SEATTLE -- Local health officials are still investigating how a Pierce County woman who hadn't traveled recently might have contracted malaria -- but however it happened, the situation involved an unusual sequence of events."
The case is rare for a few reasons, said Dr. James Miller, health officer of the Tacoma-Pierce County Health Department, which announced the news Wednesday. It's Washington state's first locally acquired case, meaning the woman was most likely bitten here by a malaria-infected mosquito, which would have gotten malaria after biting someone else with the disease, who had traveled recently and perhaps contracted malaria abroad.
If that were the case, it would be a "very unusual sequence of events" Miller said, "which is why we're doing this more extensive investigation for this case than we would typically for a travel-acquired malaria case."
The mosquito-borne disease does not spread from person to person, nor does it spread between mosquitoes, so risk to the general public is low, he said.
The patient is receiving treatment, Miller said, but declined to give more details about her condition. He noted it's common for people with malaria to be hospitalized, as the disease can cause very serious infection.
The chance of the malaria-infected mosquito biting another person in the county is "pretty low," Miller reiterated. "But of course, malaria is a serious disease. That's why we're taking this seriously and doing a full investigation."
The investigation includes, in the meantime, a plan to work with the state Department of Health to set up mosquito traps all over the county and collect data from them. It's unlikely they'll find the malaria-infected mosquito that bit the Pierce County patient, Miller acknowledged, but the data is useful in gauging how many other mosquitoes might be carrying the malaria parasite.
"The main purpose is really just to get a sense of ... the burden for the population of Anopheles mosquitoes, which is the set of species that can transmit malaria, Miller said.
Part of the investigation also looks at other patients in Washington recently diagnosed with malaria, to see if there could be any potential connections, he said.
In the United States, malaria cases are usually linked to travel, mainly from sub-Saharan African countries, and regions of South America and Southeast Asia, according to the Centers for Disease Control and Prevention. Anopheles mosquitoes, which are in Pierce County and Washington, can transmit malaria across the U.S. if they bite someone infected with the disease.
The U.S. sees about 2,000 to 2,500 cases per year, and Washington sees 20 to 70 reports of malaria in travelers each year, according to the state Department of Health. In 2023, the U.S. saw its first locally acquired malaria case in 20 years.
Ten cases were reported across four states -- Florida, Texas, Maryland and Arkansas -- from May to October 2023.
So far this year, 19 cases of malaria have been reported in Washington state, according to DOH.
Fortunately, Miller said, mosquito populations are starting to shrink in Pierce County and throughout the state at this point in the year. Mosquitoes like warm weather and being near water to breed, so they prefer early summer, he said.
When temperatures fall, populations also decline, Miller said.
Still, people should try to take basic precautions to avoid getting bit, he said, including: wearing bug spray, removing standing water in yards and considering long pants or long sleeves if in a place there are mosquitoes.
And if traveling to a country where there's risk of malaria, think about precautionary measures such as taking medication before, during and after your trip.
The disease typically causes fever, chills, body aches, headaches and fatigue. Without treatment, it can be fatal, Miller said.
If people recognize these symptoms or suspect malaria, health officials recommend seeking early testing and treatment. The disease can be treated with antimalarial drugs.