The effect of climate change on wildlife health

BELDOMENICO PM

Galway

Conferencia; Co-Operation on Health and Biodiversity 2008 Conference; 2008

Cohab initiative

major interests of humanity are intimately related to wildlife health. For decades, pathogens have been neglected as important for wildlife population dynamics, but during the last years, a number of studies have shown that they are very important for population regulation, and also they have been implicated in major population declines - and we'll see later on how the condition of individuals has an impact on host-infection dynamics. So wildlife health is very important for the conservation of spp. Most emerging diseases originate from a wildlife reservoir, so to fully understand the conditions that lead to the emergence of new zoonotic diseases we need to look at host-pathogen dynamics in the natural hosts. So wildlife health is important for public health. Also, wildlife spp may serve as an early warning for some diseases, as is the case for howler monkeys and yellow fever in south america. Because of the flux of pathogens occurring at the wildlife-livestock interface, wildlife health is also important for domestic animal health. --- Generally speaking, as it has been repeatedly mentioned in this conference, the change in climate we're experiencing is global warming and disturbance in precipitation regimes. --- So, in which ways could this affect wildlife health? The answer to that is very complex, --- as the determinants of disease incidence are several and specific to the disease in question, --- and climate change may influence every single one of these factors --- Moreover, the effect of climate change on host-pathogen interactions might be in favour of hosts, as it could release hosts from disease control by interfering with the precise conditions required by many parasites? However, in this talk we'll focus on changes in host-pathogen interactions that result DETRIMENTAL to hosts, and we'll focus mainly on vertebrate hosts. --- But we have to bear in mind that all the components of an ecosystem are interconnected, and thus the influence of climate change on animal distribution and abundance via infectious disease may be indirect. For example, the Asian chestnut blight fungus effectively extirpated the American chestnut from eastern United States forests, causing, indirectly, the extinction of several phytophagous spp, because they no longer had the resource they depended upon? --- We'll focus, however, on more direct mechanisms. And in the interests of simplification, we propose that climate change may modify the patterns of infectious disease directly basically in two ways, --- First, by making things favourable for pathogens, that is, increasing pathogen proliferation, vector proliferation, or increasing pathogen or vector subsistence And second --- By increasing the host's susceptibility to infection --- We have to acknowledge that measuring the effects of climate change is very difficult, mainly due to two reasons: first, long term data is needed. For most wildlife spp we ignore what it was like before. And therefore we lack baseline data to notice any change. and second, there are other changes occurring at the same time, which makes it difficult to dissect the effect of climate change from other changes such as, habitat destruction, pollution, changes in land use, etc?. But this will be explained in precise detail by Dr. Parmesan later on. --- Bearing that in mind, let's explore some examples of the first proposed mechanism. It is known that arthropods are favoured by heat and moisture. Therefore we expect to see an influence of climate change on vector-borne diseases. In fact, a number of vector-borne human and domestic animal diseases have increased in incidence or geographic range in recent decades. But this can be identified because these are diseases important for public health or domestic animals, and hence there are records of the occurrence of these diseases. It is impossible to know what is changing in wildlife unless there exist systematic monitoring. --- There have been quite a bit of research to try to predict the expansion of vector-borne diseases due to climate change. Largely all these efforts involve modeling. For example, a study addressed how climatic variables determined the abundance of Ixodes scapularis, the tick that transmits Lyme disease, erlichiosis and babesiosis in eastern North America? they used the current knowledge on tick biology, originated from empirical and experimental data... --- to construct this complicated theoretical model that predicted the abundance of ticks given the climatic and seasonal variables they had measured. Then they used the observed data to evaluate whether this model worked (which is called, empirical validation). --- Subsequently, in another work they used this model to simulate the expansion of the tick distribution under 2 projected climate change scenarios. So, for vector-borne diseases, we expect to see an effect of climate change on the geographical distribution of vectors, and hence in their transmitted diseases. --- A very similar situation may be observed for pathogens affecting cold-blooded hosts (ectothermic hosts), or in those that proliferate outside affected individuals, because they are more exposed to ambient temperature, as opposed to those that have a life cycle that is completed almost entirely inside a host that preserves a constant temperature. In the graph we see what would happen with a pathogen that has a threshold temperature to proliferate - indicated by the green line ? if there were a 1.5ºC increment in average temperature, the proliferation of the pathogen would be greatly affected. Notice in this case that the seasonality of the disease would also be modified, and the disease would occur earlier every year, and will remain for longer. --- Algal blooms, also known as red tides, are events in which single-celled protists, dinoflagellates, proliferate rapidly and accumulate in the water column. These events are associated with wildlife mortalities, because these algae can produce potent toxins. A systematic increase in sea water temperature has been implicated as contributing factor in red tides. --- During an episode of mass bird die-off in the Malvinas/Falkland Islands investigated by the FVP, high levels of toxins produced by these dinoflagellates were detected in sick or dead gentoo penguins. This was the first report of paralytic shelfish poisoning affecting seabirds in the South Atlantic, which might suggest that climate change may be aiding the expansion of this type of disease to higher latitudes. --- Another effect that may result from favourable conditions for pathogens is an increased intensity of parasitism or severity of infection. In the St. Kilda archipielago, Scotland, there are feral populations of Soay sheep that experience periodic mass mortalities. Although the proximate cause of death has been determined to be protein-energy malnutrition, parasites have been implicated as a contributory factor. The first observations were that the depth of the population crashes were critically dependent on the weather, --- and that large numbers of trychostrongylid nematodes were found in dead animals. --- An experimental study showed that administration of an antihelminthic reduced mortality considerably, which supported the link between parasites + death --- Trychostrongylids have a life cycle that involves several stages outside their hosts. This makes them highly vulnerable to environmental conditions. In particular, larvae are very susceptible to drying, so humidity and precipitation regimes are crucial for their subsistence outside their hosts. --- So, an emerging scenario would be that an increased precipitation resulting from climate change, would cause increased larval subsistence, and this in turn high parasite burdens, contributing to elevated mortality --- Another example showing the potential effect of climate change on intensity of parasitism is the case of botflies of the Genus Philornis parasitising nestlings in South America. Ornithologists have noticed an apparent increase in the frequency and intensity of this parasitism. --- An ongoing study conducted by the Field Veterinary Program has shown that heavy rainfall is followed by a proportionally heavy parasitism --- A preliminary analysis shows that the severity of this parasitism is explained in its majority by the amount of rain that fell 2 weeks previously. --- And precipitation has been increasing in temperate south america, and an increased intensity of parasitism of, for example, Philornis botflies, may be one of the consequences --- Recapitulating, the first mechanism proposed stated that if climate change favoured pathogens, this may result in basically three consequences: an expansion in the geographic distribution of some pathogens; changes in the seasonality of some diseases; increased severity of disease --- The second proposed mechanism is that climate change will result in an increased susceptibility to infection. For example, induced changes in host behaviour may determine increased exposure to pathogens. While some parts of the world tend to get wetter, others tend to get dryer. In Patagonia, water supplies are threatened, and this may result in concentration of individuals around water resources, thus increasing intra-specific interaction, and furthermore, water supplies may be shared with domestic animals, raising the risk of exchange of pathogens at the domestic/livestock interface. --- Another example is the effect on trophic behaviour. For instance, the reduction in sea-ice is causing a change in the behaviour and diet of walruses, which are becoming more pelagic and prey more on ringed seals, which in turn may increase the prevalence of Trichinellosis in walruses. --- Also, some vertebrate hosts may expand their distribution, and thus expose immunologically naive species to their pathogens, and also they themselves become exposed to new pathogens --- Other way in which the susceptibility of hosts may be increased, is by an effect on their intrinsic vulnerability to disease --- But how would climate change affect host resistance? Firstly, by many spp. climate change is perceived a form of stress. The effect of stress on vertebrates is well known. A cascade neuroendocrine mechanisms triggered by stress determine a reduction in the immune function. --- In addition, because wildlife species usually live on tight budgets, a number of physiological functions compete for the limited resources. An increased demand by one system results in less resources for the rest. If climate change determines that the resources become scarcer, or of poorer quality, or that other systems increase their demands, then the share left for immunological investment will be reduced. --- Recent work in field voles showed that poor condition predisposes to infections, and that these infections further decrease the condition of individuals, triggering a vicious circle that eventually ends up in death, and therefore in population declines. --- Furthermore, studying intensities of infection with T. microti, it was shown that immunosuppressed individuals were more likely to show severe infection a month later, and that those with severe infections ended up with even more immunosuppression, later on. So poor condition not only may predispose to infection, but also to infections of high intensities. Notice that this has implications also when considering an infected individual as a source of infection. --- Thus, keeping a good condition is important to avoid infections, and avoiding infections is essential to keep in good condition. If climate change impoverishes the condition of most individuals in a population, then vicious circles may be triggered and the population will decline. --- One group that's particularly sensitive to climate disturbance are amphibians. In the last decades, thousands of spp have declined worldwide, and over a hundred have disappeared. A vast majority of these have declined even in seemingly undisturbed environments. --- Different pathogens have been found at greater prevalences in declining populations. In the scientific literature, there are a number of papers incriminating the chytrid fungus as the responsible of amphibian population chrashes, but amphibians have been declining also in regions where the fungus was absent, and the fungus was found in places with no affected frogs. In declining populations where the chytrid fungus was no present, other pathogens were found at high prevalences. --- Because declines have also been associated with climatic events, a hypothesis relating climate-driven epidemics has been proposed to explain amphibian declines. --- However, the vicious circle theory also fits in this story. Global warming seems to affect the condition of toads. On the left we can see how the body condition has been dropping with the years for a particular toad species, and on the right how body condition is negatively associated with temperature. --- Also, it was observed that frog population declines are preceded by an increase in indicators of stress. The graph on the right, shows measures of stress in a population that crashed, and the one on the left shows the same measures in a "control" population. The indicator of stress used here is limb asymmetry. Notice how this indicator tends to increase before the population crashes. --- The emerging picture is that climate disturbance is affecting the condition of amphibians, which predisposes to more frequent infections, of increased severity, which triggers vicious circles with the potential to cause amphibian population declines --- so while the synergy between poor condition and infection may be proximate cause of these declines, the ultimate cause would be a condition impoverished by climate change. --- In summary, the effects of climate change on wildlife health may be several. The consequence of them all, is disrupted health dynamics --- one subject that needs to be investigated is how evolution helps organisms adapt to these rapid changes. There has been proposed that genetic shifts will modulate local effects of climate change. But as Dr. Parmesan said two days ago, there is little evidence that evolution will mitigate negative effects of climate change at the species level. Moreover, we have to consider that the speed of evolution is different among different taxa. Bacteria and viruses, for example, have the capacity to evolve rapidly, adapting to environmental changes before their hosts. This might result in a differential adaptation favouring pathogens. --- A big challenge we face is measuring these effects, so that we can understand them better, and anticipate phenomena that may threaten wildlife conservation, public health or food-animal production. --- If we are to be prepared and respond properly to mitigate or adapt to the impact of climate change, we need to develop ways of assessing the health dynamics of wildlife species, and understand the proximate and ultimate causes behind the disruption of these dynamics. Long term monitoring of wildlife health, implemented by the FVP more than 10 years ago, should be refined and expanded.