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American Miniature Schnauzer
Club, Inc.
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Disseminated
Mycobacterium Avium Complex Infection in Miniature Schnauzers
Written by: Errolyn Martin
Reviewed by:
Henry J. Baker, Professor
and Director,
Susan A. Goodman-Martin
MS DVM,
AMSC Health Committee members Kurt Garmaker, Gwen Mulheron, Patty
Ledgerwood, & Vera Potiker
Mycobacterium species cause
serious infections of humans and animals throughout the world. It is estimated that one-third of the
world’s human population is infected with Mycobacterium tuberculosis. Interestingly, only half of the people
exposed to M. tuberculosis become infected and only one-tenth of those infected
go on to develop clinical disease.
There is evidence that genetic factors are important in determining both
susceptibility to infection and whether or not there is subsequent development
of clinical disease.
Only a few Mycobacterium species produce disease in normal mammals and non-mammalian species. M. tuberculosis and M. bovus (bovine) are pathogenic mycobacteria in mammals, causing infections primarily in humans and cattle respectively. M. leprae causes leprosy in humans. Mycobacterium avium is the scientific name used for avian tuberculosis. It is named avian tuberculosis because the normal host organism is birds (Aves). The M. avium complex (MAC) is a group of mycobacteria that contains potentially pathogenic organisms. The MAC includes M. avium, M. avium subspecies (ssp.) paratuberculosis, and M. intracellulare. These normally non-disease-producing organisms are commonly present in the environment in soil, water and air. These are closely related mycobacteria capable of producing generalized disease in susceptible patients. Susceptibility to this infection in non-natural hosts, such as dogs may be due to a defect in the immune system that would render the affected individual’s immune system incapable of mounting an appropriate immune response to the pathogen. Different parts of the immune system are responsible for digesting invaders, producing antibodies, and orchestrating the body’s response to viral, fungal, protozoal, and bacterial infections. Often the various components of these systems overlap, but some also have very specific jobs.
M. avium and M. intracellulare are considered potentially harmful in individuals with an inadequate immune system. MAC infection is common in birds, but infection in mammals is uncommon. According to Dr. Craig Greene’s book Infectious Diseases of the Dog and Cat , MAC infections in dogs and cats have rarely been reported. One published research study involving experimental exposure of dogs to MAC by various means concluded that dogs possess a natural resistance to infection with MAC.
There are very few case reports of M. avium infection in dogs. These reports are generally concerning individual sporadic cases. There are two reports concerning a total of 5 Basset Hounds, all related by pedigree analysis to a particular dog. There are two reports in the literature of Mycobacterium avium infections in Miniature Schnauzers. One report describes an infection of three litter mates; the second report describes an individual Schnauzer. All four of these dogs were related by pedigree analysis to a particular dog currently under suspicion of being a genetic source for the potential immune system defect allowing a MAC infection. The three litter-mates in the first report were from a litter of five and lived in separate homes from puppy-hood. The other two littermates did not become infected with MAC.
An alarmingly increasing number
of Miniature Schnauzers have been diagnosed with generalized MAC infection in
the past 6 years. A total of 49 cases of MAC in Miniature Schnauzers have been
reported. Most cases are from the
These recorded cases of MAC infection and now more recently
discovered cases in related dogs, prompted an investigation to define a
potential immune system defect which would allow infections by this normally
non-pathogenic organism. Initial
experiments at the
1. Veterinary literature indicates that dogs in general are innately resistant to MAC infection. Dogs are exposed to this pathogen in the environment without becoming infected.
2. Veterinary literature that outlines research of experimental exposure of dogs to MAC. Results indicate that dogs who were experimentally exposed to MAC via ingestion developed neither generalized nor localized infection.
3. MAC organisms, especially M. avium, are in the environment, in soils and water, and in bird feces, and can remain viable in the environment for at least 2 years. In spite of this seemingly unavoidable exposure risk, the only reported cases of MAC in Miniature Schnauzers trace to a common bitch with the probability that the MAC-susceptibility trait was passed through her offspring.
4. There are 6 litters from which two or more siblings have become affected with disseminated disease. Most of these siblings lived in separate homes.
5. There are 5 stud dogs that have produced two or more affected offspring when bred to two or more different dams.
6. A test-breeding of the son of an affected animal back to his dam, (who was the full sister of the affected dog), produced one male and one female offspring. The female became naturally infected (not experimentally infected) and died from generalized MAC infection at about 1.5 years of age. The two offspring were housed together and exposed to the same environment for their entire lives. The offspring lived in the same environment as the parents until the age of 6 months. Both parents of this litter are still living, and healthy. The dam is over 8 years old and the sire is about 6 years old.
7. Two pedigrees that trace to the bitch in item 3 also suggest the defect may be present in low frequency in other parts of the breed population, due to common ancestry.
Based on statistical analysis of the genetic pedigree and results of very limited test-breeding, a potential immune system defect appears to be inherited as a simple autosomal recessive disorder or possibly, a polygenic recessive mode of inheritance. An X-linked mode of inheritance has been ruled out due to the almost equal representation of both sexes. A dominant mode of inheritance is unlikely, as the vast majority of parents of affected dogs were not themselves affected.
The primary clinical sign
of MAC infection is lymph node enlargement. The organism also
infiltrates the lymph system affecting the liver and spleen, resulting in liver
and spleen enlargement. Lymph
nodes, bone marrow, and other organs become infected as well. The more common causes of generalized
lymph node enlargement such as lymphoma may result in a missed diagnosis of
Mycobacterial infection unless a lymph node biopsy is performed with
identification of an acid-fast stain characteristic of Mycobaterial
infection. If a diagnosis of
mycobacteriosis is obtained, a sample should be cultured and evaluated to
determine the exact pathogen present. General symptoms that may not be
observed in all cases include lethargy, vomiting, lack of appetite, fever, not
eating, lameness, blood in the stool, diarrhea, pale mucous membranes, and
abdominal distension. Once the
infection is established in a dog, it is always fatal. There has been no reported
case of remission of a MAC-infected dog.
Some dogs perish within weeks of diagnosis, a very few have undergone
multi-drug treatment protocols and lived for another few months or even over a
year. Thus far, all animals that
have been infected have eventually perished from the disease.
There are myths that
associate MAC infections with an animal’s coat color, allergic
conditions, and reproductive status.
Affected dogs have been of both sexes, and both salt/pepper and
black/silver coat color. Some affected
animals have had allergic processes and some have not. Some have been intact
show dogs and some have been spayed/neutered companion dogs. None of the affected animals were
reported to have had a previous chronic illness.
MAC is potentially infectious for humans, but to date no recorded cases of a human contracting this infection from an affected dog have been reported. Healthy individuals are probably not at risk, because humans generally have the same innate immunity to this pathogen as dogs are thought to possess. Persons with HIV/AIDS, Diabetes, Cancer, or other immune-compromising conditions, and very young children, and elderly individuals should consult with their doctor if they have had a dog diagnosed with MAC or they are considering pursuing medical treatment for a MAC-infected dog. Although antibiotic resistance has not been reported for this type of Mycobacterial infection, it is a very serious problem with M. tuberculosis in humans and can be a risk factor for humans infected with MAC.
Currently a genetics team
is working with Miniature Schnauzer DNA provided last year through the efforts
of the AMSC Health Committee and the MSC (
Update
The research efforts at
Dr. Giger is requesting blood samples from affected
Miniature Schnauzers. He can be contacted at giger@vet.upenn.edu . To report an affected dog or for more
information on submitting blood samples please contact Dr. Giger at giger@vet.upenn.edu , Errolyn Martin,
AMSC Health Committee, at 334-703-0166, or email at chattelane@juno.com , or Kurt Garmaker at
kgarmaker@aol.com
