The pathogenesis of disseminated M. avium infection in dogs is poorly understood. A mechanism involving immunosuppression has been proposed, but immunologic studies
have not revealed abnormalities in T-cell function or CD4+ to CD8+ ratios.1
An infection may develop after a young pup, whose immune system is not fully competent, eats contaminated food.1 (There is an increased danger when feeding a raw diet containing swine or poultry viscera or lymphoid organs. The organism
cannot survive in properly processed commercial food.) Ingested organisms may be phagocytized by macrophages in Peyer's patches,
persisting and replicating until reaching sufficient numbers to produce overt disease.1
Birds and swine are the primary reservoirs of M. avium.2 Large numbers of organisms are shed in the feces of infected birds and can remain viable in water and soil for at least
two years.2 The source of the infection in this case was not determined. The owner reported that the dog had not been exposed to wild
or pet birds, chickens, or swine, but the property where the dog resided had been a farm before the dog's owner had purchased
it. The owner did not know what type of livestock had been maintained on the farm.
Parvovirus enteritis infection had been diagnosed when the dog was about 10 weeks old, shortly after it had been adopted from
a humane shelter. Immune impairment and intestinal damage caused by the parvovirus infection may have allowed the Mycobacterium species in the environment to enter the dog's body and incubate in the lymph nodes draining the digestive tract. Alternatively,
the puppy may have been infected with M. avium before its adoption, and the parvovirus infection may have enhanced the organism's ability to establish itself in the host.
Therapy for generalized M. avium infection is often prolonged and ineffective.2 One dog was treated with a combination of clofazimine, ciprofloxacin, and rifampin with limited success, and a dog that
received continuous therapy with enrofloxacin, clarithromycin, and clofazimine remained in remission for two years before
experiencing a relapse.2
Two more common differential diagnoses for generalized lymph node adenopathy in dogs are malignant lymphoma and histoplasmosis.
Clinical signs of M. avium infection are variable but include anorexia, depression, diarrhea, fever, generalized lymphadenopathy, pale mucous membranes,
and weight loss.1-3 Additionally, respiratory signs, lameness, and pathological fractures may be seen.3 Mycobacteriosis should be included as a differential diagnosis for intermittent or persistent diarrhea, especially when
accompanied by lymphadenopathy. Acid-fast stains should be performed on smears made from swabs of rectal mucosa and fine-needle
aspirates or histologic sections of lymph nodes.1
Large numbers of M. avium and closely related organisms may be present in the intestinal tract and mesenteric lymph nodes and can be shed in feces.
So infected dogs should not have contact with young children and immunocompromised people and pets.
The author thanks Katherine L. Nicholson, DVM, PhD, Lakeside Animal Hospital, 5206 Lakeside Ave., Suite #6, Richmond, VA 23228,
for providing this patient's history and laboratory data and for interpreting these data. Dr. Nicholson's present address
is Department of Pharmacology and Toxicology, School of Medicine, Virginia Commonwealth University, Richmond, VA 23298.
1. Horn B, Forshaw D, Cousins D, et al. Disseminated Mycobacterium avium infection a dog with chronic diarrhea. Aust Vet J 2000;78:320-325.
2. Greene CE, Gunn-Moore DA. Mycobacterial infections. In: Greene CE, ed. Infectious diseases of the dog and cat. 2nd ed. Philadelphia, Pa: WB Saunders Co, 1998;313-321.
3. Bauer N, Burkhardt S, Kirsch A, et al. Lymphadenopathy and diarrhea in a miniature schnauzer. Vet Clin Pathol 2002;31:61-64.
This case report was provided by Neil Allison, DVM, DACVP, Virginia Department of Agriculture and Consumer Services, 1100
Bank St., Suite 600, Richmond, VA 23239. Dr. Allison’s present address is Experimental Pathology Laboratories, Incorporated,
P.O. Box 12766, Research Triangle Park, NC 27709.