Most people infected with mosquito-borne West Nile virus don’t experience any symptoms at all. However, the tiny percentage of cases that do end up in the hospital total hundreds of millions of dollars in medical costs and lost productivity.

Published earlier this week in the American Journal of Tropical Medicine and Hygiene, researchers estimated that the cumulative cost of reported hospitalized cases of West Nile virus between 1999 and 2012 totaled $778 million. The study is the first to examine the economic burden of West Nile on infected patients who were hospitalized with conditions such as fever, encephalitis, meningitis and acute flaccid paralysis.

West Nile virus first showed up in the United States in 1999 and has since spread to every state in the country, except for Hawaii and Alaska. Less than one percent of the more than 37,000 documented cases of West Nile virus have resulted in serious neuroinvasive disease such as encephalitis and meningitis, which affect the brain and surrounding tissue. The virus is typically spread when a mosquito feeds from an infected bird and then passes it on to humans. There are no vaccines nor antivirals to prevent or treat the disease, so public health efforts typically focus on reducing the mosquito population and educating people about how to protect themselves against mosquito bites.

Lead study author J. Erin Staples told me the findings — which she characterized as a conservative estimate — could help local officials determine the cost effectiveness of public health and vector control interventions.

“By gathering short- and long-term costs, these data can be used by policy-makers, public health officials, researchers and those in academia and industry to better understand the impact of West Nile virus in their location,” said Staples, a medical epidemiologist within the Arboviral Disease Branch of the Centers for Disease Control and Prevention.

To conduct the study, Staples and her colleagues examined the initial hospital costs as well as lost productivity costs among 80 patients hospitalized with West Nile virus in Colorado in 2003. Of the 80 patients, 38 were followed for five years to gather long-term costs and six patients died. Patients who developed acute flaccid paralysis (partial to full-body paralysis) due to West Nile virus incurred the largest initial and long-term costs. People who experienced paralysis or developed encephalitis had “significantly” higher costs than those who had meningitis or fever. For example, the median initial cost for a person with West Nile virus-induced paralysis was nearly $21,000, while a patient with non-neuroinvasive West Nile virus disease experienced an average expenditure of about $4,500.

In terms of lost productivity, higher costs were found among those with meningitis than among those with encephalitis or acute flaccid paralysis. (Though Staples noted that because encephalitis patients tended to be older, lost productivity costs were subsequently lower.) When extrapolated to determine a nationwide average between 1999 and 2012, researchers came to a total cost of $778 million in health care expenditures and lost productivity. Staples emphasized that the study only calculated costs for people who ended up in a hospital, noting that among people who develop a fever and whose cases are reported to CDC, 80 percent don’t get hospitalized.

“The proportion of people (with West Nile virus) who get a fever is very underestimated in our data,” she told me.

In addition, Staples and her study co-authors — Manjunath Shankar, James Sejvar, Martin Meltzer and Marc Fischer — wrote that the “extrapolated costs do not include the costs that were incurred by public health or those related to mosquito control efforts, which are often 25 percent to 50 percent of the total cost during an outbreak response.”

Forecasting when, where and in what severity the vector-borne disease will hit a region is quite challenging (we’ve written previously about what makes West Nile virus control so difficult here). The spread of West Nile virus is affected by so many factors — weather, human behavior, the type and number of birds in an area — that Staples said “we can’t predict how many cases we’ll see — we don’t know if it’ll be a bad year or a large year.” That uncertainty is reflected in the caseload numbers reported to CDC as well: Total West Nile virus cases went from more than 9,800 in 2003 to about 1,300 in 2008 to about 5,600 in 2012. But like most public health issues, surveillance is still key. (CDC uses a system known as ArboNET to track West Nile virus.)

“National surveillance efforts are critical to determining where and when outbreaks of mosquito or tick-borne diseases occur,” Staples said in an American Society of Tropical Medicine and Hygiene news release about the new economic burden study. “Being able to react quickly to an outbreak and put in place preventive measures…is essential to limiting both the public health threat and the long-term economic cost of vector-borne infectious diseases.”

While CDC provides Epidemiology and Laboratory Capacity grants to states to support vector control activities such as surveillance and testing of animal reservoirs, that funding has dramatically decreased from more than $34 million in 2002 to $9.2 million in 2012. In a 2013 survey of health departments conducted by the American Mosquito Control Association, 88 percent of respondents said current funding levels are limiting their ability to deal with diseases such as West Nile virus.

“If you have good surveillance systems in place, you can better know when (a disease) may spill over into humans, and it can help you act more quickly and prevent more cases,” Staples told me. “Staying diligent is the best way to mitigate West Nile virus cases and deaths.”

To access the full West Nile cost burden study, click here.

Kim Krisberg is a freelance public health writer living in Austin, Texas, and has been writing about public health for more than a decade.