Malaria may seem like a disease from bygone days to many people in the United States.
But a new study published today in the American Journal of Tropical Medicine and Hygiene finds that, every year, more than a thousand patients are hospitalized in the U.S. for malaria infections -- virtually all contracted in other countries -- with some turning deadly.
While malaria used to be endemic in the U.S., the disease, which is usually spread through infected mosquitoes, was effectively eradicated in the states by the 1950's, according to the study authors.
However, malaria is still a massive health problem worldwide with 212 million cases reported globally each year, causing approximately 429,000 deaths, according to the World Health Organization. People who travel outside the U.S. remain susceptible to the disease.
"The number of imported malaria cases has steadily increased in the United States," the study authors wrote. "Similar to other countries that eliminated malaria, this increase has mostly occurred among returned travelers, as well as among foreign visitors and immigrants from malaria-endemic countries."
Malaria is a parasitic disease primarily spread by mosquitoes to humans. Symptoms may appear vague at first including fever, chills and other flu-like symptoms. If untreated, the disease can be fatal. Those traveling to areas where the disease is endemic are at higher risk, though they can take prophylactic medication to reduce the chances of infection.
To understand how people in the U.S. are affected by malaria, researchers from various institutions including the University of California Los Angeles, Los Angeles County Department of Public Health and the U.S. Centers for Disease Control and Prevention (CDC) studied national patient data. They found an average of 1,469 annual hospitalizations for malaria in the U.S. between 2000 to 2014.
Researchers found that between 2000 to 2014 there were 22,029 total malaria-related hospitalizations; 4,823 of the cases were designated as "severe," with 182 deaths reported. They used hospital discharge data from the U.S. Department of Health and Human Services' Nationwide Inpatient Sample, which contains about 20 percent of hospital discharge records nationwide.
This group of malaria patients often required multiple days in the hospital. They spent 4.36 days, on average, with a mean bill of $25,789 for all hospitalizations. Men accounted for about 60 percent of these malaria cases and more than than half, 52.5 percent, were black. The highest number of cases -- a combined 71 percent -- were reported in the southern and northeast regions of the U.S.
The actual number of malaria cases may be higher, since some people may not come to the hospital for treatment. The authors estimate an average of 2,128 people may have malaria each year in the U.S.
High numbers of imported malaria increase the chance of a local outbreak, as well. Between 1957 and 2015 there have been "63 outbreaks of locally transmitted mosquito-borne malaria," according to the CDC.
"There are elements of this that are perhaps surprising," said Dr. William Schaffner, infectious disease expert at Vanderbilt University Medical School. "Over 22,000 admissions for malaria in hospitals in the U.S. ... I wouldn't have thought it was that large."
Because many doctors learn about malaria in medical school, but rarely see live cases, Schaffner said, diagnosing the disease at an early stage can be difficult. Patients coming into the ER for treatment may be "the first case they've ever seen."
And though few people contract malaria within the U.S., the study authors note that remains a challenge for treatment.
"Despite the reduction of malaria incidence in developing countries, malaria continues to be an important public health problem in the United States," the authors said. "Despite its elimination in the early 1950s, and the disease burden remains substantial."