Experimental infection of badgers (Meles meles) with Mycobacterium bovis

Quantitative interferon-gamma responses predict future disease progression in badgers naturally infected with Mycobacterium bovis

The diagnosis and control of Mycobacterium bovis infection (bovine tuberculosis: TB) continues to present huge challenges to the British cattle industry. A clearer understanding of the magnitude and duration of immune response to M. bovis infection in the European badger (Meles meles) – a wildlife maintenance host – may assist with the future development of diagnostic tests, and vaccination and disease management strategies. Here, we analyse 5280 diagnostic test results from 550 live wild badgers from a naturally-infected population to investigate whether one diagnostic test (a gamma interferon release [IFNγ] assay, n = 550 tests) could be used to predict future positive results on two other tests for the same disease (a serological test [n = 2342 tests] and mycobacterial culture [n = 2388 tests]) and hence act as an indicator of likely bacterial excretion or disease progression. Badgers with the highest IFNγ optical density (OD) values were most likely to subsequently test positive on both serological and culture tests, and this effect was detectable for up to 24 months after the IFNγ test. Furthermore, the higher the original IFNγ OD value, the greater the chance that a badger would subsequently test positive using serology. Relationships between IFNγ titres and mycobacterial culture results from different types of clinical sample suggest that the route of infection may affect the magnitude of immune response in badgers. These findings identify further value in the IFNγ test as a useful research tool, as it may help us to target studies at animals and groups that are most likely to succumb to more progressive disease.

Association of quantitative interferon-γ responses with the progression of naturally acquired Mycobacterium bovis infection in wild European badgers (Meles meles)

Bovine tuberculosis is one of the biggest challenges facing cattle farming in Great Britain. European badgers (Meles meles) are a reservoir host for the causal agent, Mycobacterium bovis. There have been significant recent advances in diagnostic testing for tuberculosis in humans, cattle and badgers, with the development of species-specific assays for interferon-γ (IFN-γ), an important cytokine in tuberculous infections. Using data collected from longitudinal studies of naturally infected wild badgers, we report that the magnitude of the IFN-γ response to M. bovis antigens at the disclosing test event was positively correlated with subsequent progression of disease to a seropositive or excreting state. In addition, we show that the magnitude of the IFN-γ response, despite fluctuation, declined with time after the disclosing event for all badgers, but remained significantly higher in those animals with evidence of disease progression. We discuss how our findings may be related to the immunopathogenesis of natural M. bovis infection in badgers.

Mycobacterium bovis infection in the Eurasian badger (Meles meles): the disease, pathogenesis, epidemiology and control

Eurasian badgers (Meles meles) are an important wildlife reservoir of tuberculosis (Mycobacterium bovis) infection in Ireland and the United Kingdom. As part of national programmes to control tuberculosis in livestock, considerable effort has been devoted to studying the disease in badgers and this has lead to a rapid increase in our knowledge of tuberculosis in this host. Tuberculosis in badgers is a chronic infection and in a naturally-infected population the severity of disease can vary widely, from latent infection (infection without clinical signs and no visible lesions) to severe disease with generalized pathology. The high prevalence of pulmonary infection strongly supports the lungs as the principal site of primary infection and that inhalation of infectious aerosol particles is the principal mode of transmission. However, other routes, including transmission via infected bite wounds, are known to occur. The ante-mortem diagnosis of infection is difficult to achieve, as clinical examination and immunological and bacteriological examination of clinical samples are insensitive diagnostic procedures. Because infection in the majority of badgers is latent, the gross post-mortem diagnosis is also insensitive. A definitive diagnosis can only be made by the isolation of M. bovis. However, to gain a high level of sensitivity in the bacteriological examination, a large number of tissues from each badger must be cultured and sensitive culture methods employed. The transmission and maintenance of M. bovis in badger populations are complex processes where many factors influence within-population prevalence and rates of transmission. Badger social structures and the longevity of infected animals make them an ideal maintenance host for M. bovis infection. Badgers are directly implicated in the transmission of infection to cattle and the inability to eradicate the disease from cattle is, in part, a consequence of the interactions between the two species. A detailed understanding and knowledge of the epidemiology and pathogenesis of the disease are recognized as fundamental for devising new strategies to control infection with a view to limiting interspecies transmission. Vaccination, in spite of formidable challenges, is seen as the best long-term strategy option and studies with captive badgers have shown that vaccination with M. bovis bacillus Calmette-Guérin (BCG) induces protection when delivered by a variety of routes. Continued research is required to develop effective technologies to control the disease both in badgers and cattle. A combination of strategies, which employ the optimal use and targeting of resources, is likely to make a significant contribution towards eradication of the disease.

Tuberculosis in badgers; a review of the disease and its significance for other animals

This review examines the current state of knowledge of aspects of tuberculosis in the badger. The gross pathology and pathogenesis are elaborated as well as the immune mechanism, diagnosis of infection and excretion and viability of infected products. The epidemiology in badgers is considered, as is the significance of infection in this species for other wildlife species as well as domestic animals sharing the same habitat. Trials of the effects of the removal of badgers on the occurrence of tuberculosis in cattle are summarised. It is concluded that badgers are well adapted as the primary host of bovine tuberculosis in parts of Britain and much of Ireland.

Immunological approaches to the control of tuberculosis in wildlife reservoirs

Attempts to eradicate tuberculosis from cattle and farmed deer in some countries have been frustrated by the existence of wildlife reservoirs of Mycobacterium bovis infection. Possum control programmes in New Zealand using poisons have shown clearly that the brushtail possum is an important source of infection for cattle and farmed deer, and the sum of evidence strongly suggests that badgers serve as a source of infection for cattle in the UK. Bovine tuberculosis can only be eradicated from these countries by controlling M. bovis infection in both wildlife and domestic animals. The most promising options for control of M. bovis infection in wildlife in the longer term include the development of a tuberculosis vaccine for wildlife and a strategy for biological control of possums. The aim of this review is to address the problems and approaches involved in the control of wildlife tuberculosis from an immunological perspective.

A lymphocyte transformation assay for the detection of Mycobacterium bovis infection in the Eurasian badger (Meles meles)

The Eurasian badger (Meles meles) is a significant wildlife reservoir of Mycobacterium bovis in Great Britain. Improved control strategies against the disease in badgers require the development of diagnostic tests and vaccines. Here, we report the development of a comparative lymphocyte transformation assay (LTA) using bovine and avian tuberculin as antigen to detect cell-mediated responses in M. bovis-infected badgers. In a pilot study, the performance of this assay was compared with the existing indirect ELISA assay for the detection of tuberculous badgers. The sensitivity of the Comparative LTA was 87.5% compared with 62.5% for the indirect ELISA whereas the ELISA test gave a greater specificity (100% compared with 84.6% for the comparative LTA). Preliminary evidence suggests that for the comparative LTA, the blood may be stored overnight prior to testing and that this procedure might improve the specificity of the assay without compromising the sensitivity.

Vaccination of the badger (Meles meles) against Mycobacterium bovis

This review summarizes current knowledge relating to the potential development of a vaccine against Mycobacterium bovis (M. bovis) for use in badgers. The serious economic loss incurred from bovine tuberculosis in Ireland is highlighted. The problems presented to national bovine tuberculosis eradication programmes by wildlife reservoirs of M bovis, in particular the badger and the brushtail possum, are outlined. Badger and possum ecology and the epidemiology of M. bovis infection in these species are compared in relation to control strategies. Current information regarding the immune responses of badgers and possums to M. bovis infection is presented. Vaccine types, carrier and adjuvant systems, available vaccines for human and animal tuberculosis and delivery to wildlife species are described. The feasibility of vaccinating badgers against M. bovis infection and the possible options for a vaccination strategy are discussed.

The Effect of Translocation on a Social Group of Badgers (Meles Meles)

A social group of six badgers (Meles meles) (four adults and two cubs) was translocated from urban Bexhill, East Sussex, in August 1993 to a 1216m2 electrified enclosure in a part of Suffolk largely unoccupied by badgers. Three adult badgers (SY2, SY5 and SY6) escaped from the release site prior to the removal of the perimeter fence on 10 December and established a sett near a village, 2.9km from the release site. In January 1994, the remaining adult (SY4) left the release site and moved 1.8km to the grounds of a youth detention centre. The cubs did not desert the site as readily as the adults.

Home-range sizes for two adult females, SY4 and SY6, remained relatively constant, while that of adult male SY2 increased from 50ha in February to nearly 400ha in April. The range of SY2 overlapped parts of the ranges of the two females, although SY4 and SY6's ranges never overlapped.

The percentage volume of scavenged food in the diet increased monthly between February and April which corresponded to increased garden activity over this period. Earthworms were the most important item in the diet. The establishment of both main setts near housing and the preference for foraging in gardens suggests that badgers released into novel environments may search for familiar habitats.

It is concluded that translocation can successfully establish badgers at new locations. However, translocation as a solution to problems caused by badgers must only be viewed as a last resort, not least due to the potential for disease spread.

The epidemiology ofMycobacterium bovisinfection in brushtail possums (Trichosurus vulpeculaKerr) in the Hauhungaroa Ranges, New Zealand

The prevalence of Mycobacterium bovis infection in the wild possum population around the perimeter of the Hauhungaroa Ranges, New Zealand, was determined by a cross-sectional study, and risk factors associated with tuberculosis were identified. Of 6083 possums necropsied, 128 (2.1%) showed gross lesions suggestive of tuberculosis infection, and 76 (1.25%) were subsequently confirmed as tuberculous on histopathological examination. Considering only traplines where tuberculosis was detected, adult possums were 1.9 times as likely to be infected as immature animals, and the total prevalence was 5.4% in males compared with 3.9% in females. Adult females were 3.64 times as likely to be infected as immature females, whereas there was no significant age difference for males (odds ratio = 1.46, p=O.29). Immature males were 3.12 times as likely to be infected as immature females. Possums in poor condition were more likely to be found infected than possums in good condition. Tuberculous possums were found in 27 local clusters of infection. The correlation between the prevalence of tuberculosis in possums in zones and the incidence of tuberculosis in cattle on adjoining properties was 0.4 (p<O.05).

The occurrence of Mycobacterium bovis infection in cattle in and around an area subject to extensive badger (Meles meles) control

The occurrence of Mycobacterium bovis infection in cattle herds during the period 1966-92 in two geographically related areas in South-West England is compared. In one area comprising 104 km2 all badgers were systematically destroyed from 1975-81, after which recolonization was allowed; in the other, comprising 116 km2, small scale, statutory badger removal operations were undertaken from 1975 onwards where specific herds were detected with M. bovis infection. In the area with total clearance, no further incidents with M. bovis isolation occurred from 1982-92. Survival analysis and proportional hazards regression indicated that the risk of herds being identified with infection was less once badgers had been cleared from their neighbourhood, whereas it was greater in herds with 50 or more animals, and once cattle in a herd had responded positively to the tuberculin skin test, even though infection with M. bovis was not confirmed subsequently. The study provides further evidence that badgers represent an important reservoir of M. bovis infection for cattle and that badger control is effective in reducing incidents of cattle infection with M. bovis if action is thorough and recolonization is prevented.

Tuberculosis in the brushtail possum (Trichosurus vulpecula) after intratracheal inoculation with a low dose of Mycobacterium bovis

Six possums were each inoculated with approximately 125 colony-forming units of Mycobacterium bovis via a cannula inserted per os into the trachea. Five other possums were sham inoculated and housed separately. At postmortem examination 55 to 57 days after inoculation, all six infected possums showed extensive macroscopical lesions of tuberculosis in the lungs and bronchial lymph nodes and some also had lesions in the liver, kidney, spleen and hepatic lymph nodes. Mycobacterium bovis was isolated from all of these possums. No evidence of M. bovis infection was detected in the five control animals. In the M. bovis-infected possums, microscopical examination of organs and lymph nodes demonstrated a much wider distribution of lesions than did macroscopical examination. The location of early lesions indicated that the paracortical region of the lymph nodes, the marginal zone and periphery of lymphoid follicles in the spleen, and the cortex of the kidney were predilection sites for lesions resulting from haematogenous spread of infection. This method for reproducing bovine tuberculosis in the possum was more suitable for experimental studies than previously published methods.

Production and characterization of a monoclonal badger anti-immunoglobulin G and its use in defining the specificity of Mycobacterium bovis infection in badgers by western blot

A mouse monoclonal anti-badger IgG antibody was produced to investigate the specificity of the antibody response of badgers infected with Mycobacterium bovis. The monoclonal antibody generated was directed against badger IgG heavy chain and appeared to be species restricted, reacting only with badger and dog IgGs but not cat, rabbit, mouse, guinea pig, bovine or ferret IgGs. This monoclonal antibody detection system functioned well in both ELISA and Western blot analyses and was successfully used to investigate the humoral response of the badger to M. bovis infection. Sera from infected badgers detected a 25 kDa antigen which was not detected by sera from M. bovis culture-negative animals. This antigen was conserved in all field strains of M. bovis tested and seroconversion to it was detected during experimental infection. The immunodominance of this antigen in the badger during infection with M. bovis suggests that this 25 kDa polypeptide is a suitable candidate on which to base an antibody detection test for M. bovis infection.

Mycobacterium bovis in the European badger (Meles meles): epidemiological findings in tuberculous badgers from a naturally infected population

This study investigates the course of tuberculosis in a naturally infected badger population, its impact on the population and the risk of spread to other species in the light of capture data and post-mortem findings from 47 tuberculous badgers, stratified by age group and sex, accrued since 1975. The findings are compared with those for 260 badgers from the same population in whom no evidence of infection was detected. Detailed estimates of seasonal variations in bodyweight for uninfected male and female cub, yearling and adult badgers are presented and compared to the weights at post-mortem examination of the tuberculous badgers, in whom poor condition and weight loss were the principal presenting signs. Lesions were seen especially in the lungs and associated lymph nodes, and in the kidneys. Organisms were detected intermittently in faeces, urine, sputum and discharging bite wounds. Infected animals could survive for nearly 2 years and produce cubs successfully.

The haematological values of European badgers (Meles meles) in health and in the course of tuberculosis infection

Captive, healthy, adult badgers have blood containing haemoglobin at 13.3 g/dl, and 8.4 x 10(12)/l red cells with an MCV of 46.2 fl and an MCH of 15.6 pg. They have 5.1 x 10(9) white cells/l of which 3.29 x 10(9) are polymorphs, 1.49 x 10(9) are lymphocytes, 0.26 x 10(9) are monocytes, 0.07 x 10(9) are eosinophils and 0.01 x 10(9) are basophils. These values are somewhat less in adult animals just trapped from the wild, and are lower still in wild cubs. Changes associated with tuberculosis are a rise, and then a fall in red blood count and white blood count, an increase in the proportion of polymorphs and monocytes and a fall in lymphocytes late in the disease. This picture is similar to that seen in widespread, disseminated, tuberculin negative, tuberculosis in humans, a type of disease similar to that occurring in many badgers. BCG vaccination of badgers did not produce any measurable change in the blood picture.

The immunological consequences of challenge with bovine tubercle bacilli in badgers (Meles meles)

Optimal conditions were determined for performing antibody measurements (ELISA), lymphocyte transformation tests and, to some extent, skin tests in badgers. These parameters, together with the bacteriological and pathological studies reported previously (Pritchard et al. 1987), were used to follow the course of intradermal and intratracheal challenge of badgers with bovine tubercle bacilli. Two challenge doses were used for each route of infection and two animals received each dose. None of the four animals challenged by the intratracheal method showed any evidence of infection, suggesting that adult badgers may have some resistance to challenge by this method. All four animals challenged intradermally developed lesion of tuberculosis. Immunologically the disease passed through three phases. There was an early phase in which lymphocyte transformation to whole BCG steadily and significantly increased, and skin tests to tuberculin became positive but there was little change in antibody levels. This was followed by an intermediate phase of variable skin responses, fluctuating lymphocyte transformation and significant increase in antibody levels. The final phase, which was only seen in two animals with extensive disease, was associated with changing skin reactions and falling lymphocyte responses, together with a sudden increase in antibody levels. This paper presents the first formal evidence of cell-mediated immunity to tuberculosis in the badger, which may delay onset and prolong the survival of challenged animals.