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Case History: A tadpole morbidity and mortality event affecting an estimated 1,000 Green Frog (Lithobates clamitans) tadpoles occurred in June of 2017 at a lake in Wisconsin, USA.

Sick tadpoles were observed swimming in circles on the surface in one direction only, swimming upside down, and unable to swim forward or down. Large numbers of dead tadpoles were seen on the shore and on the bottom of the lake.

Gross Findings: On external examination of a Gosner stage 41 tadpole, no significant abnormalities were seen (Fig. 1A).  On internal examination, fat bodies were present, and the tadpole was in good nutritional condition. A moderate amount of transparent fluid (ascites) was present in the coelomic cavity. The liver and spleen were markedly pale and enlarged (Fig. 1B). There was irregular thickening of the intestinal wall and the mesonephros were moderately pale and enlarged. The gastrointestinal tract was full of apparently normal ingesta. 

Left side show underside of tadepole and right side shows liver and spleen of tadpole.
Figure 1. Gosner stage 41 Green Frog (Lithobates clamitans) tadpole. (A) There are no apparent external abnormalities. (B)The liver and spleen are markedly pale and enlarged.

Histopathological Findings: In multiple organs, including the liver, spleen, and kidney, there is acute, severe, multifocal to diffuse necrosis associated with myriad numbers of infiltrating 4- to 6-µm spherical spores (Figs. 2A and 2B). Inflammation is minimal to non-existent.

Photomicrographs of liver from a Green Frog tadpole
Figure 2. Liver from a Green Frog (Lithobates clamitans) tadpole. (A) Over 90% of the organ is infiltrated, effaced and replaced by protozoal spores (Perkinsea) admixed with necrotic debris. (B) Perkinsea spores are 4 to 6-µm in diameter spherical structures with thick, deep basophilic walls, and granular pale basophilic cytoplasm. H&E stain.  

Ancillary Diagnostic Tests: Pathogenic Perkinsea Clade of frogs, the etiologic agent of Severe Perkinsea Infection of frogs, was detected in the liver by PCR and sequencing of the small subunit (18S) rRNA gene.

Morphologic Diagnosis: Acute, severe, diffuse hepatocellular necrosis with intralesional protozoal spores.

Disease: Severe Perkinsea Infection

Etiology: Perkinsea is a protozoan parasite belonging to the phylum Perkinsozoa. Within this phylum, Perkinsea protists are classified as a distinct group referred to as the Pathogenic Perkinsea Clade of Frogs.

Distribution: Widespread in North America, in subtropical, temperate, and boreal habitats. Severe Perkinsea Infection has also been documented in a population of captive Hyla arborea (European tree frog) in the United Kingdom.

Seasonality: Severe Perkinsea Infection occurs in tadpoles, and seasonality is therefore dependent on the breeding habits of the affected species. In temperate regions, mortality events tend to occur between June and September; in subtropical regions, events may occur year-round.

Host range: Severe Perkinsea Infection has been documented in Lithobates spp. (Green Frog), Hylidae sp. (such as Pseudacris crucifer (Spring Peeper)), Acris gryllus (Southern Cricket Frog), Rana spp. ((such as Rana sylvatica (Wood Frog) and Rana servosa (Dusky Gopher Frog)).

Transmission: Unknown, however spores are highly resistant and capable of remaining viable for long periods of time. Therefore, an environmental source of infection has been postulated.

Clinical signs: Erratic swimming, buoyancy abnormalities (unable to dive), bloating, gaping, and lethargy have been reported.

Diagnosis:

  1. Compatible histopathologic findings (multiorgan necrosis with organs infiltrated by myriad numbers of extra- and intracellular protists consistent in morphology with Perkinsea protozoa).
  2. Positive PCR testing followed by identification by sequencing of the small subunit (18S) rRNA gene from one or more internal organs.

Public health concerns: None currently identified.

Wildlife population impacts: Severe Perkinsea Infection has been associated with tadpole mass mortality events in the USA and is believed to be the third most common infectious disease of amphibians in North America. It has been estimated that >95% of infected tadpoles die during these events. This disease is therefore of concern to conservationists and continues to be studied.

References:

  • Isidoro-Ayza M, Lorch JM, Grear DA, Winzeler M, Calhoun DL, Barichivich WJ. 2017. Pathogenic lineage of Perkinsea associated with mass mortality of frogs across the United States. Sci Rep. 7(1):10288. https://doi.org/10.1038/s41598-017-10456-1.
  • Isidoro-Ayza M, Grear DA, Chambouvet A. 2019. Pathology and Case Definition of Severe Perkinsea Infection of Frogs. Vet Pathol. 56(1):133-142.
  • Smilansky V, Jirků M, Milner DS, Ibáñez R, Gratwicke B, Nicholls A, Lukeš J, Chambouvet A, Richards TA. 2021. Expanded host and geographic range of tadpole associations with the Severe Perkinsea Infection group. Biol Lett. 17(6):20210166. https://doi.org/10.1098/rsbl.2021.0166.

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