These Are the World’s Most Dangerous Emerging Pathogens, According to WHO

You may not know their names, but health officials are concerned about the epidemic potential of these illnesses

Marburg Virus
Marburg virus is one of the pathogens the WHO recently identified as most dangerous. Springer Medizin/Science Photo Library/Corbis

International officials recently gathered to discuss one of the biggest threats facing humanity—and this wasn’t the Paris climate talks. As Science’s Kai Kupferschmidt reports, the setting was Geneva, Switzerland and the task was the selection of a shortlist of the world’s most dangerous emerging pathogens. These diseases are considered by a World Health Organization (WHO) committee of clinicians and scientists to be the pathogens “likely to cause severe outbreaks in the near future, and for which few or no medical countermeasures exist.” Here’s the WHO’s list, and what you should know about these scary diseases:

Crimean Congo hemorrhagic fever

This tick-borne fever got its name from the Crimea, where it first emerged in 1944, and the Congo, where it spread in 1969. Now, it can be found all over the world, though it primarily occurs in Asia. The disease is often misnamed “Asian Ebola virus” for its fast-moving effects, which include enlargement of the liver, fever, aching muscles and vomiting.

Outbreak News Today’s Chris Whitehouse writes that CCHF is currently spreading across India, where agricultural workers are often exposed to diseased, tick-bearing animals. According to the WHO, outbreaks of the disease can have a fatality rate of up to 40 percent. There is no vaccine for CCHF, but at least one has been shown to be effective in animals.

Ebola virus disease

It’s no surprise to see Ebola virus disease on the list—it has been ravaging African countries for decades, with widespread outbreaks throughout West Africa and the recent resurgence in Liberia. Also known as Ebola hemorrhagic fever, the disease has an average fatality rate of 50 percent, but has been as high as 90 percent in some outbreaks.

Though it is still unclear exactly how the virus is transmitted, scientists believe that bats serve as a natural “reservoir” for Ebola, which is then transmitted through some form of contact. There are no current licensed vaccines, but clinical trials for at least two are underway.

Marburg hemorrhagic fever

In 1967, a mysterious disease broke out in Europe, killing laboratory workers who had been exposed to monkeys from Uganda. The cause, Marburg virus, was named after the German city where it was first detected and is a filovirus—a family of viruses that include Ebola.

Marburg virus has broken out only sporadically since the 1960s, but occurs in people who have spent time in caves frequented by Rousettus bats. Marburg causes a rash, malaise and bleeding and is often misdiagnosed. There is no current vaccine or treatment.

Lassa fever 

First diagnosed in Benin, Lassa fever can be difficult for doctors to diagnose and only becomes symptomatic in 20 percent of the people who become infected, according to the WHO. When it does strike, patients can move from mild effects like a slight fever to, in more severe cases, hemorrhaging, encephalitis and shock. But the fever’s most devastating and common complication is deafness. About a third of all Lassa patients lose some or all of their hearing.

Lassa fever is primarily found in West Africa and is contracted when people come into contact with the waste of infected Mastomys rats or the bodily fluids of those with the disease. Though the antiviral drug ribavirin may be effective in Lassa fever cases, there is no current vaccine.

MERS and SARS coronavirus diseases

Middle East Respiratory Syndrome (MERS) and Severe Acute Respiratory Syndrome (SARS) have had their fair share of media coverage. They’re members of the coronavirus family—viruses that usually cause upper respiratory illness. Though transmission seems to come from infected camels, the diseases are both easy to catch from infected peoples’ coughs or sneezes.

Both conditions emerged relatively recently. SARS broke out in Asia in 2003, but the global outbreak was contained and no cases have been reported since 2004. The news isn’t that great concerning MERS: The disease, which started in Saudi Arabia in 2012, broke out again in South Korea this year. The WHO reports that 36 percent of reported patients die. Health officials tell SciDevNet that it’s unlikely a vaccine will be developed anytime soon.

Nipah and Rift Valley fever

The final two entries on the WHO’s list are viruses from animals—Nipah virus infection and Rift Valley fever. Nipah was first identified in 1998 when Malaysian pig farmers fell ill. To stop the outbreak, the Malaysian government ordered euthanasia over a million pigs. Even so, the virus later showed up in Bangladesh and India. Nipah causes brain inflammation, convulsions and even personality changes. 

Rift Valley fever originated with Kenyan sheep farmers in 1931 and has since been identified in outbreaks throughout Africa. The disease is spread by handling diseased animal tissue, drinking infected milk or being bitten by infected mosquitos. However, the WHO has never documented a case of human-to-human transmission. The disease causes symptoms similar to meningitis and can be hard to detect in its early stages. Though most people get a milder version of the disease, others aren’t so lucky. Around eight percent of patients get ocular disease, brain inflammation and could eventually die. Neither Nipah nor Rift Valley fever have currently-approved human vaccines.

Though the diseases on this list were identified as the most likely to cause widespread epidemics, the WHO also designated three other diseases as “serious”: chikungunya, severe fever with thrombocytopenia syndrome, and Zica. Diseases like Malaria and HIV/AIDS were not included because of already established disease control and research into treatment and prevention.

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