Bovine tuberculosis control

Question 1

What bacteria causes Bovine TB?

Answer 1

Mycobacterium bovis

(Maybe M. Orygis in India)

Question 2

What is the cost to the global economy of bovine TB?

Answer 2

$3 billion

Question 3

What animals can bovine TB infect?

Answer 3

Includes cattle, humans, badgers, sheep, camelids, wild boar, deer etc.

Question 4

What are the key features of a bovine TB infection?

Answer 4

Pulmonary disease, with involvement of draining lymph nodes

Question 5

Transmission routes of bovine TB between cattle

Answer 5

Ill defined

Nasal secretions (direct contact)
Aerosols

Question 6

Diagnosis of bovine TB in cattle

Answer 6

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)

Question 7

Routine bovine TB testing of cattle

Answer 7

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

Question 8

Ante-mortem diagnostic techniques for bovine TB

Answer 8

Delayed-type hypersensitivity testing
- insensitive but specific

Gamma interferon assay improves sensitivty

Culture of M. bovis
- gold standard but insensitive

Antibody tests
- poor sensitivity

Question 9

Delayed-type hypersensitivity reaction (cell-mediated immune response) to M. Bovis extract

Answer 9

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.

Question 10

Antibody tests for Bovine TB

Answer 10

Poor sensitivity

+problems with cross-reactions, disease status, etc.

Question 11

Epidemiology of bovine TB in the UK

Answer 11

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)

Question 12

Most commonly affected organs in bovine TB

Answer 12

Lungs

Pleura

Bronch-mediastinal LNs

Head LNs

Udder

Supra-mammary LNs

Liver

Hepatic LNs

Mesenteric LNs

Skin TB

Other (including kidney/spleen)

Question 13

WHT and RHT in bovine TB testing

Answer 13

Whole herd test and Reactor herd test

Question 14

Results of badger culling

Answer 14

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%

Question 15

Cattle vaccination for bovine TB

Answer 15

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

Question 16

Recurrence of bovine TB

Answer 16

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

Question 17

Annual herd prevalence of bovine TB

Answer 17

16-18% in current annual testing areas - mostly recurrently infected farms

Question 18

What % of bovine TB is detected in slaughterhouses?

Answer 18

22%

Question 19

Vaccination of cattle for bovine TB

Answer 19

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

Question 20

Edge area measures

Answer 20

Enhanced breakdown follow-ups/surveillance

To prevent geographic expansion of infected areas

Enhanced cattle control measures

Question 21

Badger culling

Answer 21

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