Cryptosporidiosis 

Human cryptosporidiosis is caused by infection with the Apicomplexa protozoans of the genus Cryptosporidium. Human illness was formerly thought to be caused by a single species, but molecular studies have demonstrated that several different species cause human cryptosporidiosis. Among the more common species are Cryptosporidium hominis, for which humans are the only natural host, and Cryptosporidium parvum, which infects bovines as well as humans.^{[1, 2]} (See Etiology.)

Cryptosporidiosis mainly affects children. It causes a self-limited diarrheal illness in healthy individuals. Cryptosporidiosis is also recognized as a cause of persistent diarrhea in children and of severe, prolonged diarrhea in persons with acquired immunodeficiency syndrome (AIDS). The source of most endemic cryptosporidiosis cases is human-to-human fecal-oral transmission, but infection may also result from animal-to-person transmission and waterborne transmission. Major outbreaks resulting from waterborne transmission have been recorded.^[3] (See Etiology, Epidemiology.)

Outbreaks of cryptosporidiosis should be detected by vigilant observation for increased case numbers at primary and public health care levels. (See Epidemiology.)

Also see Pediatric Cryptosporidiosis.

Cryptosporidium oocysts are highly infectious, requiring only 10¹ to 10³ oocysts to cause human disease (50% infectious dose, 10²). The oocysts are infectious immediately after excretion, and the life cycle of the parasite produces forms that reinvade the intestine. The location of the parasite in the intestine is intracellular but extracytoplasmic, which may contribute to the marked resistance of Cryptosporidium species to treatment. Large numbers of oocysts are excreted and are resistant to harsh conditions, including chlorine at levels usually applied in water treatment.

The mechanism by which Cryptosporidium causes diarrhea includes a combination of increased intestinal permeability, chloride secretion, and malabsorption, which are all thought to be caused by the host response to infection. In immunocompetent persons, the infection is usually limited to the small intestine. In persons with AIDS or certain congenital immunodeficiencies, the biliary tract may be involved.

Incidence in the United States

The frequency of cryptosporidiosis has not been well-defined. About 30% of the adult population of the United States is seropositive. The number of cases diagnosed has increased with improved diagnostic testing, with over 10,500 cases reported in 2008.^[4] Still, most laboratories do not routinely test for this organism, and many laboratories use insensitive tests when testing for Cryptosporidium.^[1] Studies have documented cryptosporidiosis in about 4% of stools sent for parasitologic examination.^[5] More recently in studies using polymerase chain reaction (PCR) tests, Cryptosporidium species have been diagnosed in 6% of American travelers to Mexico.^[6]

Prior to the availability of combination antiretroviral therapy, approximately 10-15% of patients with AIDS developed cryptosporidiosis over their lifetime. Like other opportunistic infections, the prevalence of cryptosporidiosis in AIDS patients has dropped dramatically.

International incidence

In developing countries, most people are infected as children. For example, studies in Brazil documented an infection rate of 90% for children under age 5 years who were living in slums. Serologic and stool studies have documented high rates of infection in Latin America, Africa, the Middle East, and South Asia. Overall, about 13% of stool studies submitted for parasitologic studies in developing countries reveal Cryptosporidium oocysts.

In persons with AIDS, the rate of cryptosporidiosis is higher in developing countries, ranging from 12-48% of persons with AIDS who have diarrhea.^{[1, 7]}

Age predilection

The peak incidence of cryptosporidiosis is in children younger than 5 years. Infection is rare in immunocompetent adults in developing countries but can occur in persons with AIDS.

Prolonged diarrhea (ie, >1 mo) and biliary disease indicate a poor prognosis in persons with AIDS.

Sclerosing cholangitis, acalculous cholecystis, papillary stenosis, and pancreatitis may develop with biliary involvement.

Patients with AIDS may develop respiratory tract infections.

Rare cases of pancreatitis have been recorded in immunocompetent patients.

Cryptosporidiosis is an important cause of persistent diarrhea in developing countries. Children with persistent diarrhea develop worsening malnutrition, which may result in cognitive and fitness problems that persist for years.^[8]

Chronic cryptosporidiosis may be complicated by biliary tract disease, malabsorption, and death in individuals with AIDS and malnourished children.^[9]

Encourage immunocompromised patients to consider using 1-μm water filters when drinking tap water.

Instruct patients to boil or filter water in countries with a high risk of transmission.

Instruct immunocompromised patients to avoid newborn animals (eg, calves, lambs), including domestic animals, and people with diarrhea. New pets for patients with AIDS should be older than 6 months and should not have diarrhea.

Instruct patients with AIDS, daycare workers, food handlers, and healthcare workers to avoid fecal-oral spread by wearing gloves and washing their hands after contact with human feces. Spread can occur after activities such as changing diapers.