Microsporidia Infection

Microsporidia are intracellular single-celled organisms found in a wide range of vertebrate and invertebrate species. The parasites replicate to produce a final product, a spore, which is remarkable for its small size and internal complexity. These microorganisms are found in rodents, rabbits, foxes, primates, wild carnivores, and dogs. In E. cuniculi, the existence of three strains ("mouse strain," "rabbit strain," "dog strain") was established using immunological and molecular methods. Dogs seem to be susceptible to the "dog strain" only.2 This parasite may be an under-diagnosed cause of fatal canine neurological or renal disease.3

Human microsporidiosis, a serious disease of immunocompetent and immunosuppressed people, can be due to zoonotic and environmental transmission of microsporidian spores. Waterborne microsporidian spores of species that infect people can originate from common waterfowl, which usually occur in large numbers and have unlimited access to surface waters, including waters used for production of drinking water.

The parasite replicates and invades the brain, kidneys and other organs. In some clinically ill dogs, hepatitis has been diagnosed with parasites found in liver cells and blood vessels. Mild inflammation of the heart muscle (myocarditis) and eye lesions were occasionally reported.5 Dogs become seropositive (show presence of antibodies or other immune markers in serum, that indicate prior exposure to a particular organism or antigen) between 32 and 39 days after infection. The most common clinical sign of canine microsporidian infection is an encephalitis-nephritis syndrome in puppies under 10 weeks of age, often resulting in death from a progressive neurologic disease.

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Chronic kidney disease has also been reported in adult dogs. Neurologic signs may include behavior changes, increased vocalizing, weakness and incoordination, progressing to rear limbs paralysis and weight loss. Dehydration and pus-filled discharge from the eyes are sometimes observed. These clinical signs are most frequently confused with neurologic signs associated with canine distemper virus infection, as well as rabies and toxoplasmosis. Dams producing infected litters generally do not have clinical signs of the disease. It is probable that young puppies with disease become infected from their mothers who may not have clinical signs of the disease. Infected puppies as well as adult dogs with no signs of disease shed spores in the feces and urine.1 Surviving clinically ill puppies shed spores in their urine and feces for an extended period of time.5


  1. Dogs, Zoonoses, and Public Health. Calum N. L. Macpherson, Francois-X. Meslin, Alexander I. Wandeler
  2. Cryptosporidiosis and Microsporidiosis. Franz Petry
  3. Encephalitozoon cuniculi Infections in Dogs: A Case Series. Karen F. Snowden, DVM, PhD, Barbara C. Lewis, DVM, MS, Diplomate ACVP, Jay Hoffman, DVM, PhD, Diplomate ACVP and Joanne Mansell, DVM, MS, Diplomate ACVP. In: Journal of the American Animal Hospital Association 45:225-231 (2009)
  4. Microsporidian Species Known To Infect Humans Are Present in Aquatic Birds: Implications for Transmission via Water? Anna Slodkowicz-Kowalska, Thaddeus K. Graczyk, Leena Tamang, Szymon Jedrzejewski, Andrzej Nowosad, Piotr Zduniak, Piotr Solarczyk, Autumn S. Girouard, and Anna C. Majewska. In: Applied and Environmental Microbiology, July 2006, p. 4540-4544, Vol. 72, No. 7 0099-2240/06/$08.00+0 doi:10.1128/AEM.02503-05
  5. The Microsporidia and Microsporidiosis. Murray Wittner, Louis M. Weiss

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