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Wild animals experience intense and thrilling lives. Between their moments of rest or scavenging, they are either stalking prey or avoiding being hunted themselves. Yet, alongside the immediate dangers of poaching and habitat destruction, a quieter threat is emerging: disease. Some illnesses can ravage wild animal populations, leaving destruction in their wake.
10. Ebola
Ebola has gained worldwide notoriety, especially due to recent outbreaks in Africa. First identified in 1976 in the Democratic Republic of Congo, Ebola was once known as Ebola hemorrhagic fever. Bats and other animals are suspected of spreading this lethal disease, which can easily be transmitted from one individual to another through bodily fluids.
Upon infection, the initial symptoms include a fever and sore throat. Soon after, more severe signs will appear: bleeding both externally and internally, organ failure affecting the kidneys and liver, and, ultimately, death. The disease has devastated numerous human populations, but it has also found its way into a species closely related to us: the great apes.
Africa boasts a rich diversity of primates, with large populations of chimpanzees in Guinea and western lowland gorillas across Central Africa. In just four decades, Ebola has claimed the lives of about one-third of these species, pushing the gorillas toward the brink of extinction.
Years of repeated epidemics have led to entire populations disappearing. While the global focus has rightfully been on the human toll, researchers have been working to understand the devastating effects of Ebola on the great apes, often overlooked in the chaos. With so much attention on saving humanity, the question arises: will we have the energy and resources left to save our distant animal relatives? Many experts remain hopeful.
9. Canine Distemper
Dog owners are likely familiar with this one. Canine distemper is a highly dangerous viral infection with no known cure. It is extremely contagious, spreading through the air or even by simple touch. Initially, the virus hides in the body, causing a high fever, before it attacks more aggressively, ultimately leading to the canine's death.
Dogs affected by this virus show symptoms such as bloodshot eyes, fatigue, coughing, and vomiting. As it progresses through the nervous system, the dog may experience seizures and paralysis. This highlights the importance of keeping your pets properly vaccinated.
Despite its name, canine distemper doesn’t limit itself to dogs. A variety of wild carnivores, including big cats, wolves, and foxes, have also fallen victim to it. Siberian tigers, already threatened by habitat destruction and poaching, are facing the additional danger of this virus, which may speed up their extinction.
In one instance, a population of these tigers plummeted from 38 individuals down to just nine. Even those in zoos are not safe from the disease, as outbreaks are occurring in feline enclosures. The virus affects their brains, making them fearless, and if the seizures don’t kill them, poachers and villagers often do.
The virus frequently leads to brain damage, causing affected wild tigers to venture closer to human settlements and roads. It is also causing a decline in African wild dog populations. Its ability to spread quickly and easily makes it especially dangerous to breeding packs.
8. Sylvatic Plague
Most people know that the bubonic plague devastated Europe in the 1340s, causing the deaths of around 25 million people. However, what many are unaware of is that a similar disease, sylvatic plague, has recently emerged and destroyed many prairie dog colonies on the Fort Belknap Reservation.
Over 3,000 acres have been affected by this outbreak. While it spells disaster for the prairie dogs, it also has dire consequences for the predators that rely on them. The black-footed ferret, one of the most endangered mammals, depends entirely on prairie dogs. They either use their burrows for shelter or hunt them for food to feed their young. If the prairie dogs perish, the ferrets may face the same fate.
Thankfully, there may be hope in tackling this issue. Prairie dogs have a fondness for peanut butter, which is being used as a surprising weapon against the plague. Researchers are mixing vaccines into peanut butter and spreading it throughout the prairie dog habitat. In addition to vaccinating the wild ferrets and spraying flea-killing insecticide around the burrows, all that can be done now is to hope that the ecosystem will recover and thrive.
7. Chronic Wasting Disease
Populations of deer and elk that were once on the verge of extinction have made remarkable strides. The Key deer, a species native to Florida, were down to just 27 individuals in 1957, but their numbers have now grown to approximately 800. However, other species, like the Virginia white-tailed deer, continue to struggle due to factors like car accidents, human interaction, and a disease known as chronic wasting disease (CWD).
CWD, similar to mad cow disease, targets the brain and is always fatal. The afflicted animals undergo severe physical decline and exhibit behavioral changes. They may walk in repetitive patterns, with their heads and ears hanging low, while drinking excessively and salivating. Unfortunately, the disease is always fatal, but quarantine and depopulation measures can help control its spread.
6. Whirling Disease
Fish in rivers face an unseen threat—an incapacitating disease that forces them to swim in endless circles. This condition mainly affects species in the trout and salmon families, deforming their cartilage and twisting their bodies, making it difficult for them to feed or evade predators.
In more vulnerable fish, the disease can be fatal. The culprit, Myxobolus cerebralis, is a parasite that first arrived in America from Europe in the 1950s and has since spread across 25 states. Once the parasite infests a stream, it becomes nearly impossible to remove without causing extensive damage to the ecosystem.
Whirling disease has had a catastrophic impact on coldwater fisheries in North America. Although this parasite is tough, anglers and boaters can take steps to help curb its spread. It's crucial not to transport live fish between lakes, and equipment should be thoroughly cleaned of debris before leaving infected areas. While whirling disease is not as deadly as Ebola or canine distemper, there is still hope for protecting North America's trout and salmon populations.
5. PBFD Virus
PBFD virus (also known as psittacine beak and feather disease) targets parrots, cockatoos, macaws, and lovebirds. In addition to domestic birds, this virus also affects more endangered species, such as the New Caledonian rainbow lorikeets, which spread the disease to neighboring birds as they fly.
This is a devastating disease. Young birds rarely survive, but if they do, the virus can linger in their feathers and be passed on to others. It spreads rapidly. Direct contact, inhalation, contaminated surfaces, and feather dust are all potential means of transmission, leading to catastrophic effects.
The symptoms of this disfiguring virus are unmistakable. Birds suffer irreversible feather damage, molting, deformed beaks, lesions, and weight loss. There is currently no cure. However, as with all diseases affecting animals or humans, researchers are tirelessly working to find solutions. Conservationists are dedicated to finding ways to protect endangered wild bird species, and vaccines may not be too far in the future.
4. Sarcoptic Mange
Sarcoptic mange is caused by mites that burrow into the skin, leading to intense itching and hair loss. Bacterial infections often follow. If not treated, this disease results in death after severe weight loss and damage to vital organs.
The disease spreads through direct contact with infected animals or their carcasses, as well as in burrows previously inhabited by an infected animal. Foxes often carry the disease, marked by cracked, bare skin and scaly patches.
However, it also affects wombats, lynxes, wolves, and many other animals. Sarcoptic mange can devastate small, at-risk wildlife populations. In fact, it has eradicated all of the red foxes on Bornholm Island in Denmark.
3. RHDV
Australia has faced a persistent plague for many years, but it's not one of the biblical variety—it's a rabbit plague. Capable of breeding large numbers of offspring annually, wild rabbits are wreaking havoc on the continent’s natural balance by consuming vast amounts of vegetation, driving away other species, and damaging the environment.
Rabbits were first introduced to Australia in 1788 when settlers aimed to recreate a European landscape. By the 1920s, their population had exploded to 10 billion. Efforts to control them with fire and poison were unsuccessful, but the rabbit hemorrhagic disease virus (RHDV) has proven to be the game changer. It has already wiped out 60% of the rabbit population.
This was a turning point. With fewer rabbits, native plants started to recover, and animals such as kangaroos and even endangered rodents began to return. As ecologist Reece Pedler put it, “It’s become pretty clear that something major has changed.”
The absence of overgrazing by rabbits allowed vegetation to flourish in the dry landscape, offering shelter to native wildlife. Additionally, the decline of rabbits reduced the numbers of cats and foxes that had preyed upon them, further aiding the recovery of endangered species. It seems that not all viruses harm wildlife—RHDV has had a surprisingly positive impact.
2. Endangered Parasites
When the topic of endangered species arises, parasites are rarely considered. They aren't exactly endearing or friendly, and, more often than not, they are carriers of fatal diseases—or at least most of them are.
Kakapos, those elusive, flightless green birds from New Zealand, now number fewer than 200. With dedicated breeding and conservation efforts, they are slowly making a comeback from the edge of extinction. Sadly, it's too late for Stringopotaenia psittacea, a tapeworm, to avoid disappearing forever.
As the number of its kakapo hosts dwindles, it has vanished forever. Lice from California condors and parasites from the now-extinct spotted kiwis have also met untimely ends. But why fret? After all, parasites are typically known to bring about disease, suffering, and death.
However, what many don't realize is that not all parasitic species are harmful. Take the Eurasian oystercatcher chicks, which perished after being exposed to fewer parasites. This has led scientists to theorize that some level of parasitic presence might actually contribute to building immunity. Moreover, early exposure to these parasites can help animals resist them later in life, whereas lack of exposure could cause issues with reproduction.
So, if S. psittacea were to make a comeback, it could play a role in bolstering the kakapo population. To date, of all parasitic species, only one has earned a spot on the International Union for Conservation of Nature's red list: the critically endangered pygmy hog-sucking louse. While they may not be as endearing as pandas or seals, perhaps parasites deserve more recognition.
1. Herpes
Herpes is already taking a toll on the human population, but what happens when it affects wild creatures? For the past few years, 17 percent of sea lions have been diagnosed with a cancer that strikes females in the cervix and males in the penis and prostate.
Surprisingly, the culprit is a disease many are familiar with. This particular herpes virus causes cancer that is already deadly in its own right. It paralyzes sea lions by attacking their spinal cords, leading to their death or washing up on shorelines.
The Marine Mammal Center in Sausalito has already witnessed numerous fatalities, including one involving a male sea lion with its lower body paralyzed and hind flippers swollen due to cancer. “It’s truly heartbreaking to witness,” said Dr. Frances Gulland, the center's director of veterinary science.
Sea lions aren’t the only marine animals at risk. Several beluga whales have been found with intestinal tumors linked to industrial pollutants. Contaminants found in their fish and blubber may be to blame and can also be transferred from mother to offspring.
“These creatures are apex predators,” explained Gulland. “Like humans, they consume anchovies, squid, salmon, and mussels—they act as sentinels for human health because they share our ecosystems and prey. We might witness effects in sea lions before they show up in humans—they could serve as an early warning.”
