Cats and dogs as hosts of Blastocystis – What is the evidence?

Blastocystis is one of the most common intestinal parasites observed in human and non-human hosts. Recent meta-analyses have indicated a potential role for pets such as dogs and cats as reservoir hosts of Blastocystis, but the data underpinning this hypothesis are of mixed quality. Reviewing data for 45,894 samples tested for Blastocystis by DNA-based methods and 11,908 subtype observations, a model was developed for calculating indices that could be used for evaluating individual species as natural hosts of Blastocystis, based on weighted products of positivity rates and subtype distributions. Data from cats and dogs were analysed, using other well-sampled hosts (pig, cattle, sheep, goat, and human) as references. Data from cats and dogs meeting the inclusion criteria were entered into the model. The overall positivity rates for pigs, cattle, sheep, goats, humans, dogs, and cats were 40 %, 40 %, 35 %, 28 %, 25 %, 6 %, and 5 %, respectively, with statistically significant lower positivity rates in cats and dogs (p < 0.0001). Indices indicating Blastocystis specificity to host ranged between 0.16 (humans) and 0.49 (cattle) for the reference hosts, whereas indices for cats and dogs were only 0.01 and 0.02, respectively. Finally, indices for ST specificity to host were higher for reference hosts (range, 0.66-0.93) than for cats (0.62) and dogs (0.56). Taken together, the analyses indicate that cats and dogs are not natural or reservoir hosts of Blastocystis and that the sporadic subtype pattern observed in these hosts might indicate exposure to Blastocystis through contaminated water/feed, including Blastocystis colonizing prey animals.