“Cats have passed TB to humans for the first time,” the Daily Mail reports. Authorities are closely monitoring the situation and the risk of further transmission has been described as “very low”.
The headline is based on the news that two people in England have developed tuberculosis (TB) after contact with a domestic cat infected with Mycobacterium bovis (M. bovis). This bacterium is a leading cause of TB in cattle and a less common cause, in other species.
This is newsworthy as these are the first documented cases of cat-to-human transmission anywhere in the world.
Between December 2012 and March 2013, a veterinary practice in Newbury, Berkshire, diagnosed nine cases of M. bovis infection in domestic cats. Two people who had contact with these cats were found to have an active TB infection.
Cat owners are advised to not be anxious. Though TB can be spread from animals to humans, the risk of cat owners or their families becoming infected is thought to be very low. If owners have a pet that is unwell they should consult a vet.
When cases of M.bovis in animals are diagnosed, current animal health legislation in England requires vets to notify the Animal Health and Veterinary Laboratories Agency (AHVLA) who then inform Public Health England (PHE).
The local AHVLA, Health Protection Team and veterinary surgery will provide support based on the individual circumstances.
What is the risk to people?
PHE, the agency responsible for public health in England, offered screening to 39 people who were identified to have had contact with the infected cats as a precautionary measure. Two cases of active TB were identified, both of whom were confirmed to have been infected with M. bovis and are responding to treatment.
A further two cases of “latent TB" were also identified. Latent TB means that the people had been exposed to TB at some point but they did not have active disease. PHE said it was not possible to confirm whether the two cases of latent disease were caused by M. bovis or something else.
What is TB?
Tuberculosis (TB) is a chronic infectious disease. It is caused by a group of bacteria within the Mycobacterium tuberculosis complex (a related “family” of species). TB can affect nearly all warm-blooded mammals, including farm animals, wildlife, pets and humans.
TB can infect any part of the body, but most commonly occurs in the lungs (pulmonary tuberculosis).
Most cases of TB in humans are due to a mycobacterium called Mycobacterium tuberculosis. Another mycobacterium that causes TB is M. bovis, which most commonly causes TB in cows. However, M. bovis can infect other mammals, including humans. M. bovis accounts for less than 1% of the total of human TB cases diagnosed in the UK every year. People working closely with livestock or regularly drinking unpasteurised milk have a greater risk of exposure.
A small number of M. bovis infections in pets, mostly cats, have been recorded. M. bovis infection is rarely recorded in dogs. In 2013, there were 16 cases in domestic cats (of 60 examined) and one case in a domestic dog (of nine examined).
How do pets catch TB?
Pets can become infected in a number of ways, including:
- by mouth – for example, by drinking unpasteurised infected cows’ milk or eating carcasses of infected animals
- breathing in respiratory droplets from other infected animals
- bite wounds – either from being bitten by an infected animal or if a wound gets infected by bacteria present in the environment
What are signs of TB in pets?
TB infection in pets can cause a serious long-standing disease. Signs of TB in pets include coughing, wheezing and weight loss. Lumps, abscesses or bite wounds that fail to heal, especially those around the head and neck, can also be caused by TB and are most frequently seen in infected cats.
The nine cats that were diagnosed with M. bovis between December 2012 and March 2013 in Newbury had a lack of appetite, non-healing or discharging infected wounds, evidence of pneumonia and different degrees of lymphadenopathy (lymph nodes of abnormal size).
The clinical signs of TB infection in pets are not unique and can be similar to other infections.
How can TB be spread from pets to humans?
The most likely routes of transmission would be:
- via respiratory droplets from infectious pets with signs of respiratory infection, such as coughing
- from contaminated environment
- via ingestion (by mouth) following handling of pets with cutaneous (skin) tuberculous lesions
- via contamination of unprotected cuts in the skin
PHE has assessed the risk of transmission of M. bovis from cats to humans as being very low.
How is TB treated in pets?
The choice of treatment of your pet – if it has TB – is a decision for you to make in consultation with your vet. However, the AHVLA and PHE caution that:
- there are no drugs licensed in the UK for the treatment of mycobacterial infections in animals, meaning that the “recommended” regimens for cats are based on limited clinical experience rather than randomised controlled trials. This means that they may not work or may have other health risks
- treatment involves prolonged courses of multiple drugs, which can be difficult to administer to pets
- a high rate of treated pets become unwell again
If pets remain infected despite treatment it can increase the risk of developing antibiotic-resistant strains of M. bovis, and the infection risk continues. This means that for public health reasons if a pet is diagnosed with culture-confirmed M. bovis infection, the most sensible course of action is usually to have the animal “put to sleep” (euthanised).
How is TB treated in humans?
Treatment for TB depends on which type you have, although a long course of antibiotics is most often used.
While TB is a serious condition that can be fatal if left untreated, deaths are rare if treatment is completed.
What happens next?
The relevant authorities will continue to monitor the situation carefully. While the spread of TB from cats to humans is certainly unusual, and perhaps unique, it is certainly not something to panic about.
There are an estimated 8,500 cases of TB every year, so this new impact of cat to human transmission on the public health burden is tiny.
The most effective methods of reducing your risk of catching any sort of disease from a pet is to always wash your hands after handling them, make sure their immunisations are up to date, and keep their fur clear. Read more about pet hygiene.