How do diseases jump from infecting animals to infecting humans?

A zoonotic disease is a pathogen that passes from an animal to a human, such as lyme disease from ticks.
A zoonotic disease is a pathogen that passes from an animal to a human, such as lyme disease from ticks.
Lauree Feldman/Getty Images

"Zoonotic." It sounds like the dream state one might enter while hanging out with animals. Unfortunately, it's more sinister than that. A zoonotic disease is a pathogen that passes from an animal to a human. Think rabies, or, to go way back in history, the bubonic plague. But how does a disease jump an entire species?

Zoonotic diseases spread to humans via contact with animals or insects that might be carrying various bacteria, parasites, viruses or fungi that can then take up residence in unsuspecting people. About 75 to 80 percent of infectious diseases affecting humans these days are zoonotic in origin, and more than 200 zoonotic diseases exist that are capable of transferring from animals to humans [sources: CDC, Watson and Hastwell]. Here are just a few examples of diseases that spread from animals or insects to humans:

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  • Lyme disease
  • Malaria
  • Dengue fever
  • Anthrax
  • Ebola virus
  • Lyme disease
  • E. coli
  • West Nile virus
  • Rocky Mountain spotted fever

Disease ecologists call the transfer of disease from animal to human a "spillover." Scientists still have much to discover about why and how these spillovers happen, but they know a few things. A virus has one simple goal: survival. Sometimes a virus can make the spillover from an animal to a human host easily, as in the case of rabies or the bubonic plague. The virus has always been the same, even in between species.

However, many viruses must mutate — change their genetic structure — to jump to a new species and make themselves at home. In the past, most animal viruses were content to stay in the animal population because they simply had nowhere else to go. But increased urbanization means that humans have encroached on animal and insect territories. Plant and animal habitats are being destroyed, species are disappearing, and biosystems and genomes are mutating and adapting. We live closer to animals — and other humans — than ever before. And that means viruses spread faster.

An animal or insect can transmit disease in countless ways. Here are a few examples:

  • Being bitten be an infected insect, like a tick or mosquito
  • Being bitten or scratched by an infected animal, like a dog or bat
  • Eating improperly prepared meat from an infected animal
  • Inhaling pathogens from an infected animal
  • Handling an infected animal or its excrement

Next, we'll look at some specific cases of viruses jumping from animals to humans.

Zoonotic Viruses on the Move

In our everyday lives, the most common way for zoonotic disease to spread is through mosquito or tick bites. Here's a gross look at B. burgdorferi, the bacteria that cause Lyme Disease.
In our everyday lives, the most common way for zoonotic disease to spread is through mosquito or tick bites. Here's a gross look at B. burgdorferi, the bacteria that cause Lyme Disease.
© CDC/PHIL/Corbis

Scientists have been studying Middle Eastern respiratory (MERS) virus, a new virus that first presented in an older Saudi Arabian man in June 2012 [source: Brown]. He died from pneumonia and respiratory failure with symptoms resembling SARS, but the viruses didn't match.

The race was on to identify the source of this new virus. After sequencing the virus's genome, scientists found its closest relative belonged to a virus found in bats in rural Saudi Arabia. There are several areas where bats roost near people and feed on nearby fruit. So, it's highly likely the MERS virus moved from bats to humans, probably transmitted through excrement on or near the fruit trees. As of fall 2014, MERS had infected more than 800 people and killed nearly 300 [source: Watson and Hastwell].

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Another virus that made the jump from animals to humans is HIV. Scientists believe the spillover began when humans began populating the forests in West Africa to find food. Demand for meat led to slaughter of chimpanzees, a reservoir for HIV (meaning that they carry the virus but aren't affected by it). However, that virus can jump to a different species and then cause complete chaos. People who ate chimpanzee meat, or slaughtered the chimps and got the infected blood in their own wounds, contracted the virus. The virus moved slowly and was not identified until the outbreak in the late 1980s in the United States [source: Laurance].

Scientists and researchers keep an eye on on current and emerging infectious diseases through the World Health Organization (WHO) in an attempt to stop the spread of dangerous animal-to-human viruses before they become epidemic.

Some animals and insects are more likely to carry infectious disease than others. These include:

The best way to avoid contracting a virus from an animal is prevention — taking steps to avoid the risk of infection. Everyone should take care to prevent catching a virus, but people with compromised immune systems must be even more vigilant. This includes children under age 5, the elderly, pregnant women and those with autoimmune system diseases.

In our everyday lives, the most common way for zoonotic disease to spread is through mosquito or tick bites. Using window screens, insect repellent and protective clothing helps reduce the chance of bites. So does removing any standing water around your home, as it's a breeding ground for mosquitoes.

Other places people may come into contact with infected animals include petting zoos, animal displays, farms, state fairs and wooded areas. Be sure to wash hands thoroughly if you touch any animals, and avoid direct contact with them out in nature. If you are bitten or scratched by an animal you don't know, get medical attention immediately and report the animal to authorities.

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Sources

  • Brown, Kristen V. "How a virus spreads from animals to humans." SFGate.com. Aug. 14, 2013. (Sept. 24, 2014) http://www.sfgate.com/health/article/How-a-virus-spreads-from-animals-to-humans-4729807.php
  • Centers for Disease Control and Prevention. "CDC and Zoonotic Disease." March 19, 2014. (Sept. 24, 2014) http://www.cdc.gov/24-7/cdcfastfacts/zoonotic.html
  • Laurance, Jeremy. "Deadly animal diseases poised to infect humans." The Independent. Jan. 4, 2010. (Sept. 24, 2014) http://www.independent.co.uk/news/science/deadly-animal-diseases-poised-to-infect-humans-1856777.html
  • Smith, Tara C. "What Is the World's Most Dangerous Animal?" Slate. Dec. 4, 2012. (Sept. 24, 2014) http://www.slate.com/articles/health_and_science/pandemics/2012/12/origins_of_new_diseases_zoonotic_pandemics_come_from_bats_birds_monkeys.html
  • Watson, Charles and Annie Hastwell. "How often do deadly diseases jump from animals to humans?" ABC Science. July 30, 2014. (Sept. 24, 2014) http://www.abc.net.au/science/articles/2014/07/30/4056579.htm