Coccidiosis is a broad term describing an infection caused by protozoan parasites belonging to the subclass Coccidia, which includes many species that affect animals. While classic coccidia parasites of livestock and poultry, such as those in the genus Eimeria, are highly host-specific and generally do not pose a threat to human health, humans do become ill from related parasites. These human-infecting parasites cause a diarrheal illness that is clinically and biologically similar to animal coccidiosis. They can be transmitted to people from certain animals, linking the human illness back to the general term “coccidiosis.”
Clarifying the Microbes Responsible for Human Illness
The infections commonly referred to in humans are caused by specific parasites, primarily Cryptosporidium species and Cystoisospora belli. These organisms belong to the same biological group, the Apicomplexa, as the animal-specific coccidia. They are obligate intracellular parasites that invade the lining of the small intestine. Cryptosporidium is the more widespread human pathogen, with species like C. hominis and C. parvum being the main culprits. C. parvum is especially relevant as it is a zoonotic species that infects both humans and various mammals, including cattle.
Cystoisospora belli, previously known as Isospora belli, is also a member of the Coccidia group but is primarily considered a human-to-human infection, with no significant animal reservoir demonstrated. The classification of these parasites places them within the Eimeriida order. Unlike the Eimeria species found in chickens or cows, these human pathogens have adapted life cycles that allow them to infect human hosts effectively.
Transmission Routes and Risk Factors
Transmission of these coccidian parasites occurs primarily through the fecal-oral route, involving the ingestion of environmentally hardy oocysts shed in the feces of infected individuals or animals. Cryptosporidium oocysts are particularly resilient, surviving for long periods outside a host and resisting typical chlorine levels used for water disinfection. This resistance makes contaminated drinking water and recreational water, such as swimming pools, common sources of large-scale outbreaks.
Infection can also occur through direct contact with infected animals; young calves and goats are recognized sources of C. parvum infection in humans. Foodborne outbreaks are common, often involving contaminated produce, unpasteurized cider, or raw milk. Person-to-person spread is frequent, especially in settings with poor hygiene, such as childcare centers or healthcare facilities. Individuals with weakened immune systems, particularly those with advanced HIV infection or organ transplant recipients, face a greater risk of severe and chronic disease from these parasites.
Clinical Presentation and Medical Management
Symptoms of cryptosporidiosis and cystoisosporiasis typically begin after an incubation period and involve gastrointestinal distress. The most common presentation is profuse, watery diarrhea, which may be accompanied by abdominal cramps, nausea, vomiting, and low-grade fever. In people with healthy immune systems, the illness is generally self-limiting, with symptoms lasting from a few days up to several weeks before resolving naturally. The primary danger for these individuals is dehydration from fluid loss.
For individuals who are severely immunocompromised, the illness can become chronic and severe, leading to malnutrition and weight loss. Diagnosis requires laboratory testing, typically involving the detection of oocysts in a stool sample. Specialized techniques used include the modified Kinyoun acid-fast stain or enzyme immunoassay antigen tests. Treatment for otherwise healthy individuals focuses on supportive care, meaning aggressive oral rehydration and electrolyte replacement to combat fluid loss.
The prescription medication nitazoxanide is approved for treating cryptosporidiosis in people over one year of age with healthy immune systems and can shorten the duration of diarrhea. However, for immunocompromised patients, nitazoxanide’s effectiveness is limited. The most successful intervention is often the restoration of the immune system, such as through effective antiretroviral therapy in HIV patients. Supportive management also includes a lactose-free diet for chronic cases, as the infection can cause temporary lactose intolerance.
Strategies for Minimizing Exposure
Prevention of these infections relies heavily on strict hygiene and caution regarding environmental contamination. Thorough handwashing with soap and water is necessary after using the restroom, changing diapers, handling animals, or before preparing food. Alcohol-based hand sanitizers are generally ineffective against the resilient oocysts of Cryptosporidium.
Water safety is another measure, involving avoiding the ingestion of untreated water from ponds, streams, or lakes, and exercising caution with recreational water. Since the parasite is highly resistant to chlorine, individuals with diarrhea should avoid swimming in public pools for at least two weeks after symptoms resolve. When traveling or during local water advisories, boiling water for at least one minute is an effective way to inactivate the oocysts. Food safety includes avoiding the consumption of unpasteurized milk or cider, and thoroughly washing all fresh fruits and vegetables.