Luis F. Barroso II and Richard L. Guerrant
Cryptosporidium species are tiny (2–6 μm), obligate intracellular parasites related to other coccidian protozoan, including Toxoplasma, Cyclospora, Isospora, Plasmodium, Eimeria, and Sarcocystis. Cryptosporidium species primarily infect the gastrointestinal tract of a variety of vertebrate hosts, including humans.1 Host range is largely a function of species, as a given species of parasite most efficiently maintains infection within a few species of hosts.
Over 20 species of Cryptosporidium have been described, but there is some debate as to the exact number of species.2 Two species, C hominis and C parvum, account for nearly all of the disease caused by Cryptosporidium species in humans and are the species of public health significance. C parvum was formerly divided into two genotypes (genotypes I and II), but current classification separates genotype I into a separate species known as C hominis.3
Cryptosporidium completes its life cycle within a single host1,11 (Fig. 345-1). Infection occurs after ingesting the sporulated, thick-walled oocysts. Excystation occurs in the small intestine after exposure to bile salts and pancreatic enzymes, releasing four sporozoites. These sporozoites penetrate a surface epithelial cell in the intestinal mucosa and form an intracellular parasitophorous vacuole. They then differentiate into uninuclear trophozoites, which undergo asexual replication (merogony) to form type I meronts. The type I meront can then autoinfect other surface epithelial cells or differentiate into a type II meront. The type II meront then undergoes gametogomy, producing both microgametocytes and macrogametocytes. These gametocytes fertilize to produce oocysts. The life cycle is complete when the oocysts undergo sporogomy, resulting in infectious sporozoites within the oocysts. Approximately 80% of the oocysts produced in this fashion are environmentally resistant, thick-walled cysts that are excreted in the feces. The remaining 20% are thin-walled cysts that undergo another autoinfective stage. The autoinfectious stages are important features in the parasite’s life cycle and accounts for persistent and occasionally severe disease, even when a low inoculum of cysts are ingested.
Cryptosporidia are ubiquitous in the environment and are found in several sources of untreated surface water. During the AIDS epidemic, cryptosporidia were recognized as a significant enteric pathogen, causing severe disease in patients with advanced AIDS. Currently, studies show that the incidence of AIDS-associated cryptosporidiosis has declined with the advent of highly active antiretroviral therapy (HAART).12,13
The prevalence of disease is higher in developing countries due to less sanitary conditions that promote fecal-oral transmission and the lack of safe water sources. The seroprevalence in developing countries is as high as 75% by age 4, in contrast to the United States, where seroprevalence is approximately 15%.14 Cryptosporidiosis is more common in children than adults. Daycare attendance is a risk factor, and outbreaks have been reported in childcare settings, likely through secondary person-to-person transmission.15,16 The principal mode of transmission is fecal-oral.
The oocyst is particularly resistant to disinfectants, including chlorine, bleach, ethanol, and many other hospital, industrial, and household chemicals.1,11,17 However, 6% hydrogen peroxide is an effective cysticide. Cryptosporidium is resistant to standard chlorine concentrations in public water systems, and it often escapes filtration systems. The largest waterborne outbreak occurred in 1993 in Milwaukee, Wisconsin, with approximately 400,000 people infected and 69 deaths.18
In the immunocompetent host, Cryptosporidium primarily infects the proximal small bowel. The exact mechanism whereby Cryptosporidium causes diarrhea is unknown. The organism disrupts epithelial tight junctions in several in vitro epithelial cell models,21 resulting in a loss of barrier function. Experiments have failed to reveal any exotoxin or virulence factor produced by Cryptosporidium, although the organism clearly causes malabsorption. Pathological findings include loss of intestinal epithelium, villous atrophy with loss of epithelial architecture and surface area, and infiltration of the lamina propria with mononuclear and polymorphonuclear cells.11 Various stages of the parasite can be seen immediately below the brush border of epithelial cells in biopsy samples.
FIGURE 345-1. Cryptosporidium life cycle. The infectious sporulated oocyst is ingested by a susceptible host. Exposure to digestive enzymes and bile salts releases four motile sporozoites that immediately attach and invade adjacent epithelial cells. The sporozoites develop in a parasitophorous vacuole that is intracellular but extracytoplasmic; there, they form trophozoites that differentiate through nuclear division into schizonts. Type I schizonts contain 6 to 8 nuclei that mature into 6 to 8 merozoites, which then infect neighboring cells and continue the asexual multiplication cycle or develop into type II schizonts. Type II schizonts develop into four type II merozoites that initiate the sexual cycle. Individual type II merozoites produce either macrogamonts or micro-gamotes. Nuclear division in the microgamont results in numerous microgametes, which are released and fertilize the macrogamont. The resulting zygote then develops into an oocyst. Oocysts are released into the lumen and pass out with the feces to infect another host, thus completing the life cycle. Up to 20% of oocysts can autoinfect the host by excysting before excretion; this autoinfectious cycle coupled with the asexual multiplication of type I merozoites can lead to a high organism burden in a susceptible host and high shedding rate of oocysts. (Adapted with permission from Smith, Trends Parasitol. 2005;21(3):133-142.)
The mechanisms of recovery and immunity to cryptosporidiosis has not been fully elucidated. An intact cell-mediated immune system with CD4 lymphocytes and interferon is crucial for recovery from disease. Animal models support a pivotal role of interferon gamma, with IFN-γ knock-out models developing severe infections that proceed to chronic forms.22–24 Humoral immunity is not completely protective but does decrease the severity of illness and shedding of oocysts. Secretory antibodies are beneficial, as is hyperimmune colostrum.25
Symptoms of cryptosporidiosis begin between 2 to 10 days (average 7 days) after becoming infected with the parasite. The most common symptom is watery diarrhea, which varies among individuals with some being entirely asymptomatic. Symptoms generally persist for 1 to 2 weeks. Occasionally, patients recover and then experience a recurrence of symptoms before the illness ends.
Severe cryptosporidiosis occurs in immunosuppressed patients, including those with advanced AIDS, cancer, hypogammaglobulinemia, severe combined immunodeficiency, and bone marrow and solid organ transplants. Such patients may have severe, life-threatening illness with intractable diarrhea and severe volume loss. Symptoms often last for months to years; wasting, hypovolemia, and weight loss is common. In general, the severity of the diarrheal illness correlates with the severity of immunosuppression.
Children with underlying malnutrition tend to have prolonged episodes of disease with growth shortfalls.26,27 Multiple diarrheal episodes in early childhood have been associated with decreased height-for-age scores and decreased cognition and language skills, particularly semantic fluency.26-28
In immunocompromised patients, fluid loss can be massive and can exceed 10 to 20 liters per day. Parenteral hydration is usually required to maintain euvolemia in the immunosuppressed host. Other clinical findings include fever, nausea, vomiting, and crampy abdominal pain. Myalgia, malaise, headache, and other flulike symptoms have also been reported.
Extraintestinal infection with Cryptosporidium is rare and usually occurs only in severely immunocompromised hosts. In particular, advanced AIDS is associated with biliary cryptosporidiosis, which results in AIDS cholangiopathy.29 This is due to the synergistic interaction of HIV and Cryptosporidium infection, whereby the HIV Tat-1 protein enhances Cryptosporidium-induced apoptosis in cholangiocytes.30Biliary cryptosporidiosis manifests as acalculous cholecystitis, sclerosing cholangitis, and hepatitis. The clinical presentation includes fever, upper-right quadrant pain, jaundice, nausea, vomiting, diarrhea, hyperbilirubinemia, and elevated liver enzymes. Radiographically, the disease resembles sclerosing cholangitis. Pancreatic cryptosporidiosis may also occur.
Respiratory cryptosporidiosis is a rare event and nearly always requires concomitant intestinal disease.20 It manifests as cough, shortness of breath, wheezing, croup, and hoarseness. Diagnosis is made through isolation of oocysts from sputum or tracheal aspirates.
The diagnosis of intestinal cryptosporidiosis is established by identifying the characteristic oocysts in the stool. The oocysts may be visualized using a modified acid-fast stain, although this method lacks sensitivity.31 The diagnosis is almost never made on routine ova and parasite stool examination. Even with acid-fast staining, the potential for missing this and other coccidian parasites is high. Enzyme-linked immunosorbent assays (ELISA) are commercially available but lack specificity. The current diagnostic standard is monoclonal immunofluorescent staining to visualize the parasite in stool preparations.
PCR-based molecular techniques for detecting oocysts in stool and water have been developed with increased sensitivity and more rapid turnaround times.32,33 Finally, immunomagnetic beads have been developed to allow rapid identification of oocysts in stool and water.34
Extraintestinal cryptosporidiosis remains an elusive diagnosis and requires demonstration of the parasite from an extraintestinal source. This is usually accomplished via biopsy, with the parasite appearing on light microscopy with hematoxylin-eosin stains or electron microscopy.
Until recently, there were no good treatment options for cryptosporidiosis. Three placebo-controlled studies have demonstrated that nitazoxanide (> 12 years of age, 500 mg, 4–11 years of age, 200 mg, 1–3 years of age 100 mg, all twice daily for 3 days) leads to more rapid cessation of diarrhea, and more frequent eradication of the organism compared to placebo in HIV-negative adults and children.35,36 In severely malnourished children with chronic cryptosporidiosis, cure rates were lower but many resolved with retreatment.
In patients with HIV higher doses, or more prolonged therapy (2 weeks) was superior to placebo among those with CD4+ T-cell counts greater than 50/mm3, but was not better than placebo in those with lower CD4+ counts.37
Paromomycin and azithromycin also in vitro activity against the parasite. Paromomycin (1 g bid) in combination with azithromycin (600 mg qd) has been shown to be effective in case series that largely originate from the pre-HAART AIDS era.20,38 Hyperimmune bovine colostrum reduces clinical symptoms and shedding of oocysts, but this is considered experimental therapy.39
The cornerstone of therapy in immunosuppressed hosts is restoring immune function. In patients with AIDS, this is accomplished via HAART therapy, which serves two roles: (1) the restoration of CD4 count allows the host to respond to and eliminate infection, and (2) some HAART drugs, particularly protease inhibitors, may have some in vitro activity against the parasite.38,40 The clinical relevance of the latter point is unknown. In bone marrow and solid organ transplants, decreasing immune suppression, if at all tolerated, greatly enhances cure rates.
Rehydration is also crucial, as large volume fluid loss is a frequent complication of disease. This often requires parenteral fluid supplementation. Peptidomimetic agents (octreotide and vapreotide) and antimotility agents (loperamide, opiates, and atropine) may help to control diarrheal symptoms.
In cases of biliary cryptosporidiosis in severely immunocompromised individuals, drug therapy has been effective. Endoscopic decompression of the biliary system and stenting is helpful in cases of sclerosing cholangitis.
Decreasing exposure to oocysts is crucial for immunosuppressed populations and is sensible for immunocompetent populations. Exposure largely occurs through a waterborne source, so all potentially contaminated water must be treated before consumption. Unfortunately, standard chlorination has no effect on oocysts, and even iodine-based water-purification tablets are modestly effective at best. The most reliable way to purify surface water sources involves boiling it for at least 1 minute, which thoroughly inactivates most pathogens, including Cryptosporidium. Filters with resolutions of less than 1 μm are theoretically effective but are easily clogged and may be bypassed if damaged. In general, immunosuppressed persons should be advised to avoid all untreated surface water sources and potentially risky activities such as camping. Bottled water from a reliable source offers a safe alternative, although its transport may be cumbersome. Recreational water has been linked to public outbreaks.41 Public pools may be contaminated by an infected person and therefore, the CDC advises that infected individuals avoid swimming during and for two weeks following a diarrheal illness, since they may be shedding oocysts.41