Outbreak of bovine tuberculosis featuring anergy to the skin test, udder lesions and milkborne disease in young calves

Limited usefulness of the IS6110 touchdown-PCR in blood for tuberculin skin test false-negative cattle with serological response to Mycobacterium bovis

Ante-mortem diagnosis of bovine tuberculosis (bTB) is based mainly on the tuberculin skin test (TST) and the ɣ-IFN release assay (IGRA). Some infected animals escape screening tests, thus, limit herd sanitation. Previous reports have suggested a predominant pattern of multi-organ lesions attributable to Mycobacterium bovis (the causative agent of bTB) bacteraemia. A case-control study was conducted to investigate blood PCR as an alternative tool for improving ante-mortem detection of TST false-negative bovines. Cases comprised 70 TST false-negative bovines (cases), which were serology positive, and controls included 81 TST positive bovines; all of them confirmed as infected with M. bovis. Detection of the IS6110 target through touchdown blood-PCR (IS6110 TD-PCR) was performed. The positivity of the blood-PCR was 27.2% in the control group. This performance was similar to the 15% obtained among cases (p = 0.134). Most cases identified by the IS6110 TD-PCR exhibited focalized lesions (p = 0.002). Results demonstrated that blood-PCR could detect TST false-negative cattle, even if they are negative for IGRA. Considering that cases exhibited humoral response to M. bovis, further studies conducted in a pre-serological stage could provide evidence about the real contribution of the technique in herds.

Seroprevalence and risk factors associated with bovine tuberculosis in cattle in Eastern Bhutan

Bovine tuberculosis (bTB) is a chronic zoonotic disease affecting cattle of all age groups including wild animals. It poses a significant threat to public health and high economic losses to dairy farmers. While the disease has been eradicated from most of the developed countries through extensive surveillance, testing and culling strategy, it is endemic in Africa, Asia, and the Middle East countries. Currently, there is limited research regarding the prevalence of bTB in cattle in Bhutan. This study aimed to determine the seroprevalence of bTB in cattle in six districts of eastern Bhutan. A two-stage probability proportional to size (PPS) sampling strategy was used to determine the number of animals from which serum samples needed to be collected in each district and sub-district. All farms and cattle for sampling were randomly selected from the data in the annual livestock census of 2020. The samples were tested using bTB ELISA test kit. The seroprevalence and their 95% confidence intervals were calculated. Logistic regression models were constructed to assess the influence of various individual animal and environmental risk factors (breed, age, sex, source of animal, body condition scores of animals, respiratory system status) associated with sero-positivity in animals. The study revealed an apparent seroprevalence of 2.57% (25/971 cattle; 95% CI:1.58-3.57), with an estimated true seroprevalence of 0.91% (95% CI: 0.0-2.81). However, none of the variables were found to be significantly associated with bTB seroprevalence in cattle. We recommend, further sampling and employment of confirmatory testing to fully ascertain the extent of bTB in the cattle herds in eastern Bhutan for prevention and control.

A study of the incidence of bovine tuberculosis in the wild red deer herd of Exmoor

Bovine tuberculosis (bTB) is an infectious disease which thrives at the wildlife-livestock interface. Exmoor has the largest herd of wild red deer (Cervus elaphus) in England, and also a large number of dairy and beef farms. The population, health and well-being of the herd are managed by a combination of hunting with hounds and by stalking. This study used a serological assay to determine the incidence of bTB in the population of 106 wild red deer of Exmoor, the relationship between regional deer densities and the presence of bTB in deer, and domestic cattle. The overall number of bTB positive deer was 28.3% (30/106). Stalked deer had a slightly higher incidence of bTB (19 out of 55, 34.55%) than hunted deer (11 out of 51, 21.57%). There was no clear pattern of distribution except for one region which showed an incidence of 42.22% compared with 16.4% in the remainder of the moor. There was little difference in the incidence of bTB between male and female animals. The age of animals in the study ranged from < 1 year to over 10 years. There was no clear difference in the incidence across the age range (< 1 year- > 10 years) with the exception of a particularly high incidence in those animals aged 1 year or less. There was a significant correlation between the presence of deer with bTB and the number of farms reporting bTB positive cattle, but not between the regional population of red deer and bTB in deer or cattle.

Prevalence of Mycobacterium bovis in milk on dairy cattle farms: An international systematic literature review and meta-analysis

Bovine tuberculosis, caused by Mycobacterium bovis (M. bovis), is a globally distributed chronic disease of animals. The bacteria can be transmitted to humans via the consumption of unpasteurised (raw) milk, thus representing an important public health risk. To investigate the risk of zoonotic transmission of M. bovis via raw milk, this study systematically reviewed published studies to estimate the prevalence of M. bovis in on-farm bulk-tank milk (BTM) and individual cow's milk (IM) by meta-analysis. In total, 1,339 articles were identified through seven electronic databases and initially screened using titles and abstracts. The quality of 108 potentially relevant articles was assessed using full texts, and 67 articles comprising 83 studies (76 IM and 7 BTM), were included in the meta-analysis. The prevalence of M. bovis in IM and BTM was summarised according to the diagnostic test used, and the tuberculin skin test (TST) infection status of the individual cows (for IM) or herds (for BTM). Heterogeneity was quantified using the I-squared statistic. Prediction intervals (95% PIs) were also estimated. For IM, the overall prevalence was summarised at 5% (95%CI: 3%-7%). In TST positive cows, prevalence was summarised at 8% (95%CI: 4%-13%). For BTM, the overall prevalence independent of individual herd TST infection status was summarised at 5% (95%CI: 0%-21%). There was considerable heterogeneity evident among the included studies, while PIs were also wide. Inconsistency in the quality of reporting was also observed resulting in missing information, such as the TST infection status of the individual animal/herd. No study reported the number of M. bovis bacteria in test-positive milk samples. Several studies reported the detection of M. tuberculosis and M. africanum in milk. Despite international efforts to control tuberculosis, this study highlights the risk of zoonotic transmission of M. bovis via unpasteurised milk and dairy products made using raw milk.

Long-term efficacy of BCG vaccination in goat herds with a high prevalence of tuberculosis

Vaccination of goats against tuberculosis (TB) has been promoted as an ancillary tool for controlling the disease in infected livestock herds. A three-year trial to assess the efficacy of BCG vaccine was carried out in five goat herds. At the beginning of the trial (month 0), all animals were tested for TB using thee different diagnostic tests. Animals negative to all tests were vaccinated with BCG and all replacement goat kids were also systematically vaccinated throughout the trial. All animals were tested by Interferon-gamma release assay (IGRA) using vaccine compatible reagents at months 6, 12, 24, and 36. The risk factors for TB infection were also evaluated. At the end of the study, four out of five farms showed variable reductions of the initial prevalence (93.5%, 28.5%, 23.2%, and 14.3% respectively), and an overall incidence reduction of 50% was observed in BCG vaccinated goats, although adult vaccinated goats showed higher incidences than vaccinated goat kids. The unvaccinated positive animals remaining in herds and adult BCG vaccinated goats significantly enhanced the risk of infection in vaccinated animals. A systematic vaccination of goats with BCG, together with the removal of positive unvaccinated animals, may contribute to reducing the TB prevalence in goat herds.

Bovine tuberculosis prevalence and risk factors in selected districts of Bangladesh

A cross-sectional survey was conducted in selected districts of Bangladesh to estimate the prevalence of bovine tuberculosis (bTB), and to identify the risk factors for bTB. We included 1865 farmed cattle from 79 herds randomly selected from five districts. Herd and animal level data were collected using semi-structured interviews with cattle herd owners. The single intradermal comparative tuberculin test (SICTT) was used to estimate the prevalence of bTB. The risk factors were identified using mixed-effect multiple logistic regression analyses. The overall herd and animal level prevalences of bTB were estimated to be 45.6% (95% Confidence Interval [CI] = 34.3–57.2%) and 11.3 (95% CI = 9.9–12.8%), respectively, using the OIE recommended >4 mm cut-off. The true animal level prevalence of bTB was estimated to be 11.8 (95% Credible Interval = 2.1–20.3%). At the herd level, farm size, bTB history of the farm and type of husbandry were significantly associated with bTB status in univariable analysis. Similarly, age group, sex, pregnancy status and parity were significantly associated with bTB at cattle level. However, in multivariable analysis only herd size at the herd level and age group and pregnancy status at the cattle level were significant. Compared to a herd size of 1–10, the odds of bTB were 22.8 (95% CI: 5.2–100.9) and 45.6 times (95% CI: 5.0–417.7) greater in herd sizes of >20–50 and >50, respectively. The odds of bTB were 2.2 (95% CI: 1.0–4.5) and 2.5 times (95% CI: 1.1–5.4) higher in cattle aged >3–6 years and > 6 years, compared to cattle aged ≤1 year. Pregnancy increased the odds of bTB by 1.7 times (95% CI: 1.2–2.4) compared to non-pregnant cattle. Taken together, the results suggest high herd and animal level prevalence of bTB in these 5 districts, with the greatest risk of bTB in older and pregnant cattle within large herds (>20), and highlight an urgent need for continued surveillance and implementation of bTB control programs in Bangladesh.

Genotype Diversity of Mycobacterium bovis and Pathology of Bovine Tuberculosis in Selected Emerging Dairy Regions of Ethiopia

Bovine tuberculosis (bTB) is endemic in Ethiopia with higher prevalence in cattle, particularly in the central parts. Spread of Mycobacterium bovis (M. bovis) to wider regions is inevitable in uncontrolled conditions. This study was conducted to explore the pathology, characterize M. bovis strains, and describe genotypic diversity to demonstrate possible epidemiological links in emerging dairy areas of Ethiopia, namely, Mekelle and Gondar. Twenty-seven bTB positive cattle identified by the Single Intradermal Comparative Cervical Tuberculin (SICCT) test were subjected to post-mortem inspection to determine lesion distribution and pathological score. Samples from tissues with visible tuberculous or suspected non-visible lesions were processed and cultured following a standard protocol. Isolates identified as M. bovis by Region of Difference (RD)-based Polymerase Chain Reaction (PCR) were also spoligotyped to determine their spoligotype patterns. Post-mortem inspection of visceral organs indicated bTB suggestive lesions in 41% of the animals, with 25% being in the lungs. Lymph nodes from 77% of the animals had lesions. Fifty-five isolates identified from 24 of the slaughtered animals were confirmed as M. bovis. No other mycobacterial species were isolated. Spoligotyping classified strains from 21 of these animals into seven spoligotype patterns: SB0133, SB0134, SB1176, SB2233, SB2290, SB2467, and SB2520. More than one spoligotype were identified from five of these animals, and none of the last four spoligotypes had been reported in Ethiopia before. SB0134 was the most predominant type (47%) followed by SB0133 (25.5%). SB0133, SB2290, SB2467, and SB1176 are spoligotypes lacking spacers 3–7, characteristics of M. bovis strains of the African 2 (Af2) clonal complex, while SB0134, SB2233, and SB2520 do not belong to any of the established clonal complexes and likely to have a different evolutionary history. Despite a small sample size, the present study showed strain diversity with multiple genotypes identified in a single herd and even within a single animal, and the genotypes showed no sign of geographical localization, which could be a consequence of significant movement of bTB diseased cattle around the country, spreading the disease. Therefore, any future control programme of bTB in Ethiopia needs to address the risks of cattle movement.

Characterization of potential superspreader farms for bovine tuberculosis: A review

Background

Variation in host attributes that influence their contact rates and infectiousness can lead some individuals to make disproportionate contributions to the spread of infections. Understanding the roles of such ‘superspreaders’ can be crucial in deciding where to direct disease surveillance and controls to greatest effect. In the epidemiology of bovine tuberculosis (bTB) in Great Britain, it has been suggested that a minority of cattle farms or herds might make disproportionate contributions to the spread of Mycobacterium bovis, and hence might be considered ‘superspreader farms’.

Objectives and Methods

We review the literature to identify the characteristics of farms that have the potential to contribute to exceptional values in the three main components of the farm reproductive number ‐ R_f: contact rate, infectiousness and duration of infectiousness, and therefore might characterize potential superspreader farms for bovine tuberculosis in Great Britain.

Results

Farms exhibit marked heterogeneity in contact rates arising from between‐farm trading of cattle. A minority of farms act as trading hubs that greatly augment connections within cattle trading networks. Herd infectiousness might be increased by high within‐herd transmission or the presence of supershedding individuals, or infectiousness might be prolonged due to undetected infections or by repeated local transmission, via wildlife or fomites.

Conclusions

Targeting control methods on putative superspreader farms might yield disproportionate benefits in controlling endemic bovine tuberculosis in Great Britain. However, real‐time identification of any such farms, and integration of controls with industry practices, present analytical, operational and policy challenges.

Prevalence of bovine tuberculosis and its associated risk factors in the emerging dairy belts of regional cities in Ethiopia

Bovine tuberculosis (BTB) has become an economically important disease in dairy herds found in and around Addis Ababa City and is emerging in regional cities like Gondar, Hawassa and Mekelle because of the establishment of dairy farms in the milk sheds of these cities. A cross-sectional study to estimate the prevalence of BTB and identify associated risk factors was conducted between February 2016 and March 2017. A total of 174 herds comprising of 2,754 dairy cattle in the cities of Gondar, Hawassa and Mekelle were tested using the Single Intradermal Comparative Cervical Tuberculin (SICCT) test. Data on herd structure, animal origin, body condition, housing condition, farm hygiene, management and biosecurity practices were collected using a pre-tested structured questionnaire. Generalized Linear Models (GLM) and Generalized Linear Mixed Models (GLMM) were used to analyze the herd and animal level risk factors, respectively. The herd prevalence was 22.4% (95% CI: 17-29%) while the animal prevalence was 5.2% (95% CI: 4-6%) at the cut-off >4 mm. The herd prevalence rose to 65.5% (95% CI: 58-72%) and the animal prevalence rose to 9% (95% CI: 8-10%) when the severe interpretation of >2 mm cut-off was applied. The mean within-herd prevalence in positive farms at the cut-off >4 mm was 22.7% (95% CI: 15-31%). At the herd level, the analysis showed that herd size, farm hygiene, feeding condition and biosecurity were significantly associated with BTB status, while new cattle introductions showed only borderline significance and that age of farm, housing condition, farmers' educational status and animal health care practice were not significant. At the animal level, the results showed that age and animal origin were identified as significant predictors for BTB positivity but sex and body condition score were not related to BTB status. Descriptive analysis revealed that herds having 'BTB history' showed slightly higher likelihood of being BTB positive compared to farms having no previous BTB exposure. In conclusion, this study showed relatively lower average prevalence in the emerging dairy regions as compared to the prevalence observed in and around Addis Ababa City, warranting for implementation of control program at this stage to reduce or possibly stop further transmission of BTB.

Performativity and a microbe: Exploring Mycobacterium bovis and the political ecologies of bovine tuberculosis

Mycobacterium bovis, the bacterium responsible for causing bovine tuberculosis (bTB) in cattle, displays what I call ‘microbial performativity’. Like many other lively disease-causing microorganisms, it has an agency which is difficult to contain, and there is a need for fresh thinking on the challenges of dealing with this slippery and indeterminate microbe. As a practising veterinary scientist who side-stepped mid-career into a parallel training in the social sciences to view bTB from an alternative perspective, I create an interdisciplinary coming-together where veterinary science converges with a political ecology of (animal) health influenced by science and technology studies (STS) and social science and humanities scholarship on performativity. This suitably hybridized nexus creates a place to consider the ecologies of a pathogen which could be considered as life out of control. I consider what this means for efforts to eradicate this disease through combining understandings from the published scientific literature with qualitative interview-based fieldwork with farmers, veterinarians and others involved in the statutory bTB eradication programme in a high incidence region of the UK. This study demonstrates the value of life scientists turning to the social sciences to re-view their familiar professional habitus—challenging assumptions, and offering alternative perspectives on complex problems.

Potential for rapid antibody detection to identify tuberculous cattle with non-reactive tuberculin skin test results

BACKGROUND

Bovine tuberculosis (TB) control programs generally rely on the tuberculin skin test (TST) for ante-mortem detection of Mycobacterium bovis-infected cattle.

RESULTS

Present findings demonstrate that a rapid antibody test based on Dual-Path Platform (DPP^®) technology, when applied 1-3 weeks after TST, detected 9 of 11 and 34 of 52 TST non-reactive yet M. bovis-infected cattle from the US and GB, respectively. The specificity of the assay ranged from 98.9% (n = 92, US) to 96.0% (n = 50, GB) with samples from TB-free herds. Multi-antigen print immunoassay (MAPIA) revealed the presence of antibodies to multiple antigens of M. bovis in sera from TST non-reactors diagnosed with TB.

CONCLUSIONS

Thus, use of serologic assays in series with TST can identify a significant number of TST non-reactive tuberculous cattle for more efficient removal from TB-affected herds.

The effectiveness of parallel gamma-interferon testing in New Zealand's bovine tuberculosis eradication programme

In bovine tuberculosis (bTB) eradication programmes, especially where prevalence is low, sensitivity of testing in infected herds must be maximised to reduce the possibility of recrudescence of prior infection and the risk to other herds via animal movement. The gamma-interferon (γ-IFN) assay applied in parallel with intradermal tuberculin testing has been shown to increase test sensitivity. The aim of this work was to substantiate this effect in the field. A retrospective observational study was conducted on 239 New Zealand cattle breeding and dairy herds with bTB infection between 1 July 2011 and 1 September 2015 to evaluate the outcomes of new policy introduced in 2011. The investigation defined the number and proportion of reactors (animals testing positive and slaughtered) found with lesions of bTB in intradermal caudal fold testing (CFT) and parallel γ-IFN testing, at the breakdown test or first whole herd test after breakdown, WHT(1), and at the final or projected final whole herd test, WHT(F). Parallel γ-IFN testing was used in 26.8% of the 239 herds at WHT(1), and 430 animals in 49 herds were deemed reactors. One hundred and sixty (37.2%) of these reactors from 32 herds were found to have bTB lesions, despite having been negative to caudal fold testing. These 160 infected animals accounted for 29.6% of all infection found at WHT(1). At WHT(F), parallel γ-IFN testing was conducted on 93 herds and detected a total of 122 reactors in 49 herds, in addition to those found by CFT. Twenty-one of these reactors, from 13 herds, had bTB lesions at slaughter, accounting for 67.7% of all reactors found with bTB at WHT(F). Eleven of these 13 herds would have had their movement restrictions revoked based on a negative herd CFT alone, and could potentially have caused outward transmission of bTB to other herds, as well as experiencing recrudescent breakdowns. We conclude that γ-IFN testing in infected herds, in parallel with intradermal tuberculin testing, is a valuable tool in a bTB eradication programme, as it enables higher test sensitivity at both herd and animal level. The use of the γ-IFN test over a risk cohort early in a breakdown assists in removal of early infection and some cases of anergy to intradermal tuberculin testing. Parallel γ-IFN with compulsory slaughter of reactors should be considered in breeding and dairy herds in conjunction with tuberculin testing before movement control is revoked, and will assist in achieving TB freedom on a herd level and nationally.

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.

Characterization of effector and memory T cell subsets in the immune response to bovine tuberculosis in cattle

Cultured IFN-γ ELISPOT assays are primarily a measure of central memory T cell (Tcm) responses with humans; however, this important subset of lymphocytes is poorly characterized in cattle. Vaccine-elicited cultured IFN-γ ELISPOT responses correlate with protection against bovine tuberculosis in cattle. However, whether this assay measures cattle Tcm responses or not is uncertain. The objective of the present study was to characterize the relative contribution of Tcm (CCR7⁺, CD62L^hi, CD45RO⁺), T effector memory (Tem, defined as: CCR7^-, CD62L^low/int, CD45RO⁺), and T effector cells (CCR7^-, CD62L^-/low, CD45RO^-), in the immune response to Mycobacterium bovis. Peripheral blood mononuclear cells (PBMC) from infected cattle were stimulated with a cocktail of M. bovis purified protein derivative, rTb10.4 and rAg85A for 13 days with periodic addition of fresh media and rIL-2. On day 13, cultured PBMC were re-stimulated with medium alone, rESAT-6:CFP10 or PPDb with fresh autologous adherent cells for antigen presentation. Cultured cells (13 days) or fresh PBMCs (ex vivo response) from the same calves were analyzed for IFN-γ production, proliferation, and CD4, CD45RO, CD62L, CD44, and CCR7 expression via flow cytometry after overnight stimulation. In response to mycobacterial antigens, ~75% of CD4⁺ IFN-γ⁺ cells in long-term cultures expressed a Tcm phenotype while less than 10% of the ex vivo response consisted of Tcm cells. Upon re-exposure to antigen, long-term cultured cells were highly proliferative, a distinctive characteristic of Tcm, and the predominant phenotype within the long-term cultures switched from Tcm to Tem. These findings suggest that proliferative responses of Tcm cells to some extent occurs simultaneously with reversion to effector phenotypes (mostly Tem). The present study characterizes Tcm cells of cattle and their participation in the response to M. bovis infection.

An ecological and comparative perspective on the control of bovine tuberculosis in Great Britain and the Republic of Ireland

Disease ecology involves a systematic approach to understanding the interactions and evolution of host-pathogen systems at the population level, and is essential for developing a comprehensive understanding of the reasons for disease persistence and the most likely means of control. This systems or ecological approach is being increasingly recognised as a progressive method in disease control and is exploited in diverse fields ranging from obesity management in humans to the prevention of infectious disease in animal populations. In this review we discuss bovine tuberculosis (bTB) in Great Britain (GB) within a disease ecology context, and suggest how a comparative ecological perspective helps to reconcile apparent conflicts with the evidence on the effectiveness of badger culling to assist in the control of bTB in GB and the Republic of Ireland (ROI). Our examination shows that failure of past measures to control bTB and the disparity in outcomes of badger culling experiments are the result of a complex relationship amongst the agent, host and environment, i.e. the episystem, of bTB. Here, we stress the role of distinctive bTB episystems and badger culling trial design in the ambiguity and resulting controversy associated with badger culling in GB and ROI. We argue this episystem perspective on bTB control measures in cattle and badger populations provides a useful and informative perspective on the design and implementation of future bTB management in GB, particularly at a time when both scientific and lay communities are concerned about the ongoing epidemic, the cost of current control measures and the execution of future control procedures.