Selective breeding can contribute to bovine tuberculosis control and eradication

Bovine tuberculosis (bTB) persists in many countries having a significant impact on public health and livestock industry finances. The incidence and prevalence of new cases in parts of the UK and elsewhere over the past decades warrant intensified efforts towards achieving Officially Tuberculosis Free (OTF) status in the respective regions. Genetic selection aiming to identify and remove inherently susceptible animals from breeding has been proposed as an additional measure in ongoing programmes towards controlling the disease. The presence of genetic variation among individual animals in their capacity to respond to Mycobacterium bovis exposure has been documented and heritability estimates of 0.06-0.18 have been reported. Despite their moderate magnitude, these estimates suggest that host resistance to bTB is amenable to improvement with selective breeding. Although relatively slow, genetic progress can be constant, cumulative and permanent, thereby complementing ongoing disease control measures. Importantly, mostly no antagonistic genetic correlations have been found between bTB resistance and other animal traits suggesting that carefully incorporating the former in breeding decisions should not adversely affect bovine productivity. Simulation studies have demonstrated the potential impact of genetic selection on reducing the probability of a breakdown to occur or the duration and severity of a breakdown that has already been declared. Furthermore, research on the bovine genome has identified multiple genomic markers and genes associated with bTB resistance. Nevertheless, the combined outcomes of these studies suggest that host resistance to bTB is a complex, polygenic trait, with no single gene alone explaining the inherent differences between resistant and susceptible animals. Such results support the development of accurate genomic breeding values that duly capture the collective effect of multiple genes to underpin selective breeding programmes. In addition to improving host resistance to bTB, scientists and practitioners have considered the possibility of reducing host infectivity. Ongoing studies have suggested the presence of genetic variation for infectivity and confirmed that bTB eradication would be accelerated if selective breeding considered both host resistance and infectivity traits. In conclusion, research activity on bTB genetics has generated knowledge and insights to support selective breeding as an additional measure towards controlling and eradicating the disease.

Quantitative decision making in animal health surveillance: Bovine Tuberculosis Surveillance in Belgium as case study

Despite eradication and control measures applied across Europe, bovine tuberculosis (bTB) remains a constant threat. In Belgium, after several years of official bTB-free status, routine movement testing, as currently practiced, revealed itself inadequate to detect some herds affected by sporadic breakdowns. The aim of this study was to assess different surveillance system components that strike a balance between cost and effectiveness and to identify sustainable alternatives, which substantiate a bTB-free claim while ensuring early detection and acceptance by various animal health stakeholders. For this pupose, a stochastic iteration model was used to simulate the current surveillance system's expected performance in terms of detection sensitivity and specificity. These results were then descriptively compared with observed field results. Second, the cost and effectiveness of simulated alternative surveillance components were quantified. Sensitivity analyses were performed to measure key assumptions' impacts (i.e. regarding diagnostic tests and true prevalence). The results confirmed discrepancies between the observed and simulated expected performance of bTB surveillance in Belgium. Second, simulated alternatives showed that interferon gamma (IFN-γ) and serological testing with antibody-enzyme linked immunosorbent assay (Ab-ELISA) targeting at-risk herds would enable an increase in the overall cost effectiveness (sensitivity and specificity) of the Belgian bTB surveillance system. Sensitivity analyses showed that results remained constant despite the modification of some key assumptions. While the performance of the ongoing bTB surveillance system in Belgium was questionable at the time of the study, this exercise highlighted that not only sensitivity but specificity also are key drivers of surveillance performance. The quantitative approach, taking into consideration various stakeholders' needs and priorities, revealed itself to be a useful tool in allowing evidence-based decision making for future tuberculosis surveillance in Belgium, in line with the international standards.

Association between results of diagnostic tests for bovine tuberculosis and Johne's disease in cattle

BACKGROUND

Bovine tuberculosis (bTB) diagnosis is impaired by numerous factors including cross-reactivity with Mycobacterium avium subspecies paratuberculosis, which causes Johne's disease (JD). In addition, the effect of repeated bTB-intradermal testing on the performance of JD diagnostic tests is not fully understood. This study aimed to evaluate the impact of repeated bTB-intradermal tests under field conditions in Spain on the JD serological status of cattle.

METHODS

bTB-positive herds (n=264) from Castilla-y-Leon region were selected and matched with officially tuberculosis-free control herds. The association between JD and bTB status at the herd level was assessed using conditional logistic regression and, in herds with both JD-positive and bTB-positive animals, a Bayesian hierarchical mixed-effect model was used for individual-level analysis.

RESULTS

A significantly higher risk of being JD positive (OR: 1.48; 95 per cent CI: 1.01 to 2.15) was found for bTB-positive herds compared with controls. Individual results indicated that cattle tested more than three times per year, within the last 90 days and more than 12 months were more likely to be JD positive. A skin test-related boost in antibody response could be the cause of an apparent increase of the sensitivity of the JD-absorbed ELISA.

CONCLUSION

The results demonstrate the interaction between bTB repeated testing and JD individual and herd-level results and this improved knowledge will facilitate the design of more effective control programmes in herds coinfected with two of the most important endemic diseases affecting cattle in Spain.

Assessment of the specificity of a gamma-interferon test performed with specific antigens to detect bovine tuberculosis, after non-negative results to intradermal tuberculin testing

In cattle herds in France, cervical skin tests (STs) using simple intradermal tuberculin (SIT) are performed to detect bovine tuberculosis (bTB). When positive results are found on ST screening, the herd is considered to be ‘under suspicion’ and confined, raising economic issues. The suspicion can be lifted by carrying out a single intradermal cervical comparative test (SICCT) at least six weeks later.The authors conducted an experimental study in France between 2013 and 2015 to assess the accuracy of the gamma-interferon test (IFN-γ), used in series after a non-negative result to ST screening, and to study the possibility of replacing the SICCT performed six weeks later by an IFN performed within a few days. Data were collected concerning 40 infected and 1825 bTB-free animals from herds with non-negative results to ST screening. This study showed that the IFN-γ test based on specific antigens and performed within a few days of a non-negative result to the ST has higher sensitivity than the SICCT performed six weeks later and equal specificity. The IFN test is more convenient to perform; however, it is more expensive. The IFN-γ test based on MIX antigens may be a useful alternative to the SICCT, to shorten the confinement period of suspect herds without underdetecting bTB.

The intractable challenge of evaluating cattle vaccination as a control for bovine Tuberculosis

Vaccination of cattle against bovine Tuberculosis (bTB) has been a long-term policy objective for countries where disease continues to persist despite costly test-and-slaughter programs. The potential use of vaccination within the European Union has been linked to a need for field evaluation of any prospective vaccine and the impact of vaccination on the rate of transmission of bTB. We calculate that estimation of the direct protection of BCG could be achieved with 100 herds, but over 500 herds would be necessary to demonstrate an economic benefit for farmers whose costs are dominated by testing and associated herd restrictions. However, the low and variable attack rate in GB herds means field trials are unlikely to be able to discern any impact of vaccination on transmission. In contrast, experimental natural transmission studies could provide robust evaluation of both the efficacy and mode of action of vaccination using as few as 200 animals.

Bovine tuberculosis is an infectious disease of livestock and wildlife in many parts of the world. It also can spread to humans. In the United Kingdom (UK), infected cattle and badgers contribute to its spread. To control bovine tuberculosis, cattle are tested and infected animals are slaughtered. Badgers in areas near cattle are killed to keep their populations small and reduce the likelihood of them infecting cattle. These control strategies are very controversial. Testing and slaughtering cattle is expensive and many people object to badger culling.

Developing a vaccine that would protect cattle against bovine tuberculosis is a potential alternative approach being investigated by the UK government. But such a vaccination is currently illegal in Europe because vaccinated animals may test positive for infection, creating confusion. Tests for bovine tuberculosis exist, but these DIVA (short for “Differentiates Infected from Vaccinated Animals”) tests are not yet licensed for use in the UK. The European Union (EU) said it would consider relaxing its laws against bovine tuberculosis vaccination if the UK government is able to prove a vaccine is effective on farms.

Now, Conlan et al. show that the specific field trials recommended by the EU would have to be extremely large to show a benefit of vaccination. Mathematical models were used to calculate how many cattle herds a bovine tuberculosis vaccine study would need to show that it protects cattle from infection, reduces transmission of the disease, and saves farmers money. Conlan et al. show that a study including 100 herds would be large enough to prove the vaccine protected individual animals. But a trial would have to include 500 herds to show that vaccination saves farmers money.

Because transmission of bovine tuberculosis is slow in the UK, trials on working farms are unlikely to be able to measure whether vaccination reduces the spread of the disease. Instead, Conlan et al. show that smaller, less expensive experiments in controlled settings would be able to estimate the effects of bovine tuberculosis vaccination on transmission.

These results informed the UK government decision to delay farm-based studies of a bovine tuberculosis vaccine until a DIVA test is available. If vaccination and the use of a DIVA test can be proven to be effective enough to replace test and slaughter policies it could be a huge economic boon to farmers, particularly those in lower income countries.

Retrospective study of bovine tuberculosis in Gombe Township Abattoir, Northeastern Nigeria

Tuberculosis (TB) is a major global public health problem. The World Health Organization (WHO) estimates that there are 8 million new cases and 3 million deaths in humans due to the disease each year. A retrospective abattoir study of bovine tuberculosis in Gombe Township Abattoir in Northeastern Nigeria was carried out from 2008 to 2015 using the abattoir records. Out of 154,562 slaughtered heads of cattle examined at postmortem, 1202(0.78%) had typical tuberculous lesions and overall total prevalence rate of 0.78%. The annual prevalence was significantly higher in 2015(1.04%) with the lowest records in 2011(0.34%). The annual prevalence rate showed intermittent increase in levels above the expected prevalence and there was a statistically significant difference in the relative annual prevalence rate of the disease in the abattoir. The distribution and occurrence of the disease in rainy season were higher than that in the dry season. The disease also appeared to be more prevalent in the rainy season 792(65.90%) compared with the dry season 410(34.10%) and the difference was statistically significant (χ2 = 15.45, p < 0.05). It was concluded that bovine TB is prevalent in Northeastern Nigeria, with indications of epidemics status in Gombe state.

Mycobacterial species causing pulmonary tuberculosis at the korle bu teaching hospital, accra, ghana

SUMMARY OBJECTIVE

Characterize mycobacterial species causing pulmonary tuberculosis (PTB) at the Korle-Bu Teaching Hospital in Ghana.

DESIGN

Sputum smear positive samples, two (2) from 70 patients diagnosed as having tuberculosis, after they had consented, were collected from the Korle-Bu Teaching Hospital Chest Clinic between January and July 2003.

SETTING

Korle-Bu Teaching Hospital Chest Clinic, Accra.

RESULTS

Sixty-four mycobacterial isolates were obtained and confirmed as members of Mycobacterium tuberculosis complex by colonial morphology and conventional biochemical assays. Forty-seven (73%) were M. tuberculosis, the human strain, 2 (3%) M. bovis, the bovine strain, 13 (20%) M. africanum I (West Africa type), and 2 (3%) M. africanum II (East Africa type).

CONCLUSION

The results indicate that, there are various strains causing PTB at the Korle-Bu Teaching Hospital and of great concern is M. bovis, which mostly causes extra-PTB in humans but found to cause PTB in this study. This calls for the need to conduct a nationwide survey using both conventional and molecular techniques to characterize various mycobacterial species causing TB in Ghana. This will result in better understanding of the various strains circulating in the country and inform individual TB treatment regimen especially the inclusion or exclusion of pyrazinamide.

Potential application of new diagnostic methods for controlling bovine tuberculosis in Brazil

Bovine tuberculosis, a chronic infection in cattle caused by Mycobacterium bovis, remains an economic and public health problem for several countries. Due to its economic impact on international trade, contagious nature, and implications for human health, global programs to eradicate the disease were implemented worldwide. Those programs are based on slaughtering PPD-reactive animals. Despite the National Programs in Brazil, complete eradication has not been achieved, and the disease remains, albeit at a lower prevalence. The central purpose of this review is to address diagnostic tests for tuberculosis. Considering the course of the infection in cattle, at least two tests, ideally complementary to one another, may be necessary for an adequate diagnosis: the first based on the cellular response, and the second capable of identifying anergic animals by detection of specific anti-M.bovis antibodies.