7.5: Parasites and Diseases
Parasites and diseases have always been an important natural factor in regulating the ecology of wildlife, especially in wild populations that have become unsustainably large. Today, however, human activities are facilitating increased spread and transmission of parasites (Box 7.4) and other pathogens, sometimes even creating conditions for epidemics to develop (Figure 7.13). Consequently, parasites and diseases have become a major threat to wildlife, including those already suffering under low population sizes and densities.
Vincent Dietemann
Agroscope, Swiss Bee Research Center,
Schwarzenburgstrasse, Switzerland.
vincent.dietemann@alp.admin.ch
Modern society imposes increased pressure on animals and plants to secure the food needed for a growing human population. Honeybees especially contribute to crop productivity in a crucial way thanks to their pollination services. Unfortunately, the number of bee colony losses has surged in recent years in several regions of the world (Goulson et al., 2015), worrying scientists, politicians, and the public. Some regions in Africa, however, have maintained healthy domesticated and wild honeybee colonies (Pirk et al., 2016), which continue to pollinate flowers and enhance the production of many fruit and vegetable crops.
The power of comparisons
Comparative studies have always been important to biological research. Studying a model organism or system under different conditions allows scientists to identify how these organisms or systems react and adapt to their environment. This can even be done at the continental scale and could help us understand the effect of human pressure exerted on honeybees. In general, beekeeping in North America and Europe has been widely industrialized, involving large-scale operations and modern technology, whereas in Africa beekeeping has remained small-scale and mostly low-tech. This gives us an opportunity to determine the effects of beekeeping management and trade on honeybee health.
Different contexts
The varroa mite (Varroa destructor) originally parasitised the eastern honeybee (Apis cerana). In the wake of global honeybee trade, this parasite has invaded most regions of the world that are home to the western honeybee (Apis mellifera) and resulted in major colony losses (Figure 7.E). Although eastern and western honeybees are closely-related species, the western honeybee did not coevolve with this parasite and, thus, has few natural defences against it, with colonies dying within a few years after infestation. Consequently, only those colonies treated against the parasite by beekeepers can survive, and most wild honeybee populations have been decimated. However, there have been exceptions: colonies of the western honeybee in the southern parts of Africa are resistant to the parasite, and large wild populations remain. Several international teams have now turned their attention to resistant African honeybee populations to understand the basis of their survival (Strauss et al., 2016). Researchers hope to use this knowledge to promote the breeding of surviving colonies in currently susceptible populations both within and outside of Africa.
Africa also differs strongly in land use and crop management techniques that are likely to influence honeybees’ nutrition and health. In many areas, small-scale farming prevails, with a lower use of pesticides than in other areas of the world. Understanding how pesticide and other chemical use, as well as how nectar and pollen variety and quality, impact these pollinators will be key to their survival. Therefore, the rest of the world may learn how to maintain healthy honeybees from Africa. Africa, in return, might benefit from global efforts to maintain sustainable pollination services and promote food security.
One way in which humans elevate the impact of parasites and diseases on wildlife is by exposing native species to harmful organisms that they have never previously encountered, and thus have no evolved coping mechanisms. For example, population declines and extirpations of about 200 frog species across the world, including in Africa (Tarrant et al., 2013; Hirschfeld et al., 2016), is due, in part, to a disease caused by the chytrid fungus (Batrachochytrium dendrobatidis). This disease, known as chytridiomycosis (Figure 7.14), affects a frog’s ability to absorb water and electrolytes through the skin (Alroy, 2015). It likely originated in the Korean Peninsula (O’Hanlon et al., 2018), and spread across the world through trade with African clawed frogs (Xenopus laevis, LC) (Weldon et al., 2004). As of yet, there is no cure for this disease, and it continues to be seen as one of the biggest threats currently facing the world’s amphibians.
Disease transmissions can also occur when humans and their pets or livestock interact with wildlife (Cumming and Cumming, 2015). For example, during the early 1990s about 25% of lions in Tanzania’s Serengeti National Park were killed by canine distemper virus which they contracted from domestic dogs living near the park (Kissui and Packer, 2004). Because of the many biological similarities between apes and humans, gorillas (Gorilla spp.), chimpanzees (Pan troglodytes, EN) and bonobos (P. paniscus, EN) are particularly vulnerable to anthroponotic diseases, such as measles, influenza, and pneumonia which can be transferred from humans to animals. But even chytridiomycosis (discussed above) can become an anthroponotic disease, transferred from frog to frog by a careless biologist that handles a healthy frog after a sick one without taking precautions against transmission. Some diseases (e.g. Ebola; flu; and tuberculosis) can be anthroponotic and zoonotic (transferred from animals to humans). While the impact of Ebola on humans in Africa is well-known, it is worth noting that gorillas suffer 90% mortality when exposed to Ebola, compared to 50% mortality in humans. In fact, it was an Ebola outbreak in 2004 that caused the western lowland gorilla (Gorilla gorilla gorilla, CR) to be classified as highly threatened by the IUCN (Genton et al., 2012).
Human activities often facilitate the emergence and spread of infectious diseases, which threaten wildlife, domestic species, and humans alike.
Humans also indirectly facilitate the transmission and spread of parasites and pathogens. While there are some exceptions (notably social insects), transmission and infection rates are typically low for wildlife living in large, complex ecosystems because they have space to move away from disease-carrying droppings, saliva, old skin, and other sources of infection. However, these natural buffers against pathogens and parasites are removed when humans confine those organisms to small areas (such as small fenced reserves) or keep them in crowded conditions. In addition to forcing those organisms to remain in close contact with potential sources of infection, crowded conditions lead to deterioration of habitat quality and food availability. Both these factors increase the organisms’ stress levels and reduce their body conditions which, in turn, lowers their resistance to parasites and diseases (reviewed in Gottdenker et al., 2014).
Human-induced extirpations indirectly facilitate the transmission and spread of parasites and pathogens, even to humans. Such is the case with schistosomiasis (also known as bilharzia), a zoonotic disease carried by a few freshwater snail species. In the 1980s, health care professionals observed an increased incidence of human schistosomiasis around Lake Malawi after overfishing depleted snail-eating fish populations, followed by decreased incidence of schistosomiasis as fish populations recovered in the 1990s (Stauffer et al., 2006). A similar situation occurred in East Africa, where the elimination of apex predators resulted in increased olive baboon (Papio anubis, LC) populations, which not only worsened crop raiding, but also increased parasite infection rates among local peoples (Brashares et al., 2010).
Parasites and diseases also threaten captive wildlife populations, including those kept at zoos and other ex situ conservation facilities (Section 11.5). Because of the proximity in which different species are kept, captive conditions may allow for easier spread of diseases. An added complication with captive populations is that some individuals may function as disease reservoirs. These individuals generally appear healthy because they are fairly resistant to the disease they carry, yet they are able to infect other susceptible individuals. Disease reservoirs frequently limit opportunities for translocation of captive populations (Section 11.2), even when dealing with threatened species. For example, well-meaning people often bring raggedy-looking yet healthy penguins in moult to rehabilitation centres, hoping the penguins will be released once “better”. Yet, those animals might never be released back in the wild to avoid the risk of transmitting diseases to wild penguin populations (Brossy et al., 1999).
The impacts of diseases are bound to become more important in the future of conservation biology, especially as growing human populations and increased competition for space increase the need for single-species management and ex situ conservation (Chapter 11). Disease management should therefore always be taken very seriously, and appropriate steps taken to avoid disease transmissions.