The role of the immune response on the survival of South American fur seals (Arctocephalus australis) to hookworm infection

Hookworms are a major cause of mortality in many otariid populations, including South American fur seals at Guafo Island, in the Chilean Patagonia. To understand the drivers of hookworm mortality its critical to know how the... [ view full abstract ]

Hookworms are a major cause of mortality in many otariid populations, including South American fur seals at Guafo Island, in the Chilean Patagonia. To understand the drivers of hookworm mortality its critical to know how the host fights and clears this infection, therefore the objective of the study was to evaluate the immune response of fur seals to hookworms (Uncinaria sp.). Between 2013 and 2016 Australian summers, we treated a randomly selected subset of hookworm-infected pups (n=160) with Ivermectin, and compared this treated group to pups with different hookworm burdens based on fecal analysis (n=330). In 2016, pups (n=35) with different hookworm infection statuses were challenged with phytohemagglutinin. Additionally, we performed necropsies on 50 hookworm-infected and 20 hookworm-free (control) pups. Most pups with severe hookworm infection (n=70 pups) had moderate leukocytosis with marked eosinophilia over the first 2 weeks of patent infection and later in the breeding season (pups 6 to 8 weeks of age) eliminated the parasite from the intestine. However, some pups (n=41) with severe infection experienced a sustained decline in lymphocytes and eosinophils between 2 and 10 weeks of age and failed to expel hookworms from the intestine. These same pups had a mild inflammatory response when challenged with phytohemagglutinin, while pups that expelled hookworms early in the season had higher inflammatory responses to phytohemagglutinin challenge. Most pups (n=34) that failed to expel the parasites died due to hookworm enteritis and bacteremia. These animals had 600 hookworms in the intestine, with parasites sometimes free in the peritoneal cavity. At histopathology, these pups had decreased numbers of T-lymphocytes and leukocytes expressing MHC-II in the small intestine when compared to controls (Student T test, p ≤ 0.002). Therefore, the fur seal pup’s immune response is critical for clearance of hookworm infection and pup survival.