Evaluation of a field test for trypanotolerance in young N'Dama cattle

Development of Cathepsin L-like Real-Time PCR Assays for the Detection of African Animal Trypanosomosis (AAT) in South Africa

African animal trypanosomosis (AAT), is an infectious parasitic disease of wildlife and livestock caused by multiple species and strains of Trypanosoma. In South Africa, it is restricted to northern KwaZulu-Natal (NKZN) and caused by Trypanosoma congolense and Trypanosoma vivax. A cross-sectional study was done to determine AAT prevalence in 384 goat samples and identify trypanosome species circulating in 60 cattle at dip tanks that are on the interface with the Hluhluwe-uMfolozi game reserve in NKZN. Both cattle and goat samples were analyzed using the buffy coat technique (BCT) and a polymerase chain reaction (PCR) assay targeting the internal transcribed spacer 1 (ITS) region. Cattle samples were further analyzed using an ITS quantitative real-time PCR (qPCR) assays designed for the detection of T. congolense, T. vivax, and T. brucei. None of the goat samples tested positive for Trypanosoma infections. The ITS qPCR assay detected Trypanosoma DNA in 30% of the cattle samples, while only 8.3% were positive with the ITS PCR and 11.7% were positive using BCT. Quantitative real-time PCR assays were designed to amplify a 98 bp, 137 bp, and 116 bp fragment of the cathepsin L-like (CATL) gene from T. brucei, T. theileri, and T. congolense, respectively. Each assay was shown to be efficient (>94%) and specific (10⁹ to 10²/10¹ copies/reaction) in the detection of Trypanosoma species. The CATL qPCR assays detected T. congolense and T. theileri infections in 33.3% of the cattle samples. The CATL qPCR assays also detected T. congolense infections in goats (23.1%) that were neither detected by BCT nor the ITS PCR. The CATL qPCR assays provide an additional, sensitive, and specific tool for Trypanosoma diagnostics. The presence of trypanosomes in goats suggests they might be potential reservoirs of infections to other livestock.

African animal trypanosomosis (nagana) in northern KwaZulu-Natal, South Africa: Strategic treatment of cattle on a farm in endemic area

African animal trypanosomosis (AAT) is caused by several species of the genus Trypanosoma, a parasitic protozoan infecting domestic and wild animals. One of the major effects of infection with pathogenic trypanosome is anaemia. Currently, the control policies for tsetse and trypanosomosis are less effective in South Africa. The only response was to block treat all infected herds and change the dip chemical to one which controls tsetse flies during severe outbreaks. This policy proved to be less effective as demonstrated by the current high level of trypanosome infections in cattle. Our objective was to study the impacts of AAT (nagana) on animal productivity by monitoring the health of cattle herds kept in tsetse and trypanosomosis endemic areas before and after an intervention that reduces the incidence of the disease. The study was conducted on a farm in northern KwaZulu-Natal which kept a commercial cattle herd. There was no history of any cattle treatment for trypanosome. All cattle were generally in poor health condition at the start of the study though the herd received regular anthelminthic treatment. A treatment strategy using two drugs, homidium bromide (ethidium) and homidium chloride (novidium), was implemented. Cattle were monitored regularly for 13 months for herd trypanosomosis prevalence (HP), herd average packed cell volume (H-PCV) and the percentage of the herd that was anaemic (HA). A total of six odour-baited H-traps were deployed where cattle grazed from January 2006 to August 2007 to monitor the tsetse population. Glossina brevipalpis Newstead and Glossina austeni Newstead were collected continuously for the entire study period. High trypanosomes HP (44%), low average H-PCV (29.5) and HA (24%) were rerecorded in the baseline survey. All cattle in the herd received their first treatment with ethidium bromide. Regular monthly sampling of cattle for the next 142 days showed a decline in HP of 2.2% – 2.8%. However, an HP of 20% was recorded by day 220 and the herd received the second treatment using novidium chloride. The HP dropped to 0.0% and HA to 0.0% by day 116 after the second treatment. The cow group was treated again by day 160 when the HP and HA were 27.3% and 11%, respectively. The same strategy was applied to the other two groups of weaners and the calves at the time when their HP reached 20%. Ethidium and novidium treatment protected cattle, that were under continuous tsetse and trypanosomosis challenge, for up to 6 months. Two to three treatments per year may be sufficient for extended protection. However, this strategy would need to be included into an integrated pest management approach combining vector control for it to be sustainable.

Trypanosomiasis challenge estimation using the diminazene aceturate (Berenil) index in Zebu in Gabon

A longitudinal study was conducted within a cattle ranch in Gabon to determine the diminazene aceturate (Berenil) index (DAI) in a group of Zebu, raised under low tsetse density; this measure providing an assessment of trypanosomiasis risk. The objective was to evaluate the trypanosomiasis pressure thus informing trypanosomiasis control methods and cattle management. Twenty female adult Zebu were monitored for 24 weeks during the dry season. Blood samples were collected on a weekly basis and subjected to parasitological and haematological analysis (n = 480), using the buffy-coat method and the packed cell volume value (PCV), respectively, infected animals were treated with a single intramuscular injection of diminazene aceturate (8 mg/kg). Twenty-nine single infectious events were recorded and a DAI of 1.45 was calculated. Two trypanosome species were identified: Trypanosoma congolense (96.2%) and Trypanosoma vivax (3.8%). The mean PCV value of the infected animals was lower (26.6) compared to non-infected animals (32.0). This study shows that DAI may be a useful tool to assess trypanosomiasis. However, this is a time-consuming method that may be improved by using randomly selected sentinel animals to adapt the chemoprophylactic schemes, hence decreasing the costs and the drug resistance risk.

Models of superinfection and acquired immunity to multiple parasite strains

Individuals in communities in which different strains of pathogen are circulating can acquire resistance by accumulating immunity to each strain. After considering susceptibility, models of infection and immunity are defined for vector-borne diseases such as malaria and trypanosomiasis. For these models the prevalence of infection, the number of infections per individual, and the mean duration of infection, increase rapidly in young individuals, but decrease in older individuals as immunity is acquired to the various strains of pathogen; the mean interval between successive infections lengthens with age. The bivariate Poisson distribution is shown to be a close approximation to some stochastic processes. The models explain observed cross-sectional patterns of age prevalence, and longitudinal patterns in which individuals typically continue to become infected as they age, albeit with decreasing frequency. In these models the time spent infected depends on parasite diversity, as well as the inoculation and recovery rates. It is shown that control measures can cause an increase in the number of infections and the prevalence of infection in older individuals, and in the average prevalence in the community, even when strain-specific immunity is life-long.

Combatting African Animal Trypanosomiasis (AAT) in livestock: The potential role of trypanotolerance

African Animal Trypanosomiasis (AAT) is endemic in at least 37 of the 54 countries in Africa. It is estimated to cause direct and indirect losses to the livestock production industry in excess of US$ 4.5 billion per annum. A century of intervention has yielded limited success, owing largely to the extraordinary complexity of the host-parasite interaction. Trypanotolerance, which refers to the inherent ability of some African livestock breeds, notably Djallonke sheep, N'Dama cattle and West African Dwarf goats, to withstand a trypanosomiasis challenge and still remain productive without any form of therapy, is an economically sustainable option for combatting this disease. Yet trypanotolerance has not been adequately exploited in the fight against AAT. In this review, we describe new insights into the genetic basis of trypanotolerance and discuss the potential of exploring this phenomenon as an integral part of the solution for AAT, particularly, in the context of African animal production systems.

Bovine trypanosomosis prevalence at the edge of Hluhluwe-iMfolozi Park, KwaZulu-Natal, South Africa

The northern KwaZulu-Natal (NKZN) region of South Africa is the southern limit of the African tsetse belt. Entomological information on Glossina brevipalpis and Glossina austeni was generated following the outbreak of trypanosomosis in cattle in 1990. However, these data have not been supported by parallel studies on epidemiology of the disease and therefore there has been no control policy in place. This study presented the first intensive investigations to address the epidemiology of trypanosomosis in NKZN. Tsetse abundance, trypanosome herd average prevalence (HAP), herd average anaemia (HAA) and herd average packed cell volume (HA-PCV) were investigated at three communal diptanks located at the edge of Hluhluwe-iMfolozi Park by monthly sampling from June 2006 - November 2007. Seasonal trypanosome surveys were conducted at seven other communal diptanks. Glossina brevipalpis prevalence was high at two of the diptanks, Mvutshini and Ekuphindisweni, but low at Ocilwane, whilst G. austeni was only collected from Mvutshini. This high and low tsetse challenge presented different disease scenarios. Cattle at Mvutshini and Ekuphindisweni had the highest HAP of 12.3% and 8.9% respectively, both significantly different (p = 0.001) from the HAP obtained from cattle at Ocilwane (2.9%). These two cattle herds also had the highest HAA, 27.7% and 33.4% respectively, whilst cattle at Ocilwane had the lowest, 11.1% (p = 0.001). Conversely, cattle at Ocilwane had the highest HA-PCV, ranging between 29.0% and 32.0%, whilst cattle at Mvutshini and Ekuphindisweni had the lowest HA-PCV (24.0% - 29.0%). By combining the data from the three diptanks (1318 observations), 62.0% of the infected cattle were found anaemic, compared to 20.0% in the uninfected group. Trypanosome seasonal surveys showed that cattle at all the seven diptanks were infected with trypanosomes; mean HAP, HAA and HA-PCV of 10.2%, 46.6% and 23.7%, respectively. This study generated information on the epidemiological factors related to the wide spread of trypanosome-infected cattle and tsetse flies. Trypanosomosis is a disease of economic importance impacting the livelihood of resource-poor farmers in NKZN.

Epidemiology of Trypanosoma vivax infection in cattle in the tse-tse free area of Lake Chad

A study was conducted in Chad to estimate the prevalence and the incidence of Trypanosoma vivax infection in herds of cattle from the Lake Chad area. The risk factors associated with disease were also identified. A random sample of 933 cattle from 17 herds was initially selected (January 1999, cold dry season). Cattle were identified by ear-tags and sampled in the rainy season (July 1999) and the cold dry season (January 2000). Each animal sampled was treated with diminazene aceturate (3.5mg/kg). Samples were examined for the presence of T. vivax using an antibody (indirect ELISA) and a parasite detection test (buffy-coat technique, BCT). Standardized questionnaires with information about the host and management practices were collected and evaluated for their association with seroprevalence (model 1) and parasitological prevalence (model 2) as indicator of host susceptibility to T. vivax infection. Risk factors were selected using two approaches: ordinary logistic regression (OLR) and generalized estimating equations (GEE) to account for within-herd correlation. The apparent prevalence was 1.6% using BCT and 42.3% with indirect ELISA. The true prevalence in the sample was estimated to (2.0%-8.0%) with two assumptions of BCT sensitivity. Overall, 58.8% (BCT) and 100.0% (ELISA) of the herds had a least one-positive animal. In January-July 1999, apparent monthly incidence was calculated at 0.24% in comparison with 0.76% for August 1999-January 2000. The true monthly incidence was estimated at 0.36%-1.43% for the first period and at 0.94%-3.78% for the second period. Risk factors associated with seroprevalence were age, race, a great number of small ruminants in the herd, and latitude and longitude of pasture area in the rainy season. Risk factors associated with BCT prevalence were duration of seasonal migration and longitude of pasture area in the rainy season. In conclusion, T. vivax is present and widely disseminated in the cattle herds of tse-tse free area of Lake Chad. Farm-level factors, particularly seasonal migration, should be considered as the main risk factors for infection and to host susceptibility to the parasite.

Quantitative phenotyping of N'Dama cattle for aspects of trypanotolerance under field tsetse challenge

Matching animal health and performance data were recorded over the 2 year period from weaning at 10 months of age on 255 N'Dama cattle in a high natural tsetse challenge situation in Zaire. Four parameters that are regarded as possible indicators of trypanotolerance, species of trypanosomes detected, length of time parasitaemic, intensity of parasitaemia (parasitaemia score), and anaemic condition as estimated by packed cell volume (PCV) values, were measured and the relative effects of changes in these parameters on trypanocidal drug requirements and growth were assessed. The effects of species of trypanosome on drug requirements and growth were directly measurable. In the case of the other three indicators, the effects on drug requirements and growth that would be brought about by a change of one standard deviation in each were calculated. This allowed comparison of similar sized changes in these three indicators that are of necessity recorded in dissimilar units. Trypanosoma vivax and Trypanosoma congolense infections had equal effects on the number of trypanocidal drug treatments required, an average of 0.61 treatments being administered to each infected post-weaner. A reduction of one standard deviation (SD) in length of time infected reduced the number of treatments required by 0.23 or 36% and an increase of 1 SD in PCV reduced the number required by 0.27 or 43%. Changes in parasitaemia score were not important. In the case of growth, a T. congolense infection reduced growth by 12.4 g day-1 or 8% more than a T. vivax infection. A reduction of 1 SD in length of time infected increased growth by 9.8 g day-1 or 6.5%, a reduction of 1 SD in parasitaemia score increased growth by 9.0 g day-1 or 6.0%, and an increase of 1 SD in average PCV increased growth by 8.4 g day-1 or 5.6%. The necessity to simultaneously measure the four criteria is clearly indicated by their approximately equal effects on the final performance trait of daily liveweight gain. Thus, absence of information on any of these criteria would significantly affect the accuracy of the estimate of an animal's overall trypanotolerance phenotype in this central African situation and reduce the progress possible in production projects involving N'Dama cattle.

Potential of trypanotolerance as a contribution to sustainable livestock production in tsetse affected Africa

Tsetse transmitted trypanosomiasis is possibly the major constraint on livestock and agriculture development in Subsaharan Africa. Control of the disease has been based on vector control as well as on the use of trypanocidal drugs to treat or prevent infection in animals. Both control methods are effective but have proven not to be sustainable. Moreover, the development of a vaccine against trypanosomiasis is unlikely to be successful in the near future. On the other hand, trypanotolerant cattle, like the N'Dama can survive and produce in tsetse affected areas without interventions. This taurine breed has been indigenous to Africa for approximately 7,000 years and forms presently about 6% of the bovine population of Africa. Generally the N'Dama are kept in the rural areas by the small-scale farmer as a multi-purpose animal. Recent studies have defined management characteristics and assessed the production potential at the village level and under ranching conditions of N'Dama cattle exposed to various levels of tsetse challenge. Furthermore, experimental infections showed conclusively the superior resistance to the effects of infection of the N'Dama cattle when compared to zebu cattle and have confirmed that trypanotolerance is innate in N'Dama cattle. Studies have been conducted on development of protective humoral and cellular responses, the regulation of parasite multiplication and control of anaemia. These studies provided tools for identifying components of trypanotolerance. The ability to resist the development of anaemia in the face of infection, has shown to be correlated with the capacity to be productive; moreover, PCV values can serve as selection criterium for trypanotolerance. Subsequently, repeatabilities and heritabilities of trypanotolerance and performance traits were estimated.(ABSTRACT TRUNCATED AT 250 WORDS)

Measurement of trypanotolerance criteria and their effect on reproductive performance of N'Dama cattle

One thousand and twenty-eight cow-year records were available from 260 N'Dama cows each having at least 2 years of monthly matching health and performance data over a 5-year period under a medium natural tsetse challenge in Gabon. Four hundred and fifty-eight calf/dam pairs were also available where the calf had been reared to weaning, both had monthly matching records and each cow had weaned at least two calves. Evaluations were carried out on effects of, and linkages between, environmental and stress factors, number and species of trypanosome infections, curative drug treatments given, anaemia measured by packed red cell volume (PCV), and performance measured by calf weaning weight, cow calving rate and cow weight change over the lactation period. Major findings were that over the period from calf birth to weaning, while calves and their dams grazing together had similar numbers of trypanosome infections detected, the Trypanosoma vivax: T. congolense ratios were very different: 1:0.7 in calves; 1:2.8 in cows. This indicated that some ability to control the development of parasitaemia following T. vivax infection might be being acquired, from weaning onwards. In cows, relationships between lowest PCV recorded and curative drug treatments given suggested that between 20 and 32% of trypanosome-infected cows were not being identified by the buffy coat parasitological diagnostic technique. The high level of curative treatment given (to 13.7% of cows over the calendar year, and to 40% of calves from birth to weaning) will have tended to reduce the variance and linkages between aspects of infection and PCV values, especially in calves. In calves, the influence of trypanosome infections, in both calf and dam, on their respective PCV values and hence on calf weaning weight was apparent. There was a 0.91 +/- 0.40 kg increase in calf weaning weight for each 1% increase in calf average PCV, and a 0.95 +/- 0.39 kg increase for each 1% increase in cow average PCV. In cows, there was a similar pathway of influence of T. congolense infection through the PCV values to calving rate--not significant with T. vivax infection. There was a 3.3 +/- 0.65% increase in calving rate for each 1% increase in average PCV. Repeatabilities of performance traits were in the normal range. Repeatabilities of numbers of trypanosome infections detected by the buffy coat technique were too low to have any practical significance. Repeatability of average PCV at 0.40 +/- 0.03 could allow PCV when infected to be used as one criterion of trypanotolerance.

Relationships between trypanosome infection measured by antigen detection enzyme immunoassays, anaemia and growth in trypanotolerant N'Dama cattle

Relationships were evaluated between trypanosome infection as measured by antigen detection enzyme immunoassays (antigen ELISA), anaemia as determined by average packed red cell volume (PCV), and animal performance as assessed by daily weight gain in 99 N'Dama cattle in Gabon exposed to natural tsetse challenge at 11.5 months of age and recorded 14 times over a 13 week period. Approximately half the animals were found to be infected for an average of five of the 14 times that they were examined: 38% with Trypanosoma congolense, 13% with Trypanosoma vivax and 49% with a mixed infection. Trypanosoma congolense infections had significant deleterious effects on animal growth, while T. vivax infections did not. Animals found on several occasions to be infected with T. congolense had significantly lower PCV values than those demonstrated to be infected on fewer occasions. No relationship was found between mean optical density (OD) values in antigen ELISA and PCV values. Animals capable of maintaining PCV values, even when antigen ELISA positive on a high number of occasions, grew at the same rate as uninfected animals. Animals that could not maintain PCV values when infected had poorer growth. Antigen ELISA has the potential to increase the efficiency of selection of trypanotolerant N'Dama cattle under tsetse challenge in the field, in three main ways. (1) Accurate identification of trypanosome species, especially in mixed species infections, clarifies relations between infection, anaemia and animal performance. (2) Detection of animals antigenaemic without patent parasitaemia could allow individuals with superior ability to control trypanosome infection to be identified. (3) More accurate measurement of the proportion of time an animal is infected allows more accurate evaluation of its anaemia control capability.

Comparative studies on N'Dama and zebu cattle following repeated infections with Trypanosoma congolense

Twenty N'Dama and eight zebu cattle were inoculated intradermally with bloodstream forms of a cloned strain of Trypanosoma congolense originating from East Africa. All inoculated cattle became parasitaemic. Zebus showed consistently higher levels of parasitaemia and lower packed red cell volume (PCV) percentages than did N'Damas. Three of the eight zebus required treatment when high numbers of trypanosomes were present in the blood and PCV values dropped below 15 per cent. None of the N'Dama cattle needed treatment. Statistical analysis was performed on the data to assess the variability of parasitaemia and PCV levels before and during infection of the N'Dama cattle. The variation in PCV values was large between individuals during the early stages of the disease and diminished as infection continued. After trypanocidal drug treatment and a recovery period of 14 months, the same animals were inoculated intradermally with T congolense bloodstream forms isolated and cloned in The Gambia. Differences in susceptibility to the ensuing disease were apparent when comparing N'Dama and zebu cattle. Five zebu cattle needed trypanocidal drug treatment, while none of the N'Damas needed drug intervention. Ranking the 20 infected N'Damas according to average PCV levels revealed that the animals responded similarly to both infections.

Use of antigen-detection enzyme immunoassays in assessment of trypanotolerance in N'Dama cattle

Antigen-detection enzyme immunoassays (ELISA) were used for the diagnosis of Trypanosoma vivax, T. congolense and T. brucei in N'Dama cattle in Gabon, Central Africa. The assays are based on monoclonal antibodies which recognise trypanosome antigens specific for each of the three species and animals were termed 'antigenaemic' when found positive by this technique but not found parasitaemic by the buffy coat technique. 148 one-year-old animals were exposed to a medium natural tsetse challenge and an average of 6 assays per animal were carried out over a 92-day period. Blood samples were routinely examined 11 times over this period and 28% of animals were detected as parasitaemic by the buffy coat technique. 90% of these were antigen-ELISA positive. More importantly, 40% of the animals with negative parasitological findings were also found to be antigenaemic. Parasitaemic animals with above-average packed-red-cell volume percent (PCV) values had 32% higher daily weight gains than those with below average, while antigenaemic animals showed no significant linkage between PCV values and weight gain. Thus only the 28% of animals with detectable parasitaemias could have been used for selection decisions based on PCV values. Antigenaemic animals grew at the same rate as negative animals and had 22% superior growth rates to parasitaemic animals. When antigenaemic animals were classified as having more ability to control parasite growth than parasitaemic animals, a significant sire effect suggested some possibility of a degree of genetic control being involved. Thus the ELISA could offer a practical possibility for selection of trypanotolerant animals based on infection criteria.

Susceptibility of N'Dama and Boran cattle to sequential challenges with tsetse-transmitted clones of Trypanosoma congolense

The susceptibility of N'Dama cattle (Bos taurus) to four consecutive infections with different tsetse-transmitted clones of Trypanosoma congolense was compared with that of Borans (Bos indicus). All animals were aged 13 months at the start of the study and had been born and raised free from trypanosomiasis under the same management and nutritional conditions, thereby limiting environmental factors that could have influenced susceptibility. While cattle of both breeds were equally susceptible to the establishment of trypanosome infections, the N'Damas exhibited superior resistance. Despite infection with virulent parasites, the N'Damas gained weight at the same rate as uninfected control animals, they did not develop anaemia to the extent that trypanocidal drug treatment was required, and all made a spontaneous recovery to normal haematological values within two to four months. In contrast, all the Borans needed treatment during the course of the four infections because of severe anaemia and showed markedly reduced liveweight gains. These clinical differences in the N'Damas were associated with two repeatable characteristics, namely, the ability to control parasitaemia and to 'resist' anaemia, processes that did not appear to be linked. Also in contrast to the Borans, the N'Damas were able to mount accelerated haemopoietic responses, resulting in the reduced severity of anaemia following a primary infection. These findings pose the question as to whether the ability to control parasitaemia and to 'resist' anaemia could be used as criteria for identifying resistant or trypanotolerant cattle.

Genetic aspects of control of anaemia development in trypanotolerant N'Dama cattle

148 one-year-old N'Dama cattle, progeny of 29 sires, were exposed for 92 days to a medium natural tsetse-trypanosome challenge in Gabon, Central Africa. Matching health and performance data were recorded on 11 occasions. Average packed red cell volume percent (PCV) and lowest PCV reached during the period were evaluated as measures of ability to control the development of anaemia. Attempts were made to systematically control other possible causes of anaemia. In animals detected as parasitaemic, those with above average average PCV values or above average lowest PCV reached had 34% and 35% respectively higher daily weight gains than those with below average. Even when not detected as parasitaemic, those with above average average PCV values or above average lowest PCV reached had 14% and 12% respectively higher gain indicating that a proportion of these animals actually were parasitaemic. When all environmental and parasitaemia information was taken into account, the heritability of growth, average PCV and lowest PCV reached was 0.39 +/- 0.31, 0.64 +/- 0.33 and 0.50 +/- 0.32 respectively. The genetic correlation between average PCV and growth was 0.70 +/- 0.42 and between lowest PCV reached and growth was 0.28 +/- 0.55. While the standard errors are large, the higher heritabilities of PCV measures compared to animal growth and the positive genetic correlations between PCV and growth do indicate an opportunity for selection on PCV when animals can be detected as parasitaemic. All heritabilities and genetic correlations increased in size when parasitaemia information was utilized in the analysis.(ABSTRACT TRUNCATED AT 250 WORDS)