Describing new species of Cryptosporidium in fish

The protozoan parasite Cryptosporidium (class Gregarinomorphea, subclass Cryptogregaria) causes a range of symptoms in humans and clinical signs in animals from asymptomatic to severe diarrhoea and death. However, relatively little information is available regarding the taxonomy, zoonotic potential and host relationships of Cryptosporidium in fish. Previous studies have indicated that extensive genetic diversity exists with piscine Cryptosporidium species and genotypes. The present study screened fish from two sources in Perth, Western Australia; Water Garden Life Fish Farm (n=233) and Vebas Aquarium (n=234) for Cryptosporidium. Intestinal and gastric tissue was dissected out and screened by PCR and Sanger sequencing using Cryptosporidium specific primers that amplify DNA at the 18S and actin loci. Samples that were positive by PCR were also screened by histology. The overall prevalence of Cryptosporidium was 4.3% (20/467, 95% CI: 2.6-6.5). Phylogenetic analyses of 18S sequences identified C. huwi (n=11), piscine genotype 2 (n=3), piscine genotype 4 (n=1) and piscine genotype 7 (n=5). In addition, ten novel sequences most genetically similar to species from the genus Goussia and a sequence from the non-parasitic alveolate Colpodella were identified. Sequences amplified at the actin locus were C. huwi (n=7), piscine genotype 2 (n=1), piscine genotype 7 (n=1) and one novel Cryptosporidium sequence. Piscine genotype 2 was most closely related to piscine genotype 4 (4.1% genetic distance) and exhibited 11.1-11.9%, 15.3% and 22.3% genetic distances from C. molnari, C. huwi and C. scophthtalmi, respectively. At the actin locus, piscine genotype 2 was again most closely related to piscine genotype 4 (7.2% genetic distance) and exhibited genetic distances ranging from 18.1% (C. molnari) to 20% (C. huwi) and 26.1% for C. scophthalmi, respectively, and 20.7%- 32% genetic distance from all other species. Phylogenetic analysis of concatenated 18S and actin sequences showed that piscine genotype 2 exhibited 14% (C. molnari) to 24.6% (C. canis) genetic distance from all other Cryptosporidium spp. Using concatenated sequences, piscine genotype 7 was most closely related to C. huwi at a genetic distance of 7.5% and exhibited 13.4% (C. molnari) to 23.1% (C. scophthalmi) genetic distances from other piscine Cryptosporidium species, with 17.9% (C. testudinis) to 22.6% (C. canis) genetic distance from all non-piscine Cryptosporidium species. Piscine genotype 2 exhibited 14.6% genetic distance from piscine genotype 7. These genetic distances at two separate loci confirm the genetic distinctness of piscine genotype 2 and piscine genotype 7 and indicate that they are likely novel species. Additionally, 10/467 (2.1%, 95% CI; 1.0-3.9) samples that were positive at the 18S locus, produced sequences most genetically similar to species from the genus Goussia, subclass Conoidasida, nine were novel sequences and were compared at the 18S locus to established species of Goussia and genetic distances between 1.9% and 14.8% were identified, adding to the diversity of this genus. Furthermore, Schyzocotyle acheilognathi, the invasive Asian fish tapeworm, was identified (n=2) by morphology infecting goldfish from a local fish farm. This is only the second report of S. acheilognathi in Western Australia as it was first discovered in 2018 by a Murdoch researcher in feral goldfish from a Lake in Joondalup. Analysis at additional loci or whole genome sequencing will shed more light on the evolutionary relationships between Cryptosporidium species, while next generation sequencing would elucidate the prevalence of mixed infections of Cryptosporidium in fish. The genetic data produced by the present study describes two piscine genotypes of Cryptosporidium (that are likely valid species) in detail and provides new genetic data on the diversity of Goussia spp. Keywords: Cryptosporidium, 18S, actin, Schyzocotyle acheilognathi, Goussi