Occurrence and subtype distribution of Blastocystis sp. in humans, dogs and cats sharing household in northern Spain and assessment of zoonotic transmission risk.

Blastocystis sp. is probably the most common enteric parasite in humans globally. Although the role of Blastocystis in human disease is still controversial, epidemiological and experimental evidence suggests that pathogenicity may be associated with certain subtypes of the protist. Since the life cycle of Blastocystis is maintained through still elusive pathways, companion animals have attracted the attention of researchers as potential reservoirs of human infections. In order to evaluate the risk of zoonotic transmission of Blastocystis, we investigated the occurrence and molecular diversity of this microorganism in human, canine and feline populations sharing temporal and spatial settings in the province of Álava, northern Spain. A total of 268 (including 179 human, 55 canine and 34 feline) faecal specimens were obtained from 63 family households during February-December 2014. Detection of Blastocystis was achieved by PCR amplification and sequencing of small subunit rRNA genes. Blastocystis was found in 35.2% (95% CI: 0.29%-0.42%) of the human stool samples analysed, but not in any of the canine or feline faecal specimens investigated. Out of the 63 PCR-positive human samples, 84.1% (53/63) were successfully subtyped, allowing the identification of the subtypes ST2 (62.3%), ST3 (17.0%), ST1 (13.2%) and ST4 (7.5%). No mixed subtype infections were identified. Blastocystis carriage was independent of the gender and region of origin of the affected individuals, but children in the age groups of >5-10 years and >10-15 years were significantly more affected by the protist. None of the risk factors considered (water-use practices, contact with livestock, contact with individual undergoing diarrhoeal episodes) were associated with increased prevalence of Blastocystis. Our data demonstrate that pet dogs and cats play a negligible role as natural reservoirs of human Blastocystis infection in this geographic region, although the applicability of these results should be corroborated in future molecular epidemiological studies.