Wild animal suffering video course – Unit 3

Diseases in nature are widespread, and they interact negatively with other factors such as weather conditions and malnutrition. In order to survive, animals often have to hide their symptoms and they aren’t always able to adequately rest and recuperate. Here, you’ll learn about common diseases and parasites and how they impact the lives of animals in the wild.

View other related videos in our course about wild animal suffering here
Visit the main page of the wild animal suffering video course here


Related pages on the topics covered in this video:

Diseases in nature
Antagonism in nature: Interspecific conflict


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Extended content of the video with references:

Also available as a chapter of our companion ebook to the video course Introduction to wild animal suffering: A guide to the issues

Diseases and parasitism

Diseases in nature

We now consider another very significant source of suffering and premature death for animals, which is disease. To understand how harmful disease can be to animals, think of the immense suffering that disease caused to human beings before the advent of modern medicine. This is the situation of animals in the wild. The harms caused by disease are worsened by lack of access to treatment and, sometimes, by lack of opportunity to rest and recuperate. In addition to their debilitating effects on the body’s ability to function and recover, illness and disease can increase the negative effects of environmental conditions and other stressors faced by wild animals. The result can be increased suffering and death.1

There are so many diseases that affect nonhuman animals in nature that they cannot all be listed here. Some of them are illnesses humans can suffer from too, like flu, pneumonia, tuberculosis, cholera, Ebola, anthrax, salmonella, diphtheria, and rabies.2 Cancer is also common in both land and marine animals.Some populations of whales suffer from cancer at similar rates to humans.3 Other common diseases that can infect animals living in the wild are distemper, chronic wasting disease, African swine fever, and a variety of fungal infections.4


Diseases in invertebrates

Most people don’t think much about how invertebrates might suffer from disease, but they contract bacterial, viral, and fungal infections just like other animals. Some are very specific to the animals they infect and don’t spread to vertebrates, but they can be treated similarly, with vaccines, antibiotics, and antifungals.5 Here are some common diseases found in land-dwelling and marine invertebrates.

Brucellosis is prevalent among the wild elk and bison populations living in the Greater Yellowstone Area. It has been estimated that more than 14,000 of those animals in the area are infected.6 Since Brucellosis can be transmitted between species, these animals in Yellowstone act as “reservoir” species for the disease. To combat this, a vaccine (RB51) has been developed for the bison population in Yellowstone.


Black death in butterflies

One major disease that affects butterflies is nuclear polyhedrosis virus, or the black death. It’s called this because affected animals become lethargic and their bodies start to decay, turning black.Their insides liquefy and ooze out of their decaying body. The virus usually strikes in the caterpillar phase. It causes a great deal of stress to the caterpillar, who will refuse to eat and may regurgitate food. The virus can take up to three days to kill the caterpillar.31The infected drops of the liquefied body spread easily onto leaves and is further spread by parasites, infecting the caterpillars who eat those leaves.7


Cricket paralysis virus

A widespread disease afflicting crickets is known as cricket paralysis virus. Infected crickets become malnourished, have trouble jumping, lose coordination, and then their legs become paralyzed and they fall on their backs, where they lie for a few days before dying. It can also infect other insects, and similar strains infect bees and flies.8

There is more interest in treating great apes because their species is generally highly valued, and also because of recent threats to human health that have spread through contact with or consumption of infected apes. Other animals may not receive the same attention, but they could be treated similarly.


Lobster shell disease

Lobsters can contract a common disease known simply as shell disease. Healthy lobsters have a slippery protective layer that prevents the shell from being eroded by bacteria. With shell disease, this barrier disappears, allowing the shell to start to erode. The disease itself is not always lethal, but it can cause the lobster distress and weakness that increases vulnerability to other harms such as injury.9


Diseases and infections in vertebrates

More is known about diseases that affect vertebrates. Vertebrate diseases tend to be easier to study because the animals are typically larger and many vertebrate diseases are known to be transmissible between a variety of vertebrates, including humans and domesticated animals. The diseases below are a sampling of common diseases in vertebrates.


Cholera in birds

Avian cholera is a common bacterial disease in birds living in both temperate and arctic climates. Many birds carry the disease, but it only becomes active when the birds are physically or emotionally stressed. Very cold weather or high water forcing birds in temperate regions to leave their homes are common stressors that can bring out the disease in infected birds. It causes weight loss,mucous discharge, diarrhea, and rapid breathing. It frequently leads to pneumonia. It can attack the liver, spleen, and skin and cause arthritis due to inflammation. Avian cholera can have a very high mortality rate, especially when it first spreads through a colony. It is spread by direct contact and by ingestion of contaminated water or soil.10



Distemper is a viral disease related to measles that attacks the gastrointestinal, respiratory, and nervous systems of mammals. It is commonly associated with dogs but also affects many animals in the wild, including raccoons, foxes, wild cats, monkeys, and seals. Infected animals can exhibit behaviors similar to those caused by rabies, including drooling, circling behavior, chewing fits, nonresponsiveness to the environment, and loss of fear of humans and other animals. It can cause fever, vomiting, convulsions, and paralysis. It is usually fatal. Those who survive may have permanent neurological damage.11


Skin diseases in amphibians, reptiles, and fishes

Amphibians are susceptible to deadly skin diseases, such as fungal infections and ranavirus. The aquatic fungal infection chytridiomycosis is one of the deadliest pathogens on record. It afflicts frogs, salamanders, and other amphibians in wet climates. The fungus eats through an animal’s skin, causes metabolic changes, and finally kills the animal by triggering cardiac arrest. In addition to the skin, lesions develop on multiple internal organs and muscles. It spreads continually from immune amphibians to those who are vulnerable.12


Sickness behaviors

Disease is more widespread in nature than many people realize. One of the reasons people misjudge the extent to which it affects animals living in the wild is that many animals have evolved to avoid showing signs of illness. Animals who look weak or vulnerable are prime targets for predators. Moreover, those who live in groups may lose social status or be abandoned and left to fend for themselves when they are least able to. Alternatively, sometimes animals selectively exhibit sickness behaviors, such as lethargy and sleepiness. This happens when the sickness behaviors are not caused by the illness itself, but rather by conserving energy to fight off an illness. Depending on the time of year and other circumstances, showing signs of illness might reduce opportunities to reproduce or make it impossible to defend valuable territory. An animal might take more time to rest and recover outside of breeding season, rather than trying to defend their territory. During breeding season, they might use their energy to reproduce and defend their nests or dens rather than on recovery efforts.13

Therefore, an animal can be suffering greatly from a disease or illness that we cannot recognize without performing medical checks. As more research is undertaken on how animals are affected by diseases in the wild, our knowledge in this area continues to grow.14 In the meantime, it is worth noting that there are recognizable behavioral signs in some animals who are experiencing fevers, including lethargy, decreased appetite, and reduced grooming, though as mentioned earlier, animals may be able to choose not to exhibit these behaviors if the cost is too high.15 Humans can also learn a lot by observing larger animals in hospitals or doing autopsies, and there are increasingly sensitive methods of non invasively detecting signs of illness in the wild.

Some animals are hard to observe at all, such as small animals who spend most of their lives hiding underground and extremely numerous tiny invertebrates. Marine animals can also be difficult to study because of their numbers and also because it’s more difficult to study them non-invasively. As a result, the amount of suffering caused by diseases in the wild is much greater than many people would imagine.

There is another often fatal threat to animals’ health that sometimes overlaps with disease. This is parasitism.


Parasitism and parasitoidism

Approximately half of all species of animals and plants are parasitic at some stage in their lifecycle; few, if any, species are not infested by any parasites. Many parasites are microbial pathogens that can harm their hosts by causing disease. Others are larger organisms, including animals. Some parasites cause little harm to animals. Some, however, cause pain and weaken them. Parasitoids ultimately kill the animals they infest.

The actions of a parasite can cause fatigue, making it harder for the host to find food and avoid predators. Some parasites castrate their hosts, leaving their other systems intact so that the host can survive, diverting the energy from reproduction into sustaining the parasite. Some parasites cause behavioral changes in their hosts (particularly intermediate hosts) that make them more susceptible to predators (final hosts).16 Intermediatehosts provide an environment for the immature parasite to develop and grow, and final hosts are where sexually mature parasites reproduce.

For example, there’s a parasitic fluke that reproduces inside of its final hosts, which are grazing ruminants like cows, and its eggs are excreted in the host’s feces. The first intermediate hosts are common snails, who consume the feces and become infested by the larval parasites. An infested snail forms cysts around the parasites, which he then excretes.These cysts are consumed by the second intermediate host: an ant. The parasite is able to take control of the ant’s behavior, forcing him to climb to the top of a blade of grass where he will be eaten by a grazing animal, in which the now mature parasites can reproduce.

Some parasites are called hyperparasites because they feed on other parasites. They are not to be confused with superparasites, which live in large populations within a single host (such as wasps whose larvae are parasites of caterpillars).17 The following are some examples of conditions caused by parasites that are prevalent among wild animals.


Sarcoptic mange

Sarcoptic mange is a skin disease caused by burrowing parasitic mites. The infestation causes an allergic reaction to the mite, resulting in intense scratching and biting. It affects several species of nonhuman mammals, including dogs, cats, coyotes, bears, and wombats. Wombats are especially badly affected by mange. It is believed that this is due to conditions inside wombat burrows being especially conducive to the survival and transmission of sarcoptic mites. Infested wombats get bloody lesions, lose hair, their skin becomes crusted and infected, and their eyes and ears become crusted over. The disease can cause blindness or deafness. In severe cases, it can lead to a slow and lingering death. This disease is believed to be one of the most painful ones afflicting nonhuman animals.


Parasitic infestations in birds


Wild birds commonly suffer from trichomonosis, a disease caused by parasites. It can be a debilitating and sometimes deadly disease that usually affects the mouth, esophagus, crop, and glandular stomach of birds as well as other organs such as the liver. Other extensively reported parasites in birds are tracheal worms. These obstruct the trachea and bronchi, resulting in major respiratory distress. In response, infested birds usually cough, sneeze, and shake their heads trying to dislodge the parasites. They may lose body mass, display anemia, and often die of starvation. Heartworms, reported in swans and geese, are similarly debilitating.


Common parasites among reptiles and amphibians

Protozoan infections

Haemopoteus, a protozoan parasite transmitted by blood-sucking insects, has been reported in various species of reptiles and amphibians, mostly turtles and tortoises. It has debilitating effects on skeletal muscles and other organs, such as the liver. One protozoan parasitic infection causes colitis, abscesses of the liver and other organs, and sometimes death. Spirorchiid trematodes infect turtles and snails, affecting major arteries and the heart. Other protozoan infections are reported in a variety of reptiles, mostly snakes and lizards, causing regurgitation, diarrhea, weight loss, and enlargement of the gastric mucosa.18


Parasitoidism among invertebrates

Ichneumonidae and Braconidae wasps

Among the best known examples of parasitoidism among invertebrates is the case of Ichneumonidae and Braconidae wasps. These animals lay their eggs in the bodies of other insects, such as caterpillars and ants. Some of these wasps are hyperparasites, laying their eggs in the bodies of other parasitic wasps. When the eggs hatch, the larvae start to eat their host alive, leaving the host’s vital organs intact until the end. Only after the edible nonvital parts of the host have been eaten is the host finally killed, probably after having endured great pain.19


1 Beldomenico, P. M.; Telfer, S.; Gebert, S.; Lukomski, L.; Bennett, M. & Begon, M. (2008) “Poor condition and infection: A vicious circle in natural populations”, Proceedings of the Royal Society of London B: Biological Sciences, 275, pp. 1753-1759 [accessed on 18 November 2019].

2 Simpson, V. R. (2002) “Wild animals as reservoirs of infectious diseases in the UK”, The Veterinary Journal, 163, pp. 128-146; Gortázar, C.; Ferroglio, E.; Höfle, U.; Wobeser, G. A. (2005) Essentials of disease in wild animals, New York: John Wiley and Sons; Frölich, K. & Vicente, J. (2007) “Diseases shared between wildlife and livestock: A European perspective”, European Journal of Wild Research, 53, pp. 241-256; Williams, E. S. & Barker, I. K. (eds.) (2008 [2001]) Infectious diseases of wild mammals, 3rd ed., New York: John Wiley and Sons; Martin, C.; Pastoret, P. P.; Brochier, B.; Humblet, M. F. & Saegerman, C. (2011) “A survey of the transmission of infectious diseases/infections between wild and domestic ungulates in Europe”, Veterinary Research, 42 (1) [accessed on 22 November 2019]; Washington State Department of Health (2019) “Animal transmitted diseases”, Illness and Disease, Washington State Department of Health [accessed on 26June 2019].

3 Martineau, D.; Lemberger, K.; Dallaire, A.; Labelle, L.; Lipscomb, T. P.; Pascal, M. & Mikaelian, I. (2002) “Cancer in wildlife, a case study: Beluga from the St. Lawrence estuary, Québec, Canada”, Environmental Health Perspectives, 110, pp.285-292; Albuquerque, T. A. F.; Drummond do Val, L.; Doherty, A. & de Magalhães, J. P. (2018) “From humans to hydra: Patterns of cancer across the tree of life”, Biological Reviews, 93, pp. 1715-1734.

4 Cole, R. A. & Friend, M. (1999) “Field manual of wildlife diseases: Parasites and parisitic diseases”, in Milton, F. & Franson, J. C. (eds.) Field manual of wildlife diseases: General field procedures and diseases of birds, Washington, D. C.: U. S. Geological Survey, pp. 188-258; Williams, E. S. & Barker, I. K. (eds.) (2008 [2001]) Infectious diseases of wild mammals, New York: John Wiley and Sons; Dantas-Torres, F.; Chomel, B. B. & Otranto, D. (2012) “Ticks and tick-borne diseases: A One Health perspective”, Trends in Parasitology, 28, pp. 437-446; Wobeser, G. A. (2013) Investigation and management of disease in wild animals, Dordrecht: Springer.

5 Raukko, E. (2018) “The first-ever insect vaccine PrimeBEE helps bees stay healthy”, University of Helsinki, 31.10.2018 [accessed on 18 august 2019].

6 Hadley, D. (2019) “Why are monarch caterpillars turning black?”, ThougtCo, July 12 [accessed on 14 August 2019].

7 Stairs, G. R. (1966) “Transmission of virus in tent caterpillar populations”, Entomological Society of Canada, 98, pp. 1100-1104.

8 Liu, K.; Li, Y.; Jousset, F.-X.; Zadori, Z.; Szelei, J.; Yu, Q.; Pham, H. T.; Lépine, F.; Bergoin, M. & Tijssen, P. (2011) “The Acheta domesticus densovirus, isolated from the European house cricket, has evolved an expression strategy unique among parvoviruses”, Journal of Virology, 85, pp. 10069-10078; Szelei, J.; Woodring, J.; Goettel, M. S.; Duke,G.; Jousset, F.-X.; Liu, K. Y.; Zadori, Z.; Li, Y.; Styer, E.; Boucias, D. G.; Kleespies, R. G.; Bergoin,M. & Tijssen, P. (2011) “Susceptibility of North-American and European crickets to Acheta domesticus densovirus (AdDNV) and associated epizootics”, Journal of Invertebrate Pathology, 106, pp. 394-399.

9 Groner, M. L.; Shields, J. D.; Landers, D. F.; Swenarton, J. & Hoenig, J.M. (2018) “Rising temperatures, molting phenology, and epizootic shell disease in the American lobster”, The American Naturalist,192, pp. E163-E177.

10 Iverson, S. A; Gilchrest, H. G.; Soos, C.; Buttler, I. I.; Harms, N. J. & Forbes, M. R. (2016) “Injecting epidemiology into population viability analysis:Avian cholera transmission dynamics at an arctic seabird colony”, Journal of Animal Ecology, 85, pp. 1481-1490; Sander, J. E. (2019) “Fowl cholera”, Merck Manual: Veterinary Manual, Nov [accessed on 8 December 2019].

11 Kameo, Y.; Nagao, Y.; Nishio, Y.; Shimoda, H.; Nakano, H.; Suzuki, K.; Une,Y.; Sato, H.; Shimojima, M. & Maeda, K. (2012) “Epizootic canine distemper virus infection among wild mammals”, Veterinary Microbiology, 154, pp. 222-229; Williams, E. S. & Barker, I. K. (eds.) (2008 [2001]) Infectious diseases of wild mammals, 3rd ed., New York: John Wiley and Sons, part 1.

12 Schelle, B. C.; Pasmans, F.; Skerratt, L. F.; Berger, L.; Martel, A.; Beukema, W.; Acevedo, A. A.; Burrowes, P. A.; Carvalho, T.; Catenazzi, A.; De la Riva, I.; Fisher, M. C.; Flechas, S. V.; Foster, C. N.; Frías-Álvarez, P.; Garner, T. W. J.; Gratwicke, B.; Guayasamin, J. M.; Hirschfeld, M.; Kolby, J. E.; Kosch, T. A.; La Marca, E.; Lindenmayer, D. B.; Lips, K. R.; Longo, A. V.; Maneyro, R.; McDonald, C. A.; Mendelson, J., III; Palacios-Rodriguez, P.; Parra-Olea, G.;Richards-Zawacki, C. L.; Rödel,M.-O.; Rovito, S. M.; Soto-Azat, C.; Toledo, L. F.; Voyles, J.; Weldon, C.; Whitfield, S. M.; Wilkinson, M.; Zamudio, K. R. & Canessa, S.(2019) “Amphibian fungal panzootic causes catastrophic and ongoing loss of biodiversity”, Science, 363, pp. 1459-1463.

13 Lopes, P. C (2014) “When is it socially acceptable to feel sick?”, Proceedings of the Royal Society of London B: Biological Sciences, 281, 20140218.

14 Barlow, N. D. (1995) “Critical evaluation of wildlife disease models”, in Grenfell, B. T. & Dobson, A. P. (eds.) Ecology of infectious diseases in natural populations, Cambridge: Cambridge University Press, pp. 230-259; Branscum, A. J.; Gardner,I. A. & Johnson, W. O. (2004) “Bayesian modeling of animal-and herd-level prevalences”, Preventive Veterinary Medicine, 66, pp. 101-112; Nusser, S. M.; Clark, W. R.; Otis, D. L. & Huang, L. (2008) “Sampling considerations for disease surveillance in wildlife populations”, Journal of Wildlife Management, 72, pp. 52-60; McClintock, B. T.; Nichols, J. D.; Bailey, L. L.; MacKenzie, D. I.; Kendall, W. & Franklin, A. B. (2010) “Seeking a second opinion: Uncertainty in disease ecology”, Ecology Letters, 13, pp. 659-674; Camacho, M.; Hernández, J. M.; Lima-Barbero, J. F. & Höfle, U. (2016) “Use of wildlife rehabilitation centres in pathogen surveillance: A case study in white storks (Ciconia ciconia)”, Preventive Veterinary Medicine, 130, pp. 106-111.

15 Hart, B. L. (1988) “Biological basis of behavior of sick animals”, Neuroscience & Biobehavioral Reviews,12, pp. 123-137

16 Gopko, M.; Mikheev, V. N. & Taskinen, J. (2017) “Deterioration of basic components of the anti-predator behavior in fish harboring eye fluke larvae”, Behavioral Ecology and Sociobiology, 71.

17 Van Alphen, J. J. & Visser, M. E. (1990) “Superparasitismas an adaptive strategy for insect parasitoids”, Annual Review of Entomology, 35, pp. 59-79; Sullivan, D. J. & Völkl, W. (1999) “Hyperparasitism: Multitrophic ecology and behaviour”, Annual Review of Entomology, 44, pp. 291-315.

18 Jovani, R.; Amo, L.; Arriero, E.; Krone, O.; Marzal, A.; Shurulinkov, P.; Tomás, G.; Sol, D.; Hagen, J.; López, P.; Martín, J.; Navarro, C. & Torres, J. (2004) “Double gametocyte infections in apicomplexan parasites of birds and reptiles”, Parasitology Research, 94, pp. 155-157; Tkach, V. V.;Snyder, S. D.; Vaughan, J. A. (2009) “A new species of blood fluke (Digenea: Spirorchiidae) from the Malayan Box turtle, Cuora amboinensis (Cryptodira: Geomydidae) in Thailand”, Journal of Parasitology, 95, pp. 743-746; Chen, H.; Kuo, R. J.; Chang, T. C.; Hus, C. K.; Bray, R. A. & Cheng, I. J. (2012) “Fluke (Spirorchiidae) infections in sea turtles stranded on Taiwan: Prevalence and pathology”, Journal of Parasitology, 98, pp. 437-439.

19 Weng, J. L. & Barrantes Montero, G. (2007) “Natural history and larval behavior of the parasitoid Zatypota petronae (Hymenoptera: Ichneumonidae)”, Journal of Hymenoptera Research, 16, pp. 327-336; Komatsu, T. & Konishi, K. (2010) “Parasitic behaviors of two ant parasitoid wasps (Ichneumonidae: Hybrizontinae)”, Sociobiology, 56, pp. 575-584.

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