Disease is described as a decrease in performance of normal functions of an individual caused by many factors, which is not limited to infectious agents. Furthermore, wildlife disease is a disease when one of the hosts includes a wildlife species. In many cases, wildlife hosts can act as a reservoir of diseases that spillover into domestic animals, people and other species. Wildlife diseases spread through both direct contact between two individual animals or indirectly through the environment. Additionally, human industry has created the possibility for cross-species transmission through the wildlife trade.Furthermore, there are many relationships that must be considered when discussing wildlife disease, which are represented through the Epidemiological Triad Model. This model describes the relationship between a pathogen, host and the environment. There are many routes to infection of a susceptible host by a pathogen, but when the host becomes infected that host now has the potential to infect other hosts. Whereas, environmental factors affect pathogen persistence and spread through host movement and interactions with other species. An example to apply to the ecological triad is Lyme disease, where changes in environment have changed the distribution of Lyme disease and its vector, the Ixodes tick. The recent increase in wildlife disease occurrences is cause for concern among conservationists, as many vulnerable species do not have the population to recover from devastating disease outbreaks.
Transmission.
Indirect.
Wildlife may come in contact with pathogens through indirect vectors such as their environment by consuming infected food and water, breathing contaminated air, or encountering virulent urine or feces from an infected organism. This type of transmission is typically associated with pathogens that are able to survive prolonged periods, with or without a host organism.
The most recognizable wildlife disease that indirectly spreads are prion disease. Prion diseases are indirectly spread due to their longevity in the environment, lasting for several months once released from a host via their excretions (urine or feces). Notable animal prion diseases include chronic wasting disease in cervids, scrapie in sheep and goats, and various types of spongiform encephalopathy including bovine (also known as mad cow disease), mink, feline, and ungulate.
Direct.
Disease can be spread from organism to organism through direct contact such as exposure to infected blood, mucus, milk (in mammals), saliva, or sexual fluids such as vaginal secretions and semen.
A prominent example of direct infection is facial tumor disease in Tasmanian devils, as these marsupials will repeatedly bite other individuals in the face during the breeding season. These open wounds allow transmission via blood and saliva in the devil's orifices.
Wildlife Trade.
A major driver for transmission between species recently is wildlife trade, as many organisms that do not typically encounter each other naturally are in close proximity. This can include places such as wet markets as well as the illegal trade of both live and dead animals and their body parts.
The most notable example of wildlife trade impacting both animal and human health is COVID-19, originating in a wet market in Wuhan, China. The originating species has been a topic of debate as it is unclear due to the variety of species found at the market, however pangolins and bats both have been absolved of blame despite initial claims.
Wildlife Disease Management.
The challenges associated with wildlife disease management, some are environmental factors, wildlife is freely moving, and the effects of anthropogenic factors. Anthropogenic factors have driven significant changes in ecosystems and species distribution globally. The changes in ecosystems can be caused by introduction of invasive species, habitat loss and fragmentation, and overall changes in the function of ecosystems. Due to the significant changes in the environment because of humans, there becomes a need for wildlife management, which manages the interactions between domestic animals and humans, and wildlife.
Wildlife species are freely moving within different areas, and come into contact with domestic animals, humans, and even invade new areas. These interactions can allow for disease transmission, and disease spillover into new populations. Disease spillover can become of great concern when considering outbreaks, not only in humans but in other wildlife species raising a concern for species preservation.
Detection.
Wildlife disease is detected primarily through surveys, for example taking samples from wildlife populations in an area to determine the prevalence of disease within a population. Prevalence is define as the percentage of a population that is diseased at a particular time. There are limitations to using this to detect disease within wildlife populations, such as all host may not show signs of disease, the sample distribution, and the disease distribution. Diseases in wildlife tend to form patches of disease throughout an entire population, which can affect the prevalence of the disease within a population. Sampling is assumed to be random, but is often opportunistic. Another form of disease detection is through observation of diseased hosts. However if some hosts within a species do not show signs of disease, this can influence the prevalence of disease detection within a wildlife population.
The reservoir of wildlife disease can also be a challenge when considering wildlife disease detection. An example of a challenge identifying the pathogen is the mass mortality event in bald eagles in southeastern United States in 1994. The challenge identifying the causative agent of disease was due to the neurotoxin being isolated from the areas of outbreak, but not when grown in the laboratory until a brominate metabolite was used. The management of wildlife diseases involve many factors, which should are all important to consider when determining the persistence of a pathogen within a population.
Surveillance and Monitoring.
Programs have begun to survey wildlife populations to better understand transmission and health impacts in the affected wildlife communities. Tools such as the Geographical Information System (GIS) can be utilized in order to keep track of individual occurrences of disease in order to create an overall image of disease prevalence and spread in a given area. Major zoonotic diseases such as rabies, COVID-19, influenza, and hemorrhagic fever are monitored to ensure both human health and safety as well as mitigation of impacts on wildlife. Proactive intervention can increase the likelihood of species survival while simultaneously preventing emerging pathogens from escalating to an epidemic. 
Prevention.
Culling.
Disease outbreaks in wild animals are sometimes controlled by killing infected individuals to prevent transmission to domestic and economically important animals. While easy and quick for disease management, culling has the consequence of disrupting ecosystem function and reducing biodiversity of the population due to the loss of individuals. Animal rights advocates argue against culling, as they consider individual wild animals to be intrinsically valuable and believe that they have a right to live. Activists favor humane methods of prevention such as vaccination or treatment via rehab centers, as these are non-lethal forms of management.
Vaccination programs.
Wild animal suffering, as a result of disease, has been drawn attention to by some authors, who argue that we should alleviate this form of suffering through vaccination programs. Such programs are also deemed beneficial for reducing the exposure of humans and domestic animals to disease and for species conservation.
The oral rabies vaccine has been used successfully in multiple countries to control the spread of rabies among populations of wild animals and reduce human exposure. Australia, the UK, Spain and New Zealand have all conducted successful vaccination programs to prevent Bovine Tuberculosis, by vaccinating badgers, possums and wild boar.
In response to the COVID-19 pandemic, it has been proposed that, in the future, wild animals could be vaccinated against coronaviruses to relieve the suffering of the affected animals, prevent disease transmission and inform future vaccination efforts.
Zoonoses.
Wild animals, domestic animals and humans share a large and increasing number of infectious diseases, known as zoonoses. The continued globalization of society, human population growth, and associated landscape change further increase the interactions between humans and other animals, thereby facilitating additional infectious disease emergence. Contemporary diseases of zoonotic origin include SARS, Lyme disease and West Nile virus.
Disease emergence and resurgence in populations of wild animals are considered an important topic for conservationists, as these diseases can affect the sustainability of affected populations and the long-term survival of some species. Examples of such diseases include chytridiomycosis in amphibians, chronic wasting disease in deer, white-nose syndrome, in bats, and devil facial tumour disease in Tasmanian devils.
Conservation.
Populations on the Decline.
When an epidemic strikes a population of organisms, the loss of individuals can be detrimental to already fragile or fragmented populations. Many disease epidemics have largely reduced the population of their host organisms, some even increasing the possibility of an endangered or extinct status.
Recovery.
While disease can ravage a population, many wildlife are resilient and can recuperate their population loss. Human intervention can also increase the chances of species recovering from epidemics via various prevention and treatment methods. Individuals that survive epidemics can repopulate, now with disease resistance present in the gene pool of that population. This will result in future generations of a species that are less susceptible to a specific disease.
References.
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