Most illnesses affecting humans don’t appear out of thin air; they often have deep roots in the animal kingdom, evolving and spreading through animal populations for millennia before making the jump to humans. Approximately 60 percent of known human infectious diseases are zoonotic, meaning they originate in animals, according to research in Faculty Reviews. While the role of animals like rats in spreading diseases such as the plague is well-known, the sources of many human illnesses are far more unexpected.
Understanding the Origins of Emerging Diseases
A significant challenge in addressing new and emerging diseases lies in their unpredictable origins. Researchers are still working to pinpoint the source of the virus responsible for COVID-19 in humans, for example. Current evidence suggests horseshoe bats likely harbored a related coronavirus initially, which may have then spread to intermediate hosts like civets or raccoon dogs before infecting humans, as detailed in a study published by Wiley.
“Every time there is a new pathogen, it comes from a host we didn’t expect,” explains Barbara Han, a disease ecologist at the Cary Institute of Ecosystem Studies. To effectively monitor and respond to emerging diseases, Han emphasizes the critical importance of studying wildlife health, as animals are frequently the source of diseases that ultimately affect humans. Without this understanding, tracing the origins of a new disease can be incredibly difficult. “Finding that smoking gun is really challenging,” she says.
Here are several examples of surprising animal reservoirs for diseases that continue to impact human health:
1. Chagas Disease: Beyond the Kissing Bug
Chagas disease, primarily spread by kissing bugs in Latin America and parts of the southern U.S., is typically considered a vector-borne illness. However, recent research indicates that Virginia opossums may also play a role in its transmission, particularly in areas like Florida. A 2022 study in PLOS Neglected Tropical Diseases found that opossums can transmit the disease, caused by the parasite Trypanosoma cruzi, to their offspring. Because opossums frequently inhabit urban environments and possess a gland secretion that can harbor the parasite, they have the potential to contaminate surrounding areas. While most human infections still occur through kissing bugs, this discovery raises the possibility of non-vector transmission, according to the University of Florida Emerging Pathogens Institute.
2. West Nile Virus: A Wider Host Range
While commonly associated with mosquitoes, West Nile virus can also infect a diverse range of hosts, including dozens of bird species, as well as alligators, llamas, and wolves, according to a study in Springer Nature. “Some of them will die, some of them won’t, but the virus load can be high,” Han notes. Although direct transmission from these unusual hosts to humans hasn’t been definitively reported, tracking such links is complex. Fortunately, humans are generally considered dead-end hosts for West Nile, meaning they are unlikely to transmit the virus to others, even through mosquito bites, according to the Centers for Disease Control and Prevention (CDC).
3. Leprosy: From Humans to Armadillos and Back Again
Leprosy, caused by the bacterium Mycobacterium leprae, is typically spread from person to person. However, the disease also established itself in nine-banded armadillos in North and South America after being brought by European settlers. For centuries, leprosy has persisted within armadillo populations, and evidence now suggests it can be transmitted back to humans. The University of Florida Emerging Pathogens Institute reports that humans handling soil contaminated by armadillo burrows and waste can contract the disease. Interestingly, research in Current Biology reveals that red squirrels served as a reservoir for leprosy in Europe as far back as the Middle Ages.
4. Tularemia: The Threat of Rabbit Fever
Despite their cuddly appearance, rabbits can carry the potentially deadly bacterium Francisella tularensis, causing tularemia, also known as rabbit fever. Transmission can occur directly from rabbits, rodents, or even bears to humans, as detailed in a study in Zoonoses and Public Health. Ticks and deer flies can also act as intermediaries, spreading the disease through infected blood. The CDC classifies tularemia as highly infectious, ranging from mild to life-threatening, and it has even been investigated for potential use as a bioterrorism agent, according to Han.
5. Flesh-Eating Vibrio vulnificus: A Coastal Danger
Consuming raw or undercooked shellfish carries several risks, including infection with Vibrio vulnificus. This bacterium can cause a severe, potentially fatal flesh-eating disease even through contact with infected shellfish via a cut or wound. Humans can also contract the pathogen by swimming in contaminated water or handling contaminated shellfish like oysters.
Disclaimer: This article is for informational purposes only and does not constitute medical advice.
Article Sources: Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:
Related reading
