Mycobacterial diseases- beyond tuberculosis

Spend much time reading TB culture reports it will not take long before you run across a diagnosis of asiaticum, fortuitum, kansasii etc…. Frankly, the vast majority of the time, mycobacteria other than tuberculosis (MOTT) are incidental findings, especially in general surveillance cases. However, these organisms can certainly cause disease, and they are rare enough that many practitioners will look towards regulatory VMO’s for answers and assistance, especially if the animal was tested due to chronic illness.

Here at NVSL we get asked a few questions pretty regularly, so it is probably worth covering the top 3. Remember, if you ever get culture result you have questions about, you are always welcome to call the laboratory and have a discussion with a Lab VMO or Microbiologist.

FAQ:

Mycobacterium… what??

What is the zoonotic risk?

And for valuable/rare zoo animals and small animals… What are my treatment options?

Mycobacterium… what??

If you are on the wrong side of 35, chances are when you were in veterinary school less than half of the currently named mycobacteria were even recognized. Mycobacteria are not chemically very reactive and consequently they kept their secrets hidden until identifying by DNA sequencing became routine. That occurred at NVSL about 8 years ago.

Even today, about 20% of the mycobacteria we culture do not match anything that is officially named. Most of the newly named and unnamed mycobacteria are saprophytes and can cause opportunistic infections, but unless histopathological lesions that support mycobacterial involvement are evident, the mycobacteria were probably just along for the ride… or were a secondary opportunist. All of the recently named mycobacteria can be searched for on the web, and the original paper naming the strain is usually available.

What is the zoonotic risk?

Without question, the genus mycobacterium contains some of the most dangerous bacterial species known, but other than the mycobacterium tuberculosis complex organisms (MTBC), few are obligate pathogens that transmit between animals and humans. Fish handlers’ disease ( and some related friends) and (armadillos to humans) are some notable exceptions. To be sure both humans and animals get the same opportunistic mycobacterial infections (i.e. M. intermedium outbreak associated with a hot tub party in humans and M. intermedium infection in a non-healing skin wound of a dog). But these infections are generally acquired from the environment. The important thing to remember, don’t automatically assume a mycobacterial disease is caused by a MOTT. Skilled pathologists can rule out MTBC in some situations, and of course culture is an option. In the UK where MTBC is more common, non-healing skin wounds in domestic cats are caused by MTBC more often than MOTTS. Here at NVSL we have not seen MTBC associated with skin infections in companion animals, but have grown M. tuberculosis out of the occasional sick parrot.

What are my treatment options?

First for MTBC in animals, treatment cannot be taken lightly, and euthanasia is preferable because of the zoonotic risk and the risk of resistance developing. You must get public health officials involved with the decision. When you are faced with a depopulation situation for bovine TB the VS memo 552.15 can help with initial guidance for other farm animals and pets.

Even though most atypical mycobacterial infections are opportunistic, as a rule they are very difficult to treat. Clients have to be carefully counseled and make sure they are able to comply with treatment regimens that can last over a year. If an animal has a disseminated mycobacterial disease, the prognosis is very poor, and usually there is an underlying condition causing the animal to be immunocompromised. Skin infections can be treated, but a culture and sensitivity should be done, knowing that a multidrug therapy lasting for months will be required.