Cryptosporidium and cryptosporidiosis

Cryptosporidum is a protozoan, a single-celled parasite that lives in the intestines of humans and other animals. The organism causes an intestinal malady called cryptosporidiosis (which is commonly called "crypto").

The members of the genus Cryptosporidium infects epithelial cells, especially those that line the walls of the intestinal tract. One species, Cryptosporidium muris, infects laboratory tests species, such as rodents, but does not infect humans. Another species, Cryptosporidium parvum, infects a wide variety of mammals, including humans. Calculations have indicated that cattle alone release some five tons of the parasite each year in the United States alone.

Non-human mammals are the reservoir of the organism for humans. Typically, the organism is ingested when in water that has been contaminated with Cryptosporidium-containing feces. Often in an environment such as water, Cryptosporidium exists in a form that is analogous to a bacterial spore. In the case of Cryptosporidium, this dormant and environmentally resilient form is called an oocyst.

An oocyst is smaller than the growing form of Cryptosporidium. The small size can allow the oocyst to pass through some types of filters used to treat water. In addition, an oocyst is also resistant to the concentrations of chlorine that are widely used to disinfect drinking water. Thus, even drinking water from a properly operating municipal treatment plant has the potential to contain Cryptosporidium.

The organism can also be spread very easily by contact with feces, such as caring with someone with diarrhea or changing a diaper. Spread of cryptosporidiosis in nursing homes and day care facilities is not uncommon.

Only a few oocytes need to be ingested to cause cryptosporidiosis. Studies using volunteers indicate that an infectious dose is anywhere from nine to 30 oocysts. When an oocyte is ingested, it associates with intestinal epithelial cells. Then, four bodies called sporozoites, which are contained inside the oocyst, are released. These burrow inside the neighbouring epithelial cells and divide to form cells that are called merozoites. Eventually, the host cell bursts, releasing the merozoites. The freed cells go on to attack neighbouring epithelial cells and reproduce. The new progeny are released and the cycle continues over and over. The damage to the intestinal cells affects the functioning of the intestinal tract.

Cryptosporidium and its oocyte form have been known since about 1910. Cryptosporidium parvum was first described in 1911. Cryptosporidiosis has been a veterinary problem for a long time. The disease was recognized as a human disease in the 1970s. In the 1980s, the number of human cases rose sharply along with the cases of AIDS .

There have been many outbreaks of cryptosporidiosis since the 1980s. In 1987, 13,000 in Carrollton, Georgia contracted cryptosporidiosis via their municipal drinking water. This incident was the first case of the spread of the disease through water that had met all state and federal standards for microbiological quality. In 1993, an outbreak of cryptosporidiosis, again via contaminated municipal drinking water that met the current standards, sickened 400,000 people and resulted in several deaths. Outbreaks such as these prompted a change in water quality standards in the United States.

Symptoms of cryptosporidiosis are diarrhea, weight loss, and abdominal cramping. Oocysts are released in the feces all during the illness. Even when the symptoms are gone, oocysts continue to be released in the feces for several weeks.

Even though known for a long time, detection of the organism and treatment of the malady it causes are still challenging. No vaccine for cryptosporidiosis exists. A well-functioning immune system is the best defense against the disease. Indeed, estimates are that about 30% of the population has antibodies to Cryptosporidium parvum, even though no symptoms of cryptosporidiosis developed. The malady is most severe in immunocompromised people, such as those infected with HIV (the virus that causes AIDS), or those receiving chemotherapy for cancer or after a transplant. For those who are diabetic, alcoholic, or pregnant, the prolonged diarrhea can be dangerous.

In another avenue of infection, some of the merozoites grow bigger inside the host epithelial cell and form two other types of cells, termed the macrogametocyte and microgametocyte. The macrogametocytes contain macrogametes. When these combine with the microgametes released from the microgametocytes, a zygote is formed. An oocyst wall forms around the zygote and the genetic process of meiosis results in the creation of four sporozoites inside the oocyst. The oocyst is released to the environment in the feces and the infectious cycle is started again.

The cycle from ingestion to the release of new infectious oocytes in the feces can take about four days. Thereafter, the production of a new generation of parasites takes as little as twelve to fourteen hours. Internally, this rapid division can create huge numbers of organisms, which crowd the intestinal tract. Cryptosporidiosis can spread to secondary sites, like the duodenum and the large intestine. In people whose immune systems are not functioning properly, the spread of the organism can be even more extensive, with parasites being found in the stomach, biliary tract, pancreatic ducts, and respiratory tract.

Detection of Cryptosporidium in water is complicated by the lack of a culture method and because large volumes of water (hundreds of gallons) need to be collected and concentrated to collect the few oocytes that may be present. Presently, oocysts are detected using a microscopic method involving the binding of a specific fluorescent probe to the oocyte wall. There are many other noninfectious species of Cryptosporidium in the environment that react with the probe used in the test. Furthermore, the test does not distinguish a living organism from one that is dead. So a positive test result is not always indicative of the presence of an infectious organism. Skilled analysts are required to perform the test and so the accuracy of detection varies widely from lab to lab.

See also Giardia and giardiasis; Water quality; Water purification