Scientists worry that Rift Valley fever could reach US

Jul 21, 2004 (CIDRAP News) – Rift Valley fever (RVF), a mosquito-borne disease that can kill humans and animals, is starting to grab the attention of American scientists because it could cause devastating outbreaks in the United States.

The hemorrhagic fever virus has had a reputation for wreaking havoc with occasional outbreaks in Africa since at least the 1930s, when it was identified in Kenya. Rift Valley fever has remained largely confined to Africa since its debut, with brief forays into the Middle East.

Experts say it's hard to measure the likelihood that the disease could reach the United States. If it did, however, it could find a home, because mosquitoes found in the US are known to be suitable hosts.

The World Organization for Animal Health (OIE) includes RVF among its List A pathogens, which means countries must report outbreaks. Such an outbreak in the United States could stop exports in their hoofprints.

"It disrupts the trade with live ruminants and meat and meat products," said Will Hueston, DVM, PhD, director of the Center for Animal Health and Food Safety at the University of Minnesota in St. Paul. "All of a sudden you've got people concerned for themselves, their pets, and the food supply."

Rift Valley fever is a cunning stalker. It can lie dormant for years in mosquito eggs in small potholes in Africa. Flooding turns potholes into puddles and sparks the disease cycle. Infected adult mosquitoes feed on animals and people, passing along RVF, according to the Centers for Disease Control and Prevention (CDC). Mosquitoes take up the virus along with blood when they bite infected people and many kinds of animals.

RVF can affect a variety of livestock—including cattle, sheep, and goats—as well as bats, rodents, and dogs. It attacks the liver and causes symptoms ranging from fevers and listlessness to hemorrhage and abortion rates approaching 100% in pregnant sheep and goats. Mortality rates are 20% to 30% in adult sheep, up to 10% in adult cattle, and much higher in lambs and calves.

People can contract the virus from mosquitoes or by exposure to infected blood or fluids. Most RVF victims suffer only flu-like symptoms with fever and muscle aches. Serious illness can cause hemorrhaging, brain inflammation, liver abnormalities, vision loss, and death. Of those people who become sick enough to seek medical attention, roughly 10% to 13% die, explained T.G. Ksiazek, DVM, PhD, chief of the Special Pathogens Branch at the CDC's National Center for Infectious Diseases. But the overall mortality rate for RVF in people is less than 1%.

It's reminiscent of another mosquito-borne illness that immigrated to the United States: West Nile virus.

West Nile has been around for centuries but found the United States only in 1999, noted C. J. Peters, MD, director of biodefense and professor of pathology, microbiology, and immunology at the University of Texas Medical Branch in Galveston. Once it reached the country, it exploited bird populations and mosquitoes to create a niche, spreading across most of the country in just 5 years.

No one knows whether RVF would prove as well-suited to life in the United States as West Nile is. But if it were, it could have a worse impact on human health. The disease sickens more exposed people and kills more people per infection than West Nile, Peters said. RVF is also more easily spread by aerosol, potentially exposing more veterinarians, lab workers, farmers, or others to the disease.

Peters said RVF ranks high among diseases in its potential for being introduced to the United States, spreading, and having a big impact. Outbreaks cut a wide swath of illness and death. RVF caused the death of roughly 100,000 sheep in Kenya in 1950-51, a large outbreak among animals and humans in Egypt in 1977, and outbreaks in Saudi Arabia and Yemen in 2000, the first known cases outside of Africa, according to the CDC.

Nobody is sure how the disease reached Saudi Arabia in 2000, Ksiazek said, although that strain is the same as one implicated in RVF in East Africa in 1997 and 1998.

The virus also makes a great aerosol, Peters said. The US army was working to weaponize RVF before the US biological weapons program was stopped in the 1970s, and the Soviets were interested in the same possibility.

Preparing for the possible arrival of RVF in the United States poses several challenges, said Peters, who is working on potential RVF vaccines. He recommends three steps the United States should take now.

First, develop a vaccine that stops the spread of RVF in livestock. There is no suitable agricultural vaccine, Peters said. Quarantines and mosquito control would also be used to stem the disease. Stopping RVF in livestock, its main reservoir, is believed to control the disease, Peters said, but he cautioned that the belief is based on limited circumstances outside of the United States.

That's why a human RVF vaccine is the second step, he added. Although the US Army made a vaccine for people in the 1960s, it was impractical for wide use, Peters said.

Third, create standardized diagnostic tools and have them on hand before an outbreak, because RVF has an incubation period of hours to days.

"This thing can move," he said. "It would be good to have diagnostic tools on the shelf."

Once a vaccine is developed, other issues will affect its use, said Hueston. The challenge is how much vaccine to develop, how to keep it viable, and how to distribute it quickly.

Prevention and awareness are critical, Hueston said. Getting medical professionals to consider whether a patient's symptoms could be RVF means reinventing a medical axiom, he added: These days, when doctors hear hoofbeats, they have to think first of horses, and then consider zebras.

"The challenge is to walk that narrow line of let's educate and be prepared while at the same time not say the sky is falling," Hueston said.

In the absence of an RVF vaccine program, public health response would rely heavily on mosquito control efforts, experts said.

"Without knowing how the virus would behave here, it's hard to know [the impact of RVF]," said David Neitzel, MS, an epidemiologist in the acute disease investigation and control division of the Minnesota Department of Health. But he said many mosquitoes and black flies could transmit RVF in the United States, potentially providing hosts for the virus to take hold under varied environmental conditions.

If Rift Valley reached the Minneapolis–St. Paul area, the Metropolitan Mosquito Control District would be "pretty well geared up to control potential vectors," Neitzel said. However, no such network covers the rest of Minnesota, and creating one would be costly.

"The amount of money it takes to do an integrated pest management program would make it cost-prohibitive to do the entire state," Neitzel said.

See also:

CDC description of RVF
http://www.cdc.gov/ncidod/dvrd/spb/mnpages/dispages/rvf.htm

CIDRAP overview of RVF

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