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Home / Case studies / #03
Case study #03
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In three cattle farms (EXP1-3) (beef or dairy cattle) the official tests according to the National Bovine Tuberculosis (TB) Eradication Program were carried out. Specifically, in the single intradermal tuberculin test (SIT), the increase in the skinfold thickness after the application of bovine tuberculin (PPD) was 4 and 5 mm in 2 animals from EXP1 and EXP3 respectively, both of which have been classified as Officially Tuberculosis-Free (OTF) to date. In EXP2, which already had a previous history of TB, 31 animals showed an skin fold thickness increase of 3 mm or more. The animals showed no apparent signs of disease.
According to the current legislation, animals that test positive for SIT must be slaughtered.
Figure 1: Application of the tuberculin intradermal test on a dairy cattle farm
(source: VISAVET-UCM)
Figure 2: Positive reaction to the SIT test (source: VISAVET-UCM)
Figure 3: Information on farms and number of reactors
The EXP1 and EXP3 positive animals were subjected to slaughter, with a sanitary vacuum recommended in EXP2. Post-mortem examination of the animals at the slaughterhouse did NOT reveal the presence of macroscopic lesions compatible with TB in the 2 animals in EXP1 and EXP3. However, granulomatous lesions were observed in some EXP2 animals.
In accordance with the current regulations, bacteriological culture was performed on all animals slaughtered from the 3 farms (EXP1=1; EXP2=54; EXP3=12). Bacterial growth was observed on specific culture media after 15-60 days of incubation, and PCR identified the isolates as MTBC species.
Molecular characterisation of the isolates (spoligotyping) provided distinct profiles (M. tuberculosis SIT2537, M. tuberculosis SIT1564/M. bovis SB0339, M. tuberculosis SIT58).
Figure 4: Application of the tuberculin intradermal test on a dairy cattle farm
(source: VISAVET-UCM)
Figure 5: Macroscopic lymphnode lesions compatible with TB in some animals of EXP2
(source: VISAVET-UCM)
Figure 6: Summary with individual information on bacteriological culture-positive animals
To determine the possible origin of the infection, an epidemiological survey was carried out on the farm, assessing, among other aspects, contact with other domestic and wild reservoirs, as well as with humans. No evidence of epidemiological links with other animal species was found and adequate biosecurity measures were in place on the farm. Subsequently, the Public Health inspection confirmed the existence of three workers with active TB in the three farms. In addition, it was demonstrated that the bacteria isolated from the animals and the workers were genetically identical.
Figure 7: One Health concept (source: OMSA)
Secondary questions
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Main question
What conclusion(s) do you draw from this case?
Options:
- Option 1: Bovine TB is caused only by M. bovis.
- Option 2: Epidemiological investigations should include all possible sources of transmission, including other species and direct or indirect contact with infected animals.
- Option 3: The absence of lesions in SIT-positive animals indicates that these animals are clearly false positives.
- Option 4: The TB status of the farm and the measures to be taken should not be affected by the isolation of M. tuberculosis in cattle.
Solution
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This case describes an outbreak of tuberculosis in cattle caused by Mycobacterium bovis and M. tuberculosis. Mycobacterium bovis is the main aetiological agent of bovine TB while M. tuberculosis mainly infects humans. However, both agents can infect humans and animals indistinctly and, in both cases, it would be a case of bovine TB according to the recommended EU definition, which identifies an outbreak of bovine TB as any outbreak in cattle caused by any of the MTBC species. What was exceptional and interesting about this case was precisely the isolation of M. tuberculosis from cattle, which is rare, especially in Europe. This mycobacteria is not well adapted to the bovine host, although it can still infect it, the immune response is therefore less intense, which can lead to a lack of reactivity in official diagnostic tests.
This lower reactivity further justifies the use of strict criteria (considering inconclusive results of 3 mm as positive) to increase sensitivity, which would be recommended in farms with a history of the disease or in regions of high prevalence. Furthermore, M. tuberculosis rarely causes detectable macroscopic lesions in infected animals, and it is unlikely that infected cattle can transmit this bacteria to other animals or humans, with a much higher risk when M. bovis is involved, as was the case in EXP2, where the only animals showing macroscopic lesions were those infected with M. bovis. The isolation of M. tuberculosis in animals is usually related to an infection of human origin (also referred to as reverse zoonosis), as in this case, where molecular epidemiology proved its usefulness as a tool to determine the origin of TB outbreaks.
The only correct answer to the question we ask is that epidemiological investigations should include all possible sources of transmission, including other species and direct or indirect contact with infected animals.
If you find this case interesting, this case was based on actual data from an outbreak that took place in Spain. All information is available in the journal Emerging Infectious Disease (Humans as source of Mycobacterium tuberculosis infection in cattle, Spain. 2011 Dec;17(12):2393-5. doi: 10.3201/eid1712.101476).
Figura 8: Possible sources of animal TB infection (source: Bernat Pérez de Val y Ana Balseiro. Pequeños rumiantes y tuberculosis en España. Albéitar 11/2019, 228:26-29)
