Intestinal protozoa are increasingly being studied because of their association with
acute and chronic diarrhoea in immunocompromised as well as immunocompetent patients.
Various community outbreaks due to contamination of water or food with these protozoa have
further highlighted their importance in public health. Among these important pathogens are
Giardia duodenalis, Entamoeba histolytica, Cryptosporidium parvum, Cyclospora cayetanensis,
Isospora belli, and microsporidia. Except for the cyst-forming G. duodenalis and E. histolytica,
the others are intracellular and form spores which are passed out with the faeces. These organisms
are also found in various animals and birds and zoonotic transmission is thought to occur. These
infections are distributed worldwide, with a higher prevalence in developing compared to
developed countries. However, the relative importance of zoonotic infections especially in
developing countries has not been studied in detail. The prevalence rates are generally higher in
immunodeficient compared to immunocompetent patients. Higher prevalence rates are also seen
in rural compared to urban communities. Most studies on prevalence have been carried out in
developed countries where the laboratory and other health infrastructure are more accessible than
those in developing countries. This relative inadequacy of laboratory diagnosis can affect
accurate estimates of the prevalence of these infections in developing countries. However, reports
of these infections in travellers and workers returning from developing countries can provide
some indication of the extent of these problems. Most studies on prevalence of amoebiasis in developing countries were based onmorphological identification of the parasite in faecal smears. As the pathogenic E. histolytica is morphologically indistinguishable from that of non-pathogenic E. dispar, estimates of amoebiasis may not be accurate. The epidemiology of human microsporidia infections is not completely
understood. Two species, Enterocytozoon bieneusi and Encephalitozoon intestinalis, are
associated with gastrointestinal disease in humans and it is believed that human to human as well
as animal to human infections occur. However, the importance of zoonotic infections has not
been fully characterised. G. duodenalis cysts, microsporidia and Cryptosporidium oocysts have
been detected in various ground water resources, but their role in community outbreaks and
maintenance of the infection has not been fully characterised. The taxonomic classification and
pathogenic potential of B. hominis are still controversial. While considered by many as yeast,
fungi or protozoon, recent sequence analysis of the complete SSUrRNA gene has placed it within
an informal group, the stramenopiles. This review covers recent published data on these zoonotic
infections and examines their public health importance in Asian countries.