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