Cyclospora and pets: Reservoir, bystander, or red herring?

Cyclospora cayetanensis has been in the news a lot lately. When veterinary professionals read about a parasite, we might naturally wonder whether animals are the source and how it might fit into our own clinical work. So, what exactly is this parasite?

To begin with, it's tiny. Single-celled, in fact. Cyclospora is an apicomplexan protist and a close relative of a familiar veterinary parasite, Eimeria. Like Eimeria, it has a direct life cycle, replicates within intestinal epithelial cells, and produces oocysts that are shed unsporulated. This intracellular replication can damage the intestinal epithelium and contribute to diarrhea. Those oocysts require at least 1 to 2 weeks in the environment to become infectious, so direct person-to-person transmission is unlikely.¹

The genus name roughly means "circle seed," a nod to its spherical oocyst. The species name, cayetanensis, honors Universidad Peruana Cayetano Heredia in Lima, Peru, where much of the early work on the parasite was performed. The suffix "-ensis" often signals an association with a geographic location or place.

As veterinary professionals, we can breathe a collective sigh of relief. Current evidence supports humans as the only major host of C. cayetanensis, and no animal reservoir has been identified. Other Cyclospora species infect nonhuman primates and have historically been described in reptiles. Although C. cayetanensis-like oocysts or DNA have occasionally been detected in some animals, these findings have not established that animals maintain or transmit the human parasite. There is no established role for dogs, cats, ruminants, horses, or birds in maintaining or transmitting C. cayetanensis^2,3,4

Although cyclosporiasis is most common in tropical and subtropical regions, the United States has experienced seasonal cases and produce-associated outbreaks for decades. In 2026, the CDC reported both a substantial national increase in cases and a large multistate outbreak in the Midwest. At the time of writing, a specific food source had not been confirmed.⁵

Transmission occurs when people ingest sporulated oocysts in fecally contaminated food or water. Fresh produce has been a frequent vehicle in previous outbreaks. The CDC recommends washing fruits and vegetables thoroughly under running water, although washing reduces the risk of foodborne illness rather than guaranteeing removal of the parasite.⁶

Cyclosporiasis is not usually life-threatening, but it should not be dismissed as merely annoying. Infection can cause frequent watery diarrhea, abdominal cramping, fatigue, weight loss, and symptoms that improve and then relapse. Untreated illness may persist for a month or longer. As of July 14, 2026, the CDC had reported 141 hospitalizations among confirmed domestically acquired cases, but no deaths. Trimethoprim-sulfamethoxazole is the treatment of choice.⁷

So, despite the parasite headlines, veterinary teams do not need to worry about dogs or cats as carriers, reservoirs, or patients with C. cayetanensis. Other Cyclospora species in primates and reptiles are a legitimate veterinary parasitology footnote, but they are a different story.

Christopher Lee, DVM, MPH, DACVPM, DACVM (Parasitology) is an Assistant Professor of Microbiology at the Lincoln Memorial University Orange Park College of Veterinary Medicine. He is a board-certified specialist in preventive medicine and veterinary parasitology. He teaches parasitology, immunology, and bacteriology. He also hosts the Vet Watch program on dvm360.

References

1. Almeria S, Cinar HN, Dubey JP. Cyclospora cayetanensis and cyclosporiasis: an update. Microorganisms. 2019;7(9):317. doi:10.3390/microorganisms7090317
2. Dubey JP, Khan A, Rosenthal BM. Life cycle and transmission of Cyclospora cayetanensis: knowns and unknowns. Microorganisms. 2022;10(1):118. doi:10.3390/microorganisms10010118
3. Eberhard ML, da Silva AJ, Lilley BG, Pieniazek NJ. Morphologic and molecular characterization of new Cyclospora species from Ethiopian monkeys: C. cercopitheci sp. n., C. colobi sp. n., and C. papionis sp. n. Emerg Infect Dis. 1999;5(5):651-658. doi:10.3201/eid0505.990506
4. Taylor MA, Coop RL, Wall RL. Veterinary Parasitology. 4th ed. Wiley-Blackwell; 2016.
5. Domestically acquired cyclosporiasis cases in multiple US states, 2026 (CDCHAN-00531). Centers for Disease Control and Prevention. July 14, 2026. Accessed July 15, 2026.
6. Preventing cyclosporiasis. Centers for Disease Control and Prevention. July 14, 2026. Accessed July 15, 2026.
7. Clinical care of cyclosporiasis. Centers for Disease Control and Prevention. February 27, 2024. Accessed July 15, 2026.