HumBio 153: Parasites and Pestilence
Dr. Scott Smith
February 25, 2009
Cryptosporidiosis is a diarrheal disease, caused by Cryptosporidium. The main species that infects humans is Cryptosporidium parvum. The parasite is spread fecal-orally, often through contaminated water. There are periodic outbreaks worldwide. Good sanitation, proper hygiene, and careful preparation of water (boiling or filtering) and food (washing and cooking) can help prevent the spread of the disease. Cryptosporidiosis generally resolves on its own in immunocompetent people. It is when Cryptosporidium parasites infect an immunocompromised person that the disease can become a serious, life-threatening, problem.
Agent (Classification and Taxonomy)
Cryptosporidiosis is caused by protozoa.
The main agent that causes the disease in humans is Cryptosporidium parvum. C. parvum has 2 genotypes, the first of which became known as C. hominis. Other agents that can infect humans include C. felis (feline cryptosporidia), C. muris (rodent cryptosporidia) and C. meleagridis.
Cryptosporidium is also known as “crypto”.
History of Discovery
1907 heralded the discovery of Cryptosporidium. It was not until 1976, however, that the first human cases were identified. One of the afflicted humans was a 3-year-old child who developed severe enterocolitis that resolved on its own. The other was a 39-year-old immunosuppressed man. His condition improved after he discontinued one of his medications, cyclophosphamide.
Clinical Presentation in Humans
There are 3 possible forms of the illness in immunocompetent people. The disease can be asymptomatic or cause acute diarrhea or persistent diarrhea that can last for a few weeks. Diarrhea is usually watery with mucus. It is very rare to find blood or leukocytes in the diarrhea. Diarrhea often ceases on its own. Other symptoms include abdominal pain, nausea and vomiting. Malabsorption and dehydration can also occur. Another clinical presentation is a low-grade fever. Anorexia can occur, as can weight loss. Symptoms generally last for 7 to 14 days, although they can persist for up to one month.
Immunocompromised people, as well as very young or very old people, can develop a more severe form of cryptosporidiosis. There are 4 clinical presentations for patients with AIDS. 4% have no symptoms, 29% have a transient infection, 60% have chronic diarrhea, and 8% have a severe, cholera-like infection. With transient infections diarrhea ends within 2 months and Cryptosporidium is no longer found in the feces. Chronic diarrhea is diarrhea that lasts for 2 or more months. The most severe form results in the patients excreting at least 2 liters of watery diarrhea per day. They can lose up to 25 liters per day. AIDS patients can have up to 10 stools per day. They experience severe malabsorption and can have 10% weight loss.
The severity of the disease in immunocompromised people can be predicted by the CD4+ T-cell count. When the count is greater than 200 cells/mm3 the disease is either asymptomatic or resolves on its own. When the CD4+ count is less than 100 it is chronic and can spread beyond the intestine. A count below 50 indicates severe disease with a sudden onset. When Cryptosporidium spreads beyond the intestine, as it can predominantly in patients with AIDS, it can reach the lungs, middle ear, pancreas, and stomach. Thus, one symptom is pain in the right upper quadrant. The parasite can infect the biliary tract, causing biliary cryptosporidiosis. This can result in cholecystitis and cholangitis. Half of AIDS patients who contract cryptosporidiosis die in less than 6 months. Many never completely eliminate Cryptosporidium from their bodies.
For cryptosporidiosis it is the oocysts that are infective. Transmission occurs fecal-orally, or when oocysts from feces are ingested. Chlorination and treated water supplies are not always sufficient to kill Cryptosporidium. Thus, the oocysts may reside in swimming pools and chlorine-treated or filtered drinking water. A person can contract the disease via direct contact with infected animals or by eating food contaminated by feces. There are often outbreaks in hospitals and day-care centers, showing that Cryptosporidium can be transmitted from one human to another. Yet another way to become infected is through exposure to feces during sexual encounters. Coming into contact with any contaminated objects or environmental surfaces can also transmit the disease.
The main reservoir for Cryptosporidium is domestic animals. Cryptosporidium parvum can reside in 150 different species of mammals such as cattle, sheep, goats, deer, mice and pigs. Non-biting cyclorrhaphan flies are another reservoir.
There is no vector for cryptosporidiosis.
The incubation period for cryptosporidiosis is 2 to 10 days, with an average of 7 days.
The oocysts are ovoid or spherical and measure 5 to 6 micrometers across. When in flotation preparations they appear highly refractile. The oocysts contains up to 4 sporozoites that are bow-shaped.
The life cycle of Cryptosporidium parvum consists of an asexual stage and a sexual stage. After being ingested the oocysts excyst in the small intestine. They release sporozoites that attach to the microvilli of the epithelial cells of the small intestine. From there they become trophozoites that reproduce asexually by multiple fission, a process known as schizogony. The trophozoites develop into Type 1 meronts that contain 8 daughter cells. These daughter cells are Type 1 merozoites, which get released by the meronts. Some of these merozoites can cause autoinfection by attaching to epithelial cells. Others of these merozoites become Type II meronts, which contain 4 Type II merozoites. These merozoites get released and they attach to the epithelial cells. From there they become either macrogamonts or microgamonts. These are the female and male sexual forms, respectively. This stage, when sexual forms arise, is called gametogony. Zygotes are formed by microgametes from the microgamont penetrating the macrogamonts. The zygotes develop into oocysts of two types. 20% of oocysts have thin walls and so can reinfect the host by rupturing and releasing sporozoites that start the process over again. The thick-walled oocysts are excreted into the environment. The oocysts are mature and infective upon being excreted. They can survive in the environment for months.
“Panels A through E show a Cryptosporidium parvum sporozoites attaching to and involving a host epithelial cell in an in vitro model of biliary cryptosporidiosis. Panels A, B, C, and D are scanning electron micrographs, and Panels E and F transmission electron micrographs. Panel A shows a sporozoite attaching to the apical membrane surface of a biliary epithelial cell. Panels B and C show a sporozoite invading a host cell and the protrusion of the epithelial-cell membrane around the parasite at its attachment site. Panels D and E show an organism being enveloped by the host-cell membrane and the formation of a vacuole. In Panel E, the zoite has made contact with the microvillous border of the epithelial cell, with its anterior end inserted into the host-cell membrane (arrow), and is in the process of being internalized. A dense band is formed where the parasite meets the epithelial cell. Panel F shows an intestinal-biopsy specimen from a patient with the acquired immunodeficiency syndrome and intestinal cryptosporidiosis. The bar represents 1 μm. The illustration of the life cycle is modified from Tzipori and Widmer, with the permission of the publisher. Panels A, B, C, and D are reprinted from Chen et al., with the permission of the publisher” (Chen et. al 1724-1725).
(Centers for Disease Control)
As few as 10 to 100 oocysts can initiate an infection. The parasite is located in the brush border of the epithelial cells of the small intestine. They are mainly located in the jejunum. When the sporozoites attach the epithelial cells’ membrane envelops them. Thus, they are “intracellular but extracytoplasmic”. The parasite can cause damage to the microvilli where it attaches. The infected human excretes the most oocysts during the first week. Oocysts can be excreted for weeks after the diarrhea subsides.
The immune system reduces the formation of Type 1 merozoites as well as the number of thin-walled oocysts. This helps prevent autoinfection. B cells do not help with the initial response or the fight to eliminate the parasite.
Histopathology of the intestine. Cryptosporidium at the surface of epithelial cells.
There are many diagnostic tests for Cryptosporidium. They include microscopy, staining, and detection of antibodies. Microscopy can help identify oocysts in fecal matter. To increase the chance of finding the oocysts the diagnostician should inspect at least 3 stool samples. There are several techniques to concentrate either the stool sample or the oocysts. The modified formalin-ethyl acetate (FEA) concentration method concentrates the stool. Both the modified zinc sulfate centrifugal flotation technique and the Sheather’s sugar flotation procedure can concentrate the oocysts by causing them to float. Another form of microscopy is fluorescent microscopy done by staining with auramine.
Other staining techniques include acid-fast staining, which will stain the oocysts red. One type of acid-fast stain is the Kinyoun technique. Giemsa staining can also be performed. Part of the small intestine can be stained with hematoxylin and eosin (H & E), which will show oocysts attached to the epithelial cells.
Kinyoun acid-fast stain. The Cryptosporidium oocysts are red.
Detecting antigens is yet another way to diagnose the disease. This can be done with direct fluorescent antibody (DFA). It can also be achieved through indirect immunofluorescence assay. Enzyme-Linked ImmunoSorbent Assay (ELISA) also detects antigens.
Polymerase chain reaction (PCR) is another way to diagnose cryptosporidiosis. It can even identify the specific species of Cryptosporidium. If the patient is thought to have biliary cryptosporidiosis then an appropriate diagnostic technique is ultrasonography. If that returns normal results the next step would be to perform endoscopic retrograde cholangiopancreatography.
Management and Therapy
In immunocompetent people cryptosporidiosis often resolves on its own. Therefore, no treatment is necessary. The main therapy consists of compensating for fluid lost from diarrhea. Pregnant women and young children are the most prone to become dehydrated. Nitazoxanide is one drug that the FDA has approved for use in immunocompetent people to combat diarrhea. Spiramycin can help shorten the amount of time oocysts are passed as well as the duration of diarrhea in children. The drug can also help treat diarrhea in patients who are in the early stages of AIDS.
In one AIDS patient from Iran, who had pulmonary cryptosporidiosis in addition to intestinal cryptosporidiosis, azithromycin and paromomycin helped to clear the infection. Parenteral octreotide acetate can help decrease the number of stools passed. Generally, though, for AIDS patients, the best treatment is to use highly active antiretroviral therapy to strengthen the immune system. For immunosuppressed patients it is helpful to stop immunosuppressants and to provide supportive therapy. A treatment for patients with biliary cryptosporidiosis who have cholangitis in addition to papillary stenosis is endoscopic sphincterotomy.
Currently research is being done in molecular-based immunotherapy. For example, synthetic isoflavone derivates have been shown to fight off Cryptosporidium parvum in vitro and in a gerbil. Derivates of nitazoxanide, which are synthetic nitro- or non nitro- thiazolide compounds, have also shown promising results in vitro.
Cryptosporidiosis is found worldwide. It causes 50.8% of water-borne diseases that are attributed to parasites. In developing countries 8-19% of diarrheal diseases can be attributed to Cryptosporidium. 10% of the population in developing countries excretes oocysts. In developed countries the number is lower at 1-3%.
The age group most affected is children from 1 to 9 years old. From 1991-2004 the incidence of the disease for 65 year olds and older in the United States (based on people covered by Medicare) was 0.27 cases per 100,000 people. For people 85 years old or older the incidence was 0.51 per 100,000 people.
(Morbidity and Mortality Weekly Report)
# of cases:
1990-2000: 10 outbreaks due to contaminated drinking water
1993: 400,000 people in Milwaukee after cattle manure contaminated the water supply
2001: Cryptosporidium parvum outbreak in Illinois at a water park
2005: 3,131 people in New York from a park spray pool
2006: 49 cases of giardiasis and cryptosporidiosis in Florida from a spraying fountain
2007: 1,902 cases in Utah from recreational water
2000: 112 British tourists in Majorca, Spain from a contaminated hotel swimming pool
2003: 391 British tourists in Majorca, Spain at a hotel
2006: 29 Americans in Ireland
2007: 25 Swedish guests in Norway at a hotel
Recent Outbreaks: 2008
Sweden: 21 guests in Stockholm at a wedding reception
United Kingdom: 29 cases in East Midlands from contaminated drinking water
United States: 389 cases of salmonellosis in Colorado due to contaminated water supply; cryptosporidium also found in water
Public Health and Prevention Strategies/Vaccines
The law requires doctors and labs to report cases of cryptosporidiosis to local or state health departments. These departments then report to the Center for Disease Control and Prevention. The best way to prevent getting and spreading cryptosporidiosis is to have good hygiene and sanitation. An example would be hand-washing. People should avoid contact with animal feces. They should also avoid possibly contaminated food and water. Additionally, people should refrain from engaging in sexual activities that can expose them to feces.
Since standard water filtration may not be enough to eliminate Cryptosporidium, to be extra safe one can boil water for at least 1 minute. If the person is above 6500 feet he or she should boil the water for 3 minutes. For milk, heating it at 71.7°C for 15 seconds pasteurizes it and can destroy the oocysts ability to infect someone. Another way to ensure that water is safe is to use a filter with a pore size of 1 micron or smaller. Filters that have been approved for “cyst removal” by the National Science Foundation (NSF) also work. One can also use bottled drinking water, which is less likely to contain Cryptosporidium, especially if the water is from an underground source.
People who have cryptosporidiosis should not swim in communal areas because Cryptosporidium can reside in the anal and genital areas and therefore be washed off. They should wait until at least two weeks after diarrhea stops before entering public water sources since oocysts can still be shed for a while. Also, they should stay away from immunosuppressed people. Immunocompromised people should take care to protect themselves from water in lakes and streams. They should also stay away from animal stools and wash their hands after touching animals. To be safe they should boil or filter their water. They should also wash and cook their vegetables.
Useful Web Links
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Jeffrey P. Davis, and Anne C. Haddix. “Cost of Illness in the 1993 Waterborne Cryptosporidium Outbreak, Milwaukee, Wisconsin.” Emerging Infectious Diseases. Vol. 9, No. 4. April 2003: 426-431.
21. Causer, L. M., T. Handzel, P. Welch, M. Carr, D. Culp, R. Lucht, K. Mudahar, D. Robinson, E. Neavear, S. Fenton, C. Rose, L. Craig, M. Arrowood, S. Wahlquist, L. Xiao, Y.-M. Lee, L. Mirel, D. Levy, M. J. Beach, G. Poquette, and M. S. Dworkin. “An Outbreak of Cryptosporidium hominis infection at an Illinois Recreational Waterpark.” Epidemiol. Infect. Vol. 134. 2006: 147-156.