Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Cryptosporidium and cryptosporidiosis

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production of melanin. Current thought is that the phenol oxi-

dase prevents the formation of charged hydroxy groups, which

can be very damaging to the yeast cell. The yeast may actually

recruit the body’s melanin producing machinery to make the

compound.

Cryptococcus neoformansalso has other enzymesthat

act to degrade certain proteins and the phospholipidsthat

make up cell membranes. These enzymes may help disrupt the

host cell membrane, allowing the yeast cells penetrate into

host tissue more easily.

Cryptococcus neoformansis able to grow at body tem-

perature. The other Cryptococcus species cannot tolerate this

elevated temperature.

Yet another virulence factor may operate. Evidence

from laboratory studies has indicated that antigens from the

yeast can induce a form of T cellsthat down regulates the

immune response of the host. This is consistent with the

knowledge that survivors of cryptococcal meningitis display a

poorly operating immune system for a long time after the

infection has ended. Thus, Cryptococcus neoformansmay not

only be capable of evading an immune response by the host,

but may actually dampen down that response.

If the infection is treated while still confined to the

lungs, especially in patients with a normally operative immune

system, the prospects for full recovery are good. However,

spread to the central nervous system is ominous, especially in

immunocompromised patients.

The standard treatment for cryptococcal meningitis is

the intravenous administration of a compound called ampho-

tericin B. Unfortunately the compound has a raft of side

effects, including fever, chills, headache, nausea with vomit-

ing, diarrhea, kidney damage, and suppression of bone mar-

row. The latter can lead to a marked decrease in red blood

cells. Studies are underway in which amphotericin B is

enclosed in bags made of lipid material (called liposomes).

The use of liposomes can allow the drug to be more specifi-

cally targeted to the site where treatment is most needed,

rather than flooding the entire body with the drug. Hopefully,

the use of liposome-delivered amphotericin B will lessen the

side effects of therapy.

See alsoFungi; Immunomodulation; Yeast, infectious

CRYPTOSPORIDIUM AND

CRYPTOSPORIDIOSISCryptosporidium and cryptosporidiosis

Cryptosporidum is a protozoan, a single-celled parasite that

lives in the intestines of humans and other animals. The organ-

ism causes an intestinal malady called cryptosporidiosis

(which is commonly called “crypto”).

The members of the genus Cryptosporidiuminfects

epithelial cells, especially those that line the walls of the intes-

tinal tract. One species, Cryptosporidium muris, infects labo-

ratory 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, Crypto-

sporidium exists in a form that is analogous to a bacterial

spore. In the case of Cryptosporidium, this dormant and envi-

ronmentally 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 drink-

ing 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 cryp-

tosporidiosis. Studies using volunteers indicate that an infec-

tious 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 neigh-

bouring 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 con-

tracted 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 cryp-

tosporidiosis, 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 qualitystandards 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 chal-

lenging. No vaccinefor cryptosporidiosis exists. A well-func-

tioning immune systemis the best defense against the disease.

Indeed, estimates are that about 30% of the population has

antibodies to Cryptosporidium parvum, even though no symp-

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