Hammondia hammondi

Approximately 20% of the US population is infected with the intracellular parasite Toxoplasma gondii. T. gondii is a member of the phylum Apicomplexa, which includes multiple human and animal pathogens, including the causative agents of malaria and cryptosporidiosis. A unique feature of this parasite compared with its apicomplexan relatives is the ability to infect, and cause disease in, nearly all warm-blooded animals studied to date, including birds. Hammondia hammondi is a coccidian parasite, the closest relative of T. gondii that also has its sexual cycle in cats. This parasite was once thought to be a strain of T. gondii, but it is now accepted as a distinct species1.

Live tissue cysts of Hammondia Source: Redescription of Hammondia hammondi and its differentiation from Toxoplasma gondii (naldc.nal.usda.gov)

The definitive host of Hammondia hammondi is the cat. Intermediate hosts include rats, mice, deer mice, rabbits, hamsters, pigs, goats, and dogs. This parasite does not infect humans. The life cycle is similar to that of T. gondii, except that there is no extraintestinal phase of the cycle in the cat, the intermediate hosts become infected by ingesting oocysts, and cats become infected by ingesting intermediate hosts.2 The oocysts of both species (T. gondii and H. hammondi) are regarded as undistinguishable morphologically and serologically.3 H. hammondi cysts are rarely found in the brain, even though this parasite needs to reach the definitive host to complete its life cycle.4

No disease has yet been associated with H. hammondi.


  1. Hammondia hammondi, an avirulent relative of Toxoplasma gondii, has functional orthologs of known T. gondii virulence genes
  2. Flynn's Parasites of Laboratory Animals. David G. Baker (editor)
  3. Prevalence and Risk Factors of Intestinal Parasites in Cats from China
  4. Chronic Toxoplasma Infection Modifies the Structure and the Risk of Host Behavior



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