(Reuters Health) - U.S. public health officials declared victory over malaria in 1951, but the mosquito-borne disease continues to infect and kill American travelers, a new study shows.

From 2000 through 2014, more than 22,000 people with malaria were hospitalized in the U.S., including 182 who died from it, according to estimates published in the American Journal of Tropical Medicine and Hygiene.

Using hospitalization data, researchers found an average of more than 2,100 U.S. malaria cases a year – nearly double the number previously reported.

Most of those hospitalized for malaria in the U.S. are men. A majority are black, most likely foreign-born individuals who traveled abroad, frequently to their countries of origin, where they often fail to realize they are no longer immune to the deadly parasitic disease, lead author Diana Khuu said in a phone interview.

“We need to encourage travelers to seek pre-travel advice and to use personal protective measures against mosquitoes and anti-malarial medications when they travel to malarious countries,” said Khuu, an epidemiologist at the UCLA Fielding School of Public Health.

“Because a lot of people are not using these preventive measures, it’s resulting in a lot of hospitalizations and in half a billion dollars in hospital charges from 2000 to 2014,” she said.

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Insecticide-treated bed nets have been credited with spurring a 41 percent drop in worldwide malaria incidence and a 62 percent drop in worldwide malaria deaths over the past 15 years.

Despite the progress, malaria infected an estimated 212 million people and killed an estimated 429,000 in 2015. Nearly 90 percent of cases were in children under age 5 in Africa, where malaria claims a child’s life every two minutes, according to a statement from the National Institutes of Health this week on World Malaria Day, April 25.

Meanwhile, the number of imported malaria cases has steadily increased in the U.S.

Dr. Stephen Gluckman of the Perelman School of Medicine at the University of Pennsylvania in Philadelphia sees six to 10 patients a week who travel to malaria-risk areas in Africa, India, Southeast Asia and parts of South America.

He attributes the increase in U.S. malaria cases to an increase in Americans traveling to developing parts of the world where the disease continues to threaten lives.

He advises people traveling outside the U.S. to consult a “bona fide” travel clinic before departing. “There are several options and significant nuances in advice for preventing malaria,” he said in an email.

Checking the U.S. Centers for Disease Control and Prevention website for travelers (bit.ly/2k74gQd) also can be helpful, but Gluckman, who was not involved with the study, believes individualized advice is superior.

He warned that people infected with malaria might carry the disease for months or years before falling ill. In the meantime, an infected pregnant woman could pass malaria to her unborn child, and a mosquito could bite an unsuspecting infected person and pass malaria on when it bites another person.

Moreover, Gluckman warned, there is “a very large group of at-risk people who rarely get screened.”

“These people grew up in developing parts of the world but now live in the U.S.,” he said. “When returning to visit their country of origin, they very rarely seek any prevention advice because they grew up in a community that did not have access to prevention.”

“They are difficult to reach but clearly could be accessed by the proper public health programs in the communities where they live in the U.S.A.,” he said.

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People who grew up in malaria-endemic areas, especially those who were exposed as children, do gain some natural protection, but they still can get malaria and therefore need to take preventive steps. In addition, spending long periods in a malaria-free area, like the U.S., can reduce immunity and render people more susceptible to infection.

“You can develop immunity,” Khuu said. “But after you move away, that immunity wanes. If they go back to visit, they might not use the preventive measures because they might think they’re still immune.”

And why do men appear to be more vulnerable, comprising 60 percent of the U.S. cases? Khuu believes they are less likely to seek pre-travel advice, less likely to use mosquito repellent and take anti-malarial medication, and their sweat might be more attractive to mosquitoes.

SOURCE: bit.ly/2oH8XyC American Journal of Tropical Medicine and Hygiene, online April 24, 2017.

(This story changes "foreigners" to "foreign-born individuals" in para 4.)