Cryptosporidium parvum

Cryptosporidium parvum, a single-celled animal, i.e., a protozoa, is an obligate intracellular parasite. It has been given additional species names when isolated from different hosts. It is currently thought that the form infecting humans is the same species that causes disease in young calves. The forms that infect avian hosts and those that infect mice are not thought capable of infecting humans. Cryptosporidium sp. infects many herd animals (cows, goats, sheep among domesticated animals, and deer and elk among wild animals). The infective stage of the organism, the oocyst is 3 um in diameter or about half the size of a red blood cell. The sporocysts are resistant to most chemical disinfectants, but are susceptible to drying and the ultraviolet portion of sunlight. Some strains appear to be adapted to certain hosts but cross-strain infectivity occurs and may or may not be associated with illness. The species or strain infecting the respiratory system is not currently distinguished from the form infecting the intestines.

Nature of Acute Disease:

Intestinal, tracheal, or pulmonary cryptosporidiosis.

Nature of Disease:

Intestinal cryptosporidiosis is characterized by severe watery diarrhea but may, alternatively, be asymptomatic. Pulmonary and tracheal cryptosporidiosis in humans is associated with coughing and frequently a low-grade fever; these symptoms are often accompanied by severe intestinal distress.

Infectious dose–Less than 10 organisms and, presumably, one organism can initiate an infection. The mechanism of disease is not known; however, the intracellular stages of the parasite can cause severe tissue alteration.

Diagnosis of Human Illness:

Oocysts are shed in the infected individual’s feces. Sugar flotation is used to concentrate the organisms and acid fast staining is used to identify them. A commercial kit is available that uses fluorescent antibody to stain the organisms isolated from feces. Diagnosis has also been made by staining the trophozoites in intestinal and biopsy specimens. Pulmonary and tracheal cryptosporidiosis are diagnosed by biopsy and staining.

Associated Foods:

Cryptosporidium sp. could occur, theoretically, on any food touched by a contaminated food handler. Incidence is higher in child day care centers that serve food. Fertilizing salad vegetables with manure is another possible source of human infection. Large outbreaks are associated with contaminated water supplies.

Relative Frequency of Disease:

Direct human surveys indicate a prevalence of about 2% of the population in North America. Serological surveys indicate that 80% of the population has had cryptosporidiosis. The extent of illness associated with reactive sera is not known.

Course of Disease and Complications:

Intestinal cryptosporidiosis is self-limiting in most healthy individuals, with watery diarrhea lasting 2-4 days. In some outbreaks at day care centers, diarrhea has lasted 1 to 4 weeks. To date, there is no known effective drug for the treatment of cryptosporidiosis. Immunodeficient individuals, especially AIDS patients, may have the disease for life, with the severe watery diarrhea contributing to death. Invasion of the pulmonary system may also be fatal.

Target Populations:

In animals, the young show the most severe symptoms. For the most part, pulmonary infections are confined to those who are immunodeficient. However, an infant with a presumably normal immune system had tracheal cryptosporidiosis (although a concurrent viremia may have accounted for lowered resistance). Child day care centers, with a large susceptible population, frequently report outbreaks.

Food Analysis:

The 7th edition of FDA’s Bacteriological Analytical Manual will contain a method for the examination of vegetables for Cryptosporidium sp.

Selected Outbreaks: For more information on recent outbreaks see the Morbidity and Mortality Weekly Reports from CDC.

Education and Background Resources:

Literature references can be found at the links below. Loci index for genome Cryptosporidium parvum

Available from the GenBank Taxonomy database, which contains the names of all organisms that are represented in the genetic databases with at least one nucleotide or protein sequence.

Genetic Polymorphism Among Cryptosporidium parvum isolates: Evidence of Two Distinct Human Transmission Cycles

FSIS Parasites and Foodborne Illness Resource page

Cryptosporidium parvum, cause of the disease cryptosporidiosis (KRIP-toe-spo-RID-e-O-sis), is a one-celled, microscopic parasite, and a significant cause of waterborne illness worldwide. It is found in the intestines of many herd animals including cows, sheep, goats, deer, and elk.

Molecular Structural Data:

None currently available.

FDA Regulations or Activity:

Current recovery methods are published in this FDA methodology reference. The FDA continues to actively develop and improve methods of recovering parasitic protozoa and helminth eggs from foods.

Extra links:

Control and Prevention
Source of Infection
Emerging Infectious Disease 1(2)1995
Emerging Infectious Disease 3(1)1997
Emerging Infectious Disease 3(4)1997
Bacteriological Analytical Manual.
CDC Case Definition
What is a “Case Definition”?
Overview of Public Health Surveillance