Abstract
Toxoplasma gondii is a significant protozoal disease threat to endangered Hawaiian birds and marine mammals. To estimate the prevalence of T. gondii from 1997–2019 in Hawaiian cetaceans, we tested tissues from 26 stranded spinner dolphins (Stenella longirostris) and 29 stranded individuals representing 15 cetacean species. DNA from tissue extracts was screened by nested PCR assay targeting the Toxoplasmatinae internal transcribed spacer 1 of the nuclear ribosomal DNA and of 312 tissue samples tested, two spinner dolphins were positive in all tissues. One spinner dolphin was an adult male that stranded on Hawai’i Island in 2015 and the other was a calf stranded on the island of Oah’u in 2019 suggestive of vertical transmission. The PCR positive spinner dolphins had correlative microscopic lesions including myocarditis with extracellular and intracellular protozoal cysts and tachyzoites, necrotizing lesions in the brain with protozoal cysts and bronchointerstitial pneumonia with intra-endothelial tachyzoites. Discrete positive immunohistochemistry staining for T. gondii was observed in adrenal gland, brain, liver and lung. PCR-RFLP analysis of ten genetic markers revealed the ToxoDB genotype #24, recently described from feral swine (Sus scrofa) in Hawai’i whereas different Toxoplasma genotypes were found in studies of monk seals, Hawaiian goose and crow.1–5 These cases represent the first report of Toxoplasma genotypes in cetaceans in Hawai’i and are the second and third reports of disseminated T. gondii infection in stranded spinner dolphins in the last 30 years. When using low carcass recovery rates in Hawai’i for spinner dolphins to extrapolate these findings, T. gondii is likely responsible for at least 60 spinner dolphin deaths in Hawaiian waters over the past three decades. Nearshore species, like Hawaiian spinner dolphins, may be at increased risk from this parasite in marine coastal waterways via sewage systems, storm water drainage and freshwater runoff.
Acknowledgements
The authors wish to thank the Pacific Islands Marine Mammal Stranding Response Network volunteers for stranding response efforts to recovery and transport spinner dolphin carcasses for necropsy. This work was funded by the National Marine Fisheries John H. Prescott Marine Mammal Rescue Assistance Grant Program and by the National Marine Fisheries Pacific Islands Regional Office.
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