Bovine tuberculosis control Flashcards
What bacteria causes Bovine TB?
Mycobacterium bovis
(Maybe M. Orygis in India)
What is the cost to the global economy of bovine TB?
$3 billion
What animals can bovine TB infect?
Includes cattle, humans, badgers, sheep, camelids, wild boar, deer etc.
What are the key features of a bovine TB infection?
Pulmonary disease, with involvement of draining lymph nodes
Transmission routes of bovine TB between cattle
Ill defined
Nasal secretions (direct contact)
Aerosols
Diagnosis of bovine TB in cattle
Routine slaughterhouse surveillance
Routine testing of cattle and slaughter of diagnosed animals
Pre-movement testing of cattle
Detailed follow up of herds that have had infections
Ante-mortem diagnostics (insensitive)
Routine bovine TB testing of cattle
Six monthly or annual testing of all animals in high incidence areas, four yearly testing of breeding stock in low incidence areas
Pre-movement testing of cattle being moved from or in high incidence areas
Ante-mortem diagnostic techniques for bovine TB
Delayed-type hypersensitivity testing
- insensitive but specific
Gamma interferon assay improves sensitivty
Culture of M. bovis
- gold standard but insensitive
Antibody tests
- poor sensitivity
Delayed-type hypersensitivity reaction (cell-mediated immune response) to M. Bovis extract
Single intradermal comparative cervical tuberculin test -SICCT
Sensitivity 50-95%
Specificity 99.98%
Gamma interferon assay improves sensitivity but relatively poor specificity (and cost) makes test unsuitable for routine surveillance. Now widely used in badger cull areas and in low risk area of England. Can lead to farms becoming classified as infected when they are not.
Antibody tests for Bovine TB
Poor sensitivity
+problems with cross-reactions, disease status, etc.
Epidemiology of bovine TB in the UK
Very heterogeneous across UK
Just cattle to cattle transmission in low incidence / ‘free’ Scotland and N England
Burden of wildlife challenge in SW England and parts of Wales
Badgers infection estimated to be associated with 50% of herd breakdowns in ‘trial’ areas (substantial cattle to cattle cycling within this figure)
Most commonly affected organs in bovine TB
Lungs
Pleura
Bronch-mediastinal LNs
Head LNs
Udder
Supra-mammary LNs
Liver
Hepatic LNs
Mesenteric LNs
Skin TB
Other (including kidney/spleen)
WHT and RHT in bovine TB testing
Whole herd test and Reactor herd test
Results of badger culling
Reduced incidence by ~25% in culled regions
But incidence increased by ~12% in peripheral regions due to perturbation
Led to suggestion that average rate of reduction in average sized culling region is 16%
Cattle vaccination for bovine TB
Massively hampered by need to continue testing vaccinated animals
Lack of available DIVA
Cost-benefit analysis suggests that it might not be feasible to get farmer uptake
Recurrence of bovine TB
50% of herds infected have breakdown in previous 3 years, 40% within 2 years
Could be due to
§ Persistence in cattle
§ Reintroduction from wildlife
§ Reintroduction from cattle movement
30% have ‘prolonged’ breakdowns
Up to 20% may have persistent infection when cleared, not all will lead to breakdowns
Most recurrence is due to reintroduction
Annual herd prevalence of bovine TB
16-18% in current annual testing areas - mostly recurrently infected farms
What % of bovine TB is detected in slaughterhouses?
22%
Vaccination of cattle for bovine TB
BCG- attenuated strain of M. Bovis
Less efficient than tuberculin testing
Experimentally: significant protection of calves neonatally vaccinated at 12 months
No significant protection at 24 months
Vaccination with BCG sensitises animals to tuberculin
Licensed for use in badgers -reduced risk of infection, not yet demonstrated to have an impact on the incidence of disease in cattle
Subject to complex rules with EC. Currently illegal, but legislation could be changed.
Unclear quite how much vaccination would interfere with testing
Edge area measures
Enhanced breakdown follow-ups/surveillance
To prevent geographic expansion of infected areas
Enhanced cattle control measures
Badger culling
Conclusion of ISG in 2006 said no meaningful contribution but was not really accepted
Estimation that culling in largish areas would result in 16% reduction overall
Anything that can do even this is a method that needs to be considered
However doesn’t mean you need to accept it on a ethical basis
Policy is still that intensive culling should be phased out eventually
