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Mycobacterium bovis

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This is one of two species of the ‘tubercle bacilli’ (the other is M. tuberculosis) that are able to cause tuberculosis.

Unlike M. tuberculosis, M. bovis infects cattle and other animals, and so the disease can be spread to humans via contaminated milk and meat. We gave TB to cows 40-50,000 years ago. From the same genus as leprosy – which diverged from it 40m years ago, long before there were humans to get it…

Growth and Control Survival Inactivation (CCPs and Hurdles) Clinical Notes Reservoirs / Sources Plague and Pestilence
A key characteristic of the organism is that it is very slow growing, and so given the shelf life of foods that it has been associated with – such as unpasteurised milk and raw meat – growth in foods is unlikely to be significant. The organism is a microaerophile.

Temperature

Survival is better under cool conditions, e.g. survived in cow faeces for 5 months in winter and 2 months in summer.

Water Activity

Survives dry conditions well.

Temperature

Inactivated by normal pasteurisation. An inoculum of 105 / ml cells became undetectable after 30 minutes at 63.5ºC in whole milk (low temperature, long-time pasteurisation). In meat products the D time at 61°C was 1 min, while at 55°C it was approximately 10 min. Treatment at 65°C for 5 min gives a 5 D kill.

Sanitisers / Disinfectants

Ortho-phthalaldehyde applied at the minimum effective concentration result in a 6 log reduction in 5 minutes. A study of 14 hospital disinfectants found chlorine dioxide, 0.8% hydrogen peroxide plus 0.06% peroxyacetic acid, glutaraldehydes (2% alkaline and 2% acidic, a phenolic and chlorine (approx. 1,000 ppm) and an iodophor were effective, some quaternary ammonium compounds and 0.13% glutaraldehyde plus 0.44% phenol plus 0.08% phenate were ineffective. On their own chlorhexidine diacetate and cetylpyridinium chloride are relatively ineffective, but their action may be improved in the presence of ethambutol.

Radiation

Inactivated by sunlight.

Don’t forget to read the disclaimer!

Incubation

In airborne infections and in immunocompetent people the incubation period can be years, while in the immunosupressed it may be months. Cases of the gastrointestinal form can occur after reactivation of infections that must have occurred many years earlier.

Symptoms

Fever, chills, weight loss, abdominal pain, diarrhoea or constipation. Other symptoms depend on the organs infected. Symptoms may last for months or years. The organism enters the body via the intestinal tract in foodborne infections, and primary infection is set up in the associated lymph nodes to form ‘tubercles’. The infection is often contained at that point, but it can also spread to other parts of the body to cause illness. The reverse can be true in that pulmonary disease can spread to the intestinal region.

Condition

Intestinal tuberculosis or tuberculous enteritis.

Toxins

Does not produce toxins.

At Risk Groups

Immunosupressed people are especially at risk of either acute infection or reactivation of an infection acquired in the past. In countries where infection is uncontrolled children are at greater risk of infection.

Long Term Effects

The course of the disease is long term and may result in death.

Dose

The infectious dose for organisms ingested (as opposed to inhaled) is probably very high (millions of organisms).

Treatment

Multiple antibiotic treatment is required to be administered over protracted periods. This is because the organism may have antibiotic resistance and this will not be apparent for long periods because of the slow growth of the organism. The antibiotics currently used are rifampicin, isoniazid and ethambutol. Infected lymph nodes can be removed. Multiple drug resistant forms have caused outbreaks among AIDS patients that resulted in deaths of all those infected.

Human

Humans are a reservoir of the organism, but human to human infection occurs only rarely.

Animal

Cattle and other animals are the main reservoirs of the organism.

Food

Meat and milk derived from infected animals may contain the organism. Tubercles are detectable post-mortem in food animals, and infection can also be detected using an immunological test.

Environment

Can persist and remain infective in the environment for long periods.

Transmission Routes

Can be by respiratory aerosols between humans and animals. To a lesser extent also transmitted by milk and meat derived from infected animals. Apparently not transmitted by the waterborne route.

Outbreaks

The last outbreak attributed to contaminated milk occurred in the UK in 1959.

Epidemiological Data

A study in Ireland on disease caused by M. bovis between 1983 and 1992 found that most cases could be attributed to the prior consumption of milk in the pre-pasteurisation era.

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