Parasitic infections caused by protozoan parasites and intestinal helminths are among the most prevalent human infections in developing countries. In developed countries, however, protozoan parasites are more frequent causes of gastrointestinal infections than helminths. Intestinal parasites (which are principally small intestine pathogens, though the large intestine may also be involved) are responsible for substantial morbidity and mortality in endemic countries.
Cryptosporidium is a genus of parasites that has emerged as a considerable cause of diarrheal disease around the world. It has serious health consequences for young and malnourished children that live in endemic areas, as well as patients with significant impairment of T-cell number and/or functions. This includes patients with acquired immunodeficiency disorder (AIDS).
Even though several different species of this Apicomplexan protozoan parasite have been identified in humans, Cryptosporidium hominis and Cryptosporidium parvum are responsible for more than 90% of intestinal cryptosporidiosis in humans. Sub-par treatment options, and the ability of these organisms to survive and transmit through water, make them a significant public health threat.
America's Deadliest Parasite Outbreak
Cryptosporidium organisms can infect humans by three main routes. These include:
- contaminated raw food ingredients from abattoirs or farms
- contaminated water from unprocessed sources (such as wells and rivers) or from treatment plants for water
- direct transmission from infected hosts (such as pets, pests and unhygienic food handlers)
This parasite has been found in every region of the world, with the exception of Antarctica. Infection is more commonly observed in warm or moist months, and therefore it often peaks in late summer and early autumn. Studies conducted in England have shown that Cryptosporidium parvum infection rates peak in the spring, while those with Cryptosporidium hominis peak in autumn.
Oocysts of Cryptosporidium are the infective forms of this parasite and are relatively resistant to environmental conditions. They can remain infectious for at least six months in moist conditions, though their viability decreases swiftly with desiccation. Oocysts can also be killed by heat, including by microwave heating and pasteurization.
It has been shown that the required infectious dose is low, and shows variability among different isolates of Cryptosporidium. It may range from 10 oocysts to approximately 1000 oocysts. Still, even a single oocyst may result in infection in a proportion of exposed individuals, especially if they are immunocompromised.
Intestinal Cryptosporidiosis in Developing and Industrialized Countries
The greatest burden of intestinal cryptosporidiosis is found among children in developing countries, although the estimates vary greatly even among surveys from the same geographic region, making quantification difficult. The main problems encountered during research include faulty study designs, inaccurate or insensitive diagnostic methods, and, sometimes, unknown underlying HIV status.
For example, by using acid-fast staining of fecal smears, Cryptosporidium was found in only 2% of children with diarrhea younger than five years of age attending health centers in Ghana. However, it was almost 9% in a similar population from Ghana with the use of molecular methods such as real-time polymerase chain reaction (PCR).
Seroprevalence studies have been a convenient way to demonstrate that infection is more widespread than previously thought. In the United States, seropositive rates range from 17% to 54%, reaching 70% in children living next to the Mexican border. Rates were also higher in South-Eastern Europe, as well as in some developing countries. The HIV epidemic in sub-Saharan Africa has unquestionably enhanced the burden of disease.
In industrialized countries Cryptosporidium is not a common cause of acute sporadic diarrhea, but is considered a leading cause of waterborne outbreaks. The most notable example is the outbreak of intestinal cryptosporidiosis in Milwaukee in 1993, which affected more than 400 thousand people who were using the municipal water supply.
A recent review of all published parasitic protozoan outbreaks around the world showed that Cryptosporidium was responsible for the majority of outbreaks, approximately 60%, with most reports coming from Europe, North America and Australia. Many of these outbreaks are under the radar, even in countries with well-established surveillance systems. For this reason, many cases that are considered sporadic may actually be part of an unrecognized outbreak.