American Miniature Schnauzer
Mycobacterium Avium Infection and Miniature Schnauzers
Appearing in the March, 2001 issue of the AMSC’s newsletter, AMSCope
Tuberculosis, a highly contagious disease, is caused by mycobacterium bovis (bovine) and mycobacterium tuberculi (human) and is characterized by the formation of large granulomas particularly in the lungs in human beings. Mycobacterium avium (M. avium) is an "opportunistic" mycobacterium species that is not normally pathogenic. M. avium is called an "opportunistic" pathogen because it does not normally cause disease in dogs or humans unless there is an acquired immune deficiency or a genetic defect or disease that predisposes an individual to M. avium infection. There has been resurgence in M. avium in people because of AIDS due to HIV infection. In addition there are several genetic diseases in people that result in selective predisposition to M. avium and other intracellular parasites such as Listeria or Salmonella. Individuals with AIDS are susceptible to a variety of infectious diseases in addition to M. avium. M. avium is a common cause of serious disease because M. avium survives in the environment for a long time outside of a mammalian host, unlike M. bovis or M. tuberculi. M. avium is, in fact, found just about anywhere in the environment (creeks, soils, lakes, swamps) regardless of geography or climate. Therefore, it is easy to see how dogs would be more readily exposed to M. avium than to M. bovis or M. tuberculi.
The genetic diseases in human beings that result in selective susceptibility to M. avium are caused by mutations in the IFN-gamma, Interleukin-12 or TNF-alpha genes, which are essential for killing intracellular bacteria. The surprising characteristic of these genetic diseases is that patients do not have an increased incidence of other types of infectious diseases.
It is entirely possible that certain dog breeds could have mutations in the same/similar genes, which could result in a genetic predisposition to M. avium infection. M. avium infection is an uncommon disease in dogs with most cases reported in Basset Hounds, Doberman Pinchers, and Miniature Schnauzers. There have been 4 cases in Miniature Schnauzers that are reported in veterinary literature. Importantly, we are aware of at least an additional 10 cases of confirmed M. avium infection in Schnauzers in the last three years. In reality there may be many more additional cases of M. avium which have not been confirmed or may have been misdiagnosed as lymphoma. Generalized lymph node enlargement, which is seen with M. avium infection, can easily be confused with lymphoma without histologic diagnosis using special stains for "acid fast" bacteria.
There is strong circumstantial evidence that the apparent increased incidence of M. avium infection in Miniature Schnauzers is due to an inherited defect that results in defective mechanisms for killing of intracellular bacteria. The fact that M. avium is an opportunistic pathogen, that there are limited breeds with proven M. avium infection, and the lack of a chronic history of a variety of infectious diseases in dogs with M. avium all point to an underlying genetic disease that results in increased sensitivity to M. avium following environmental exposure. The relatively equal number of cases in male and female schnauzers suggests that it is not sex-linked but could be autosomal recessive or autosomal dominant with incomplete penetrance.
M. avium infection is treated with a combination of antibiotics for months or longer. It is difficult to affect a cure although long term remission can occur but may not be possible in dogs with a genetic defect that causes ineffective killing of intracellular bacteria. There are no reported cases of animal to human transmission of tuberculosis or M. avium infection. However, individuals with diabetes mellitus, AIDS, pregnant women, or individuals which otherwise have a compromised immune system may be at increased risk for M. avium and should be extremely cautious when handling dogs with M. avium infection.
Although the genetic basis for the increased susceptibility of Miniature Schnauzers to M. avium is not identified, our laboratory is actively trying to develop a screening test to enable us to identify susceptible dogs, and ultimately a genetic test which would permit identification of carriers. To date we have been able to eliminate a few candidate genes as possible locations of mutation. Understanding the genetic basis of M. avium infection is also important from a comparative biomedical perspective because of significant human health concerns.
Owners of affected Miniature Schnauzers should contact Dr. Lothrop. We do not need blood samples from the general breed population at this time.
Contact: Dr. Clint Lothrop
Scott Ritchey Research Center
College of Veterinary Medicine
Auburn University, AL 36849