Cryptosporidiosis

Etiology

Cryptosporidium species are intracellular protozoan parasites belonging to the phylum Apicomplexa, which also includes other human disease-causing protozoans, eg, Toxoplasma, Plasmodium, Babesia, Cyclospora, and Cystoisospora. Currently, at least 44 Cryptosporidium species are known to infect a variety of vertebrates, including mammals, reptiles, birds, and fish.[1] Cryptosporidium hominis and Cryptosporidium parvum are the 2 predominant species causing human disease, accounting for up to 95% of infections. Cryptosporidium hominis infects only humans, whereas Cryptosporidium parvum can infect both animals and humans. Molecular studies have also identified other species as causes of human disease, eg, Cryptosporidium cuniculus, Cryptosporidium meleagridis, Cryptosporidium felis, and Cryptosporidium canis; however, these species are typically found only in the significantly immunosuppressed or malnourished.[2] Transmission of Cryptosporidium occurs by the ingestion of cysts in the environment through the fecal-oral route. This can occur through human-to-human contact, zoonotic exposure, or by consuming contaminated food or water. 

Epidemiology

Cryptosporidiosis has a global distribution and is recognized as one of the leading causes of diarrhea across all age groups. Cryptosporidium oocysts are acquired through fecal-oral transmission, most often by ingesting contaminated water or by direct contact with infected persons and animals.[2] Between 2017 and 2022, Cryptosporidium accounted for 74.4% of worldwide waterborne protozoan outbreaks, followed by Giardia at 17.1%. Recreational water and public swimming pools accounted for 92% of water sources, likely because Cryptosporidium is small enough to pass through most standard filters and is chloride-resistant.[3] In terms of foodborne transmission, there have been reports of outbreaks from apple cider, unpasteurized milk, and vegetables.[4][5][6] There are also reports of zoonotic transmission to farmers and veterinary personnel, as cattle, sheep, goats, and pigs can be affected.[7] Person-to-person transmission has been well described in households of infected individuals and in daycare centers, mostly involving Cryptosporidium hominis.[8][9] Sexual transmission has also been described, particularly in men who have sex with men.[10] Infection is generally more common during the warmer months, when recreational water activities are ongoing, and at other times of the year when water runoff from pastures occurs.[11]

Cryptosporidium infections are more prevalent in resource-poor countries. A systematic review and meta-analysis demonstrated a global pooled prevalence of cryptosporidiosis of 7.6%, with an average of 4.3% in developed countries versus 10.4% in developing countries. The highest infection prevalence was observed in Mexico, Bangladesh, and Nigeria at 69.6%, 42.5%, and 34.0%, respectively.[12] The higher prevalence in resource-poor countries is likely secondary to poor hygiene practices, overcrowding, intensive animal farming, open defecation, and a greater number of people living with HIV. Interestingly, outbreaks have occurred less often in resource-poor compared to resource-rich countries, thought to be due to higher immunity from repeated exposure in areas with high endemicity.[13][14] Per the Centers for Disease Control and Prevention summary report, a worldwide estimate of 823,000 cryptosporidiosis cases occurs annually, with 12,609 cases reported in the United States in 2022, of which 45.5% occurred in males and 54.1% in females.[CDC. Cryptosporidiosis NNDSS Summary Report for 2022] 

Cryptosporidiosis is known to affect 3 main epidemiological groups: immunocompetent individuals, immunocompromised individuals, and children. For the immunocompromised, people living with HIV have been found to have higher rates of infection, particularly when the CD4+ T cell count is less than 200 cells/µL. In Ghana, a study found a prevalence of 26% in HIV-positive patients with CD4+ T cell count less than 200 cells/µL. Among individuals with CD4+ T cell count less than 50 cells/µL, the prevalence of infection increased to 46%. In patients with CD4+ T cell counts above 200 cells/µL, the prevalence of Cryptosporidium infection was comparable to that of those who were HIV-negative at 6%.[15] The transplant population, including solid organ transplants and bone marrow transplants, has also been found to have higher rates of infection compared to the immunocompetent, with most data coming from kidney transplant recipients.[16] In children, cryptosporidiosis is known to be a major cause of prolonged diarrhea leading to malnutrition, particularly in developing countries. Furthermore, a clear association between income class and age of infection has been noted. In lower- to middle-income classes, children are generally affected by age 2 years. In a prospective case-control study known as the Global Enteric Multicenter Study (GEMS), cryptosporidiosis was found to be the second leading cause of diarrheal illness in children younger than 2 years of age in South Asia and sub-Saharan Africa, with an estimated 7.6 million cases annually and 200,000 deaths attributable to infection.[17] In contrast, children of the higher-income class generally acquire infection at age 5 or older, thought to be due to better hygiene practices that lead to lower rates of fecal-oral transmission in infancy.[13]

Cryptosporidium is also a well-recognized protozoan cause of traveler’s diarrhea, though less common than bacterial pathogens and Giardia. In large surveillance studies of returning travelers, Cryptosporidium accounts for approximately 2% to 11% of protozoal infections, with higher prevalence in those returning from South-Central Asia and sub-Saharan Africa. Children and immunocompromised travelers are at particular risk for more severe or prolonged disease, and over 14% of travelers with cryptosporidiosis may require hospitalization.[18][19]

Pathophysiology

Cryptosporidium is an intracellular yet extracytoplasmic pathogen that completes its entire life cycle within a single host, reproducing both sexually and asexually. The infective stage is known as the oocyst, which is the hardy, spore-form of the parasite found in the environment. Once picked up by a host through fecal-oral transmission, oocysts primarily traverse to the distal small bowel and proximal colon. The oocysts then become activated in the gastrointestinal tract, releasing 4 sporozoites. The sporozoites invade epithelial cells, causing the host membrane to fold over the parasite and form a parasitophorous vacuole. This vacuole allows the parasite to survive and replicate. Although intracellular, the vacuole is located outside the cytoplasm of the cell, which helps it evade host defenses that typically degrade intracytoplasmic vacuoles, such as phagocytosis, antigen-antibody interactions, and protein binding.[20] Inside the vacuole, sporozoites undergo asexual reproduction to become trophozoites, which then produce merozoites.

Merozoites invade new cells, continue asexual reproduction, or begin sexual reproduction, leading to sporulation and the formation of new oocysts. Some oocysts become thin-walled and continue the autoinfectious cycle, while others become thick-walled and are excreted back into the environment through feces.[2] The thick-walled oocysts are particularly resilient to multiple environmental stressors, allowing them to survive up to 6 to 8 months outside a host, especially in aquatic environments. They can also survive in temperatures ranging from -20 to 60 °C, with a study showing oocysts remained infectious after 24 weeks of storage at 0 to 20 °C.[21] Studies have demonstrated that oocysts become inactivated at -20 °C or lower after 1 week and can also be killed by microwave heating and pasteurization.[22] Oocysts are also known to be chlorine-resistant, and their small size can evade many filtration systems not designed to capture organisms 1 µm or smaller, thereby facilitating Cryptosporidium outbreaks in public swimming pools and recreational waters.[23]

Histopathology

Cryptosporidium is found on the brush border of intestinal epithelial cells. Oocysts appear basophilic on hematoxylin and eosin stain and are acid-fast positive with Ziehl–Neelsen and Kinyoun stain. They are smaller (4-6 µm), which helps differentiate them from other acid-fast-positive intracellular protozoa, including Cyclospora (8-10 µm) and Cystoisospora (25-30 µm). The location of the parasitophorous vacuole also helps differentiate between the protozoa: Cryptosporidium is extracytoplasmic, found on the surface of the enterocyte, whereas Cyclospora is intracytoplasmic. Infected intestinal epithelium undergoes rapid turnover, causing dysplasia. Crypt hyperplasia and villous atrophy also occur, leading to the disruption of intestinal architecture. This loss of barrier integrity contributes to a decrease in intestinal absorptive function and an increase in membrane permeability.[24]

Toxicokinetics

Studies in seronegative, healthy volunteers inoculated with varying doses of Cryptosporidium oocysts have shown that as few as 10 to 100 oocysts can cause infection. The median time to disease onset after inoculation is about 7 days, with a range of 2 to 28 days. Infected hosts have been observed to excrete anywhere from 10,000 to 1 billion oocysts, with oocyst shedding typically lasting 7 to 30 days. Higher inoculum doses showed no statistically significant differences in shedding duration, illness duration, or disease severity. This suggests it is not necessarily the amount of organisms that leads to virulence, but rather that host factors and strain differences may play a pivotal role.[25][26] For example, in contrast to seronegative volunteers, seropositive volunteers required an inoculum more than 20-fold higher and showed less oocyst shedding, suggesting that host response can reduce the number of oocysts produced in those who have been previously infected.[27]

History and Physical

Cryptosporidiosis has a wide clinical spectrum ranging from asymptomatic shedding of oocysts to severe, cholera-like diarrheal illness. In symptomatic individuals, clinical symptoms vary with epidemiological factors and the host's immune status. In immunocompetent individuals in high-income countries, symptoms are generally mild, presenting as nonbloody diarrhea. Diarrhea can be associated with other typical coexisting symptoms of gastroenteritis, including vomiting, dehydration, fatigue, abdominal cramping, low-grade fevers, and anorexia.[28] 

In the HIV population, the clinical spectrum can also vary from being asymptomatic and self-limiting to severe and persistent. Individuals with HIV who have CD4+ T cell counts less than 50 cells/µL are at increased risk for extraintestinal manifestations of Cryptosporidium, specifically within the pancreatic and biliary systems, including acalculous cholecystitis, sclerosing cholangitis, and pancreatitis.[29] Cryptosporidium also is known to infect transplant patients, with main symptoms being diarrhea, abdominal pain, weight loss, and vomiting. Extraintestinal infections within the biliary system in kidney transplant patients have also been reported.[16] Children who are affected also tend to have watery diarrhea, vomiting, dehydration, and may develop signs of malnutrition.

Evaluation

Cryptosporidium infections can be diagnosed through microscopy, immunoassay, and molecular testing. Standard stool microscopy methods can easily miss oocysts due to their small size; however, specialized techniques improve detection, including modified acid-fast staining and immunofluorescence. Cryptosporidium oocysts are smaller (4-6 µm), which helps differentiate them from other acid-fast-positive intracellular protozoa, including Cyclospora (8-10 µm) and Cystoisospora (25-30 µm). Oocysts stain bright pink or red on a blue-green background with modified acid-fast staining. Studies have shown sensitivities ranging from 37% to 79.1% with the Ziehl-Neelsen stain and from 66.7% to 91.6% with Kinyoun's stain. Immunofluorescence microscopy highlights oocysts in bright green on a black background and is known to be more sensitive than acid-fast staining, making it the gold standard for microscopy testing.

Immunoassays, eg, direct fluorescent antibody and enzyme-linked immunosorbent assay, are also used, with specificity ranging from 91% to 100%. However, the sensitivities of commercially available immunoassays have been found to vary widely, ranging from 47% to 100%. As a result, molecular methods have been increasingly utilized for their high sensitivity and specificity. Several multiplex polymerase chain reaction (PCR) kits are available in the United States that include Cryptosporidium testing, such as Luminex xTAG gastrointestinal pathogen panel, BioFire FilmArray, and BD Max parasitic panel. These panels are noted to have a sensitivity of 95% to 100% and a specificity of 99.6% to 100%.[30][31] 

Oocyst shedding in the stool can be intermittent rather than continuous; therefore, organisms can be missed on a single test.[32] If suspicion of cryptosporidiosis remains high despite a negative initial test, sending up to 3 separate stool samples over a few days is reasonable to increase the chances of detecting actively shedding oocysts. Ultrasound (US), CT, and ERCP/MRCP are the primary imaging modalities for evaluating hepatobiliary and pancreatic cryptosporidiosis, with ultrasound being the best initial diagnostic test when biliary disease is suspected. The most common findings include biliary duct dilatation, wall thickening, and features of sclerosing cholangitis. The radiologic findings are nonspecific and may resemble those seen in cytomegalovirus infection and microsporidiosis.[33]

Treatment / Management

The mainstay treatment for cryptosporidiosis is supportive therapy with fluid and electrolyte replacement. Nutritional status should be monitored closely and supplemented as needed, particularly in children and patients who are malnourished at baseline. Antimotility agents, eg, loperamide, can be used in mild to moderate disease, although their efficacy in severe disease is limited. People living with HIV who are not on HIV treatment should have highly active antiretroviral therapy initiated as soon as possible, given the importance of the cellular immune system to control the infection, which is fundamental to symptom improvement and parasite clearance in this population.[34][CDC. Guidelines for the Prevention and Treatment of Opportunistic Infections in Adults and Adolescents with HIV. 2023]

To date, no antiparasitic drug has demonstrated reliable cure rates, especially in the immunocompromised host. Nitazoxanide is the only FDA-approved medication for the treatment of cryptosporidiosis. Studies have shown improvements in symptoms and clearance of parasitemia in immunocompetent individuals treated with nitazoxanide. Yet these results are not consistently observed in immunocompromised hosts or malnourished children, underscoring the importance of an intact immune system in clearing infection. Duration of use is not well-defined. Those who are immunocompetent have shown improvement within 3 to 5 days, while the immunocompromised generally require treatment for at least 2 weeks, sometimes months, depending on symptom improvement.

The current recommendations for nitazoxanide in moderate-to-severe or persistent disease (greater than 2 weeks) are 3 days in immunocompetent patients and 14 days in immunocompromised patients. Paromomycin is a non-FDA-approved alternative medication to consider if nitazoxanide is not available. Although studies have demonstrated increased parasite clearance with paromomycin, there is significant variation in clinical response, and it has been shown to be inferior to nitazoxanide. Few studies have used azithromycin and rifaximin in the HIV population and immunocompromised children with favorable clinical results. This has led to the use of combination therapy with azithromycin or rifaximin plus nitazoxanide or paromomycin in diseases that are refractory to monotherapy, or don´t have access to those drugs. Other medications, eg, clarithromycin, spiramycin, and clofazimine, have not demonstrated clinical benefit in patients with cryptosporidiosis.[35][36]

As in HIV infected patients, where restoring immunity is critical with ARV drugs, in solid-organ transplant patients, reduction of immunosuppression is considered a cornerstone of management because cellular immune recovery is critical for parasite clearance, and persistent immunosuppression is associated with prolonged or relapsing infection and worse outcomes. The evidence base for this approach is primarily case series and expert consensus, as controlled trials are lacking. However, strategies with some positive results have been reported in the medical literature, eg, tapering or discontinuing mycophenolate mofetil, reducing calcineurin inhibitors (especially tacrolimus), or switching from tacrolimus to cyclosporine.[37][38]

Differential Diagnosis

The differential diagnosis of diarrheal illness caused by cryptosporidiosis includes infectious and noninfectious etiologies, including:

• Bacterial
  • Campylobacter spp
  • Clostridium difficile
  • Escherichia coli
  • Salmonella spp
  • Shigella spp
  • Vibrio spp, including cholerae and noncholerae species
  • Yersinia enterocolitica
• Viral
  • Adenovirus
  • Astrovirus
  • Cytomegalovirus (including biliopancreatic symptoms)
  • Norovirus
  • Rotavirus
  • Sapovirus
• Protozoan
  • Cyclospora cayetanensis
  • Cystoisospora belli
  • Entamoeba histolytica
  • Enterocytozoon bieneusi
  • Giardia lamblia
• Noninfectious
  • Tumors (Colorectal cancer, carcinoid)
  • Celiac disease
  • Inflammatory bowel disease
  • Irritable bowel syndrome
  • Small intestinal bacterial overgrowth

Prognosis

In the immunocompetent, gastrointestinal symptoms of Cryptosporidium infection are usually self-limiting after 1 to 2 weeks, with a median duration of 7 days. Studies have reported that up to one-third to one-half of cases with initial symptom resolution recur, defined as the return of diarrhea after at least 2 days of normal stools.[39] Immunosuppressed individuals are more likely to have a chronic course, sometimes with diarrhea lasting longer than 6 weeks. The degree of immunosuppression plays a large role in prognosis. For instance, in the HIV population, both the severity and chronicity of disease seem to correlate with CD4 count, highlighting the importance of T-cell immunity.

In those with CD4+ T cell counts greater than 150 cells/µL, disease is often mild and self-limiting, similar to that in the immunocompetent population. In contrast, HIV-positive individuals with a CD4+ T cell count less than 100 cells/µL may develop more of a chronic illness with frequent bulky stools, weight loss, and malabsorption. A minority will develop fulminant disease, defined as greater than 21 stools per day, particularly those with a CD4+ T cell count less than 50 cells/µL.[28] Initiation of highly active antiretroviral therapy in the HIV patient infected with Cryptosporidium has often been shown to provide resolution of disease once their CD4+ T cell count reconstitutes.[40] Severely immunosuppressed patients who go on to develop biliary involvement tend to have higher mortality with shortened survival times.[41]

Complications

Prospective cohort studies that follow patients for up to 10 years after Cryptosporidium infection have demonstrated long-term sequelae in both adults and children. In adults, those with a history of cryptosporidiosis were 3 times more likely to report consistent abdominal symptoms and 2 times more likely to report consistent joint pain. New onset of irritable bowel syndrome, inflammatory bowel disease, and lactose intolerance were also reported at higher rates.[42] Other nonintestinal sequelae commonly seen include eye pain, intermittent headaches, dizziness, and fatigue.[43] In contrast, patients who acquired infection in childhood were less likely to report ongoing abdominal pain or joint pain.

The main complication of childhood infection is malnutrition. Studies have shown that even in subclinical or asymptomatic infection, cryptosporidiosis in children has been associated with growth stunting, wasting, and becoming underweight.[44][45] The severity of malnutrition has been linked with infection at age 2 years or younger and living in resource-poor countries. A few reasons have been speculated for why malnutrition is more severe in children who reside in resource-poor countries, including exposures to higher inoculum infections, a tendency for multiple exposures, and children being more malnourished at baseline.[46] In kidney transplants, severe cases of diarrheal illness have led to elevated tacrolimus levels and graft failure.[16]