A two-for-one benefit from vaccinating seabirds

When it comes to the threats that affect wildlife conservation, the usual suspects are anthropogenic in nature: habitat loss, overfishing, circadian disruption, physical barriers to movement, and so on. But another important factor to account for is infectious disease. Wildlife in general, and birds in particular, play an important role in the ecology of pathogens, and are also susceptible to the spillover of diseases from domestic animals.

Infectious disease is of course a normal part of wildlife ecology, but if a species or population is already stressed by the plethora of anthropogenic stressors they might face, then a disease could lead to extinctions or declines in local populations, losses of biodiversity, and shifts in community composition. Anthropogenic stressors could also weaken the immune system of an animal, making them more susceptible to a disease that they’d otherwise fight off with little effort.

But most studies on the dynamics of the immune response have focused on humans, livestock, poultry, and the model species most often found in scientific laboratories. With the exception of humans, those are species that typically “live fast.” In some ways, that makes a great deal of sense; lots of effort is invested into the species that we eat, and it’s easier to study species with short lifespans. Long-term studies of long-living species are logistically difficult.

But that doesn’t mean they’re impossible, and researcher Raul Ramos from Universite Montpellier in France set out to add to the body of research on immunity in long-lived species. He turned to birds, since they’re a highly threatened group of animals. In particular, he focused on Cory’s shearwaters, a long-lived seabird. Because they, like other long-lived species, have extended childhoods and low reproductive rates, even small reductions in survival caused by diseases can have severe consequences for a population.

For a colonial breeding species like the shearwaters, parasites that are directly transmitted between individuals can be devastating. Some of those pathogens, like Newcastle disease virus (NDV), are also prevalent in domestic poultry. As a result, efficient vaccines have been developed. What Ramos and his colleagues set out to do was see whether the vaccine, which was developed for chickens, would be effective to protect Cory’s shearwaters. He monitored the vaccinated individuals for six years. In addition, he assessed whether the chicks of vaccinated mothers acquired any benefit from their mothers’ inoculations. If the mothers passed antibodies on to their offspring, that would represent the first step in a process of natural selection against the pathogen. In addition, by monitoring both vaccinated mothers and their offspring, the researchers could potentially identify the tradeoffs between antibody persistence in the mothers and other physiologically demanding processes, such as those involved in egg laying, incubation, and chick rearing.

At the start of the breeding periods in 2008 and 2010, 90 female Cory’s shearwaters at a colony in Spain’s Canary Islands were captured. After a brief blood withdrawal, the birds were injected either with the vaccine or with an inert saline solution as a control. They also verified that the population of birds they were studying was not naturally exposed to the Newcastle disease virus. Any antibodies they showed were therefore a result of the vaccination rather than a naturally acquired defense. In addition, 16 of the individuals vaccinated in 2008 were re-vaccinated in 2010 to see whether they benefited from the second exposure to the antigen. Importantly, the vaccination had no detrimental effects, at least in the short term, on bird survival or capturability.

Over the study’s six years, the researchers collected 355 blood samples from 105 females. The control females were negative for NDV-related antibodies, while the vaccinated individuals had high levels of antibodies starting around three months after the injection. Over the six years, antibody levels declined, as would be expected, though they was still higher at the end of the study for vaccinated birds than for those birds that had not been vaccinated. Those who were given the booster shot after 2 years had higher levels of antibodies after an additional four years (six years total) than those who were only injected once. Incredibly, the mothers were able to transfer their antibodies to their chicks, even five years after they were vaccinated! And the chicks of those mothers who received the booster also had higher antibody levels and the chicks of those vaccinated just once.

The findings confirm that antibodies can persist following vaccination in a long-lived seabird, that they can persist even while the mothers are engaged in breeding and rearing their offspring, and that they’re present in chicks at least for several weeks after hatching. By relying on their mother’s antibodies for protection against infection, chicks can focus their physiological attention on growth. Not having to divert their energy to mounting their own immune response might allow newborn chicks to grow faster.

Life history theory suggests that there’s a tradeoff between investing in immunity and in other physiologically demanding activities, suggesting that species with different life histories should have different strategies for immunity. Longer-lived species, for example, might skew their immunity towards the acquired, rather than innate, variety. Innate immunity is highly costly but seems a reasonable strategy for a fast living species, where individuals may be challenged by a pathogen just once in their short lives. Meanwhile, acquired immunity reduces the costs of multiple encounters with the same pathogens over a longer period of time, something that’s more likely in longer-lived species. The results of this study are consistent with the theoretical assumptions behind life history theory.

This study isn’t just interesting to life history theorists, because seabirds as a group are highly endangered. While Cory’s shearwaters seem to be doing okay in most of their range, other related seabirds, like petrels and albatrosses, are under greater risk of extinction. Previous modeling of those populations hasn’t included information about the dynamics of immunity, so this sort of information may allow researchers and conservationists to more accurately forecast population trends in these at-risk species. “Our results suggest that considering the prolonged persistence of some protective antibodies in long-lived colonial species could greatly increase the potential usefulness of such specific vaccines as management tools,” write the researchers. – Jason G. Goldman | 26 December 2014

Source: Raul Ramos, Romain Garnier, Jacob González-Solís & Thierry Boulinier (2014). Long Antibody Persistence and Transgenerational Transfer of Immunity in a Long-Lived Vertebrate, The American Naturalist, 184 (6) 764-776. DOI: http://dx.doi.org/10.1086/678400

Header image: Daniel Valverde/Flickr, CC BY-NC-ND 2.0.

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