Abstract

Infection due to Otostrongylus circumlitus often causes mortality in stranded Northern elephant seals (NES: Mirounga angustirostris) rescued by The Marine Mammal Center (Sausalito, CA). Otostrongyliasis causes 12.1% of strandings and accounts for 37% of the causes of death in NES at the Center.¹ Mortality occurs as a result of larval migration through vessels causing pulmonary vasculitis and thrombosis before the parasite achieves patency in bronchioles.² Thus, fecal parasitology tests are unable to detect the parasite.³ Often, by the time clinical signs appear, severe coagulopathy and inflammation have developed, preventing treatment and causing death.³ Additionally, most studies on NES otostrongyliasis have focused on the clinicopathologic presentation as well as limited descriptions of the vascular and pulmonary lesions. Thus, there is a need for a more comprehensive understanding of the pathologic lesions related to otostrongyliasis, which may provide insight into early indicators of infections and previously unexplored interventions.

The goal of this study is to determine the prevalence and characterize the previously undocumented non-pulmonary lesions of otostrongyliasis in NES, as well as determine the correlation between clinical pathology and histology. We hypothesize that Otostrongylus-infected NES will have acute renal tubular necrosis caused by hemoglobinuria and be detectable by changes in serum chemistry. We also hypothesize that there will be hemosiderosis, centrilobular passive congestion, and other previously undocumented lesions in the liver and other organs.

A retrospective search from 2021–2022 in the center’s database identified NESs as O. circumlitus-infected (n=20) and controls (n=9) based on necropsy findings. Antemortem blood samples nearest to the time of death were collected from O. circumlitus-infected NESs and the control group. Bloodwork comparisons revealed the otostrongyliasis group had a significantly higher total WBC count, indicating systemic inflammation (p<0.001), and significantly higher bilirubin suggestive of RBC lysis (p=0.008) though post-hepatic and hepatic causes cannot be ruled out. There was no difference in RBC count between the groups (p=0.64), while the PLT value was significantly lower in the otostrongyliasis group (p=0.001), consistent with thrombotic diseases. Sodium was significantly lower in the otostrongyliasis group (p<0.001). Histological comparisons between the study group (n=15) and the control group (n=8) showed that O. circumlitus-infected NESs have a relative risk of 3.0 (p=0.0027) in acute tubular necrosis, a relative risk of 1.88 (p=0.0520) in tubular casts, and a relative risk of 1.313 (p=0.369) in arteriosclerosis in the kidney. In the liver, O. circumlitus-infected NESs have a relative risk of 2.143 (p=0.3341) in passive congestion and a relative risk of 3.214 (p=0.274) in hypercellularity. There was no significance in the prevalence of hemosiderosis in the spleen. In conclusion, the renal, hepatic, and clinicopathology changes are secondary to otostrongyliasis-related RBC lysis and inflammation.

The future direction of the study is to perform specific staining to further characterize the lesions, perform a comprehensive description of the pathogenesis in a larger group, describe the differences in histopathology and clinical pathology between anthelminthic-treated NES and an untreated control cohort, as well as introducing the higher sensitivity scoring system with special stains and analysis of liver enzymes.

Acknowledgments

The authors wish to thank The Marine Mammal Center clinical and pathology team, staff, volunteers, and donors who made this research possible. NOAA permit #24359.

*Presenting author
+Student presenter

Literature Cited

1.  Colegrove KM, Greig DJ, Gulland FM. Causes of live strandings of northern elephant seals (Mirounga angustirostris) and Pacific harbor seals (Phoca vitulina) along the central California coast. Aquat Mamm. 2005;31(1):1–10.

2.  Gulland FM, Beckmen K, Burek K, Lowenstine L, Werner L, Spraker T, Dailey M, Harris E. Nematode (Otostrongylus circumlitus) infestation of northern elephant seals (Mirounga angustirostris) stranded along the central California coast. Mar Mamm Sci. 1997;13(3):446–458.

3.  Sheldon JD, Hernandez JA, Johnson SP, Field C, Kaye S, Stacy NI. Diagnostic performance of clinicopathological analytes in Otostrongylus circumlitis-infected rehabilitating juvenile northern elephant seals (Mirounga angustirostris). Front Vet Sci. 2019;6:134.