When you think about contagious diseases, the image that’s conjured is probably not a romantic one. Spanish flu, polio, the Black Death – while they’ve all shaped our history, they’re not exactly prime fodder for light-hearted musicals or sitcoms. And while many of these diseases we’ve successfully conquered as a society, we still see many diseases that do not have a cure yet.

Over the last few years, we’ve become intimately acquainted with the concept of diseases jumping from animals to humans. And while we’ve generally been more focused on what those diseases mean to us, they’ve shaped the very evolution of the species they first originated in. Over time, many species of wildlife have been fighting what’s known as an evolutionary arms race. Sometimes it’s against other wildlife species. Other times, it’s against something else, like a disease. In this battle, wildlife are constantly fighting to stay one step ahead of the disease in order to not go extinct, while the disease is fighting to spread to more and more individuals. 

While this is often a completely natural cycle, humans have involved ourselves in many of these cycles. More recently, with our ongoing encroachment and destruction of species habitats, we’ve messed up the balance between some of the species involved. Here are some examples of relationships between wildlife and their diseases that we’ve begun to play a role in. 

White-Nose Syndrome

While introducing new things to the United States has given the locals many useful resources, like a wide variety of food crops, it has also brought devastating consequences to many native species of plants and animals. One example of that is a disease called white-nose syndrome.

Although the fluffy white fungus perched on the nose of this bat may look cute at first, it is deadly. White-nose syndrome is caused by the fungus Pseudogymnoascus destructans, and it’s thought to have been brought to the US around 16 years ago from Europe. This fungus can be spread to bats by other bats, as well as cave surfaces and even human clothing. When bats hibernate during the winter, they lower their body temperatures and metabolic rates in order to last through the winter without having to wake up to feed. When a bat catches the fungus, it wakes up more often during hibernation, using up more and more valuable energy reserves each time. Eventually, the bat runs out of energy and starves to death. 

White-nose syndrome has already killed millions of bats, and as of right now there is no cure. Fortunately, conservation officials and researchers are working around the world to understand how to protect bats from this deadly disease.

Chronic Wasting Disease 

Chronic wasting disease is a disease that affects animals like deer and elk, and yes, it does sound as bad as the name suggests. Chronic wasting disease is categorized as a prion disease, which is a gradual neurodegenerative disease where the brain is fatally damaged. It can remain in the environment for a very long time, although it is also highly contagious between animals, so it usually doesn’t take long to spread. It can take over a year for symptoms like weight loss and lack of coordination to appear and take over the animal, but it is always fatal in the end. 

Chronic wasting disease is unique in the fact that it was first discovered in deer at a research facility in Colorado in the 1960s, and over time, it has spread to captive and wild populations across the continent. In wild populations, infection rates are generally low, but they can skyrocket in captive populations where animals are at a higher density. As the disease has shown up in more areas, many decisions have been made to cull animals in an attempt to prevent the spread. Until we can get chronic wasting disease under control, areas where the disease has been recorded have recommended guidelines for people managing and interacting with animals in order to prevent the disease from spreading even further. Currently, there is no cure, and although it is not shown to be transmittable to humans, it is still not recommended to consume meat from an animal that tests positive for chronic wasting disease out of caution. 

Chytridiomycosis 

While white-nose syndrome is certainly a strong contender for the worst diseases for biodiversity loss, chytridiomycosis has it beat. Chytridiomycosis is a disease caused by the fungus Batrachochytrium dendrobatidis. Chytridiomycosis has already caused the extinction of hundreds of species of amphibians, and today, it’s still threatening the existence of hundreds more. This fungus is spread through water, and easily enters an amphibian’s skin to wreak havoc. Signs of chytridiomycosis include lethargy, anorexia, shedding skin, and eventually death.

Like the other diseases here, there is not yet a known cure for this disease, and unfortunately, it can very easily be spread by humans in addition to the environment, so it is very hard to prevent. Some governments have even made it illegal to spread amphibians from one area to another, as what’s seen as a kind gesture by good-willed humans can actually result in the spread of the disease. If we aren’t able to find a way to stop this disease soon, the impact on amphibian biodiversity may be catastrophic. 

Toxoplasmosis 

While the other diseases mentioned so far have been horrifying, none of them have been infectious to humans. Our next disease, Toxoplasmosis, changes that. 

Toxoplasma gondii is a parasite that has multiple life cycle pathways, depending on who- or what- the hosts are. In the simplest pathway, the adult parasite lays eggs, or oocysts, inside of the definitive host, a feline, and then the oocysts are shed in the cat’s feces. Over time, this contaminates a nearby food or water source. Eventually, a small animal that has ingested contaminated food or water, called the intermediate host, is eaten by a cat, and the cycle repeats. In this pathway, the parasite generally doesn’t harm the feline host.

Another way that Toxoplasma gondii can spread involves a few more species, including humans. In this pathway, humans can get the parasite from coming in contact with cat feces or contaminated food or water sources containing oocysts, or they can get it from eating meat from an animal, such as a cow or pig, that has been infected as an intermediate host. In this pathway, with a human at the end, the life cycle of the parasite is broken. 

Toxoplasmosis infects millions of people, and most of the time doesn’t lead to symptoms or concerns. However, it is still considered an important foodborne illness to be aware of, especially for anyone who is pregnant or has a compromised immune system. Fortunately, there are efficient treatments for toxoplasmosis, and symptoms often go away quickly. 

There are so many wildlife diseases that we are still grappling with today. In a world where climate change is already leading to so many extinctions, finding treatments for wildlife diseases may be incredibly helpful in the fight against biodiversity loss.

---

Holly Albrecht is currently an environmental educator, and holds a Bachelors degree in Conservation Biology from the University of Arizona. She is passionate about ichthyology, herpetology, and using education and outreach to get people interested in conservation and the natural world.