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The Threat of Emerging and Re-emerging Infectious Diseases to Wildlife

In this 6th interview with renowned wildlife biologist Dr. Michael Hutchins, we discuss the challenges facing vanishing species and other threatened free-ranging and captive populations of wildlife due to emerging and re-emerging infectious diseases.

Jordan: Zoonoses and anthroponoses may be confusing terms to some of our readers. Can you define these terms and any others that may help us better understand infectious disease dynamics?  What constitutes an Emerging Infectious Disease and who studies these disease agents?

Ebola (Nat Geo)
Ebola (Nat Geo)

Michael: Before responding to your questions, first let me say that I am not a wildlife veterinarian, epidemiologist or medical doctor, so my knowledge of these issues comes from many years of interacting with experts in these fields.  Simply put zoonoses are diseases that can pass from animals to humans. Some examples include rabies, Murine typhus fever, hookworm and Ebola. I’m not surprised about the confusion over various terms.  I’ve seen two definitions of anthroponoses: (1) diseases that pass only between humans and (2) diseases that can pass from humans to animals.  An example of the former would be the common cold; examples of that latter would be measles and tuberculosis.  An emerging infectious disease (EID) is generally defined as a disease that has recently appeared in a population or that has been known for some time but is rapidly increasing in incidence or geographic range. Some examples include the, Ebola virus, Monkey Pox, SARS, HIV/AIDS, bird flu (H5N1) and Lyme disease.

Such diseases are of great concern to public health officials, wildlife managers and conservationists (http://www.nps.gov/public_health/zed/documents/Emerging%20Infectious%20Diseases%20of%20Wildlife–.pdf).  EIDs that impact human health, pets, and food animals have received a great deal of attention from the scientific community over the last century. However, EID’s in wildlife populations have only recently received much attention. In fact, EID’s in wildlife species are considered fairly unreported to date.  In 2007, I was invited to give the plenary talk to the joint annual meeting of the American Association of Wildlife Veterinarians and American Association of Zoo Veterinarians. The focus of my speech was on the need for more cooperation between wildlife veterinarians and wildlife biologists and managers and more attention to the role of disease in wildlife population dynamics.  This has become even more critical as, with human population growth and encroachment on wildlife habitats, interactions among wildlife, domestic animals and humans are on the rise.  With international commerce and the frequent movement of animals and people, our planet has essentially become a giant petri dish which could result in the emergence and spread of new and virulent diseases (Sleeman, J. and Gilin, C. 2012, Ills in the pipeline: Emerging infectious diseases and wildlife.  The Wildlife Professional 6(1) 28-32).  Realization of this has led to the “One World, One Health” concept (http://oneworldonehealth.org/).  A meeting of experts organized by The Wildlife Conservation Society in 2004, led to 12 recommendations intended to establish a “more holistic approach to preventing epidemic / epizootic disease and for maintaining ecosystem integrity for the benefit of humans, their domesticated animals, and the foundational biodiversity that supports us all.”

That being said, disease is a natural part of our world and plays an important role in controlling populations and in natural selection. Intervening in every case may therefore be problematic. A good example of this is the incidence of conjunctivitis (pink-eye) in North American bighorn sheep. Wildlife managers have struggled as to whether or not they should intervene during outbreaks of this disease. Bighorn sheep that contract this malady, that is easily treatable in domestic animals, cannot see well and often fall to their deaths while navigating rugged terrain. However, some sheep are less susceptible to the disease. If wildlife managers were to intervene and provide treatment and, as a result, individual sheep that were susceptible to conjunctivitis survived and reproduced (passing along this susceptibility to their offspring), then intensive (and expensive) treatment might have to go on forever. Thus, intervening in such cases may disturb natural processes that will eventually strengthen the population over time.  On the other hand, a number of outbreaks of ‘pink-eye’ have also occurred in bighorn sheep that were traceable to goats or other domestic livestock. In those cases, treating the bighorn sheep might be justifiable, and more importantly, preventing disease by limiting contact between domestic and wild species would probably benefit all concerned.  This example illustrates the complexity involved in decision-making.    However, what if the species in question is threatened or endangered?  What if the disease is not naturally-occurring and was introduced either by humans or invasive species? Clearly then the ethical and biological circumstances are different and the conservation ethic would permit intervention. A good example is the treatment of measles in endangered mountain gorillas—a disease that was introduced by humans (http://www.examiner.com/article/gorillas-the-mountains). In some cases, tourists are now being asked to wear surgical masks to prevent the transmission of diseases, such as measles and pneumonia, from humans to wild gorillas (http://wwwnc.cdc.gov/eid/article/17/4/pdfs/10-0883.pdf).  A vaccine against Sylvatic plague has now been developed for use on endangered black-footed ferrets (http://www.nwhc.usgs.gov/disease_information/sylvatic_plague/publications/protecting_black-footed_ferrets.pdf).  Plague has been a major factor in the near extinction of the ferret and has made it difficult to maintain reintroduced populations once established.  In addition, wildlife managers also drop flea powder on prairie dog colonies, the main prey of the black-footed ferret to help suppress outbreaks of the disease (http://www.fort.usgs.gov/Research/research_tasks.asp?TaskID=2123).  This helps to control flea populations, the main vector of Sylvatic plague, which ferrets contract from eating infected prairie dogs. Such interventions are also justified when the risk to human health is high. For example, efforts are now being made to control the incidence of rabies in suburban wildlife by dropping baits containing oral vaccines, which are subsequently ingested by wildlife (http://www.aphis.usda.gov/wildlife_damage/nwrc/research/rabies/index.shtml; http://www.ncbi.nlm.nih.gov/pubmed/22947155).

The U.S. government has been particularly concerned about the possible introduction of the H5N1 virus, also known as bird flu or avian influenza. A great deal of time and effort has been spent monitoring the condition of migratory birds coming into the country and predicting where incursions are most likely to occur (http://www.sciencedaily.com/releases/2013/03/130314141333.htm).  A pandemic outbreak could be devastating to both humans and wild birds, although there has been much work recently to develop a human vaccine against the disease (http://www.sciencedaily.com/releases/2008/10/081019184800.htm).

Jordan: Generally, we think of emerging infectious diseases as a recent phenomenon, but rinderpest virus or cattle plague, as it is called, decimated populations of artiodactyls (even-toed ungulates) from domestic cattle to wild bovids, cervids, and suids, etc. as early as the beginning of the 18th century. The disease, which was enzootic (endemic) in Asia, was spread via the transport of cattle to Africa during European colonization of the continent. But only in the last two years has the disease been considered globally eradicated.   Why was this global eradication such a difficult feat?

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Wood bison (Photo by Jordan Schaul)

Michael: The eradication of rinderpest, a viral disease related to measles, was an extremely difficult task and involved intensive global cooperation and action, which was not possible until relatively recently (http://www.theworld.org/2011/06/eradication-rinderpest/). This devastating disease first appeared in Central Asia and then spread throughout Asia to Europe and Africa with the movement of domestic cattle. In the late 1890’s, rinderpest was responsible for the deaths of 95% of the domestic cattle in many parts of Africa and took a tremendous toll on the continent’s wildlife as well.  In 1924, a committee was founded to coordinate a global campaign against the disease. In the late 1950s, a British scientist, Walter Plowright, developed the first effective vaccine. However, it wasn’t until 1994 that the goal of total eradication seemed feasible. Many governments wanted to see the disease eradicated and the United Nation’s Food and Agriculture Organization (FAO) established a global rinderpest eradication program. Scientists distributed kits to test for the disease and asked farmers to kill and bury any infected animals. Healthy cows in surrounding areas were vaccinated against the disease. The program was highly successful. The last known case of rinderpest occurred in Kenya in 2001.  However, no one wanted to declare the disease eradicated for fear that it would emerge again. Only now, after more than a decade of intensive surveillance, are scientists confident that the virus is gone.  It should be noted that rinderpest and smallpox are the only two diseases that have been totally eradicated through global action, which testifies to the difficulty of such an undertaking. In each case, the key was global cooperation.  Had any cooperator or government fallen down on the job, we would still have these diseases with us.

Jordan: Currently, brucellosis and tuberculosis jeopardize wild bovid conservation around the globe.   Can you talk about some of ”spillover” effects and disease dynamics as they relate to domestic and wild populations of ungulates and other taxa?

Yes, the One World, One Health concept recognizes the potential for “spillover” of diseases between wild and domestic animals. For example, brucellosis has been a huge concern to cattle ranchers.  The disease, which is caused by the bacterium Brucella abortus, causes abortion or premature calving in cattle, usually between the fifth or eighth month of pregnancy.  Given the damage that it does in terms of loss of calves, loss of milk and weight loss in female cattle, ranchers consider it to be one of the more destructive of cattle diseases. (http://www.state.nj.us/agriculture/divisions/ah/diseases/brucellosis.html).  Despite efforts to control this disease by state, federal and international agencies, reservoirs of the disease still exist in wild herbivores, such as bison and elk. One of the last known areas in which the disease exists is in the Greater Yellowstone Ecosystem. Considerable controversy has been generated by the lethal control of wild bison as they leave the park, where they could potentially come into contact with domestic cattle (http://www.aphis.usda.gov/animal_health/animal_dis_spec/cattle/downloads/cattle-bison.pdf).  Yellowstone has been a major source of genetically pure wild bison for reintroduction into other areas. In order to overcome the risk of brucellosis transmission, the U.S. National Park Service has been maintaining and breeding a population of bison known to be brucellosis free.  However, similar efforts have not been proposed for elk.

Brucellosis, also known as Mediterranean fever, Malta fever, Crimean fever, Bang’s disease, and undulant fever, is actually fairly common in humans, resulting in more than 500,000 cases annually worldwide (http://www.medicinenet.com/brucellosis/article.htm).  Symptoms may include fever, joint pain, cough and a variety of others.

Tuberculosis (TB) is another disease that is transmittable between wildlife, domestic cattle and humans.  It is caused by three specific types of bacteria that are in the genus Mycobacterium: M. bovis, M. avium, and M. tuberculosis. Bovine TB, caused by M. bovis, can be transmitted from livestock to humans and other animals, including bison and elk (http://www.aphis.usda.gov/animal_health/animal_diseases/tuberculosis/). The disease has nearly been eliminated in livestock, but it occasionally crops up.  In its late stages, common symptoms include emaciation, a low–grade fluctuating fever, weakness and loss of appetite. This may be accompanied by a moist cough that is worse in the morning, during cold weather or exercise. Eventually, the animals may become extremely emaciated, develop acute respiratory distress, and die (http://www.cfsph.iastate.edu/Factsheets/pdfs/bovine_tuberculosis.pdf).

An important point to be made here is that both TB and Brucellosis were brought to North America with domestic cattle and it was cattle that infected wildlife.  Neither of these introduced diseases are a threat to wildlife at a population level.  However, some of the control measures proposed could significantly impact wildlife populations. Despite the perceptions of ranchers, both diseases are comparatively minor sources of economic loss and both can be tolerated more readily than completely eradicated. The key is to work to prevent contact between wildlife and domestic livestock, which reduces the risk of transmission, as opposed to eradication across vast landscapes.

Jordan: Introduced populations of several vertebrate and invertebrate species serve as reservoirs of parasitic, bacterial, and viral agents that have decimated native species.  Can you talk about some of these pathogens and their hosts?

Michael: Yes, introduced species can bring pathogens with them for which native species have not developed immunity; the results can be devastating. The best examples come from the introduction of exotic plant diseases. The once ubiquitous American chestnut tree has almost gone extinct due to the introduction of chestnut blight that was introduced to the United States from imported Japanese chestnut trees (http://www.dbc.uci.edu/sustain/global/sensem/patel297.html).  An illustrative example of an exotic pathogen threatening endemic wildlife comes from Hawaii. Being so isolated in the Pacific, Hawaii never had mosquitos until they were in introduced during the 1830s. In the early 1900s, avian malaria, caused by the parasite Plasmodium relictum, arrived with a shipment of exotic songbirds. What followed was the near decimation of the islands diverse endemic bird fauna.  Indeed, avian malaria, transmitted by mosquitos, is thought to be the primary limiting factor to the recovery of Hawaii’s endemic forest bird populations. Many species that were once common are now highly endangered and found only in high elevation areas, above the thermal limits of mosquitoes and disease transmission (http://fwcb.cfans.umn.edu/courses/nresexotics3002/GradPages/Avian_Malaria_Hawaii/default.htm ). .

Newly-encountered pathological organisms themselves can be considered invasive species. A relatively recent example is the West Nile Virus, an emerging disease that is transmitted by mosquitoes and affects wildlife and humans. About one in 150 people infected with WNV will develop severe illness. Symptoms can include high fever, headache, stiffness, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness and paralysis. The initial introduction of this disease decimated  many bird populations (http://www.post-gazette.com/stories/news/environment/west-nile-virus-has-profound-effect-on-birds-485470/).  In fact, there has been great concern about the impact of West Nile Virus on a wide variety threatened and endangered species, including birds, mammals, reptiles and amphibians (http://news.ucdavis.edu/search/news_detail.lasso?id=7050).

A well-documented example of an introduced pathogen of significance for human health was the Monkey Pox outbreak of 2003.  Many people throughout a three state area (Illinois, Indiana, and Wisconsin) suddenly came down with an unusual severe pustular rash and a variety of other symptoms.   The source of the infections was subsequently identified as the Monkeypox Virus, a disease endemic to West African rainforests.   The pathogen was introduced into the United States through the exotic pet trade.  All of the victims had been in contact with pet prairie dogs, which had acquired the disease when they were housed together with Gambian giant rats (http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5223a1.htm).

Jordan: Are there any examples of diseases that are placing species in immediate danger of extinction?

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Tasmanian Devil (Photo by Michael Hutchins)

Michael: Yes, one example I can think of is the highly contagious Tasmanian devil facial tumour disease (DFTD), a cancer, which causes 100 per cent mortality in the endangered marsupials. The facial cancer spreads when the animals bite each other’s faces during fighting.  The disease kills its victims in a matter of months.  Indeed, it has already wiped out the majority of the population with field sightings reduced by 85 per cent.  Fortunately, efforts are underway to study and understand this disease and to develop an effective vaccine against it (http://www.sciencedaily.com/releases/2013/03/130311173627.htm). Hopefully, such efforts will be successful in reversing the rapid downward population trend.  It is also being speculated that natural selection may be underway that may favor less aggressive devils, which would logically be less likely to contract the disease through fighting (http://www.sciencedaily.com/releases/2012/09/120903221037.htm).  Interestingly, it was recently confirmed that disease was not a factor in the extinction of the devil’s cousin, the Tasmanian wolf, whose numbers declined through human persecution (http://www.sciencedaily.com/releases/2013/01/130131095310.htm).

Another example is the so-called White-nosed Syndrome in bats, which is currently sweeping across eastern North America and virtually killing every bat in its path. The disease, which is caused by the fungus Geomyces destructans, infects the muzzle, ears and wings of afflicted hibernating bats (http://www.nwhc.usgs.gov/disease_information/white-nose_syndrome/).  The pathogen now appears to have been introduced from Europe, perhaps on the boots or equipment of unsuspecting cave enthusiasts (spelunkers). University and federal scientists are desperately seeking some way to put a halt to the epidemic, but no silver bullet has been found yet (http://www.aboutmyplanet.com/environment/white-nose-bat-syndrome-continues-without-solution-in-u-s/).  One possible solution is to build artificial caves for bats that do not provide conditions for the growth of the fungus (http://www.aboutmyplanet.com/environment/white-nose-bat-syndrome-continues-without-solution-in-u-s/).

Last, but not least, this interview would not be complete without mentioning the deadly chytrid fungus that is impacting populations of amphibians around the world. The chytrid fungus, Batrachochytrium dendrobatidis, was found in 1998 to be responsible for parasitizing and killing its amphibian hosts (through the often-fatal resultant disease called chytridiomycosis). Spread of this disease has been implicated in the worldwide decline of amphibians and may have already contributed to the loss of several species, such as the attractive golden frog in Central America and unusual mouth-brooding frog in Australia (http://wamu.org/news/11/08/02/fungus_pushes_frogs_towards_extinction.php).  Efforts to combat this threat have fallen short, particularly when it comes to treating the disease in nature (http://www.sciencedaily.com/releases/2011/06/110620094856.htm).  Spread of the disease may have been facilitated by a global trade in amphibians, primarily for the pet trade and for the food industry (http://www.sciencedaily.com/releases/2009/11/091119135642.htm).  There is hope that some species or individuals will prove resistant to the disease and provide a basis for recovery (http://www.sciencedaily.com/releases/2008/07/080715204750.htm).  The recent successful cloning of one extinct species (Australia’s mouth-brooding frog) has given hope to conservationists that at least some species can be revived (http://www.sciencedaily.com/releases/2013/03/130315151044.htm).  Accredited zoological parks are also working cooperatively to try to address this pressing conservation issue (http://www.aza.org/amphibian-conservation-and-education-resources/) by implementing captive breeding and associated research and educational programs.

Jordan: Syndemics occur as a result of synergistic interactions among pathogenic agents, which result in exacerbated health implications for diseased individuals. Are syndemics common in wildlife populations?

Yes, various pathogens when contracted simultaneously can exacerbate the impact on wildlife health. One of the examples I am familiar with is the interaction of two parasitic infections in marine mammals: Toxoplasma gondii, a parasitic organism that must pass through the gut of cats and Sarcocystis neurona, a parasite that is thought to be transmitted by opossums. Both organisms enter the marine environment through freshwater runoff following heavy rains.  Both result in neuroinflammatory disease, which may cause death.   According to one of the scientists working on the project, “The most remarkable finding of our study was the exacerbating role that S. neurona appears to play in causing more severe disease symptoms in those animals that are also infected with T. gondii.” Among animals for which necropsy had suggested parasitic infection as the primary cause of death, the co-infected animals were more likely to display evidence of severe brain tissue inflammation than those infected by either S. neurona or T. gondii alone. This is therefore a classic case of the synergy that can occur between different pathogens with devastating effects on afflicted animals. Toxoplasmosis has also been implicated in human disease, including behavioral abnormalities (http://www.sciencedaily.com/releases/2009/03/090311085151.htm; http://rsbl.royalsocietypublishing.org/content/8/1/101.abstract?sid=d5e27e3c-ac2e-4f21-8f52-b65190a0e563 ; http://www.medscape.com/viewarticle/771145; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2526142/).

Jordan: Zoological institutions house living collections of exotic animals, including many endangered species, which are often propagated in conservation breeding programs. As a consequence of housing diverse fauna in such close proximity these conservation breeding centers have to be particularly vigilant with regard to the introduction of EID’s.  Why is this and what is at stake?

Michael: Yes, zoological institutions have to be particularly vigilant with regard to wildlife health, particularly since the animals housed within typically originate from many sources.  In addition, the diversity of organisms displayed can mean that many diseases can be present in the collection.  Also, little may be known about the health challenges facing some species, especially those that are rare or endangered.  Indeed, some animals can be carriers of diseases and show no outward symptoms.  Furthermore, animals cannot communicate how they are feeling to keepers or veterinarians, except through subtle behavioral cues. This is why keepers are trained in disease prevention, not only for their own protection, but also for the health of the animals in their care. Keepers generally wear latex gloves and face masks when working around primates. Because of their genetic similarity, these animals have the highest potential for disease transmission.  Working with certain types of animals (e.g. vampire bats) may require rabies vaccine.  Because keepers often move between various animal exhibits or holding facilities, they are asked to step in tubs of disinfectant when moving in and out of buildings. Staff veterinarians give animals regular health checkups and keepers keep a close eye on their charges and note any changes in behavior that might be indicative of a health problem.  Some zoos employ pathologists or epidemiologists who can delve into the impacts of certain diseases on animals and the methods and rapidity by which various diseases spread.  Modern zoological institutions have well-equipped hospitals for the care and treatment of the animal collection. Many are outfitted with the latest medical technology used on humans, including x-rays, ultrasound, antibiotics, etc.

What is at stake? The risk of disease outbreaks is one reason that entire populations of threatened or endangered animals are not all keep in a single facility, but rather spread out among many.  This is the concept of not keeping all of your eggs in one basket.  It would obviously be a tragic situation if all remaining members of an endangered species were at a single facility during a disease outbreak and all were lost. As responsible employers, zoological institutions are also concerned with the health and safety of their keeper staff. This highlights the need for appropriate training and procedures for disease prevention.  There is also a risk that captive breeding for reintroduction programs will introduce diseases into wild populations.  Risk assessments and extensive testing are therefore necessary to protect wild populations from disease introductions (http://www.aza.org/reintroduction/). Unfortunately, there are examples of this occurring. Captive-bred Mallorcan midwife toads released into the wild in 1991 were infected with the deadly chytrid fungus (see above).  Measures to screen the health of the toads did not pick up the fungus, because it was not known to science at the time. (http://www.sciencedaily.com/releases/2008/09/080922122427.htm). An upper respiratory disease is thought to have been transmitted to free-ranging populations of the endangered desert tortoise through the release of infected captive-bred animals (http://www.jstor.org/discover/10.2307/20095276?uid=3739704&uid=2&uid=4&uid=3739256&sid=21101904144731).

What are some of the associations between climatic conditions and infectious disease as they relate to wildlife populations and the conservation of imperiled species?  Can you include water borne diseases in your discussion, as well blood or bodily fluid borne diseases?

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(Nat Geo)

Michael:  Climate change could present some very significant challenges in terms of the spread of disease and its impact on human, domestic animal and wildlife health. With climate change, some areas are expected to become cooler and wetter and other areas are expected to become warmer and dryer and everything in between. Changing environmental conditions may favor various disease organisms or their vectors. For example, seasonally warmer conditions may facilitate the spread of mosquitoes—major vectors of diseases, such as West Nile Virus and avian malaria, just to name a few.  Aquatic environments and fauna may be particularly vulnerable to climate change.  Many fish can only live optimally within a narrow range of temperatures. Deviation from this norm could weaken immune systems and aid the spread of disease or even cause mortality outright (http://www.usgs.gov/newsroom/article.asp?ID=2267#.UUpkbk3D-Uk). A number of water borne and other pathogens, such as botulism (http://minnesota.publicradio.org/display/web/2013/03/09/environment/invasive-species-may-be-killing-loons) and red tide (http://www.highbeam.com/doc/1G2-3079000215.html), may also become considerably more frequent with a warming planet, and the impact on both terrestrial and aquatic organisms could be considerable.  Harmful algal blooms, which deplete oxygen and produce toxins in aquatic environments, are also likely to be more prevalent under warmer conditions (Miller, M., Kudela, R., and Jessup, D.A. 2012. When marine ecosystems fall ill. The Wildlife Professional 6(1): 44-48.

As a parting thought, diseases are part of our natural landscape. However, with the advent of large-scale commerce and air travel, the ease with which diseases can move around the world has increased exponentially. This is precisely why organizations such as the Centers for Disease Control, World Health Organization and others must remain forever vigilant.  Emergent diseases have important implications for conservation, as the introduction of a novel disease could easily wipe out a small, isolated population, either in nature or in captivity.  It is important to note that the factors driving species to extinction are cumulative. If a species is already heavily impacted by habitat loss, invasive species, pollution or other factors, diseases could deliver the final blow.  This is precisely why greater collaboration between wildlife veterinarians, epidemiologists, pathologists and conservation professionals will be critical going forward.  In addition, more resources, both federal and private, will need to be directed toward finding solutions.

Comments

  1. Dr. Kassa Demissie Abdi
    University
    March 13, 7:44 am

    Quite important

  2. Dr. Kassa Demissie Abdi
    Ethiopia
    March 13, 7:43 am

    I found the article to be informal and quite explanatory.