In a new study published in the journal Nature Communications, researchers provides the first scientific evidence that another family of deadly viruses may have “jumped” from fruit bats to humans in Africa, prompting experts to caution that earlier surveillance is needed to prevent another epidemic.
The family of viruses in question is known as henipaviruses, which cause rare infections – sometimes harmless, but sometimes extremely deadly. Some strains have mortality rates close to 90 percent, which is similar to those for Ebola.
Two members of the henipavirus family, Nipah and Hendra, were first identified in Australia and Southeast Asia around 20 years ago. Thought to be confined to undeveloped locations near bat habitats in these areas, a recent study found that the common fruit bat species across Africa are carrying several related viruses.
Zoonosis is the process by which a virus is spread from one species to another, and researchers employ this method to understand how viruses become threats.
Though most diseases infect animals first, serving as long-term homes for a virus – as in the case of HIV, SARS, influenza and Ebola – often a “reservoir species” such as a bat or pig is not harmed by the virus. Sometimes, however, the virus becomes deadly when it “jumps” into humans.
The reason for this change is that our immune systems are different and are unable to sufficiently control the virus.
Dr. Benhur Lee, lead study author and the Ward-Coleman Chair in Microbiology at the Icahn School of Medicine at Mount Sinai in New York, NY, says the ongoing Ebola outbreak in West Africa underscores the need for better methods of tracking emerging and re-emerging infectious diseases:
“The tragedy of Ebola, which also jumped into humans from bats in Africa, argues that we must heighten our surveillance of viruses on the verge of spillover from animals into humans. HIV, SARS and West Nile virus were also unknown until they emerged.”
Findings suggest a bat-to-human jump
In their latest study, Dr. Lee and colleagues investigated whether the henipaviruses have been jumping into humans in Africa – as humans spread into traditional bat habitats – by examining sets of bat and human blood serum samples for indications of previous exposure.
To do this, the team created a sensitive, specific test that employs antibodies – proteins that play an important role in how the human immune system recognizes viruses.
Once a person is exposed to a virus, he or she will forever have antibodies that are “primed” to recognize that virus. As such, antibody tests can show whether populations have been exposed to a specific virus, even after recovery.
Using almost 500 human serum samples collected from 13 rural locations across southern Cameroon in Africa, the researchers detected antibodies that signified previous henipavirus exposure in 48% of bat samples and 3-4% of 227 of the human blood samples.
Dr. Lee says their findings show there has been a bat-to-human jump in Africa:
“Our study found the first evidence – written in the immune cells of people living in our African study area – that humans have been exposed to henipaviruses, and also that the risk of infection goes up with exposure to a bat’s bodily fluids.”
He adds that their results support the theory that henipavirus infections are more common than previously thought, suggesting they were hidden as undiagnosed brain infections or blamed on malaria, yellow fever or typhoid, which can cause comparable fevers and are endemic in many parts of Africa.
Contact with bodily fluids of infected animals a likely source of infection
Though only seven people tested positive for henipavirus antibodies from their sample, the researchers found that three of these seven individuals came from the same village. With three people testing positive from a village where only 12 people took part in the study, that makes the potential exposure rate 25%.
What’s more, the three positive individuals were all bat butchers, suggesting close contact with bodily fluids of infected animals is required for cross-species infection.
“It’s important to note that much broader surveillance efforts will be required before we can make generalizations about the risks of henipavirus spillover across Africa,” says Dr. Lee.
“On the other hand,” he adds, “for a disease that is not supposed to exist, a 3-4% henipavirus infection rate in a village would be highly significant if seen in other villages. The prevalence of HIV in Cameroon is 5%, and it’s perceived globally as an African epidemic.”
Brad Schneider, PhD, director of Laboratory Sciences at Metabiota, a company that created the surveillance system used to detect spillovers in the study, warns:
”The high mortality rates seen with some strains of henipaviruses in humans and the ability of the viruses to spread efficiently among non-reservoir hosts such as pigs and horses, pose a threat to global health and economy.
Active surveillance is critical to developing strategies that help prevent a widespread outbreak.”