Blastocystis

(endorsed 2011)

Guideline

No guideline value has been set for Blastocystis in drinking water and its inclusion in routine monitoring programs is not recommended.

A multiple barrier approach from catchment to tap is recommended to minimise the risk of contamination. Protecting catchments from human and animal wastes is a priority. Operation of barriers should be monitored to ensure effectiveness and that microbial health-based targets are being met.

General description

Although Blastocystis was first described in the early 1900s, its pathogenicity and taxonomy remains uncertain. Blastocystis has been detected in a range of animal hosts but speciation has not been established. Isolates from humans are generally referred to as B. hominis, while isolates from other animals are referred to as Blastocystis spp. However, there is some evidence that Blastocystis spp. may not be host-specific and that animal-to-human transmission is possible. A survey in Malaysia showed that animal handlers and abattoir workers had an increased risk of infection (Rajah et al. 1999).

Blastocystis hominis is probably the most common protozoan detected in human faecal samples worldwide. Reported prevalence ranges from 2% to 50%, with the highest rates reported for developing countries with poor environmental hygiene (Stenzel and Boreham 1996). Infection appears to be more common in adults than in children.

While prevalent, the pathogenicity of B. hominis is controversial because of the non-specific symptoms. There have been contradictory reports on clinical significance: some reports suggest pathogenicity, but the frequency of asymptomatic infections is very high (Stenzel and Boreham 1996).

Although not confirmed experimentally, faecal-oral transmission is considered to be the main mode of infection. Blastocystis cysts have been detected in sewage (Suresh et al. 2005). These cysts could be environmentally persistent in a similar fashion to other protozoan cysts, but there are no data on its survival in the environment. The role of drinking water as a source of Blastocystis infections has been suggested but not established (Leelayoova et al. 2004).

Australian significance

An Australian study found no correlation between clinical symptoms and infection with Blastocystis hominis (Leder et al. 2005). There is no evidence of waterborne transmission in Australia.

Preventing contamination of drinking water

Control measures applied to other infectious protozoa will also reduce risks associated with Blastocystis. The likely source of Blastocystis is faecal waste, and prevention of contamination of water sources by human and animal waste is a priority. There is little information on the removal and/or inactivation of Blastocystis by water and wastewater treatment processes. The morphology of Blastocystis varies over a broad range, but faecal cysts can be as small as 3-10 µm in diameter. These should be removed by filtration in a similar manner to Cryptosporidium oocysts (4-6 µm in diameter). It has been reported that Blastocystis cysts are relatively resistant to chlorine (Suresh et al. 2005).

Method of identification and detection

A method for concentrating cysts has been reported together with an in vitro culture method (Suresh et al. 2005). Clinical samples are examined by light microscopy.

Health considerations

The pathogenicity of B. hominis has not been established. A broad range of symptoms has been attributed to B. hominis, including watery or loose stools, diarrhoea, abdominal pain, cramps and nausea; however, as mentioned above, the frequency of asymptomatic infections is very high (Stenzel and Boreham 1996).

Derivation of guideline

No guideline value is proposed for Blastocystis and inclusion in routine verification monitoring programs is not recommended. The focus should be on monitoring of control measures, including prevention of contamination by human and animal waste, and (where used) filtration. Escherichia coli is not a reliable indicator for the presence/absence of Blastocystis.

References

Leder K, Hellard ME, Sinclair MI, Fairley CK, Wolfe R (2005). No correlation between clinical symptoms and Blastocystis hominis in immunocompetent individuals. Journal of Gastroenterology and Hepatology, 20:1390-1394.

Leelayoova S, Ramsin R, Taamasri P, Naaglor T, Thathaisong U, Mungthin M (2004). Evidence of waterborne transmission of Blastocystis hominis. American Journal of Tropical Medicine and Hygiene, 70:658-662.

Rajah Salim H, Suresh Kumar G, Vellayan S, Mak JW, Khairul Anuar A, Init I, Vennila GD, Saminathan R, Ramakrishnan K (1999). Blastocystis in animal handlers. Parasitology Research, 85:1032-1033.

Stenzel DJ, Boreham PFL (1996). Blastocystis hominis revisited. Clinical Microbiology Reviews, 9(4):563-584.

Suresh K, Smith HV, Tan TC (2005). Viable Blastocystis cysts in Scottish and Malaysian sewage samples. Applied and Environmental Microbiology, 71:5619-5620.