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

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

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).

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

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:

Granulomatous lesions:

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.

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.

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

A variety of mammalian species are susceptible among livestock, wild, and domestic animals (O’Reilly and Daborn, 1995).

Susceptible species, in addition to humans, include:

Livestock:

• Cattle (main host)
• Goats
• Pigs
• Sheep

Wildlife:

• Badgers
• Deer

Domestic animals:

• Cats
• Dogs

(WARNING: This post contains a graphic image!)

What is bovine tuberculosis (bTB)?

• Highly infectious, chronic respiratory disease
• Caused by the bacteria, Mycobacterium bovis
• Results in granulomatous lesions in respiratory tract and infected organs, and potentially death
• Affects livestock, wildlife, and domestic animals, as well as humans
• Has potential to greatly damage livestock industries as infected cattle must be culled to prevent widespread outbreak

Mycobacterium bovis

• Bacterium
• Replicates slowly
• Spreads through aerosal droplets or through fecal-oral route