Blastocystis sp. is ubiquitous in avian, mammalian and human hosts and propagates
in either neutral or slightly alkaline conditions within the host’s gastro-intestinal tract. Of the
few previous studies on this enteric protozoan parasite in feline and canine hosts, prevalence
values have been shown to range between 0 to 70.8%. In view of the close association
between humans, and canine and feline hosts as companion animals, faecal samples of 180
Felis catus and 82 Canis lupus, collected from Penang and Kuala Lumpur, Malaysia, were
initially screened by in vitro cultivation followed by molecular characterization. No positive
isolates were identified in culture but in 12 feline samples DNA barcoding detected a zoonotic
subtype Blastocystis ST1 for the first time. Consequently, avian and human isolates, which
had previously been successfully cultured, were used to investigate the impact of pH on the
viability and morphology of Blastocystis sp. The use of Trypan blue showed that the number
of viable cells increased when exposed to pH 4 and a significant increase in viability occurred
in pH values of 5 to 7. Development of Blastocystis cells in both isolates was suppressed in
media less than pH 5 followed by the disappearance of viable cells from avian isolates in
more acidic media below pH 4. Morphologically at pH 4 cells from avian isolates were less
rounded, and with wrinkled / shrunken surfaces, than the more normal rounded cells from
human isolates. On the other hand, at values below pH 3, no viable cells in human isolates
were visible. The present findings therefore confirm that gastro-intestinal pH is an important
determinant of Blastocystis viability and consequently influences the epidemiology of infection
within avian, mammalian and human hosts.