Mycobacterium bovis infection and tuberculosis in cattle

This review considers the possible events that can occur when cattle are exposed to Mycobacterium bovis and, where appropriate, draws on principles accepted for tuberculosis infection in humans and laboratory animal models. Consideration is given to the many complex factors which influence the outcome of challenge with tubercle bacilli. These include features inherent to the mycobacterium, the host and the environment. It is apparent that clinical disease probably occurs only in a relatively small, but undetermined, proportion of cattle that are exposed to Al. bovis. The majority of animals may clear infection or control the bacilli, possibly in a condition of latency. It is concluded that a better understanding of the dynamics of the events following M. bovis exposure and subsequent infection in cattle would be of significant benefit in developing new tools appropriate for disease control and to designing optimal approaches for their application.     

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.     

Immunological approaches to the control of tuberculosis in wildlife reservoirs

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

Advances in understanding disease epidemiology and implications for control and eradication of tuberculosis in livestock: the experience from New Zealand

A deteriorating tuberculosis problem in cattle and deer in New Zealand has been halted and then reversed over the last decade. Mycobacterium bovis infection in both wild and domestic animal populations has been controlled. This has been achieved by applying a multi-faceted science-based programme. Key features of this have been a comprehensive understanding of the epidemiology of tuberculosis in animals, confidence in sampling wild animal populations, effective application of diagnostic tests in cattle and deer, and the ability to map M. bovis genotypes.     

An update on bovine tuberculosis programmes in Latin American and Caribbean countries

Of the approximately 374 million cattle in Latin America and the Caribbean, 70% are held in areas where rates of Mycobacterium bovis infection in cattle are higher than 1%. The remaining 30% are in countries where infection affects less than 1% of cattle, including 62 million in countries where bovine tuberculosis infection is virtually nil. Measures for controlling bovine tuberculosis are partially or extensively applied in most of the countries in the Region. These measures are based on test and slaughter, notification, post-mortem inspection and surveillance in slaughterhouses. A coordinated production, standardization and quality control of purified protein derivatives is urgently required for use in control and eradication campaigns in order to assure reliability of reagents and comparability of data on tuberculin testing within the Region. On the basis of information from Argentina, M. bovis is estimated to cause 2% of all human cases of tuberculosis in the Region. Slaughterhouse and dairy farms workers are most-frequently infected, with infection occurring via the respiratory tract. Various in vitro assays for the diagnosis of bovine tuberculosis have been developed and/or assessed in the Region, and DNA fingerprinting has been applied for a comprehensive understanding of the epidemiology of bovine tuberculosis at the local and regional level.     

Bovine tuberculosis in Europe from the perspective of an officially tuberculosis free country: trade, surveillance and diagnostics

Switzerland has been officially free of bovine tuberculosis (OTF) since 1960. Since 1980 the control of bovine tuberculosis (bTB) has been reduced to passive abattoir surveillance. Isolated cases of bTB, partly due to reactivation of human Mycobacterium bovis infections with subsequent transmission to cattle, have been noticed in the last years. In Europe, the overall prevalence of bTB is slightly increasing. Both OTF and non-OTF countries report increases in the proportion of bTB positive cattle herds. Current bTB eradication and control programs in Europe are facing a range of challenges. Whole herd depopulation is becoming a less attractive option for economic reasons and due to animal welfare concerns. Live animal trade is increasing both at national and international levels. Regarding these tendencies and taking into account the chronicity of bTB infection, pre-movement testing is becoming increasingly important as a central tool for eradication and for protection against re-introduction of bTB. Pre-movement testing, however specifically focuses on the infection status in individuals, requiring a high level of diagnostic accuracy to correctly diagnose infected animals. Current screening tests for bTB, however, have been designed to meet demands as herd tests. This illustrates that the modification of existing and/or the development of new diagnostics for bTB might be needed. The tuberculin skin test (TST), the primary screening test for bTB may in certain situations have low sensitivity. The interferon gamma (IFN-γ) assay is accepted to be more sensitive compared to TST. Reduced specificity, however, especially in areas of low bTB prevalence raises concerns. New antigen combinations including Rv3615c, OmpATb and others have been shown to complement ESAT-6 and CFP-10 in the whole blood IFN-γ assay and resulted in improved sensitivity (compared to ESAT-6 and CFP-10) and specificity (compared to tuberculins). Lesion detection after slaughter represents a cost-effective procedure for passive surveillance of bTB, especially in areas of low prevalence or in regions free of bTB; however, its sensitivity is very low. This illustrates that trade is linked with a certain risk to re-introduce bTB in OTF regions or countries and that there may be delays in detecting a re-introduction of bTB. In conclusion, regarding the fact that some parameters linked with bTB programs are changing, the development of improved diagnostic tests with a high reliability for use as individual animal tests will be important for future eradication of bTB, in line with international commitment to high standard animal health programs.     

A review of M. bovis BCG protection against TB in cattle and other animals species

Bovine tuberculosis (TB) causes severe economic losses in livestock due to low production, animal deaths and condemnation of carcasses. It is also an important constraint in international trade of animals and animal products. A scientific committee in Great Britain in 1997 concluded that the development of a cattle vaccine would be the best option for long-term control of TB. However, vaccination of cattle currently is not accepted because the vaccine interferes with the skin reaction to the tuberculin test in the field. Efficacy of M. bovis BCG in protecting bovine and other animal species against tuberculous infection has received much study. Vaccination of cattle prevents the spread of the disease in populations by reducing the number and size of the lesions, and the load of bacteria (rather than by preventing infection). We review the literature about the efficacy of BCG in protecting cattle and other animal species against infection with field strains of M. bovis and discusses its potential use in programs of TB control in high-prevalence populations.     

New generation vaccines and delivery systems for control of bovine tuberculosis in cattle and wildlife

Advances in the understanding of protective immune responses to tuberculosis are providing opportunities for the rational development of improved vaccines for bovine tuberculosis. Protection requires activation of macrophages through stimulation of a Th 1 type immune response. Ideally, a vaccine for cattle should induce protection without causing animals to react in a tuberculin test when exposed to Mycobacterium bovis. A number of new tuberculosis vaccines including attenuated M. bovis strains, killed mycobacteria, protein and DNA vaccines have been developed and many of these are being assessed in cattle. The requirements for a tuberculosis vaccine for wildlife differ from those for cattle. The major goal of a wildlife vaccine is to prevent the transmission of M. bovis to cattle and other wildlife. Although there are a number of technical problems associated with the development of a vaccine delivery system for wildlife, attenuated M. bovis vaccines administered via oral baits or aerosol spray to possums have already been shown to reduce the severity of a subsequent M. bovis infection.     

Tuberculosis in goats: assessment of current in vivo cell-mediated and antibody-based diagnostic assays

Tuberculosis in goats, mainly caused by Mycobacterium bovis and M. caprae, is a zoonotic disease with implications for public health, as well as having an economic impact due to decreased goat production, increased mortality rates and costs of diagnosis. There is an increasing need for surveillance of tuberculosis-infected goat herds, particularly in countries that are not officially free of bovine tuberculosis, and goats sharing farms with cattle should be subjected to the official tuberculin test. In Spain, some regions have programmes for the control of tuberculosis in goats, applying the same diagnostic assays that are used for cattle. The objective of tuberculosis eradication in livestock requires adaptation of existing control strategies to include goats. As such, it is necessary to determine whether current diagnostic assays for tuberculosis in cattle will work as efficiently in the goat. This review provides an overview of current in vivo tools for diagnosis of caprine tuberculosis, including estimates of the sensitivity and specificity of tests performed in this species. The number of tested animals and co-infection with Mycobacterium avium subsp. paratuberculosis are also addressed, with the aim of demonstrating the limitations of current assays and the need for further research.     

Tuberculosis in cattle herds are sentinels for Mycobacterium bovis infection in European badgers (Meles meles): the Irish Greenfield Study

In Ireland badgers are removed in response to tuberculosis (TB) breakdowns in cattle herds (focal culling). Prevalence studies, conducted using a detailed post mortem and bacteriological examination, showed that 36-50% of badgers were infected with Mycobacterium bovis. Focal culling forms part of the medium term national strategy for the control of bovine TB in cattle and is based on the premise that badgers in areas with herd breakdowns have a higher prevalence of infection than the badger population at large. However, the hypothesis that cattle can be used as sentinels for infection in the badger population has never been formally tested. In this study we tested the hypothesis by determining the infection prevalence in badgers in areas where there had been historically, a consistently low prevalence of infection in cattle. Low cattle TB prevalence areas were defined as those herds with ≤ 2 standard reactors in the annual round of skin testing over the preceding 5 years (Greenfield sites). Using GIS, and adjusting for variation in land use, previous culling and cattle density, 198 Greenfield sites were identified and surveyed, and 138 areas with badger setts or signs of badger activity were identified. A single badger was removed from 87 sites and all were examined using detailed post mortem and bacteriological procedures. A prevalence of M. bovis infection of 14.9% was found in the Greenfield site badgers. This prevalence was significantly lower (P<0.001) than in badgers removed during focal culling (36.6%). The results validate the use of cattle as sentinels for TB in badgers and support the medium term national strategy for the control of bovine TB. The geographic variation in M. bovis infection prevalence in the Irish badger populations will be used when devising strategies for the incorporation of badger vaccination into the long term bovine TB control programme.     

Pathologic findings and association of Mycobacterium bovis infection with the bovine NRAMP1 gene in cattle from herds with naturally occurring tuberculosis

OBJECTIVE: To determine necropsy and Mycobacterium bovis culture results in cattle from herds with tuberculosis, the role of the bovine NRAMP1 gene in resistance and susceptibility to infection with M bovis, and the association between magnitude of the tuberculous lesions and various types of M bovis isolates. ANIMALS: 61 cattle from herds with tuberculosis in Texas and Mexico. PROCEDURE: 61 cattle were evaluated by necropsy; 59 had positive and 2 had negative caudal fold tuberculin intradermal test (CFT) results. Thirty-three cattle with positive CFT results were genotyped to evaluate polymorphism of the 3' untranslated region of the bovine NRAMP1 gene, using single-stranded conformational analysis, 9 were resistant to M bovis with no tuberculous lesions and negative M bovis culture results, and 24 were susceptible with tuberculous lesions and positive M bovis culture results. Isolates of M bovis were analyzed by restriction fragment length polymorphism (RFLP) on the basis of IS6110 sequences and direct-repeat fingerprinting patterns. RESULTS: 21 (35.6%; 21/59) cattle with positive CFT results had tuberculous lesions or positive culture results; in addition, 1 of 2 cattle with negative CFT results had tuberculous lesions and positive culture results. Tuberculous lesions were most common in the thorax (35/63; 55.5%) and lymphoid tissues of the head (10/63; 15.9%). Tuberculous lesions varied from 1 to 11/animal; 8 of 21 (38.1%) had solitary lesions. Associations were not found between resistance or susceptibility to infection with M bovis and polymorphism in the NRAMP1 gene or between the magnitude of the lesions and various RFLP types of M bovis isolates. CONCLUSIONS AND CLINICAL RELEVANCE: The NRAMP1 gene does not determine resistance and susceptibility to infection with M bovis in cattle.     

Identification of Mycobacterium bovis in bovine clinical samples by PCR species-specific primers.

Tuberculosis, caused by Mycobacterium bovis is emerging as the most important disease affecting cattle. Furthermore, it results in a major public health problem when transmitted to humans. Due to its difficult and non-specific diagnosis, M. bovis has been declared to be one of the etiologic agents causing significant economic loss in the cattle industry. Our group evaluated a more rapid and specific method, based on a new polymerase chain reaction species-specific primers, which amplifies a 470-base pair fragment of the M. bovis genome. A total of 275 milk-producing cows were studied by intradermal tuberculin test (ITT) which gave 184 positive and 91 negative cases. From them, 50 animals were taken from a cattle ranch free of tuberculosis. Three different samples were collected from each animal (blood, nasal mucus, and milk). Positive results were obtained from 26 animals by PCR (11.4%), 1 by bacteriological culturing (0.4%) and 1 by bacilloscopy (0.4%). This finding suggests, as in previous reports, that ITT, normally used for bovine tuberculosis detection, has the inconvenience of having a broad range of specificity and sensitivity, and the PCR technique is a more specific and sensitive test to detect infection associated with M. bovis. Therefore, we propose this PCR assay as a useful tool in the epidemiological characterization of infected animals in areas considered to be at high risk of transmission.     

Evaluation of an in vitro blood-based assay to detect production of interferon-gamma by Mycobacterium bovis-infected white-tailed deer (Odocoileus virginianus).

Tuberculosis due to Mycobacterium bovis in captive Cervidae was identified as an important disease in the United States in 1990 and prompted the addition of captive Cervidae to the USDA Uniform Methods and Rules for eradication of bovine tuberculosis. As well, M. bovis infection was identified in free-ranging white-tailed deer in northeast Michigan in 1995. Tuberculosis in both captive and free-ranging Cervidae represents a serious challenge to the eradication of M. bovis infection from the United States. Currently, the only approved antemortem tests for tuberculosis in Cervidae are the intradermal tuberculin skin test and the blood tuberculosis test (BTB). At present, the BTB is not available in North America. Tuberculin skin testing of Cervidae is time-consuming and involves repeated animal handling and risk of injury to animals and humans. This study evaluated the potential of a new blood-based assay for tuberculosis in Cervidae that would decrease animal handling, stress, and losses due to injury. In addition, a blood-based assay could provide a more rapid diagnosis. Twenty 6-9-month-old white-tailed deer, male and female, were experimentally inoculated by instillation of 300 colony-forming units of M. bovis in the tonsillar crypts. Seven, age-matched uninfected deer served as controls. Blood was collected on days 90, 126, 158, 180, 210, 238, 263, and 307 after inoculation and was analyzed for the production of interferon-gamma (IFN-gamma) in response to incubation with M. bovis purified protein derivative (PPDb), M. avium PPDa, pokeweed mitogen (PWM), or media alone. Production of IFN-gamma in response to PPDb was significantly greater (P < 0.05) at all time points in samples from M. bovis-infected deer as compared with uninfected control deer, whereas IFN-gamma production to PWM did not differ significantly between infected and control deer. Measurement of IFN-gamma production to PPDb may serve as a useful assay for the antemortem diagnosis of tuberculosis in Cervidae.     

A review of the contemporary knowledge of bovine tuberculosis and government policy in Iran

In 1931 Carpantier reported bovine TB (BTB) in Iranian cattle. Some eighty years on with a national test-and-slaughter programme in place for over four decades, the efforts to vanquish Mycobacterium bovis (M. bovis) infection in cattle have been in vain as the vast majority of the 30 Iranian provinces still have reports of BTB in their cattle herds every year. This paper reviews the present epidemiology of BTB in Iran and in the region and evaluates the success of government policy in controlling this disease.     

Implementation of two-step vaccination in the control of bovine viral diarrhoea (BVD)

Bovine viral diarrhoea (BVD) control/eradication programmes based on the test and removal of persistently infected cattle without use of vaccination were first introduced by the Scandinavian countries in the early 1990s. Within the last 10 years the programmes have proven to be very successful and have served as a blueprint for several other European regions. However, in areas with high cattle densities, intense animal trade and high BVD prevalence this control approach is risky, because there is a high probability that herds, which have been cleared of persistently infected (PI) animals and have become partly or fully susceptible to reintroduction of the virus, will come in contact with a BVD virus (BVDV) infected animal. A combination of the test and removal strategy with subsequent systematic vaccination of cattle could overcome this problem. The goals of vaccination in such a programme is protection against reintroduction of BVDV into herds free from PI cattle and foetal protection of pregnant animals accidentally exposed to the virus. Two-step vaccination is based on the use of inactivated BVDV-1 vaccine for priming followed by a live attenuated vaccine booster 4 weeks later. The immune response elicited by such a vaccination scheme has proven to be long lasting and foetal infection after challenge with BVDV-1 and BVDV-2 was prevented in pregnant animals 5 months after vaccination. These findings suggest that the implementation of a two-step vaccination in the initial phase of control programmes in addition to test and removal of PI animals in areas with high cattle densities and endemic BVD is practical and efficacious.     

The epizootiological significance of positive bacteriological findings on Mycobacterium tuberculosis and Mycobacterium bovis in humans

The relation between the active form of tuberculosis in persons working in agriculture and incidence of tuberculosis in cattle was analyzed in 1974 to 1978, i.e. in the period after the elimination of bovine tuberculosis in Czechoslovakia (in 1968). M. tuberculosis was isolated in 15 cases and M. bovis in four cases of persons employed by the farms on which the Regional Hygienic Station, Brno, was responsible for the microbiological diagnostics of tuberculosis. Direct contact with animals was demonstrated in eight patients; M. tuberculosis was isolated from seven of these patients and M. bovis from one. Seven cattle herds were exposed to spontaneous infection by M. tuberculosis and in one of them tuberculosis was not demonstrated during complex examination. In three herds the examination revealed only a sensitivity of cattle to mammalian tuberculin. In other three herds tuberculosis was detected by allergic tests, patho-anatomic examination and bacteriological examination. M. tuberculosis in cattle was detected in two herds. The occurrence of bovine tuberculosis caused by a cattle tender with a positive finding of M. bovis in sputum was demonstrated in one herd. Virulence for the tested cattle was found in one strain (isolated from a mesenterial lymph node of cattle) of the four strains of M. tuberculosis used for the experimental infection of 17 animals. On the other hand, in three strains of M. tuberculosis, trials with experimental infection demonstrated only allergy to mammalian tuberculin and changes at the sites of subcutaneous inoculation of mycobacteria of regressive nature; these mostly disappeared within 90 days from infection.     

Evaluation of two cocktails containing ESAT-6, CFP-10 and Rv-3615c in the intradermal test and the interferon-γ assay for diagnosis of bovine tuberculosis

The intradermal tuberculin tests and the interferon-gamma (IFN-γ) assay are the principal tests used worldwide for the ante-mortem diagnosis of bovine tuberculosis. The conventional reagent currently in use in these tests is purified protein derivative (PPD) tuberculin obtained from Mycobacterium bovis culture. The components of PPD are poorly characterized and difficult to standardize. To overcome this issue, antigens specific to the Mycobacterium tuberculosis complex are being studied. Here we have assessed the biological potency of ESAT-6, CFP-10 and Rv-3615c presented as peptide or recombinant protein cocktails in comparison with the standard bovine PPD used routinely in Spanish eradication campaigns. The study was performed in cattle (n=23) from a herd with natural M. bovis infection. Animals were simultaneously injected with PPD and the peptide and protein cocktails. The percentages of cattle reacting positively to single intradermal test were 60.9% (bovine PPD), 47.8% (peptide cocktail) and 60.9% (protein cocktail), with no significant difference between the actual skin fold thickness increases (p>0.05). The IFN-γ assay detected 60.9% of animals when stimulation was performed with bovine PPD, but decreased to 52.2% when stimulation was performed with the peptide cocktail and to 47.8% when stimulation was performed with the protein cocktail. However, no significant differences were found between IFN-γ responder frequencies (p>0.05). These results show a potential use of these defined reagents for in vivo tuberculosis diagnosis.     

Further evaluation of an indirect enzyme-linked immunosorbent assay for the diagnosis of bovine tuberculosis

The sensitivity and specificity of an ELISA for the detection of bovine IgG anti-Mycobacterium bovis antibodies were 73.6% and 94.1%, respectively, as determined in 53 bacteriologically confirmed tuberculous cattle and 101 healthy cattle from a tuberculosis-free area. In addition, the results of ELISA and tuberculin tests in 149 cattle were compared with those of subsequent necropsy studies. Both tests failed to detect 2 animals with tuberculous lesions and positive culture; 3/12 cattle with M. bovis isolation and no lesions, and 2/7 with atypical mycobacterial infection reacted to tuberculin, but none had antibodies; in 128 cattle with neither lesions nor mycobacterial isolation, 6 were tuberculin reactors and 7 others had antibodies. Negative results were obtained by ELISA in 21/22 paratuberculous cattle. Antibodies were not detected in 88.9% to 96.4% of 697 cattle from two tuberculin negative herds of an endemic area. In a herd with proved M. bovis infection, distribution of seropositive animals in tuberculin and non-tuberculin reactors was similar. Antibody responses to cutaneous tuberculin stimuli were observed in 4 experimentally infected cattle, but only in 2/10 healthy controls after repeated PPD stimuli. Nine controls which had either received a single tuberculin dose or none showed no increase in antibody levels. The low sensitivity of this ELISA limits its usefulness as a diagnostic tool for bovine tuberculosis eradication campaigns. However, it could be helpful in epidemiological surveillance if its efficiency to identify infected herds is demonstrated.     

BCG vaccination against tuberculosis in European badgers (Meles meles): a review

Tuberculosis (TB) is a significant animal health problem in many parts of the world, and reservoirs of infection in wild animals complicate disease control efforts in farmed livestock, particularly cattle. Badgers (Meles meles) are a significant wildlife reservoir of Mycobacterium bovis infection for cattle in the United Kingdom (UK) and Republic of Ireland (ROI). Vaccination of badgers using an M. bovis strain bacille Calmette-Guérin (BCG) vaccine could potentially be an option in the national TB eradication strategy. Wildlife vaccination has been used successfully for other diseases in wildlife species, and may have a role to play in reducing M. bovis transmission at the wildlife-livestock interface. Research to date has provided evidence that BCG is protective in badgers, and a parenteral badger BCG vaccine has been licensed in the UK. Further research is required to develop effective strategies for vaccine deployment and to determine the effect of badger vaccination on cattle TB incidence.