Fertility Control

354-welsh-badgers2

Research is currently underway by Defra (Department for Environment, Food & Rural Affairs) to develop a female oral contraceptive for badgers. Ideally, the contraceptive would be available to female badgers through oral baits and would aid in reducing population growth by preventing these females from reproducing. Mathematical models have predicted that fertility control alone would not be effective in eradicating bTB, but would need to be used in combination with other strategies such as vaccination or culling (Swinton et al., 1997).

This potential alternative is not yet available.

Management

Badger sneaking into cattle housing while foraging for food, as possible route of transmission

While management strategies alone are not likely enough to eradicate bTB, cattle management practices can be used to greatly reduce the probability of badger-to-cattle transmission.

Possible Management Practices:

  • Prevent badgers from accessing cattle housing
    • Keep buildings and gates closed, secured
    • Raising cattle feed off the ground to exclude badger access
  • Rotational grazing system
    • To reduce exposure of cattle to potentially badger-contaminated pasture
  • Fencing off badger setts

Cattle Vaccine?

Currently there is no bTB vaccine available legally for cattle.

While the Bacillus Calmette-Guérin (BCG) vaccine could potentially be used to vaccinate cattle, the current test available to detect bTB known as the Single Intradermal Comparative Cervical Tuberculin (SICCT) skin test cannot differentiate between vaccinated and infected animals. Thus, BCG vaccines for cattle are not yet approved by the European Union for use. For now, new diagnostic tests for bTB are being developed, known as the Differentiate Vaccinated from Infected Animals (DIVA) tests. Vaccinations are expected to be available for use once these diagnostic tests are fully approved in cattle, which is estimated to be within the next 10 years.

For more information, see: http://www.tbfreeengland.co.uk/vaccination/ 

Badger BCG Vaccine

Badger being injected with BCG vaccine after cage-trapped

What is it?

  • Vaccine against bovine tuberculosis for badgers
  • Known as the Badger BCG (Bacillus Calmette-Guérin)
  • Commercially available in 2010, but only available as injection

This video shows how badgers are vaccinated.

Pros?

  • Do not need to euthanize any wildlife
  • Can reduce risk of bTB-positive badgers, and decrease spread across badger social groups
  • Herd immunity, where enough individuals are protected from bTB by vaccination to prevent entire group from being infected

Cons?

  • Expensive!!
  • Need to live cage-trap badgers and check cages every morning (labour-intensive)
  • Need to give booster annually for min. 5 years
  • Does not cure infected animals
  • More effective in younger badgers, which are more likely to be hidden in badger setts

Costs?

  • Estimated over 5 years = £3,365 per vaccinated badger

Vaccinations

Vaccinations are the major alternative to badger culling to control bTB. While the use of vaccines is not effective if the animal is already infected, vaccinations are able to provide protective immunity against bTB. This is especially important in limiting the spread of bTB in susceptible populations. The following posts will describe current vaccines available to use for badgers and cattle, effectiveness, and the associated costs.

Epidemiology

Cases of bovine tuberculosis has been recorded throughout the world, with Britain among the hardest hit:

Reported bTB cases in cattle from countries around the world in 2010 (Source: OIE World Animal Health Information System)

Southwest Britain especially has an increasingly high prevalence of bTB with more than 28,000 cases in cattle reported in 2010.

Badger to Cattle Transmission

A cow’s curiosity brings it closer to a badger

While cattle are the main animals to be infected with bTB, various scientific reports such as by Krebs (1997) suggest that Eurasian badgers (Meles meles) act as a wildlife reservoir and can transmit bTB to cattle.

Badger infection rate is estimated to be ~4-6% (RSPCA), and badger to cattle transmission may account for 5.7% of the total bTB infections in Great Britain cattle. The rest of the cattle infections are thought to derive from cattle-to-cattle transmission (McDonald, 2014).

Pie chart
Percentage of bTB transmission source to cattle, according to McDonald (2014)

In order for badger to cattle transmission, certain criteria must be met:

  • susceptible cattle
  • badgers must be excreting bacteria such that cattle can encounter it
  • cattle must encounter min. infectious dose of bacteria (5 bacilli through aerosols)

While transmission is thought to occur through aerosols, feces, or urine, the exact mechanism of transmission from badger to cattle is unknown and controversial (Lodge and Matus, 2014).

Potential transmission mechanisms include:

  • Contaminated pasture (urine/feces from badger encountered by cattle)
    • Unlikely since cattle avoid contaminated feed (Benham and Broom, 1991)
  • Badger visits to cattle housing while foraging for food
    • Enhanced transmission with low ventilation housing
    • Badgers would tend to visit cattle feed stores -> possible contamination
  • Direct contact with cattle
    • When bTB in late, terminal stage, badger behaviour changes (more active and less fearful of cattle)
    • Cattle very curious
    • When combined… increased contact between badgers and cattle

Pathology

WARNING: The following post contains graphic images!

Bovine tuberculosis is often identified and characterized by large lesions on affected organs, primarily on lymph nodes and throughout the respiratory tract. These lesions greatly impair respiratory function and the overall health of the infected animals.

Diseased vs. normal lung:

bTB-infected vs. normal lungs from possums

 

Granulomatous lesions:

bTB Lesions on cattle lung

 

Infected alpaca lungs illustrate loss of respiratory function during bTB

Risk to Humans

Bovine tuberculosis can potentially infect humans, resulting in lesions across the respiratory tract and body. Humans can contract bTB from cattle primarily through drinking milk contaminated with Mycobacterium bovis. However the overall risk to humans is LOW since milk is now routinely pasteurized.

Child with facial lesions from bTB, infected from drinking contaminated milk

Other animals can potentially also spread bTB to humans during close contact such as through slaughter or veterinary care. Accidental exposure in laboratories is another possible route of infection. These routes of infection are highly unlikely though.

Infection routes for humans

Disease Progression

Bovine tuberculosis progresses from initial infection, subclinical symptoms, to a severe disease state riddled with numerous lesions. Death may eventually result.

Step 1: Infection

  • Bacteria enters respiratory system
  • Engulfed by immune cells
  • Carried to lymph nodes and throughout respiratory tract

Step 2: A silent danger 

  • Symptoms often do not appear for long periods of time following infection (subclinical infection)

Step 3: Lesions!

  • Development of numerous lesions on infected organs

Step 4: Death?

  • Weakened, emaciated state
  • May eventually result in death