For most of its history, the field of wildlife management largely ignored or dismissed the role of disease in wildlife populations. Not to say they denied the existence of disease, certainly they did not, but for many years, its role was seen as one of natural correction: when populations grew too large or dense, a disease outbreak would prune down those numbers, bringing the population back to a sustainable level. Indeed, for millennia, this was the case. Now, the study of wildlife disease and disease ecology is booming. What happened? We did.
When wild populations evolve alongside their diseases, be they viral, bacterial, parasitic, or otherwise, the two are engaged in a constant arms race. Pathogens evolve new ways to infect and spread, and hosts evolve new defenses. This dynamic breaks down, however, when a pathogen infects a host population with no previous exposure or defenses. In human history, there were the infamous smallpox outbreaks among Native Americans upon first contact with Europeans. The Pilgrims arrived in a land largely emptied of its native human inhabitants after they were nearly annihilated by infections brought by previous European traders and their rats. Without natural immunity, what we call a naive population can be very nearly exterminated by a disease that coexists in ancient balance in another population.
Things are not much different in wildlife. Wild populations exposed to new diseases carried across oceans by human travelers can face extinction when those diseases run unchecked through their ranks. We have seen this happen with the spread of avian malaria in Hawaiian songbirds, and with the Geomyces destructans fungus that causes White Nose Syndrome in North American bats but seems to cause no harm to its normal European host species.
As awareness has spread of the threat posed by disease in wildlife, people often ask why we can’t develop vaccines to combat them. The typical response is that it would not be practical. For many reasons, this is true. First, developing a vaccine is expensive and without a large scale market for the product, it’s not likely to get done. Wildlife medicine doesn’t pay well, I can tell you from personal experience, so the economics alone are daunting. Second, not every disease is well suited to vaccination. We mainly vaccinate against viruses, though some vaccines (Tetanus, Diphtheria, e.g.) target the toxins released by bacteria. Even within viruses, some are stable and relatively unchanging, while others are constantly mutating. This is why we have a standard vaccine for rabies (stable), an annually shifting one for flu (not so stable or consistent), and no vaccine for HIV (constantly mutating). But let us assume we had a vaccine on hand that could combat a particular disease threatening a population, and that we knew we could capture enough individual animals to administer it to a high percentage of the population. Even then, there are challenges.
When I was a first year vet student, I had to get a series of rabies vaccinations. We all did, given our line of work and our higher than average risk of exposure. After that initial three shot series, we were told we should have a titer checked every two years to make sure we are still protected. A titer tests the blood to see what level of antibodies are still patrolling for a particular virus. So far, every two or three years, my titer has shown that my body is still alert to the threat of rabies, and if the virus did find its way into my body, the level of antibodies in my system would be sufficient to neutralize those viral particles and destroy them.
The reason I have to be checked so often is that this response is variable. Some people never need a booster shot after receiving the series. Others do. The level of antibodies can decline over time. Knowing how long a protective level persists is critical to any vaccination plan, so when contemplating vaccinating a wild population, we must have a sense of how long the protection will last. If an animal would require annual boosters, that vaccine is not likely to be of much use long term in wildlife where capturing individuals repeatedly ranges from difficult to impossible.
The question of vaccinating wild animals must also encompass the life history strategy of the particular species. Many seabirds, for instance, are extremely long-lived, and their population structure is based on an extended juvenile or subadult phase, low annual reproduction, but a tradeoff in many, many years of reproduction (for a particularly amazing example, read about Wisdom). In such birds, if a vaccine were found to lead to multi-year protection from a serious disease, the balance could tip toward attempting it.
Thinking along just these lines, a team of researchers set out to study vaccine responses in Cory’s Shearwaters, another long-lived seabird species. Some seabirds are susceptible to Newcastle Disease Virus, a potentially deadly disease that also affects domestic chickens. Given that last point, there is a vaccine currently available for use in poultry, and these researchers administered it to shearwaters instead and then tracked not only their own bodies’ responses over a period of years, but also how the antibodies passed into the birds’ eggs and persisted in hatchlings. What they found was that the adult birds did mount an immune response to the injection and that the antibody levels in their blood rose quickly, and then gradually declined over a period of years. Some of the birds received a booster shot a couple years after the initial one, and the antibody levels rose again in response to that. Levels of antibodies passed to the chicks as the egg was forming persisted after hatching, and the higher the levels of antibodies in the mother at the time of reproduction, the higher the levels in the chicks.
Just as every virus is different, every vaccine is different, and the results from this study do not mean that all vaccines would result in persistent antibody levels for several years. Additionally, what made this multi-year study possible is the site fidelity of shearwaters and other seabirds, many of which return year after year to the same exact burrow or nest site, and can be captured and resampled reliably. For many wild animals, this type of resampling is simply impossible, as would be booster shots. Finally, this was a study aimed at determining persistence of antibody levels, not at actually protecting the population from Newcastle Disease. Generally, something around 80% or so of the population need to be vaccinated to keep a disease from spreading. To attain those kinds of levels would be incredibly labor intensive, and in many cases, not feasible. But this study is an indication of the rising profile of disease ecology in wildlife management, and an awareness that we must shift finally away from a view of disease as a natural corrector and toward viewing it as another of the anthropogenic threats we created, and that we must strive to address.