11 Wildlife Disease
A disease is any condition in which the normal structure or functions of the body are damaged or impaired. Physical injuries or disabilities are not classified as disease, but there can be several causes for disease, including infection by a pathogen, genetics (as in many cancers or deficiencies), noninfectious environmental causes, or inappropriate immune responses. Our focus in this chapter will be on infectious diseases.
Disease is a complicated relationship between factors of the agent, the host, and the environment (Figure 11.1). Different diseases require different balances and interactions of these three components. Development of appropriate, practical, and effective measures to control or prevent disease usually requires assessment of all three components and their interactions.
Figure 11.1: One triangle model shows agent, host, and environment in three corners as having equal influence. Another model shows agent and host as interdependent variables (like a balance beam), with the environment at the base of the triangle. The environment has influence on the balance of agent and host.
An agent is the thing that causes disease or injury. An agent can be chemical, biological, radioactive, or environmental. Most often we think of infectious agents such as viruses. However, for many diseases, just having the presence of an agent is not sufficient enough to cause disease. We must also consider pathogenicity (ability to cause disease) and dose (“amount of a substance available for interaction with metabolic processes or biologically significant receptors”). Some diseases, especially chronic diseases, have multiple potential causes that must be evaluated on a case-by-case basis.
The host is the individual that can get the disease. Intrinsic factors about the host influence their exposure, their susceptibility to the disease, and their response to whatever causes the disease or injury. Whether or not an exposure affects an individual is linked to how susceptible they are and the body’s response to the agent.
The environment in the epidemiological triangle refers to the extrinsic factors about the host and factors that support the agent. Poor drainage or the presence of invasive species may encourage a poor climate that encourages insects such as mosquitoes that can transmit disease.
An infectious disease is any disease caused by the direct effect of a pathogen. A pathogen may be cellular (bacteria, parasites, and fungi) or acellular (viruses, viroids, and prions) and is a type of agent. Some infectious diseases are also communicable, meaning they are capable of being spread from individual to individual through either direct or indirect mechanisms. Some infectious communicable diseases are also considered contagious diseases, meaning they are easily spread from individual to individual. The degree to which a disease is contagious usually depends on how the pathogen is transmitted, therefore contagious disease transmission is variable. For example, measles is a highly contagious viral disease that can be transmitted when an infected person coughs or sneezes and an uninfected person breathes in droplets containing the virus. Gonorrhea is not as contagious as measles because transmission of the pathogen (Neisseria gonorrhoeae) requires close intimate contact (usually sexual) between an infected person and an uninfected person.
Certain infectious diseases can be transmitted from animals to humans and are called zoonotic diseases (or zoonosis). A zoonosis is a disease that occurs when a pathogen is transferred from a vertebrate animal to a human; however, sometimes the term is defined more broadly to include diseases transmitted by all animals (including invertebrates). For example, rabies is a viral zoonotic disease spread from animals to humans through bites and contact with infected saliva. Many other zoonotic diseases rely on insects or other arthropods for transmission. Examples include yellow fever (transmitted through the bite of mosquitoes infected with yellow fever virus) and Rocky Mountain spotted fever (transmitted through the bite of ticks infected with Rickettsia rickettsii).
Viruses can infect every type of host cell, including those of plants, animals, fungi, protists, bacteria, and archaea. Most viruses will only be able to infect the cells of one or a few species of organism. This is called the host range. However, having a wide host range is not common and viruses will typically only infect specific hosts and only specific cell types within those hosts. Viruses may cause abnormal growth of the cell or cell death, alter the cell’s genome, or cause little noticeable effect in the cell.
Viruses can be transmitted through direct contact, indirect contact with fomites, or through a vector: an animal that transmits a pathogen from one host to another. Arthropods such as mosquitoes, ticks, and flies, are typical vectors for viral diseases, and they may act as mechanical vectors or biological vectors. Mechanical transmission occurs when the arthropod carries a viral pathogen on the outside of its body and transmits it to a new host by physical contact. Biological transmission occurs when the arthropod carries the viral pathogen inside its body and transmits it to the new host through biting.
Human Impacts on Wildlife 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 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.
Figure 11.2: Habitat loss and encroachment of human activities into natural areas increase the rates of transmission of infectious diseases such as influenza, rabies, canine distemper virus, and Ebola between wildlife, domestic animals, and human. This figure illustrates the infection and transmission routes of rabies. Blue arrows represent factors that leads to higher infection rates, while red allows represent factors that contribute to the spread of the disease among the three vulnerable groups. After Daszak et al., 2000, CC BY 4.0.
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 (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.
Figure 11.3: Biologists swabbing an olive striped frog (Phlyctimantis leonardi, LC) in Gabon to screen for the chytrid fungus. Frogs from Cameroon has tested positive (Baláž et al., 2012) which suggests the species might be at risk; however, thus far no ill effects have been observed. Photograph by Brian Gratwicke, https://www.flickr.com/photos/briangratwicke/4395505435, CC BY 2.0.
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).
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.
Contributors and Attributions
Modified from the following sources:
Conservation Biology in Sub-Saharan Africa: Pollution, Overharvesting, Invasive Species, and Disease. By Open Book Publishers, Chapter 7 by J.W. Wilson and R.B. Primack, licensed CC BY 4.0
Additional references and citations from the above sources can be found in References in the backmatter.
11. Wildlife Disease is shared under a CC BY-NC-SA license