DNA restriction fragment typing of Mycobacterium bovis isolates from cattle and badgers in Ireland

Mycobacterium tuberculosis complex in wildlife: Review of current applications of antemortem and postmortem diagnosis

Tuberculosis (TB) is a chronic inflammatory and zoonotic disease caused by Mycobacterium tuberculosis complex (MTBC) members, which affects various domestic animals, wildlife, and humans. Some wild animals serve as reservoir hosts in the transmission and epidemiology of the disease. Therefore, the monitoring and surveillance of both wild and domestic hosts are critical for prevention and control strategies. For TB diagnosis, the single intradermal tuberculin test or the single comparative intradermal tuberculin test, and the gamma-interferon test, which is regarded as an ancillary test, are used. Postmortem examination can identify granulomatous lesions compatible with a diagnosis of TB. In contrast, smears of the lesions can be stained for acid-fast bacilli, and samples of the affected organs can be subjected to histopathological analyses. Culture is the gold standard test for isolating mycobacterial bacilli because it has high sensitivity and specificity compared with other methods. Serology for antibody detection allows the testing of many samples simply, rapidly, and inexpensively, and the protocol can be standardized in different laboratories. Molecular biological analyses are also applicable to trace the epidemiology of the disease. In conclusion, reviewing the various techniques used in MTBC diagnosis can help establish guidelines for researchers when choosing a particular diagnostic method depending on the situation at hand, be it disease outbreaks in wildlife or for epidemiological studies. This is because a good understanding of various diagnostic techniques will aid in monitoring and managing emerging pandemic threats of infectious diseases from wildlife and also preventing the potential spread of zoonotic TB to livestock and humans. This review aimed to provide up-to-date information on different techniques used for diagnosing TB at the interfaces between wildlife, livestock, and humans.

Mycobacterium bovis: From Genotyping to Genome Sequencing

Mycobacterium bovis is the main pathogen of bovine, zoonotic, and wildlife tuberculosis. Despite the existence of programs for bovine tuberculosis (bTB) control in many regions, the disease remains a challenge for the veterinary and public health sectors, especially in developing countries and in high-income nations with wildlife reservoirs. Current bTB control programs are mostly based on test-and-slaughter, movement restrictions, and post-mortem inspection measures. In certain settings, contact tracing and surveillance has benefited from M. bovis genotyping techniques. More recently, whole-genome sequencing (WGS) has become the preferential technique to inform outbreak response through contact tracing and source identification for many infectious diseases. As the cost per genome decreases, the application of WGS to bTB control programs is inevitable moving forward. However, there are technical challenges in data analyses and interpretation that hinder the implementation of M. bovis WGS as a molecular epidemiology tool. Therefore, the aim of this review is to describe M. bovis genotyping techniques and discuss current standards and challenges of the use of M. bovis WGS for transmission investigation, surveillance, and global lineages distribution. We compiled a series of associated research gaps to be explored with the ultimate goal of implementing M. bovis WGS in a standardized manner in bTB control programs.

Control strategies for wildlife tuberculosis in Ireland

The principal domestic maintenance host for Mycobacterium bovis is infected cattle. In countries where comprehensive surveillance schemes have been applied, tuberculosis rarely affects an animal to the extent that it presents with clinical disease. In the latter stages of an eradication campaign, the aim is to maintain the disease-free status of clear herds and eliminate foci of infection in herds as well as restricting movement of infected animals from these herds, other than to slaughter. However, the eradication of tuberculosis from cattle herds may be compromised if infected wildlife species, such as Eurasian badgers (Meles meles), share the same environment and contribute to transmission of infection. The options for dealing with tuberculosis in the wildlife reservoir hosts are limited to segregation of domestic animals from the wildlife, culling of the wildlife host or vaccination. Options are further limited by conservation and social reasons, particularly where culling is concerned. In Ireland and the UK, vaccination of badgers against M. bovis, if successfully employed, could directly facilitate the completion of bovine tuberculosis eradication. Programmes of research into vaccination of badgers are being undertaken in both countries, and there is clear evidence that vaccination induces protection. Vaccine trials in captive badgers have established that the M. bovis bacille Calmette-Guérin (BCG) vaccine can induce a protective response that limits the distribution and severity of tuberculosis disease following experimental challenge. In Ireland, a large-scale field trial of oral BCG vaccination is being conducted to measure the protection generated in wild badgers subjected to natural transmission of infection and to estimate vaccine efficacy. The results will provide a framework for the development and implementation of a national strategy to address the disease in badger populations and if successful will remove this major impediment to tuberculosis eradication from cattle.

Advances in molecular diagnostics for Mycobacterium bovis

The two most important molecular diagnostic techniques for bovine tuberculosis are the polymerase chain reaction (PCR) because of its rapid determination of infection, and DNA strain typing because of its ability to answer important epidemiological questions. PCR tests for Mycobacterium bovis have been improved through recent advances in PCR technology, but still lack the sensitivity of good culture methods, and in some situations are susceptible to giving both false negative and false positive results. Therefore, PCR does not usually replace the need for culture, but is used to provide fast preliminary results. DNA typing of M. bovis isolates by restriction endonuclease analysis (REA) was developed 25 years ago in New Zealand, and remains an important tool in the New Zealand control scheme, where the typing results are combined with other information to determine large and expensive possum poisoning operations. A range of other DNA typing systems developed for M. bovis in the 1990 s have assisted epidemiological investigations in some countries but are now less commonly used. Variable number tandem repeat (VNTR) typing and spoligotyping, either alone or together, have now become the preferred approaches as they are robust and amenable to electronic analysis and comparison. Spoligotyping gives only moderate discrimination but can be easily applied to large numbers of isolates, and VNTR typing provides better discrimination than all other methods except for REA. While the current typing techniques are sufficient for most epidemiological purposes, more discriminating methods are likely to become available in the near future.

Comparison of 45 variable number tandem repeat (VNTR) and two direct repeat (DR) assays to restriction endonuclease analysis for typing isolates of Mycobacterium bovis

Restriction endonuclease analysis (REA), developed 25 years ago for genotyping Mycobacterium bovis strains, is an important tool for bovine tuberculosis control in New Zealand. While REA gives excellent discrimination, it is technically difficult to perform compared to PCR-based typing systems which are faster and simpler to operate. Genotyping of M. bovis by the use of variable number tandem repeat loci (VNTR) and spoligotyping, either alone or together, has now become the preferred approach for typing M. bovis. Here, we evaluated the widest range of VNTR loci yet investigated for M. bovis, including two VNTR loci not previously studied, one of which (4155) had particular utility for characterizing New Zealand isolates. VNTR typing provided substantial geographical resolution of 26 of the most commonly found REA types and this was improved by the addition of two PCR assays based on parts of the direct repeat (DR) locus. Overall, 68 REA types of M. bovis common in New Zealand were discriminated into 33 VNTR/DR groups by using a minimum of nine VNTR and two DR assays. These 11 VNTR/DR assays concorded for three isolates each of 45 of the REA types but showed some variation with at least one of the VNTR/DR assays for the remaining 23 REA types. Major differences were found in allelic variation of some VNTRs between isolates from New Zealand and other countries, emphasizing the importance of adapting M. bovis typing systems to suit individual countries.

The influence of veterinary epidemiology on public health: past, present and future

This paper summarizes a presentation given at the Association for Veterinary Epidemiology and Preventive Medicine sponsored Calvin W. Schwabe symposium honouring the lifetime achievements of Dr. S. Wayne Martin. While the concepts were amalgamated from many sources, the examples were primarily selected to represent areas where Wayne Martin has been an active researcher and educator. The purpose was to describe the impact of veterinary epidemiology on public health in the past and present and to consider the future of veterinary epidemiology in public health. Veterinary medicine contributes to public health not only in the area of zoonotic disease prevention and control, but also through contributions to animal health, comparative and basic medical research, and population and environmental health. Veterinary epidemiologists contribute to both research in public health and the practice of public health through a wide range of methodological approaches and via the networks of trained epidemiologists working in the area. The contributions of veterinary epidemiologists have resulted in significant improvements in human health. There are considerable challenges and opportunities facing veterinary epidemiologists working in the public health area in the future. Meeting these needs will require continued integration between veterinary and human public health research and practice, and enhanced communication of both content and context expertise.

Human Mycobacterium bovis infection in the United Kingdom: Incidence, risks, control measures and review of the zoonotic aspects of bovine tuberculosis

Amongst the members of the Mycobacterium tuberculosis complex (MTBC), M. tuberculosis is mainly a human pathogen, whereas M. bovis has a broad host range and is the principal agent responsible for tuberculosis (TB) in domestic and wild mammals. M. bovis also infects humans, causing zoonotic TB through ingestion, inhalation and, less frequently, by contact with mucous membranes and broken skin. Zoonotic TB is indistinguishable clinically or pathologically from TB caused by M. tuberculosis. Differentiation between the causative organisms may only be achieved by sophisticated laboratory methods involving bacteriological culture of clinical specimens, followed by typing of isolates according to growth characteristics, biochemical properties, routine resistance to pyrazinamide (PZA) and specific non-commercial nucleic acid techniques. All this makes it difficult to accurately estimate the proportion of human TB cases caused by M. bovis infection, particularly in developing countries. Distinguishing between the various members of the MTBC is essential for epidemiological investigation of human cases and, to a lesser degree, for adequate chemotherapy of the human TB patient. Zoonotic TB was formerly an endemic disease in the UK population, usually transmitted to man by consumption of raw cows' milk. Human infection with M. bovis in the UK has been largely controlled through pasteurization of cows' milk and systematic culling of cattle reacting to compulsory tuberculin tests. Nowadays the majority of the 7000 cases of human TB annually reported in the UK are due to M. tuberculosis acquired directly from an infectious person. In the period 1990-2003, between 17 and 50 new cases of human M. bovis infection were confirmed every year in the UK. This represented between 0.5% and 1.5% of all the culture-confirmed TB cases, a proportion similar to that of other industrialized countries. Most cases of zoonotic TB diagnosed in the UK are attributed to (i) reactivation of long-standing latent infections acquired before widespread adoption of milk pasteurization, or (ii) M. bovis infections contracted abroad. Since 1990, only one case has been documented in the UK of confirmed, indigenous human M. bovis infection recently acquired from an animal source. Therefore, for the overwhelming majority of the population, the risk of contracting M. bovis infection from animals appears to be extremely low. However, bovine TB is once again a major animal health problem in the UK. Given the increasing numbers of cattle herds being affected each year, physicians and other public health professionals must remember that zoonotic TB is not just a disease of the past. A significant risk of M. bovis infection remains in certain segments of the UK population in the form of (i) continuing on-farm consumption of unpasteurized cows' milk, (ii) retail sales by approved establishments of unpasteurized milk and dairy products and (iii) occupational exposure to infectious aerosols from tuberculous animals and their carcases.

The tuberculosis eradication programme in Ireland: a review of scientific and policy advances since 1988

A national programme to eradicate bovine tuberculosis commenced in Ireland in 1954. During the last 15-20 years, research has been conducted to address gaps in knowledge of disease epidemiology, to objectively evaluate alternative strategy options, and to critically assess the implementation of disease control strategies. This paper provides a review of scientific and policy advances in Ireland since 1988, relevant to the tuberculosis eradication programme in Ireland. There have been substantial advances in knowledge of aspects of disease epidemiology, relating to cattle-to-cattle transmission, the role of wildlife, transmission of infection from wildlife and methods to minimise wildlife-to-cattle transmission. Further, scientific advances have been made both in the detection and management of infected herds. With respect to policy, the paper describes current policy and policy advances in both the detection and management of infected herds, as well as current strategies to prevent herd breakdowns. The Irish programme is a useful example of science-informed policy in a national context.

Immunological responses of Eurasian badgers (Meles meles) vaccinated with Mycobacterium bovis BCG (bacillus calmette guerin)

Wildlife species, such as the badger (Meles meles), may act as maintenance hosts for Mycobacterium bovis and contribute to the spread and persistence of tuberculosis in associated cattle populations. Targeted vaccination of badgers against tuberculosis is an option that, if successfully employed, could directly facilitate the advancement of bovine tuberculosis eradication in affected areas. In this study, the immunological responses of a group of badgers vaccinated subcutaneously with low doses of Mycobacterium bovis bacillus calmette guerin (BCG) were measured in vitro and compared with non-vaccinated control animals over a period of 42 weeks. Peripheral blood mononuclear cells (PBMC) from badgers which had received repeated booster injections of BCG proliferated in response to culture with PPD-bovine (purified protein derivative of tuberculin). The proliferation was significantly greater than that seen in the non-vaccinated control group. In contrast, the proliferative response of PBMC from vaccinated badgers to PPD-avian declined relative to the control group. These results demonstrate that repeated vaccination of badgers with M. bovis BCG induced a population of T-lymphocytes responsive to specific antigens in PPD-bovine. Throughout the course of the study, the sera from all animals were tested (BrockTest) by an enzyme-linked immunosorbent assay (ELISA) system for the presence of antibodies to MPB83, a serodominant antigen whose expression is high in M. bovis, but very low in BCG (Pasteur). No animals at any stage showed seroconversion to the antigen, consistent with the tuberculosis-free status of the badgers under study.

The development of wildlife control strategies for eradication of tuberculosis in cattle in Ireland

Wildlife species, such as badgers, act as maintenance hosts for Mycobacterium bovis and contribute to the spread and persistence of tuberculosis in associated cattle populations. In areas in which there is a tuberculosis problem affecting a number of herds, the involvement of infected wildlife in the introduction of M. bovis infection into herds act as a constraint to eradication of the disease. Epidemiological evidence demonstrates a high prevalence of tuberculosis in badgers, and controlled studies involving comprehensive badger removal have shown that this strategy can serve to significantly reduce cattle reactor rates in the targeted areas. However, as the badger is a protected wildlife species, alternative strategies are required to combat the disease. Targeted vaccination of wildlife species against tuberculosis is an option which, if successfully employed, could directly facilitate the advancement of bovine tuberculosis eradication in affected areas. Any proposed vaccination programme would need to be undertaken against the background of an exhaustive investigation of the cattle and herd management-related factors, and take account of environmental issues.

Tuberculosis in deer: perceptions, problems and progress

Since the emergence of deer farming as an alternative farming enterprise over the past 30 years, there has been an increasing awareness of the potential threat posed by tuberculosis (TB) to domesticated deer. TB, caused by Mycobacterium bovis, has been found in deer in every country involved with deer farming. Different types of TB control policies, which vary from whole-herd depopulation to selective testing and slaughter of reactor animals, have been implemented. Extensive research has been carried out, incorporating modern microbiological and immunological concepts and advanced molecular methodologies, to find new solutions for the eradication of TB from domesticated deer. This work has resulted in valuable new insights into the aetiology, transmission, pathogenesis, diagnosis, prevention and heritability of resistance to M. bovis infection in ruminants. This knowledge has complemented the existing literature database on bovine and human TB and will provide new strategies for improved diagnosis, vaccination and selective breeding to control TB, which should be relevant for human, domestic livestock and wildlife populations.

Cattle-to-cattle transmission of bovine tuberculosis

In developed countries, Mycobacterium bovis infection in cattle is now mostly confined to the respiratory system, which reflects transmission and establishment of infection mainly by this route. A single bacillus transported within a droplet nucleus is probably sufficient to establish infection within the bovine lung. Infected cattle should always be considered as potential sources of infection, since studies have demonstrated that a significant proportion of tuberculous cattle excrete M. bovis. In general, the dynamics of M. bovis transmission are poorly understood and the conditions under which a tuberculous animal becomes an effective disseminator of infection are currently not defined although environmental contamination appears to be a less effective method of disease transmission. Field studies indicate a wide spectrum of transmission rates but generally the spread of M. bovis infection is still considered to be a relatively slow process. Slaughter of diseased cattle detected by tuberculin testing and at meat plant inspection has been shown to be an effective policy for tuberculosis eradication, provided there are no other reservoirs of infection and all involved in the cattle industry are committed to a policy of eradication. Epidemiological approaches, particularly case-control studies, seem to provide the best method for quantifying the relative importance of the various sources of M. bovis transmission to cattle and modelling techniques can be used to assist in the design of cost-effective control measures that may lead to tuberculosis eradication.

Study of restriction fragment length polymorphism analysis and spoligotyping for epidemiological investigation of Mycobacterium bovis infection

Restriction fragment length polymorphism (RFLP) analysis with probes derived from the insertion element IS6110, the direct repeat sequence, and the polymorphic GC-rich sequence (PGRS) and a PCR-based typing method called spacer oligonucleotide typing (spoligotyping) were used to strain type Mycobacterium bovis isolates from the Republic of Ireland. Results were assessed for 452 isolates which were obtained from 233 cattle, 173 badgers, 33 deer, 7 pigs, 5 sheep, and 1 goat. Eighty-five strains were identified by RFLP analysis, and 20 strains were identified by spoligotyping. Twenty percent of the isolates were the most prevalent RFLP type, while 52% of the isolates were the most prevalent spoligotype. Both the prevalent RFLP type and the prevalent spoligotype were identified in isolates from all animal species tested and had a wide geographic distribution. Isolates of some RFLP types and some spoligotypes were clustered in regions consisting of groups of adjoining counties. The PGRS probe gave better differentiation of strains than the IS6110 or DR probes. The majority of isolates from all species carried a single IS6110 copy. In four RFLP types IS6110 polymorphism was associated with deletion of fragments equivalent in size to one or two direct variable repeat sequences. The same range and geographic distribution of strains were found for the majority of isolates from cattle, badgers, and deer. This suggests that transmission of infection between these species is a factor in the epidemiology of M. bovis infection in Ireland.

Differentiation of Mycobacterium bovis isolates from animals by DNA typing

The insertion sequence IS6110 and the direct repeat (DR) specific to tuberculosis complex mycobacteria and the highly repeated DNA sequence, the polymorphic GC-rich repeat sequence (PGRS), were systematically used to identify restriction fragment length polymorphisms (RFLPs) within 210 isolates of Mycobacterium bovis. The isolates were primarily of bovine origin, but isolates from badgers, feral deer, sheep, humans, and a pig were included. The RFLP probes IS6110, DR, and PGRS individually identified 17, 18, and 18 different RFLP types, respectively, but in combination these probes identified a total of 39 different M. bovis RFLP types. The recommendations (J. D. A. van Embden, M. D. Cave, J. T. Crawford, J. W. Dale, K. D. Eisenach, B. Gicquel, P. W. M. Hermans, C. Martin, R. McAdam, T. M. Shinnick, and P. M. Small, J. Clin. Microbiol. 31:406-409, 1993) for a standardized RFLP analysis for M. tuberculosis were adapted to facilitate gel documentation, image analysis, and construction of a database of RFLP types. In the present study the same M. bovis RFLP types were evident in the various animal species included, indicating that the strains were not host restricted. Application of these techniques to defined field studies should help elucidate more accurately aspects of the epidemiology of bovine tuberculosis in different countries.

Use of DNA restriction fragment typing in the differentiation of Mycobacterium tuberculosis complex isolates from animals and humans in Burundi

SETTING

Bubanza Hospital and Veterinary Laboratory in Bujumbura, Burundi.

OBJECTIVE

To monitor the rate of Mycobacterium bovis infections among tuberculosis (TB) patients and among slaughtered cattle, and to analyse the polymorphism among deoxyribonucleic acid (DNA) fingerprints of the M. tuberculosis complex isolates.

DESIGN

135 lymph node biopsies and 35 sputum specimens from human patients, together with specimens from 46 healthy animals and 36 animals suspected for bovine tuberculosis (BTB), were cultured. Isolates were identified phenotypically and DNA fingerprints were obtained by IS6110 based restriction fragment length polymorphism.

RESULTS

119 M. tuberculosis complex isolates were obtained from 170 human specimens. M. bovis was not identified in any human sample. One out of 46 healthy animals and 14 out of 36 BTB suspected animals yielded M. bovis isolates. DNA fingerprinting revealed four to eight copies of IS6110 for all M. bovis isolates with some degree of polymorphism, and some clustering for human TB isolates. No relationship was observed between human and bovine isolates.

CONCLUSION

At present M. bovis seems to play a minor role in human TB in Burundi, despite the high prevalence of both human immunodeficiency virus infection in humans and M. bovis in cattle. DNA fingerprinting is able to differentiate between bovine isolates.

Genomic analysis of Mycobacterium bovis and other members of the Mycobacterium tuberculosis complex by isoenzyme analysis and pulsed-field gel electrophoresis

Initially, multilocus enzyme electrophoresis was used to examine genetic relationships among 63 isolates of Mycobacterium bovis and 13 other members of the M. tuberculosis complex. The isolates were divided into five electrophoretic types, with a mean genetic diversity of 0.1. The strains were genetically homogenous, indicating that members of the complex were closely related. This supported the suggestion that they should be considered as subspecies of a single species. Pulsed-field gel electrophoresis (PFGE) was then used to differentiate these isolates, as well as 59 additional isolates of M. bovis from different parts of the world. PFGE differentiated these strains into 63 patterns (53 patterns for M. bovis). Isolates of M. bovis from Western Australia (n = 46) were more homogenous than isolates from other regions. Eight strains were identified in that state, and one predominantly bovine strain was isolated from two human beings and a feral pig. Although M. bovis isolates from different parts of the world had distinct DNA patterns, some were very similar. PFGE is a highly discriminatory technique for epidemiological studies of bovine tuberculosis. For example, it allowed differentiation between isolates of M. bovis cultured from animals in separate outbreaks of tuberculosis, it suggested the transmission of infection between certain properties, and it demonstrated the existence of multiple infections with different strains at certain farms.

The epidemiology of Mycobacterium bovis infections in animals and man: a review

Tuberculosis is primarily a respiratory disease and transmission of infection within and between species is mainly by the airborne route. Mycobacterium bovis, the cause of bovine-type tuberculosis, has an exceptionally wide host range. Susceptible species include cattle, humans, non-human primates, goats, cats dogs, pigs, buffalo, badgers, possums, deer and bison. Many susceptible species, including man, are spillover hosts in which infection is not self-maintaining. In countries where there is transmission of infection from endemically infected wildlife populations to cattle or other farmed animals, eradication is not feasible and control measures must be applied indefinitely. Possible methods of limiting spread of infection from wildlife to cattle including the use of vaccines are outlined. The usefulness of DNA fingerprinting of M. bovis strains as an epidemiological tool and of BCG vaccination of humans and cattle as a control measure are reviewed. The factors determining susceptibility to infection and clinical disease, and the infectiousness of infected hosts and transmission of infection, are detailed. Reports of the epidemiology of M. bovis infections in man and a variety of animal species are reviewed. M. bovis infection was recognised as a major public health problem when this organism was transmitted to man via milk from infected cows. The introduction of pasteurization helped eliminate this problem. Those occupational groups working with M. bovis infected cattle or deer, on the farm or in the slaughter house, are more likely to develop pulmonary disease than alimentary disease. In recent years, tuberculosis in farmed cervidae has become a disease of economic as well as public health importance in several countries. Nowadays, the human immunodeficiency virus (HIV) is associated with a greatly increased risk of overt disease in humans infected with Myobacterium tuberculosis. It is believed this increased risk also occurs in the case of M. bovis infections in humans.