Blastocystosis - Transmission and Risk Factors

Transmission and Risk Factors

Humans contract Blastocystis infection by drinking water or eating food contaminated with feces from an infected human or animal. Blastocystis infection can be spread from animals to humans, from humans to other humans, from humans to animals, and from animals to animals. Risk factors for infection have been reported as following:

• International travel: Travel to less developed countries has been cited in development of symptomatic Blastocystis infection. A 1986 study in the United States found that all individuals symptomatically infected with Blastocystis reported recent travel history to less developed countries. In the same study, all hospital employees working in New York who were screened for Blastocystis were found to have asymptomatic infections.

• Military service: Several studies have identified high rates of infection in military personnel. An early account described infection of British troops in Egypt in 1916 who recovered following treatment with emetine. A 1990 study published in Military Medicine from Lackland AFB in Texas concluded symptomatic infection was more common in foreign nationals, children, and immunocompromised individuals. A 2002 study published in Military Medicine of army personnel in Thailand identified a 44% infection rate. Infection rates were highest in privates who had served the longest at the army base. A follow-up study found a significant correlation between infection and symptoms, and identified the most likely cause as contaminated water. A 2007 newspaper article suggested the infection rate of US military personnel returning from the Gulf War was 50%, quoting the head of Oregon State University's Biomedicine department.

• Consumption of Untreated Water (well water): Many studies have linked Blastocystis infection with contaminated drinking water. A 1993 study of children infected symptomatically with Blastocystis in Pittsburgh indicated that 75% of them had a history of drinking well water or travel in less developed countries. Two studies in Thailand linked Blastocystis infection in military personnel and families to drinking of unboiled and untreated water. A book published in 2006 noted that in an Oregon community, infections are more common in winter months during heavy rains. A research study published in 1980 reported bacterial contamination of well water in the same community during heavy rainfall. A 2007 study from China specifically linked infection with Blastocystis sp. subtype 3 with drinking untreated water. Recreational contact with untreated water, for example though boating, has also been identified as a risk factor. Studies have shown that Blastocystis survives sewage treatment plants in both the United Kingdom and Malaysia. Blastocystis cysts have been shown to be resistant to chlorination as a treatment method and are among the most resistant cysts to ozone treatment.

• Contaminated Food: Contamination of leafy vegetables has been implicated as a potential source for transmission of Blastocystis infection, as well as other gastrointestinal protozoa. A Chinese study identified infection with Blastocystis sp. subtype 1 as specifically associated with eating foods grown in untreated water.

• Daycare facilities: A Canadian study identified an outbreak of Blastocystis associated with daycare attendance. Prior studies have identified outbreaks of similar protozoal infections in daycares.

• Geography: Infection rates vary geographically, and variants which produce symptoms may be less common in industrialized countries. For example, a low incidence of Blastocystis infection has been reported in Japan. A study of individuals infected with Blastocystis in Japan found that many (43%, 23/54) carried Blastocystis sp. subtype 2, which was found to produce no symptoms in 93% (21/23) of patients studied, in contrast to other variants which were less common but produced symptoms in 50% of Japanese individuals. Studies in urban areas of industrialized countries have found Blastocystis infection associated with a low incidence of symptoms. In contrast, studies in developing countries generally show Blastocystis to be associated with symptoms. In the United States, a higher incidence of Blastocystis infection has been reported in California and West Coast states.

• Prevalence over Time: A 1989 study of the prevalence of Blastocystis in the United States found an infection rate of 2.6% in samples submitted from all 48 states. The study was part of the CDC's MMWR Report. A more recent study, in 2006, found an infection rate of 23% in samples submitted from all 48 states. However, the more recent study was performed by a private laboratory located in the Western US, and emphasized samples from Western states, which have previously been reported to have a higher infection rate.

Research studies have suggested the following items are not risk factors for contracting Blastocystis infection:

• Consumption of municipal water near water plant (not a risk factor): One study showed that municipal water was free of Blastocystis, even when drawn from a polluted source. However, samples taken far away from the treatment plant showed cysts. The researchers suggested that aging pipes may permit intrusion of contaminated water into the distribution system.

• Human-to-Human transmission among adults (not a risk factor): Some research suggests that direct human-to-human transmission is less common even in households and between married partners. One study showed different members of the same household carried different subtypes of Blastocystis.