Analytical validation of an immunoassay for the detection of <i>Cystoisospora</i> spp. coproantigen in canine and feline fecal samples

<i>Cystoisospora</i> species that infect dogs and cats are ubiquitous, and oocysts can be found in the feces of both subclinically infected and sick animals. Most clinical cases are diagnosed in puppies and kittens <4-mo-old. We validated a high-throughput immunoassay (Fecal Dx immunoassay; Idexx), which uses 2 monoclonal antibodies to detect <i>Cystoisospora</i> spp. coproantigen, using zinc sulfate centrifugal flotation (ZCF) as the reference method and samples from experimentally infected animals. This new genus-level coproantigen assay had equivalent sensitivity for at least 4 relevant <i>Cystoisospora</i> spp.-<i>C. canis</i>, <i>C. ohioensis-</i>like, <i>C. felis</i>, and <i>C. rivolta</i>-with no observed cross-reactivity to other protozoal parasites (<i>Giardia</i>, <i>Cryptosporidium</i>, <i>Tritrichomonas</i>, <i>Eimeria</i>) of companion animals. In comparison to ZCF, the immunoassay had a positive percent agreement of 88.5% (95% CI [86.1, 91.6]) and a negative percent agreement of 98.2% (95% CI [98.1, 98.2]). The prevalence of <i>Cystoisospora</i> spp. detected by ZCF was 1%; the prevalence identified through coproantigen testing was 2.7% (95% CI [2.6, 2.8]). Among puppies and kittens <1-y-old, the prevalence according to coproantigen testing was 7.4% (95% CI [6.6, 8.2]) for puppies and 8.2% (95% CI [7.7, 8.6%]) for kittens; the prevalence detected by ZCF was 3.6% (95% CI [3.0, 3.9]) for puppies and 2.9% (95% CI [2.4, 3.4]) for kittens. Our results validate our <i>Cystoisospora</i> coproantigen immunoassay as specific, precise, and sensitive.