Has your cat been diagnosed with a mycobacterial infection?
Feline mycobacterial infections have been documented in veterinary journals since the late 1880s, however, there is much yet to be learned about these infections.
Relatively few studies have examined large groups of cats with mycobacterial infections. These studies are generally restricted to animals in a particular country or region, and therefore may not be representative of the condition in cats in other parts of the world, especially with regards to which mycobacterial species are causing disease.
If your cat has been diagnosed with a mycobacterial infection we would love to get further details to enter into our confidential database. This information will not be shared with any third party and is purely for epidemiological purposes.
What are mycobacteria?
Mycobacteria are bacterial organisms that can cause disease in humans and animals.
Some species of mycobacteria are specialist pathogens, and their main role in life, so to speak, is to infect their host, and they only have a limited ability to survive in the environment. Examples of these species that infect cats are the tuberculous mycobacteria, such as Mycobacterium bovis (which causes bovine TB), M. microti (which causes “vole” TB), and rarely M. tuberculosis (the cause of most TB in humans). In most parts of the world bovine TB has been controlled via eradication programs, but in some areas the existence of a wild-life reservoir, e.g. the Brushtail possum in New Zealand, and the badger in the UK/Ireland, means that the infection tends to persist. M. microti is found in Western Europe, the UK, South Africa and South America, and its main reservoir appears to be voles (also known as field mice in North America), shrews, wood mice and other small rodents found in woodland areas. Cats rarely acquire M. tuberculosis, but can become exposed by humans infected with this organism. Further information regarding tuberculosis in cats can be found here.
Other mycobacterial species reside in the environment (e.g. dirt rich in organic matter, contaminated water). It is likely that most of the mycobacterial species in the environment do not cause disease in animals, but some species are more likely to cause disease in certain instances, such as members of the M. avium complex. Further information regarding environmental mycobacterial infections in cats can be found here.
Unfortunately, some of the mycobacteria that cause infection in cats cannot be grown in the laboratory, and their ecology is mostly unknown. These species are collectively known as the lepromatous mycobacteria, and are not the same as the organism that causes human leprosy, or Hansen’s disease (M. leprae). Examples of these species include M. lepraemurium, M. visibile and two unnamed species, one causing disease in cats in eastern Australia and NZ, and another localized to a restricted area of Victoria, Australia. Further information regarding leprosy in cats can be found here.
How do cats get mycobacterial infections?
Some species of mycobacteria are primary pathogens in healthy hosts (for example, tuberculosis [TB]), thus are likely to be obtained directly from other infected animals, such as altercations with or ingestion of infected prey, or ingestion of unpasteurized milk or infected beef/offal in the case of “bovine TB” infection.
Infection with environmental mycobacterial species are thought to be acquired via skin inoculation, perhaps via a catfight, or may be ingested in water or infected prey, or may be licked off the paws or coat. Typically it is thought that cats acquire infection with lepromatous organisms via skin inoculation.
How are mycobacterial infections diagnosed?
Because most mycobacteria reside in the environment, they have developed a tough outer layer of protective fats and waxes. This can make it difficult to see them on routine pathology slides using regular stains. It does however impart a special quality when stained with particular techniques. Mycobacteria are often called “acid-fast bacteria” (AFB), as when they are treated with hot dye, such as carbol fushin, this is driven deep into the outer coating of the cells, which when cooled gets locked in and cannot be washed away with an acid solution, unlike other bacteria. This helps the microbiologist or pathologist to identify them in smears or biopsies as magenta-coloured rods on a pale blue background (see image below).
The visualization of AFBs in a smear tells the microbiologist that there is a mycobacterium present, but not which particular species is involved. This requires identification via culture and/or genetic testing, which is typically carried out in a specialist laboratory, called a Mycobacterium Reference Laboratory. Unfortunately, the fatty coating of the outer membrane, and the fact that there are a relatively low number of genes that produce protein-manufacturing units within the cells, means that mycobacteria are typically much more slowly growing in the laboratory than other organisms – some taking up to 8-12 weeks to produce positive cultures.
The prognosis for the cat depends on a number of factors, including the causative mycobacterial species and the extent of the infection, with internal disease holding a more guarded prognosis.
Many treatment failures result from lack of compliance to the months of multidrug therapy, and the expense of treatment may also be a limiting factor in some of these cases. In some cases, the placement of an oesophagostomy tube may make the administration of medications easier (see image below).
Generally, localized infections caused by all species have a relatively favorable prognosis if treated with an appropriate combination of drugs and, if possible, surgical resection.
Since it is assumed that most cases of mycobacterial infection in cats are acquired either via inoculation or ingestion of organisms from the environment or infected prey species, the best thing owners can do to prevent these infections is to keep cats indoors. Avoiding the feeding of unpasteurized milk or offal to cats in areas of where bovine TB is found is also strongly recommended.
Despite the occasional outcry in the popular press of the risks posed to humans from TB-infected cats, it appears direct transmission from cats to people is very uncommon.
Environmental mycobacterial infections, particularly caused by the M. avium complex may cause considerable disease in humans, particularly those with immunosuppression, but it is likely that almost all of these infections are derived from environmental sources; however, potential transfer from infected cats cannot be completely discounted. There are only rare reports of these diseases being associated with cats, one being a M. fortuitum infection secondary to a cat bite to the forearm of an otherwise healthy middle-aged woman.
There appears to be little to no risk of humans acquiring M. lepraemurium from cats. However, as the ecology and transmission of other organisms associated with feline leprosy are not known at this time, it is difficult to ascertain their potential for zoonotic transfer, but it appears to be low.
Careful consideration of the zoonotic potential of any mycobacterial infection must be made before an attempt is made to treat infected animals and some authors recommend against treatment of confirmed cat cases of M. bovis and M. tuberculosis infection.
Published guidelines from the European Advisory Board on Cat Diseases (ABCD), recommend that all owners, regardless of immunological status, but especially if any member of the household is immunocompromised, should be made aware of the low, but potential risk of zoonotic transfer of feline mycobacterial diseases particularly those caused by M. bovis, M. microti and the M.avium complex, and the use of protective gloves is recommended when treating these animals.
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