Immune-mediated hookworm clearance and survival of a marine mammal decrease with warmer ocean temperatures

Abstract

Increases in ocean temperature are associated with changes in the distribution of fish stocks, and the foraging regimes and maternal attendance patterns of marine mammals. However, it is not well understood how these changes affect offspring health and survival. The maternal attendance patterns and immunity of South American fur seals were assessed in a rookery where hookworm disease is the main cause of pup mortality. Pups receiving higher levels of maternal attendance had a positive energy balance and a more reactive immune system. These pups were able to expel hookworms through a specific immune mediated mechanism and survived the infection. Maternal attendance was higher in years with low sea surface temperature, therefore, the mean hookworm burden and mortality increased with sea surface temperature over a 10-year period. We provide a mechanistic explanation regarding how changes in ocean temperature and maternal care affect infectious diseases dynamics in a marine mammal.

eLife digest

Every year off the coasts of Chile, Guafo Island becomes a nursery for South American fur seals pups. Mother fur seals leave their young on the beaches, going out at sea to hunt for fish before returning to the shore to nurse. These first few months are dangerous for young seals, with many dying because of hookworms, parasites that latch to the wall of the bowels to suck blood. However, the immune system of the pups is usually able to mount a response and fight off these parasites.

Even though the pups stay on land, their lives depend on the health of the ocean that feeds their nursing mothers. In recent years, sea temperature has been rising rapidly, which modifies winds and water currents. This can set off a chain of events that results in fewer fish being available for seals and other marine mammals to eat. Researchers know that years with warmer waters are associated with changes in the pattern of the mothers’ hunting trips, more pups’ deaths, and a weaker immune system in young fur seals. However, the mechanisms that connect these different factors are still unclear.

To explore this, Seguel et al. followed South American fur seals colonies on Guafo Island for several years, tracking the mothers’ trips and monitoring the health of the pups by looking at their levels of blood sugar, whether they carry hookworms, and certain elements of their immune system. Results showed that in years when the sea is warmer, fur seal mothers are gone hunting for longer: they spend less time nursing their young, which then grow slower. These young seals also have lower levels of blood sugar, and so they have less energy to create the immune response necessary to clear off parasitic worms. In fact, in years with warmer seas, almost half of the pups die from hookworm infections.

The work by Seguel et al. shows that warmer oceans directly weaken the immune defenses of certain marine mammals. If temperatures keep rising, infectious diseases may kill more of these animals. Further work is now needed to explore if strategies could be developed to help seal populations, for example by treating the pups with drugs that eliminate the parasites.