infection through vigorous vector control operations has been a major contributor to the marked reduction in the number of infected cattle and farmed deer herds in the past two decades. Management of TB in cattle and farmed deer in New Zealand has involved a combination of vector control, regionalisation of diagnostic testing of cattle and deer herds, abattoir surveillance and movement control from vector risk areas. Accurate diagnosis of infected cattle and deer has been a crucial component in the control programme. As the control programme has evolved, test requirements have changed and new tests have been introduced or test interpretations modified. Subspecific strain typing of M. bovis isolates has proved to be a valuable component in the epidemiological investigation of herd breakdowns to identify whether the source of infection was domestic livestock or wildlife. New initiatives will include the use of improved models for analysing diagnostic test data and characterising disease outbreaks leading to faster elimination of infection from herds. The introduction of the National Animal Identification Tracing programme will allow better risk profiling of individual herds and more reliable tracing of animal movements. TB in cattle and farmed deer in New Zealand can only be controlled by eliminating the disease in both domestic livestock and the wildlife reservoir.

The emergence of Mycobacterium paratuberculosis in farmed deer in New Zealand - a review of 619 cases

AIM

To review cases in which Mycobacterium paratuberculosis was identified in farmed deer in New Zealand.

METHODS

Case histories were reviewed where M. paratuberculosis was identified in deer by either culture or a polymerase chain reaction (PCR) test using primers from IS900.

RESULTS

Between 1986 and 2000, M. paratuberculosis was identified by bacterial culture and/or PCR in 619 farmed deer from 299 herds, representing approximately 6% of deer herds in New Zealand. Over 85% of cases were identified during the last 6 years. In 60% of the infected herds, only one infected animal was identified. The maximum number of cases identified in a single deer herd was 47, and these were identified over a period of 8 years. Only 36 (5.8%) cases came from clinically affected animals identified on farms by veterinarians. The majority (89.7%) of the 619 cases were identified from lesions in mesenteric lymph nodes, including the ileocaecal lymph nodes, identified at meat inspection as being macroscopically either typical or equivocal of bovine tuberculosis (M. bovis). While the overwhelming majority of lesions were identified in mesenteric lymph nodes, M. paratuberculosis was also identified in 27 lesions in lymph nodes of the head, especially the retropharyngeal lymph node.

CONCLUSIONS

The figures presented underestimate the true prevalence of infection with M. paratuberculosis, especially since not all suspect cases were submitted for culture or PCR. However, they do show that M. paratuberculosis appears to be spreading in farmed deer in New Zealand and highlight the possibility that Johne's disease is emerging as a potential major problem affecting this species. Identification of the organism by bacterial culture or PCR is required in many cases to distinguish lesions in mesenteric lymph nodes and lymph nodes of the head caused by M. paratuberculosis from those caused by M. bovis and M. avium.

The prevalence of disseminated Mycobacterium avium subsp. paratuberculosis infection in tissues of healthy ewes from a New Zealand farm with Johne's disease present

AIM

To determine the prevalence of disseminated Mycobacterium avium subsp. paratuberculosis (Map) infection in healthy ewes in a flock with a history of clinical Johne's disease.

METHODS

Twenty-four healthy ewes, from a large sheep and cattle farm with a history of clinical Johne's disease in the ewe flock, were randomly selected, euthanased, blood sampled, and examined at necropsy. BACTEC™ radiometric culture for Map was performed on samples of faeces, ileum, mesenteric lymph node, biceps femoris muscle and mononuclear cells in peripheral blood. Serum antibody ELISA tests were performed. Histological sections and Ziehl Neelsen (ZN) stains of impression smears of ileum and mesenteric lymph node were examined for pathological lesions characteristic of Johne's disease and acid fast organisms (AFO). Indirect quantification of Map was performed, using BACTEC radiometric growth indices measuring the time taken for the production of (14)CO(2.)

RESULTS

No histological evidence of Johne's disease or AFO was found in the ileum and mesenteric lymph nodes. Twelve of the 24 ewes (50%) had Map cultured from the ileum (n=6) and/or mesenteric lymph nodes (n=8) while none had Map cultured from the faeces, biceps femoris muscle or blood mononuclear cells. One of the 12 Map culture positive ewes was serum ELISA positive. The culture growth rates in liquid medium suggest low numbers of Map were present in the tissues of the culture positive ewes.

CONCLUSION

Fifty per cent of clinically healthy ewes exposed to Map within a Johne's infected flock were Map culture positive in the ileum and/or mesenteric lymph node(s), while the ELISA was positive in 8% of those animals (n=1). There was no faecal shedding of Map and no Map was cultured from skeletal muscle or from blood mononuclear cells suggesting that systemic Map infection, defined as positive culture of Map from skeletal muscle and/or blood, may be uncommon in healthy mixed age ewes without clinical Johne's disease.

CLINICAL RELEVANCE

ELISA serology detected 1 of 12 ewes infected with Map whilst none were detected from faecal BACTEC radiometric culture, suggesting biosecurity measures used to control the spread of Map may be of limited use. Map was not cultured from blood mononuclear cells or skeletal muscle, indicating that meat from healthy ewes, from farms where Johne's disease is present, is an unlikely source of Map exposure for humans. Further research is warranted to establish the prevalence and dissemination of Map in tissues outside the alimentary tract of healthy ewes from farms throughout New Zealand where Map is present.

Detection of Mycobacterium avium subsp. paratuberculosis in skeletal muscle and blood of ewes from a sheep farm in New Zealand

AIM

To determine whether viable Mycobacterium avium subsp. paratuberculosis (Map) is present in skeletal muscle and blood in ewes with and without Johne's disease confirmed histologically.

METHODS

A total of 51 mixed-aged ewes in poor body condition from a farm with a history of clinical Johne's disease were culled and examined at necropsy. BACTEC radiometric culture was performed on samples of skeletal muscle from the biceps femoris, mononuclear cells in peripheral blood (hereafter referred to as blood), and ileum. Histological sections and Ziehl-Neelsen (ZN)-stained impression smears of terminal ileum and mesenteric lymph nodes were examined. Ewes were defined as having confirmed Johne's disease if there was histopathological evidence typical of the disease within the ileum and adjacent lymph nodes.

RESULTS

Eighteen of 21 (86%) ewes with confirmed clinical Johne's disease were culture-positive for Map from sites peripheral to the alimentary tract, comprising 15 from skeletal muscle and 13 from blood. Five of 30 (17%) ewes that did not have Johne's disease were culture-positive, with four from skeletal muscle and one from blood. The likelihood that ewes with confirmed Johne's disease had systemic Map infection compared with ewes without was determined as OR=30 (95% CI=6.3-142.0; p<0.001).

CONCLUSION

The prevalence of Map infection of skeletal muscle and blood in ewes with confirmed Johne's disease was 71% and 62% respectively, and in unaffected ewes was 13% for muscle and 3% for blood.

CLINICAL RELEVANCE

Skeletal muscle and blood are potential sources of exposure of humans to Map, and the risk appears higher from sheep with Johne's disease.

Serological survey of Toxoplasma gondii and Campylobacter fetus fetus in sheep from New Zealand

AIM

To determine the prevalence of antibody titres to Toxoplasma gondii and Campylobacter fetus fetus in sheep from New Zealand.

METHODS

As part of a free screening service, unsolicited blood samples were supplied by veterinarians wishing to gauge the exposure of their clients' ewe flocks to T. gondii and C. fetus fetus. Blood samples were submitted from mixed-age ewes throughout New Zealand, from 2006 to 2009, that had not been vaccinated for T. gondii and C. fetus fetus. A total of 2,254 sera were serologically titrated for T. gondii and 3,429 for C. fetus fetus. A latex agglutination kit available commercially was used to quantify antibodies to T. gondii, and an agglutination test developed in-house was used for C. fetus fetus. For T. gondii, titres of ≥1:16 and ≥1:64 were used to define a positive response, and for C. fetus fetus a titre of ≥1:10 was defined as positive. A flock was defined as positive if ≥1 ewe had a positive titre.

RESULTS

Of the sera tested for T. gondii, 1,917/2,254 (85%) were positive, using a titre of ≥1:16, and 1,384/2,254 (61%) with a titre of ≥1:64. All 198 ewe flocks tested were seropositive to T. gondii, at a titre of ≥1:16, and all but three were at a titre of ≥1:64. A bimodal distribution was evident in the prevalence of titres to T. gondii suggesting that a percentage of titres ≤1:64 may have been non-specific. Of the sera tested for C. fetus fetus, 1,644/3,429 (48%) were positive to at least one of the four test antigens at titre of ≥1:10. Only 34/298 (11%) flocks tested for C. fetus fetus were completely seronegative. The percentage of seropositive ewes to both T. gondii and C. fetus fetus was significantly higher in the North Island than the South Island.

CONCLUSIONS

The study demonstrated that exposure to these two important infectious abortifacients was both considerable and widespread. Minimum titres were postulated to establish a 'cut-off' for a positive result and to allow comparison with past and future studies. The bimodal distribution evident for T. gondii suggested a titre of 1:64 may be an appropriate cut-off. The widespread on-farm exposure probably stimulates the immune response of vaccinated ewes.

CLINICAL RELEVANCE

Further studies are required to confirm the clinical significance of flock-based antibody responses, and to validate their use in identifying recently aborted ewes, especially where there are no aborted fetuses for examination.

Serology ofCampylobacter fetus fetusstrains from four outbreaks of ovine abortion

As a result of a Ministry of Agriculture & Fisheries survey on ovine abortions, 76 isolates of Campylobacterfetus fetus were obtained. These isolates were from four farms in the southern Hawkes Bay, with an abortion incidence of 2.8% to 9.1%. Antisera to eight different strains of C. fetus fetus were made in rabbits. Strains were then examined using whole cell tube agglutination tests and sensitised Staphylococcal Protein A slide agglutination tests. Heat labile antigens were examined by absorbing antisera with heat-treated bacteria. Two broad serogroups were found, but within-group variation was demonstrated by cross-absorbing antisera. The isolates from one farm were all of a single broad serogroup. Both serogroups were found on the remaining three farms. Evidence for the presence of two major serogroups was also obtained by immobilisation tests and antigen analysis by gel diffusion.

Mycobacteria isolated from deer in New Zealand from 1970–1983

Mycobacterium bovis was isolated from 504 deer from 1970 to 1983. It was first isolated from feral red deer (Cervus elaphus) in New Zealand in 1970, and from farmed deer in 1978. Cervine tuberculosis has emerged as a significant problem in farmed deer and in 1983 M.bovis was found on 40 different farms. Thirty-five isolates of Mycobacterium avium-intracellulare have been cultured from deer but were associated with clinical disease in only four cases. Mycobacterium nonchromogenicum, Mycobacterium diernhoferi, Mycobacterium gastri, Mycobacterium chelonei, Mycobacterium smegmatis and Mycobacterium vaccae were isolated from deer but were not considered to be pathogenic.

Experimental tuberculosis in red deer (Cervus elaphus)

This study was designed to investigate experimental Mycobacterium bovis infection of red deer (Cervus elaphus). Three intravenously inoculated deer (dose 10 microg-1000 microg) developed miliary tuberculosis of the lungs and all died within 28 days of being infected. No clinical illnesses were observed in four subcutaneously (dose 1 microg-1000 microg) and three intratracheally (dose 10 microg-100 microg) inoculated deer. At the conclusion of the experiment six weeks post inoculation, these seven animals reacted to 2 mg/ml of bovine purified protein derivative. The principal lesions in the intravenously inoculated deer were in the lungs which had multiple foci of necrosis containing very large numbers of acid fast bacilli. A gradation of changes was seen in the subcutaneously inoculated deer. The animal receiving the 1 microg dose only had lesions at the injection site and the draining prescapular lymph node. Deer receiving higher doses also had histopathological changes in the lungs and liver. Microscopic changes in the intratracheally infected animals were restricted to the thoracic cavity. The ability of the deer to controlled infection was related to the route of inoculation.

Mycobacterium aviuminfection in a farmed deer herd

Tuberculous lesions were identified over a 2-year period in 36 clinically normal red deer from a single herd. The lesions were only present in the retropharyngeal lymph nodes and lymph nodes draining the intestinal tract, indicating infection by the oral route. Mycobacterium avium was isolated from 27 of 29 lesions examined by bacterial culture. Grossly and histologically, the lesions were indistinguishable from those caused by Mycobacterium bovis. DNA restriction endonuclease analysis revealed that all the 26 M. avium isolates available for examination had identical cleavage patterns. These patterns were identical to a New Zealand M. avium serotype 2 isolate from a pig and were very similar to a reference strain of M. avium serotype 2. The DNA examinations indicated that the deer were infected from a common source that was not identified.

ExperimentalMycobacterium bovisinfection of cattle: Effect of dose ofM. bovisand pregnancy on immune responses and distribution of lesions

Groups of 18-month-old cattle were inoculated intratracheally with 5 x 10(5) colony forming units (high dose) or 500 colony forming units (low dose) of Mycobacterium bovis to determine an appropriate dose to induce lesions similar to those seen in the natural disease. An additional group of 21-28 weeks pregnant cattle were inoculated with the high dose of M. bovis to determine if pregnancy increased the susceptibility of cattle to M. bovis infection. By 23-24 weeks after challenge, the high dose of M. bovis had induced extensive lung lesions, and tuberculous lesions were observed in the lymph nodes of the head, neck, and thoracic and abdominal cavities. In contrast, the low dose of M. bovis induced predominantly small lesions (< 1 cm diameter) which were localised to the lungs and pulmonary lymph nodes. The lesions induced by the low dose were similar to those seen in the natural disease in cattle. The majority of the high dose group cattle produced strong antibody responses to M. bovis culture filtrate, while only one low dose animal produced a detectable response. All of the M. bovis-inoculated cattle produced strong cellular immune responses to bovine PPD (skin test and interferon-gamma responses). Pregnancy did not appear to affect the susceptibility to M. bovis infection, and immune responses of the cattle in this group at the end of the study were similar to those in the high dose non-pregnant group. However, from the first test after calving, the interferon-gamma responses of peripheral blood cultures to bovine PPD were low compared with the responses prior to calving.

Age susceptibility of red deer (Cervus elaphus) to paratuberculosis

The aim of this study was to measure the relative susceptibility of three age classes of red deer (Cervus elaphus) to the various manifestations of infection and disease states due to Mycobacterium avium subsp. paratuberculosis (MAP) after experimental oral challenge. Three groups of seronegative female deer (30 three-month-old weaners, 20 fifteen-month-old yearlings and 20 adults) received four oral doses of approximately 10(9) colony forming units (cfu) of a bovine strain of MAP. They were monitored for 50 weeks by weighing, blood sampling for immunological assays, skin testing and faecal culturing. Clinically affected animals were promptly euthanised and the remaining deer were killed at the end of the study. Necropsies were carried out and samples of intestine and associated lymph nodes were taken for culture and histopathology from all deer. Ten weaners developed clinical paratuberculosis and were euthanased 20-28 weeks post-challenge (pc). No clinical cases occurred in the yearlings or adults. All 10 clinically affected weaners had severe gross and histopathological lesions typical of paratuberculosis (Johne's disease). At slaughter, gross lesions were seen in the jejunal lymph nodes of 8/17 weaners, 2/19 yearlings, and 0/20 adults. MAP was cultured from samples of the intestine and/or lymph nodes from all 10 clinical cases and from 16/17 weaners, 19/19 yearlings and 18/20 adult hinds at slaughter. Lesion Severity Scores of deer slaughtered 50 weeks pc averaged 4.9, 3.5 and 1.1 for the weaner, yearling and adult groups, respectively. At some time over the course of the trial, 24/28 weaners were antibody positive and immediately prior to slaughter, 13/17 weaners, 15/19 yearlings and 3/20 hinds were antibody positive. There is a strong age-related resistance against clinical disease and subclinical disease, but not to infection with MAP, after heavy oral challenge.

Oral vaccination reduces the incidence of tuberculosis in free-living brushtail possums

Bovine tuberculosis (Tb) caused by Mycobacterium bovis has proved refractory to eradication from domestic livestock in countries with wildlife disease reservoirs. Vaccination of wild hosts offers a way of controlling Tb in livestock without wildlife culling. This study was conducted in a Tb-endemic region of New Zealand, where the introduced Australian brushtail possum (Trichosurus vulpecula) is the main wildlife reservoir of Tb. Possums were trapped and vaccinated using a prototype oral-delivery system to deliver the Tb vaccine bacille Calmette-Guerin. Vaccinated and control possums were matched according to age, sex and location, re-trapped bimonthly and assessed for Tb status by palpation and lesion aspiration; the site was depopulated after 2 years and post-mortem examinations were conducted to further identify clinical Tb cases and subclinical infection. Significantly fewer culture-confirmed Tb cases were recorded in vaccinated possums (1/51) compared with control animals (12/71); the transition probability from susceptible to infected was significantly reduced in both males and females by vaccination. Vaccine efficacy was estimated at 95 per cent (87-100%) for females and 96 per cent (82-99%) for males. Hence, this trial demonstrates that orally delivered live bacterial vaccines can significantly protect wildlife against natural disease exposure, indicating that wildlife vaccination, along with existing control methods, could be used to eradicate Tb from domestic animals.

Isolation of Mycobacterium bovis from brushtail possums with non-visible lesions

AIM

To determine the prevalence of Mycobacterium bovis infection in brushtail possums (Trichosurus vulpecula) that did not have macroscopic lesions of bovine tuberculosis, and to evaluate culture of pooled tissues from multiple possums as a method for determining the M. bovis-infection status of wildlife populations in New Zealand.

METHODS

Pools of selected tissues were collected from possums from four different populations known to be infected with M. bovis. Tissue pools from individual animals, and combined pools from multiple animals, were cultured for M. bovis.

RESULTS

In the four populations investigated, the prevalence of possums with macroscopic lesions confirmed by culture to be infected with M. bovis ranged from 1 to 19 (mean 31/283; 10.9)%. The prevalence of possums with non-visible lesions that were culture positive for M. bovis in the same populations ranged from 4 to 10 (mean 24/283; 8.5)%. The mean of the log10 cfu of M. bovis of the macroscopic lesions and of the culture- positive samples that did not have visible lesions was 3.85 (SE 0.26) and 1.46 (SE 0.26) log10 cfu, respectively (p<0.01). Mycobacterium bovis was cultured from pools of 30-50 animals in the four populations studied.

CONCLUSIONS

The finding of M. bovis infection in possums with non-visible lesions identified a potential deficiency of declaring possum populations free of M. bovis on the basis of absence of macroscopic lesions. The culturing of pools of selected tissues from multiple animals without visible lesions can be used to reduce laboratory costs of possum surveys without a major reduction in the ability to detect M. bovis infection.

A bovine macrophage screening system for identifying attenuated transposon mutants of Mycobacterium avium subsp. paratuberculosis with vaccine potential

Johne's disease is a chronic granulomatous enteritis in ruminants caused by Mycobacterium avium subsp. paratuberculosis (MAP). The disease is responsible for considerable economic losses in the livestock industry and in particular within the dairy sector. A more effective vaccine against Johne's disease would be of major benefit. In this study, we developed an efficient procedure for identifying mutants of MAP with reduced virulence that are potential live vaccine candidates against Johne's disease. A mariner transposon was used to create random insertional libraries in two different MAP strains (989 and k10), an effective cattle macrophage survival system was developed, and a total of 1890 insertion mutants were screened by using a 96-prong multi-blot replicator (frogger) system. Two of the transposon mutants with poor survival ability in macrophages were tested in mice. These strains were found to be attenuated in vivo, thereby validating the further use of this macrophage screening system to identify MAP mutants with potential as candidate vaccines against Johne's disease.

Efficacy, immune responses and side-effects of vaccines against Johne's disease in young red deer (Cervus elaphus) experimentally challenged with Mycobacterium avium subsp paratuberculosis

AIMS

To test the efficacy of a commercially available and an experimental vaccine against Johne's disease in young red deer (Cervus elaphus), using experimental challenge with live virulent Mycobacterium avium subsp paratuberculosis (M. ptb), measure injection-site reactions, and assess the effects of vaccination and challenge on results of subsequent skin tests and ancillary blood tests for bovine tuberculosis (Tb).

METHODS

Ninety 6-8-week-old red deer fawns were randomly allocated to three equal groups of 30, and received either a 1-ml S/C injection of either a commercially available whole-cell killed vaccine with a mineral-oil adjuvant (COM), or a live attenuated M. ptb experimental vaccine with a lipid adjuvant (EXP), or were unvaccinated controls. Ten weeks later (Week 10), all 90 fawns received an oral challenge with approximately 10(8) cfu of a bovine strain of M. ptb daily for 4 days. The fawns were regularly weighed and monitored for clinical signs of Johne's disease, and regularly blood-sampled and tested for antibodies to M. ptb, using the Paralisa test, an IgG1 ELISA, and for antibodies to Mycobacterium bovis, using a similar test. A mid-cervical tuberculin skin test (MCT) was administered at Week 23, and comparative cervical skin tests (CCTs) were administered at Weeks 37 and 57. All animals were electively killed at Week 59, injection sites inspected, gastrointestinal tracts examined for gross lesions, and samples taken for culture and histopathology.

RESULTS

There were no clinical cases of Johne's disease but, at slaughter, more gross lesions in intestinal lymph nodes were observed in Control (20%) than COM animals (0%; p<0.05). This latter group also had less severe histopathological lesions in samples of intestines and lymph nodes compared with the Control group (p<0.05), but not deer in the EXP group. Over 89% of deer in all three groups were shown by culture to be infected with M. ptb, while only 21-33% of faecal samples were culture-positive. Time to positive culture was longer for COM vs EXP and Control groups (p<0.01), reflecting fewer M. ptb organisms in samples from the ileocaecal valve (ICV) in that group. Almost all (>or=90%) deer reacted to the MCT at Week 23, and there were no significant differences between groups. One or two deer in each group were classified as Tb reactors to the CCT at Week 37, and none were classified as Tb reactors to the CCT at Week 57. At the time of challenge, over 50% of deer in the COM group were classified as positive (9/28) or suspicious (7/28) for M. ptb antibodies in the Paralisa test, one animal in the EXP group was classified as suspicious, and all the Controls were negative. From Week 23 to the end of the trial, 25/28 (89%) deer in the COM group were Paralisa-positive or -suspicious. The proportion of animals in the EXP and Control groups that were Paralisa-positive peaked at Week 39 (60% and 55%, respectively). The majority of deer in the COM group had significant levels of antibody to M. bovis 10 weeks after vaccination, while the proportion of M. bovis-antibody positive Control deer rose gradually throughout the trial, reaching 23/30 (77%) at slaughter. Injection-site lesions in COM deer ranged from 10-38 mm in diameter 4 weeks after vaccination, and then resolved. Minimal injection-site lesions were observed in EXP deer. At slaughter, 14 months after vaccination, 19/28 deer in the COM group had 5-15-mm nodules that were easily trimmed from the carcass.

CONCLUSIONS

The experimental challenge with M. ptb produced subclinical Johne's disease in the majority of deer, but did not cause any clinical disease. The number and severity of gross and microscopic lesions was significantly reduced in the COM compared with Control and EXP groups; vaccination of the EXP group did not appear to give significant protection. Deer vaccinated with the commercial vaccine are likely to give a false-positive reaction to the MCT but should have an avian reaction to the CCT, if it is carried out >12 months after vaccination. Most of the deer vaccinated with the commercial vaccine produced significant levels of antibodies against both M. ptb and M. bovis, which interfered with ancillary Tb tests. If this vaccine or similar oil-based vaccines are used on deer farms in the future, it may be advisable to only vaccinate animals destined for slaughter, that would not need to be Tb-tested, but would be 'works-monitored' for evidence of Tb instead.

Experimental infections in young red deer(Cervus elaphus)with a bovine and an ovine strain ofMycobacterium aviumsubspparatuberculosis

AIMS

To compare the virulence of a 'bovine' and an 'ovine' strain of Mycobacterium avium subsp paratuberculosis (M. ptb) in red deer (Cervus elaphus) after experimental inoculation orally, and to examine the relationship between the dose of the bovine strain given and immunological, clinical and histopathological outcomes in young red deer.

METHODS

Newly-weaned 4-month-old male red deer (n=81) were randomly assigned to one of five groups. Three groups (n=16) received high (10(9) colony forming units (cfu); HB), medium (10(7) cfu; MB) or low (10(3) cfu; LB) oral doses of a bovine strain of M. ptb, one group (n=16) received medium (10(7) cfu; MO) doses of an ovine strain of M. ptb, and a Control group (n=17) was not dosed. The HB and Control groups were grazed together, the MB and LB groups were grazed together, and the MO group was grazed alone, in separate small paddocks on a quarantined area of the farm for 45 weeks. Liveweight, clinical signs and immunoglobulin G1 (IgG1) antibody levels were monitored for up to 45 weeks. Deer affected with Johne's disease were euthanised when they showed obvious clinical signs. Unaffected deer were slaughtered at the end of the trial (Week 45), and all deer were necropsied. Faeces and tissue samples were cultured for M. ptb, and fixed tissues were examined for histopathology.

RESULTS

Between 21 and 38 weeks post-challenge (pc), 5/16 animals in the HB group developed early signs of Johne's disease and were euthanised. The remaining deer in the five groups were all apparently healthy and reached good liveweights (approximately 100 kg average), and were euthanised and examined 45 weeks pc. Three deer (two HB and one MB) had small caseous lesions in their jejunal lymph nodes (JJLNs) and one HB animal had a small caseous lesion in a retropharyngeal lymph node. The remaining animals had no grossly-visible lesions. Mycobacterium avium subsp paratuberculosis was cultured from samples from 100% of the HB and MB animals, 50% of the LB group, 69% of the MO group and all Control animals. Thus all Control deer were infected by natural transmission from the HB group but none developed signs of clinical disease. Examination of histological sections of jejunum, ileocaecal valve (ICV) and associated lymph nodes showed a gradation of severity of lesions that was positively correlated (p<0.001) with dose of the bovine strain administered; mean lesion severity scores were 4.8, 2.9 and 0.9 for HB, MB and LB groups, and 2.2 and 0.9 for the Control and MO groups, respectively. IgG1 antibody levels at the time of euthanasia were also correlated with lesion severity scores at slaughter (p<0.001).

CONCLUSIONS

The ovine strain of M. ptb used in this study was less virulent for red deer than the bovine strain. The correlation between dose of the bovine strain and the severity of lesions suggests that clinical Johne's disease in yearling red deer likely results from high oral challenge with a bovine strain whilst they are young. The minimum oral infective dose may be close to 10(3) cfu for this bovine strain.

Oral vaccination of brushtail possums with BCG: Investigation into factors that may influence vaccine efficacy and determination of duration of protection

AIMS

To determine factors that may influence the efficacy of an oral pelleted vaccine containing Mycobacterium bovis bacille Calmette-Guérin (BCG) to induce protection of brushtail possums against tuberculosis. To determine the duration of protective immunity following oral administration of BCG.

METHODS

In Study 1, a group of possums (n=7) was immunised by feeding 10 pellets containing dead Pasteur BCG, followed 15 weeks later with a single pellet of live Pasteur BCG. At that time, four other groups of possums (n=7 per group) were given a single pellet of live Pasteur BCG orally, a single pellet of live Danish BCG orally, 10 pellets of live Pasteur BCG orally, or a subcutaneous injection of live Pasteur BCG. For the oral pelleted vaccines, BCG was formulated into a lipid matrix, and each pellet contained approximately 107 colony forming units (cfu) of BCG, while the vaccine injected subcutaneously contained 106 cfu of BCG. A sixth, non-vaccinated, group (n=7) served as a control. All possums were challenged by the aerosol route with a low dose of virulent M. bovis 7 weeks after vaccination, and killed 7-8 weeks after challenge. Protection against challenge with M. bovis was assessed from pathological and bacteriological findings. In Study 2, lipid-formulated live Danish BCG was administered orally to three groups of possums (10-11 per group), and these possums were challenged with virulent M. bovis 8, 29 or 54 weeks later. The possums were killed 7 weeks after challenge, to assess protection in comparison to a non-vaccinated group.

RESULTS

The results from Study 1 showed that vaccine efficacy was not adversely affected by feeding dead BCG prior to live BCG. Feeding 10 vaccine pellets induced a level of protection similar to feeding a single pellet. Protection was similar when feeding possums a single pellet containing the Pasteur or Danish strains of BCG. All vaccinated groups had significantly reduced pathological changes or bacterial counts when compared to the non-vaccinated group. In Study 2, oral administration of Danish BCG induced protection against challenge with M. bovis, which persisted for at least 54 weeks after vaccination. Some protection was observed in possums challenged 54 weeks after vaccination, but this protection was significantly less than that observed in groups vaccinated 29 or 8 weeks prior to challenge. There was a strong relationship between the proportion of animals producing positive lymphocyte proliferation responses to M. bovis antigens and protection against challenge with M. bovis.

CONCLUSIONS

Factors considered potentially capable of interfering with vaccination, including feeding dead BCG to possums prior to feeding live BCG, feeding multiple doses of BCG at one time, and changing strains of BCG, were shown not to interfere with the acquisition of protective immune responses in possums. Protection against tuberculosis was undiminished up to 29 weeks after vaccination with BCG administered orally. It is concluded that vaccination of possums by feeding pellets containing BCG is a robust and efficient approach to enhance the resistance of these animals to tuberculosis.

Use of a polymerase chain reaction to subtypeMycobacterium aviumsubspeciesparatuberculosis, an increasingly important pathogen from farmed deer in New Zealand

AIMS

To review the number of microbiologically-confirmed cases of Johne's disease in farmed deer since 2000, and determine the prevalence of the bovine and ovine subtypes of Mycobacterium avium subsp paratuberculosis (M. paratuberculosis), using a highly specific polymerase chain reaction (PCR) test on samples from infected herds.

METHODS

The number of cases of M. paratuberculosis in farmed deer identified by culture or IS900 PCR was documented. A highly specific PCR test was applied to subtype M. paratuberculosis from BACTEC 12B cultures selected on the basis of one culture per deer herd, to give a wide coverage of herds in New Zealand.

RESULTS

From January 2001 to October 2005, M. paratuberculosis was isolated from 1,141 farmed deer, and has now been identified by microbiological testing in over 600 deer herds in New Zealand. The bovine subtype of M. paratuberculosis was shown by a highly specific PCR test to be present in 91/95 herds examined; the ovine subtype was found in the remaining four herds.

CONCLUSIONS

Since 2000, there has been a substantial increase in both the number of microbiologically-confirmed cases of Johne's disease in farmed deer and the number of infected herds. Johne's disease is now widespread and common in deer herds throughout New Zealand. Whilst the bovine subtype of M. paratuberculosis predominates in deer herds in New Zealand in which Johne's disease has been confirmed, the occasional finding of the ovine subtype highlights the need to consider both sheep and cattle as potential sources of infection for farmed deer.

Advances in understanding disease epidemiology and implications for control and eradication of tuberculosis in livestock: the experience from New Zealand

A deteriorating tuberculosis problem in cattle and deer in New Zealand has been halted and then reversed over the last decade. Mycobacterium bovis infection in both wild and domestic animal populations has been controlled. This has been achieved by applying a multi-faceted science-based programme. Key features of this have been a comprehensive understanding of the epidemiology of tuberculosis in animals, confidence in sampling wild animal populations, effective application of diagnostic tests in cattle and deer, and the ability to map M. bovis genotypes.

Oral delivery of lipid-encapsulated Mycobacterium bovis BCG extends survival of the bacillus in vivo and induces a long-term protective immune response against tuberculosis

The success of oral-route vaccination using Mycobacterium bovis bacille Calmette-Guérin (BCG) relies on delivery of live, actively metabolising bacilli to confer protection. Here, we describe that lipid-microencapsulation can extend the in vivo survival of bacilli when fed to mice, and can induce a long-lasting protective immune response. Feeding mice with lipid-encapsulated BCG (L-BCG) resulted in greater recovery of viable BCG bacilli from the mesenteric lymph nodes (MLN) compared to mice fed non-encapsulated BCG. A time-course study indicated persistence of viable BCG bacilli in MLN up to 30 weeks post-vaccination, similar to the duration of viable BCG recovery from the spleen following subcutaneous vaccination. The persistence of viable bacilli in the MLN of L-BCG mice invoked long-lasting systemic cell-mediated immune reactivity, with responses similar to those observed in subcutaneously-vaccinated mice. Further, L-BCG-vaccinated mice showed a high degree of protection against aerogenic challenge with virulent M. bovis at 30 weeks post-vaccination, with significant reductions in lung and spleen pathogen burdens. This study identifies that lipid-encapsulation of live BCG bacilli can facilitate increased in vivo survival and immunogenicity of the vaccine in orally-vaccinated mice, and highlights protection via this route for up to 7 months post-immunisation.

A report of tuberculosis in cats in New Zealand, and the examination of strains of Mycobacterium bovis by DNA restriction endonuclease analysis

Mycobacterium bovis was isolated from diagnostic samples from 57 cats submitted to New Zealand Animal Health Laboratories from 1974 to 1986. With six exceptions, these cats came from suburban and rural areas of New Zealand where M.bovis was also present in feral and wild animals, especially the brush-tailed possum. Tuberculous skin lesions were seen in 33 (58%) of the cats. Histologically, these lesions had some similarities to those of cat leprosy. Included in the 57 cats was a group of 12 tuberculous animals which were diagnosed in a suburban veterinary practice over a 3 month period. When these 12 M. bovis isolates were examined by DNA restriction endonuclease analysis, they were found to be identical. This evidence, together with the relatively short period during which the cases occurred, suggested that these cats were exposed to a single source of infection.

Response of the North Island brown kiwi, Apteryx australis mantelli and the lesser short-tailed bat, Mystacina tuberculata to a measured dose of rabbit haemorrhagic disease virus

Four North Island brown kiwis and six lesser short-tailed bats were inoculated intramuscularly with 300 000 rabbit lethal doses of rabbit haemorrhagic disease (RHD) virus. No clinical abnormalities were observed in the kiwis and bats throughout the study period. Although no viraemia was detected in any of the kiwis, all four birds produced a serological response to RHD virus above the positive cut-off by 14 days after inoculation, and in two of the birds, antibodies persisted for over 5 months. Two kiwis were killed 48 days after inoculation. Their tissues were examined for lesions, and for the presence of persistent virus by both reverse transcription polymerase chain reaction and by inoculation of tissue suspensions into rabbits. No gross or histological lesions suggestive of a viral infection were detected and tests for detection of virus were negative. The serological response in the kiwis was probably due to the birds responding to viral antigen in the inoculum rather than to multiplication of the virus. None of the bats showed a serological response to RHD virus above the positive cut-off by 14 days after inoculation and the results of the pathological and virological examinations were negative.

Serology and semen culture for the diagnosis of Brucella ovis infection in chronically infected rams

The serological response to Brucella ovis and the shedding of the organism in semen was followed for a period of 13-14 months in 42 naturally infected rams. Most rams remained chronically infected and excreted the organism in their semen throughout the investigation. B. ovis was isolated from 87.9% of the semen samples from the infected rams. The most common sites from which B. ovis could be isolated at necropsy were the epididymides and accessory sexual glands. In one ram the organism was isolated from lung, spleen, kidney and iliac lymph nodes. Three rams ceased to shed B. ovis in their semen during the course of the investigation. Seventy-five (11%) of 686 sera from infected rams were negative in the complement fixation test (CFT) although 76% and 77% of CFT-negative sera were positive in the gel diffusion precipitin test (GDT) and enzyme labelled immunosorbent assay (ELISA) respectively. The high incidence of CFT-negative infected rams was due to the selection for the investigation of many rams with histories of negative or vacillating CFT titres. Sera from five rams which never shed B. ovis in their semen reacted erratically in the three serological tests. The five rams were from heavily infected flocks and were kept in contact with infected rams throughout the investigation.

Vaccination of cattle with Mycobacterium bovis culture filtrate proteins and CpG oligodeoxynucleotides induces protection against bovine tuberculosis

Culture filtrate protein (CFP) vaccines have been shown to be effective in small animal models for protecting against tuberculosis while immunisation with these types of vaccines in cattle has been less successful. A study was conducted in cattle to evaluate the ability of selected adjuvants and immunomodulators to stimulate protective immune responses to tuberculosis in animals vaccinated with Mycobacterium bovis CFP. Seven groups of cattle (n=5) were vaccinated with M. bovis CFP formulated with either Emulsigen or Polygen adjuvant alone or in combination with a specific oligodeoxynucleotides (ODN), polyinosinic acid: polycytidylic acid (poly I:C) or poly I:C and recombinant granulocyte-macrophage colony stimulating factor. Two additional groups were vaccinated subcutaneously with BCG or non-vaccinated. In contrast to the strong interferon-gamma (IFN-gamma) responses induced by BCG, the CFP vaccines induced strong antibody responses but weak IFN-gamma responses. The addition of CpG ODN to CFP significantly enhanced cell-mediated responses and elevated antibody responses to mycobacterial antigens. Of the CFP vaccinated groups, the strongest IFN-gamma responses to CFP vaccines were measured in animals vaccinated with CFP/Emulsigen+CpG or CFP/Polygen+CpG. The animals in these two groups, together with those in the BCG and non-vaccinated groups were challenged intratracheally with virulent M. bovis at 13 weeks after the first vaccination and protection was assessed, by examination for presence of tuberculous lesions in the lungs and lymph nodes, 13 weeks later at postmortem. While BCG gave the best overall protection against tuberculosis, significant protection was also seen in animals vaccinated with CFP/Emulsigen+CpG. These results establish an important role for CpG ODN in stimulating protective Th1 responses to tuberculosis in cattle and indicate that a sub-unit protein vaccine can protect these animals against tuberculosis.

Mycobacterial diseases of deer

The most significant mycobacterial diseases of free-living, captive and farmed deer are bovine tuberculosis, caused by Mycobacterium bovis, Johne's disease (paratuberculosis), caused by Mycobacterium avium subsp paratuberculosis (basonym M. paratuberculosis), and avian tuberculosis, caused principally by M. avium subsp avium. The first case of M. bovis infection in farmed deer was identified in New Zealand in 1978. In 1983, a voluntary scheme was introduced in New Zealand to control tuberculosis in farmed deer, followed by a compulsory tuberculosis control scheme in 1990. The primary control measure is the slaughter of infected animals, detected by skin testing and blood testing, together with movement control and vector control. The number of infected deer herds peaked in the mid 1990s at over 160 herds, but by 30 June 2002 this had been reduced to 79 (1.45%), and to 67 (1.23%) by June 2003. Deer-to-deer transmission occurs, but the majority of herd breakdowns are believed to be from infected vectors. Factors likely to affect the susceptibility of deer include age, environment, population density, exposure and genetics. Avian tuberculosis occasionally causes clinical disease in wild, captive and farmed deer in New Zealand and overseas. Mycobacterium intracellulare, and subspecies of M. avium other than M. paratuberculosis, are widespread throughout New Zealand and are thought to be largely responsible for the high level of sensitisation to avian purified protein derivative (PPD), which is used for comparison purposes in tuberculosis skin testing of deer in this country. Infections with these organisms are usually subclinical in farmed deer, although M. avium subsp avium commonly causes lesions in retropharyngeal, mesenteric and ileocaecal lymph nodes. These lesions cause problems because of their gross and microscopic similarity to those due to M. bovis infection. Birds and domestic animals are most likely to become infected via environmental contamination of food, water, bedding litter or soil, while carnivores or scavengers may also become infected by ingesting infected carcasses. Johne's disease has been reported in deer in the wild and in zoos, especially in North America, the United Kingdom (UK) and Europe. Since first being confirmed in farmed deer in New Zealand in 1979, the incidence of Johne's disease has increased steadily. To date, M. paratuberculosis has been identified in >600 farmed deer on 300 properties. The majority of cases have been identified from suspected tuberculous lesions submitted from deer slaughter plants. Clinically, Johne's disease in deer is similar to the disease in sheep and cattle, with typical signs of loss of weight and condition, and diarrhoea. However, outbreaks of Johne's disease frequently occur in young red deer, 8-15 months of age, whereas the clinical disease in sheep and cattle is sporadic and usually affects adults 3-5 years of age. The disease is characterised by a chronic granulomatous enteritis and lymphadenitis, especially affecting the jejunum and ileum and the mesenteric lymph nodes. Deer affected subclinically may have lesions in these lymph nodes at slaughter, which are grossly indistinguishable from those due to bovine tuberculosis. Because of the antigenic similarity between M. intracellulare and all the subspecies of M. avium, including M. paratuberculosis, the diagnostic tests for Johne's disease lack sensitivity and specificity, making control difficult.

Surveillance of wildlife forMycobacterium bovisinfection using culture of pooled tissue samples from ferrets (Mustela furo)

AIM

To compare culture results of homogenates of pooled lymph nodes from individual ferrets with and without macroscopic lesions of bovine tuberculosis for the presence of Mycobacterium bovis, and to determine whether homogenates from 10-30 ferrets could be combined and cultured without loss of sensitivity as a possible method for improving cost-effectiveness of surveillance for M. bovis infection in wildlife populations.

METHODS

Numbers of colony forming units (cfu) of M. bovis present in cultures of homogenates of pooled lymph nodes from individual ferrets known to be infected and having no visible lesions (NVL) or macroscopic lesions consistent with bovine tuberculosis were determined. Prevalences of M. bovis infection in populations of ferrets in the Marlborough region of the South Island of New Zealand were determined by culturing homogenates of pooled lymph nodes from individual animals. Samples from homogenates from North Canterbury were combined to form pools representing 10, 20 and 30 animals and also cultured for M. bovis.

RESULTS

Fewer M. bovis cfu were isolated from ferrets with NVL (mean=0.77 log10) compared with ferrets with macroscopic lesions (mean=3.22 log10; p<0.05). The mean prevalence of infection in eight different surveys involving 427 ferrets from the Marlborough region was 18% (range 8-44%), which included a small number of animals with macroscopic lesions of tuberculosis. Pooling of samples from up to 30 different ferrets with NVL did not reduce the sensitivity of detecting M. bovis infected populations.

CONCLUSION

Culturing of pools of lymph node samples detected a significant proportion of M. bovis-infected ferrets that would otherwise have gone unnoticed based on samples that had only macroscopic lesions. Culturing of samples pooled from up to 30 different ferrets could provide significant cost savings in surveys of wildlife for the presence of M. bovis infection without any apparent loss of sensitivity.

The efficacy of live tuberculosis vaccines after presensitization with Mycobacterium avium

The variable efficacy of BCG in humans has been extensively documented but its cause is still not well understood. One possible reason for this variation is the effect of presensitization with environmental mycobacteria. To investigate in guinea pigs the effects of presensitization with well characterized Mycobacterium avium strains on the vaccine efficacy of BCG and of two recently developed, avirulent strains of Mycobacterium bovis. Two strains of M. avium containing the DNA insertion element IS901 (M. avium+) and two strains not containing this element (M. avium-) were inoculated subcutaneously or by oral administration into guinea pigs to assess their virulence in these animals and their ability to induce delayed type hypersensitivity to tuberculins. Subsequently, groups of guinea pigs presensitized with orally administered M. avium+ and M. avium- and a control group were vaccinated with BCG, or one of two newly attenuated strains of M. bovis. All groups were then challenged by the aerosol route with virulent M. bovis. Vaccine efficacy was assessed 5 weeks later by the presence of macroscopic lesions and bacterial counts of spleen and lung. No macroscopic lesions were observed in any of the guinea pigs inoculated with strains of M. avium+ or M. avium- and all animals gave delayed-type hypersensitivity skin-test reactions to avian PPD. In the vaccine experiment, presensitization with orally administered M. avium+ alone produced a low level of protection against subsequent challenge with virulent M. bovis. In the absence of presensitization with M. avium or after presensitization with an M. avium- strain, BCG and two attenuated strains of M. bovis produced significant levels of protection. No additional protection was observed in lungs of guinea pigs presensitized with M. avium+ and subsequently vaccinated with BCG. In contrast, both newly attenuated strains of M. bovis induced significant protection in lungs after such presensitization. Presensitization of guinea pigs by the oral administration of M. avium+ provides a model for testing vaccines under conditions where the efficacy of BCG has been compromised by prior sensitization with environmental mycobacteria.

Vaccine and skin testing properties of two avirulent Mycobacterium bovis mutants with and without an additional esat-6 mutation

SETTING

Molecular techniques are now available to develop new live tuberculosis vaccines by producing avirulent strains of the Mycobacterium tuberculosis complex with known genes deleted.

OBJECTIVES

Determine if removal of esat-6 from new live tuberculosis vaccines with known attenuating mutations affects their vaccine efficacy and if it could enable the development of discriminating diagnostic tests.

DESIGN

Remove the esat-6 gene by allelic exchange from two illegitimate mutants of Mycobacterium bovis that had previously been shown to have similar vaccine efficacy to BCG in a guinea pig vaccination model. Determine the effect this removal has on virulence, vaccine efficacy and skin test reactivity in guinea pigs.

RESULTS

Two double knockout strains of M. bovis were produced and their virulence and vaccine efficacy were compared to their parent strains. Removal of the esat-6 gene had no significant effect on vaccine efficacy. In skin tests, animals inoculated with the double knockout strains reacted to PPD but not ESAT-6, whereas those inoculated with the parent strains had similar skin test reactivity to both PPD and esat-6.

CONCLUSION

Removal of esat-6 from new live tuberculosis vaccine candidates has no significant effect on vaccine properties but does enable the use of skin tests to distinguish between vaccination and infection.

Use of mycobacterial peptides and recombinant proteins for the diagnosis of bovine tuberculosis in skin test-positive cattle

More accurate tests are required to test cattle which have reacted positively in the tuberculin skin test. For this purpose, a range of mycobacterial antigens, MPB59, MPB64, MPB70, MPB83, ESAT-6 and CFP10, were used either as recombinant proteins or as synthetic peptides in the whole blood interferon-gamma (IFN-gamma) test. Groups of uninfected cattle with typical 'non-specificity' problems were targeted, in particular animals with skin tuberculosis, animals vaccinated against Johne's disease and animals that were positive in the standard purified protein derivative (PPD)-based IFN-gamma test. The two study groups consisted of 74 Mycobacterium bovis-culture positive animals and 72 uninfected animals, all of which tested positive in the caudal fold tuberculin skin test eight to 28 days before the blood test. The use of combinations of ESAT-6 and CFP10 antigens, either as recombinant proteins or peptides, detected similar percentages of M bovis-infected animals as the PPD-based IFN-gamma test, but produced significantly fewer false positive reactions. The PPD-based IFN-gamma test was very effective in differentiating animals vaccinated against Johne's disease that were skin-test positive from those with bovine tuberculosis, and the use of PPD or specific mycobacterial antigens minimised the number of false positive reactions in animals with skin tuberculosis.

A DNA prime-live vaccine boost strategy in mice can augment IFN-gamma responses to mycobacterial antigens but does not increase the protective efficacy of two attenuated strains of Mycobacterium bovis against bovine tuberculosis

The Mycobacterium bovis bacille Calmette-Guérin (BCG) vaccine has variable efficacy for both human and bovine tuberculosis. There is a need for improved vaccines or vaccine strategies for control of these diseases. A recently developed prime-boost strategy was investigated for vaccination against M. bovis infection in mice. BALB/c and C57BL/6 mice were primed with a DNA vaccine, expressing two mycobacterial antigens, ESAT-6 and antigen 85 A and boosted with attenuated M. bovis strains, BCG or WAg520, a newly attenuated strain, prior to aerosol challenge. Before challenge, the antigen-specific production of interferon-gamma (IFN-gamma) was evaluated by ELISPOT and antibody responses were measured. The prime-boost stimulated an increase in the numbers of IFN-gamma producing cells compared with DNA or live vaccination alone, but this varied according to the attenuated vaccine strain, time of challenge and the strain of mouse used. Animals vaccinated with DNA alone generated the strongest antibody response to mycobacterial antigens, which was predominantly IgG1. BCG and WAg520 alone generally gave a 1-2 log10 reduction in bacterial load in lungs or spleen, compared to non-vaccinated or plasmid DNA only control groups. The prime-boost regimen was not more effective than BCG or WAg520 alone. These observations demonstrate the comparable efficacy of BCG and WAg520 in a mouse model of bovine tuberculosis. However, priming with the DNA vaccine and boosting with an attenuated M. bovis vaccine enhanced IFN-gamma immune responses compared to vaccinating with an attenuated M. bovis vaccine alone, but did not increase protection against a virulent M. bovis infection.

Vaccination with DNA vaccines encoding MPB70 or MPB83 or a MPB70 DNA prime-protein boost does not protect cattle against bovine tuberculosis

SETTING

Bovine tuberculosis is a problem in a number of countries and protection of cattle by vaccination could be an important control strategy.

OBJECTIVES

To determine the ability of DNA vaccines, which express the mycobacterial antigens MPB83 and MPB70 and a DNA prime-protein boost strategy to stimulate immune responses in cattle and protect against bovine tuberculosis.

DESIGN

Groups of cattle (n=10) were vaccinated with MPB83 DNA, MPB70 DNA, or MPB70 DNA followed by MPB70 protein or injected with BCG or control plasmid DNA. Animals were challenged intratracheally with virulent Mycobacterium bovis at 13 weeks and protection assessed 17 weeks later at postmortem.

RESULTS

In contrast to the strong cellular immune responses induced by BCG, the DNA vaccines induced minimal interferon-gamma (IFN-gamma) and interleukin-2 (IL-2) responses. Cattle primed with MPB70 DNA and boosted with MPB70 protein induced a strong antibody response and a weak IFN-gamma response. BCG gave significant reduction in four pathological parameters of disease while the DNA vaccines and MPB70 DNA/protein did not protect animals against challenge with M. bovis. Moreover, cattle vaccinated with MPB70 DNA/protein had a significantly higher proportion of animals with severe lung lesions (>100 lesions) than the MPB70 DNA alone or the control group. Increased bovine PPD-specific IL-4 mRNA expression in cattle, post-challenge, correlated with the presence of tuberculous lung lesions.

CONCLUSION

Vaccination of calves with MPB70 or MPB83 DNA vaccines or with a more immunogenic MPB70 DNA prime-protein boost strategy did not induce protection against bovine tuberculosis.

Production of avirulent mutants of Mycobacterium bovis with vaccine properties by the use of illegitimate recombination and screening of stationary-phase cultures

A better tuberculosis vaccine is urgently required to control the continuing epidemic. Molecular techniques are now available to produce a better live vaccine than BCG by producing avirulent strains of the Mycobacterium tuberculosis complex with known gene deletions. In this study, 1000 illegitimate recombinants of Mycobacterium bovis were produced by illegitimate recombination with fragments of mycobacterial DNA containing a kanamycin resistance gene. Eight recombinant strains were selected on the basis of their inability to grow when stationary-phase cultures were inoculated into minimal medium. Five of these recombinants were found to be avirulent when inoculated into guinea pigs. Two of the avirulent recombinants produced vaccine efficacy comparable to BCG against an aerosol challenge in guinea pigs with M. bovis. One of these recombinants had an inactivated glnA2 gene encoding a putative glutamine synthetase. Transcriptional analysis showed that inactivation of glnA2 did not affect expression of the downstream glnE gene. The other recombinant had a block of 12 genes deleted, including the sigma factor gene sigG. Two avirulent recombinants with an inactivated pckA gene, encoding phosphoenolpyruvate carboxykinase which catalyses the first step of gluconeogenesis, induced poor protection against tuberculosis. It is clear that live avirulent strains of the M. tuberculosis complex vary widely in their ability as vaccines to protect against tuberculosis. Improved models may be required to more clearly determine the difference in protective effect between BCG and potential new tuberculosis vaccines.

New generation vaccines and delivery systems for control of bovine tuberculosis in cattle and wildlife

Advances in the understanding of protective immune responses to tuberculosis are providing opportunities for the rational development of improved vaccines for bovine tuberculosis. Protection requires activation of macrophages through stimulation of a Th 1 type immune response. Ideally, a vaccine for cattle should induce protection without causing animals to react in a tuberculin test when exposed to Mycobacterium bovis. A number of new tuberculosis vaccines including attenuated M. bovis strains, killed mycobacteria, protein and DNA vaccines have been developed and many of these are being assessed in cattle. The requirements for a tuberculosis vaccine for wildlife differ from those for cattle. The major goal of a wildlife vaccine is to prevent the transmission of M. bovis to cattle and other wildlife. Although there are a number of technical problems associated with the development of a vaccine delivery system for wildlife, attenuated M. bovis vaccines administered via oral baits or aerosol spray to possums have already been shown to reduce the severity of a subsequent M. bovis infection.

Tuberculosis in free-ranging wildlife: detection, diagnosis and management

Mycobacterium bovis is emerging as an important pathogen of free-ranging wildlife in which it is a potential source of infection for domestic animals and a threat to valuable wildlife species. This review examines the procedures for the detection, diagnosis and management of M. bovis in wildlife populations. The ante-mortem detection of M. bovis infection in wildlife is difficult, due to the common occurrence of subclinical infections and the deficiencies of the currently available diagnostic tests. Serological tests are insensitive, while tests measuring cell-mediated immune responses show promise, but have not been sufficiently developed for routine use in most species. The diagnosis of M. bovis in free-ranging wildlife relies on post-mortem examination supported by histopathology and microbiology. A feature of M. bovis infections is the variation in the appearance and distribution of lesions in the different host species. Bacterial culture remains the gold standard for diagnosis of tuberculosis, while histopathology is limited by the frequent inability to distinguish lesions caused by M. bovis from those produced by other mycobacterial species. Deoxyribonucleic acid (DNA) fingerprinting and advanced typing techniques are increasingly being used to unravel the epidemiology of mycobacterial infections, including tuberculosis in free-ranging wildlife. An understanding of the epidemiology is essential if procedures are to be developed for the management of tuberculosis in wildlife. Few management options are currently available, especially for protected wildlife. Vaccination is the subject of much research, but further developments are required before it can be used to control tuberculosis in any animals, let alone in free-ranging wildlife.

Natural transmission ofMycobacterium bovisinfection in captive brushtail possums (Trichosurus vulpecula)

AIMS

To examine natural transmission of bovine tuberculosis (Mycobacterium bovis infection) in captive brushtail possums (Trichosurus vulpecula) and to determine if this mode of transmission could be employed to challenge possums in vaccination studies.

METHODS

Three experiments were conducted. In Experiment 1, 11 pairs of possums were housed together in cages, one of the pair having been experimentally infected with M. bovis. Of the in-contact possums 5/11 had been vaccinated with bacille Calmette-Guerin (BCG). In Experiment 2, three susceptible possums were placed in a colony of 19 possums that had been experimentally infected with M. bovis. In Experiment 3, the four most socially active possums in each of two colonies (24 possums in one colony and 23 in the other) were experimentally infected with M. bovis, and 10 of the remaining possums in each colony were vaccinated with BCG.

RESULTS

In Experiment 1, transmission of M. bovis infection occurred in only 1/11 pairs. In Experiment 2, none of the three in-contact possums became infected. In Experiment 3, infection was transmitted to 5/20 in-contact possums in one colony and 12/19 in-contact possums in the other. The possums that became infected by natural transmission were significantly more socially interactive than those that remained free of infection (p<0.05).

CONCLUSIONS

When susceptible and infected possums were randomly mixed, the rate of transmission of M. bovis was low, but when highly sociable possums were infected the rate of transmission increased markedly. The risk of transmission was dependent on the close proximity of infected and susceptible possums and the frequency and duration of their social interactions. Natural transmission from experimentally infected to incontact possums in a colony would be a useful way of studying the pathogenesis of tuberculosis in this species, and the social behaviour of the possums studied should be taken into account. The high degree of variation in the rate of natural transmission of M. bovis infection between possums makes this mode of transmission unreliable for assessing vaccine efficacy.

Johne's disease in New Zealand: the past, present and a glimpse into the future

Johne's disease is endemic in cattle and sheep in New Zealand and is spreading in farmed deer. - Procedures for the control of Johne's disease in New Zealand have been sporadic and often ineffective. Characterisation of strains of Mycobacterium paratuberculosis using molecular biological techniques have revealed two distinct strain types. One type has been found in cattle, goats and deer, and the other in sheep, goats and deer. Vaccination has been identified as an important method for the control of Johne's disease in New Zealand but the side effects of the current oil-based vaccines severely limit their use. There is increasing international concern that M. paratuberculosis may cause Crohn's disease in humans. Demands for food free of M. paratuberculosis would have very serious consequences for the meat and dairy industries in New Zealand and overseas. The cattle, sheep, goat and deer industries need to jointly develop a strategy for the control of Johne's disease in New Zealand.

Effect of turbulent-flow pasteurization on survival of Mycobacterium avium subsp. paratuberculosis added to raw milk

A pilot-scale pasteurizer operating under validated turbulent flow (Reynolds number, 11,050) was used to study the heat sensitivity of Mycobacterium avium subsp. paratuberculosis added to raw milk. The ATCC 19698 type strain, ATCC 43015 (Linda, human isolate), and three bovine isolates were heated in raw whole milk for 15 s at 63, 66, 69, and 72 degrees C in duplicate trials. No strains survived at 72 degrees C for 15 s; and only one strain survived at 69 degrees C. Means of pooled D values (decimal reduction times) at 63 and 66 degrees C were 15.0 +/- 2.8 s (95% confidence interval) and 5.9 +/- 0.7 s (95% confidence interval), respectively. The mean extrapolated D72 degrees C was <2.03 s. This was equivalent to a >7 log10 kill at 72 degrees C for 15 s (95% confidence interval). The mean Z value (degrees required for the decimal reduction time to traverse one log cycle) was 8.6 degrees C. These five strains showed similar survival whether recovery was on Herrold's egg yolk medium containing mycobactin or by a radiometric culture method (BACTEC). Milk was inoculated with fresh fecal material from a high-level fecal shedder with clinical Johne's disease. After heating at 72 degrees C for 15 s, the minimum M. avium subsp. paratuberculosis kill was >4 log10. Properly maintained and operated equipment should ensure the absence of viable M. avium subsp. paratuberculosis in retail milk and other pasteurized dairy products. An additional safeguard is the widespread commercial practice of pasteurizing 1.5 to 2 degrees above 72 degrees C.

Use of ESAT-6 in the interferon-gamma test for diagnosis of bovine tuberculosis following skin testing

The whole blood interferon-gamma (IFN-gamma) test has proven to be a practical ancillary test for re-testing cattle for bovine tuberculosis 8-28 days following tuberculin skin testing. An improvement in the specificity of the IFN-gamma test could further reduce culling of false positive animals. The primary aim of this study was to evaluate a single mycobacterial antigen, ESAT-6 in the IFN-gamma test for use in skin test-positive cattle. These skin test-positive cattle comprised 51 Mycobacterium bovis-infected animals from tuberculosis-infected herds and 85 non-infected animals from tuberculosis-free herds. The test based on ESAT-6 had a higher specificity than the test based on purified protein derivative (PPD) tuberculin, but this was offset by a small decrease in sensitivity. Use of a lower cut-off in the ESAT-6-based test improved the sensitivity, while still maintaining a very high specificity. A secondary aim in the study was to assess the ESAT-6 and PPD-based tests for detecting bovine tuberculosis in skin test-negative animals from a persistently infected herd. The PPD-based test detected the majority of the lesioned or M. bovis-culture positive animals, while the ESAT-6-based test detected a smaller proportion. The false negatives in the IFN-gamma test from both the skin test-negative and positive groups were predominantly M. bovis-culture positive animals with no visible lesions. The current study has shown that a defined specific antigen such as ESAT-6 can markedly improve the specificity of the IFN-gamma test for re-testing skin test-positive animals. An ESAT-6-based IFN-gamma test could be particularly useful to reduce the false positive rate, yet still maintain an acceptable level of sensitivity.

Mycobacterium bovis in free-living and captive wildlife, including farmed deer

Mycobacterium bovis has been isolated from a wide range of wildlife species, in addition to domestic animals. This review examines the role played by various species in the maintenance of M. bovis in wildlife communities and the spread to domestic animals. Badgers (Meles meles), brushtail possums (Trichosurus vulpecula), deer (Odocoileus virginianus), bison (Bison bison) and African buffalo (Syncerus caffer) are examples of wildlife that are maintenance hosts of M. bovis. The importance of these hosts has been highlighted by the growing realisation that these animals can represent the principal source of infection for both domestic animals and protected wildlife species. The range of methods for controlling M. bovis in wildlife is limited. While population control has been used in some countries, this approach is not applicable in many situations where protected wildlife species are concerned. Vaccination is a potential alternative control method, although as yet, no practical, effective system has been developed for vaccinating wildlife against bovine tuberculosis. Tuberculosis caused by M. bovis has also been a problem in captive wildlife and in recently domesticated animals such as farmed deer. Control of M. bovis in this group of animals is dependent on the judicious use of diagnostic tests and the application of sound disease control principles. The advances in the development of bovine tuberculosis vaccines for cattle and farmed deer may offer valuable insights into the use of vaccination for the control of tuberculosis in a range of captive wildlife species.

An esat6 knockout mutant of Mycobacterium bovis produced by homologous recombination will contribute to the development of a live tuberculosis vaccine

SETTING

Strains of the Mycobacterium tuberculosis complex are being rationally attenuated in order to develop better tuberculosis vaccines than BCG, and it would be helpful if new vaccines lacked an immunogenic protein which could be used as a skin test reagent for determining infection status.

OBJECTIVE

To delete the esat6 gene from a virulent Mycobacterium bovis strain and determine (i) whether this mutant sensitizes guinea pigs to a skin test based on ESAT6 and (ii) what effect this has on the virulence of M. bovis.

DESIGN

An homologous recombination technique was used to produce an esat6 knockout mutant of a virulent strain of M. bovis. Guinea pigs were inoculated with either the mutant or parent strain and their reactivity in intradermal skin tests was determined to bovine purified protein derivative (PPD) and recombinant ESAT6 protein.

RESULTS

Production of an esat6 knockout strain was demonstrated by Southern blot hybridization and the polymerase chain reaction. Guinea pigs inoculated with either the esat6 knockout strain or its virulent parent had positive skin test reactions to PPD but only animal inoculated with the parent strain had positive skin test reactions to ESAT6. Gross pathology, histopathology and mycobacterial culture of tissues indicated that the knockout strain was less virulent than its parent.

CONCLUSION

If an effective live tuberculosis vaccine can be produced by inactivation of virulence genes in M. bovis, then prior or subsequent knockout of the esat6 gene could contribute to the loss of virulence and enable the development of a test to distinguish between vaccinated and infected animals.

Vaccination of cattle with Mycobacterium bovis culture filtrate proteins and interleukin-2 for protection against bovine tuberculosis

In this study vaccines prepared from culture filtrate proteins (CFP) of Mycobacterium bovis and interleukin-2 (IL-2) were tested in cattle for their capacity to stimulate immune responses and to protect against an intratracheal challenge with virulent M. bovis. Nine groups of cattle were vaccinated with combinations of different doses of CFP and bovine IL-2 mixed with a monophosphoryl lipid A (MPL) adjuvant. An additional group was vaccinated with M. bovis BCG. Immune responses in b1P-IL-2-vaccinated animals differed from those seen in BCG-vaccinated animals by inducing high antigen-specific antibody responses and low levels of gamma interferon and IL-2 released from purified protein derivative-stimulated whole-blood cultures. In a concurrent experiment, additional animals were added to the high-dose CFP-IL-2, MPL control, and BCG groups and these expanded groups of animals were challenged intratracheally with virulent M. bovis. Although the lung lesion scores were significantly lower for both the CFP-IL-2-and BCG-vaccinated groups compared to the MPL control group, the overall level of protection was greatest for the BCG-vaccinated animals. There were more animals with extrathoracic spread of disease in the CFP-IL-2 group than in the other groups. While vaccination of cattle with M. bovis CFP gave an encouraging reduction in tuberculous lesions and did not induce a delayed-type hypersensitivity response to PPD, future CFP vaccines must prevent any extrathoracic spread of disease.

Construction and screening of Mycobacterium paratuberculosis insertional mutant libraries

Mycobacterium paratuberculosis causes Johne's disease, a common wasting disease in ruminants. As a first step in studying virulence mechanisms, libraries of random mutants were produced in two M. paratuberculosis strains by using the conditionally replicating shuttle phasmid phAE94 which contains the transposon Tn5367. Two thousand mutants were screened for auxotrophy, carbon source preference, and altered cell wall. Genes interrupted by insertion were identified for seven mutants isolated from the screening process. Two mutants had insertions in putative genes involved in synthesis of the cell wall.

Immune responses induced in cattle by virulent and attenuated Mycobacterium bovis strains: correlation of delayed-type hypersensitivity with ability of strains to grow in macrophages

Comparison of immune responses induced in cattle by virulent and attenuated strains of Mycobacterium bovis will assist in identifying responses associated with resistance or susceptibility to disease. Four strains of M. bovis, one which is virulent in guinea pigs (WAg201) and three which are attenuated in guinea pigs (an isoniazid-resistant strain [WAg405], ATCC 35721, and BCG) were compared for their abilities to induce immune responses in cattle and to grow in bovine lung alveolar macrophage cultures. Extensive macroscopic lesions were found only in cattle inoculated with the virulent M. bovis strain. Strong antibody responses to M. bovis culture filtrate, as well as persistently high levels of gamma interferon and interleukin-2 released from purified protein derivative (PPD)-stimulated peripheral blood lymphocyte cultures, were observed in the cattle inoculated with the virulent strain compared to those inoculated with the attenuated strains. All cattle inoculated with the virulent strain or two of the attenuated strains (WAg405 and ATCC 35721) elicited strong delayed-type hypersensitivity responses to PPD in skin tests, while animals inoculated with BCG induced only a weak response. The three strains which produced strong skin test responses proliferated well in bovine alveolar macrophages and induced high levels of proinflammatory cytokine mRNAs compared to BCG. Our study showed that skin test responsiveness to PPD correlated with the ability of the strains to grow in alveolar macrophages rather than to their pathogenicity in cattle.