Malaria in the United States

Stephanie Richards, PhD, Medical Entomologist

What causes malaria?

Mosquitoes in the genus Anopheles are responsible for transmission of the protozoan (single-celled microorganism) that causes malaria. The species of malaria that infect humans include Plasmodium falciparum, Plasmodium vivax and Plasmodium ovale. Mosquitoes transmit the protozoan to humans and humans are considered the reservoir (e.g., if a mosquito bites an infected human, the mosquito can potentially become infected and transmit the pathogen to another human), hence traveler-imported cases into the United States can occur. A small number of cases have been reported where transmission has occurred through congenital (mother to child) means and blood transfusion.

Where is malaria?

Malaria is a global public health issue, primarily in Africa and Asia. In 2015, approximately 214 million malaria cases were documented worldwide, including 438,000 deaths (90% of cases in sub-Saharan Africa). Malaria has been difficult to control in endemic countries due to a variety of issues, including lack of infrastructure for mosquito surveillance and control programs; however, insecticide-treated bed nets and indoor residual spraying can be used to suppress mosquito populations and protect public health.

Malaria has been difficult to control in endemic countries due to a variety of issues, including lack of infrastructure for mosquito surveillance and control programs. Insecticide-treated bed nets and indoor residual spraying can be used to suppress mosquito populations and protect public health; however, insecticide resistance is of increasing concern.

Is malaria a risk to residents of the U.S.?

Widespread local transmission of the malaria protozoan has not been experienced in the U.S. since the 1950s. In the U.S., we currently primarily have traveler-imported malaria cases (approximately 1,500–2,000 malaria cases per year). For comparison, from January 1, 2015–September 28, 2016, the U.S. experienced 3,565 imported cases of Zika. However, periodically, there are locally transmitted (victim did not travel) cases of malaria. From 1957–2015, there were 63 outbreaks of locally transmitted malaria cases in the U.S. that included a traveler-imported case leading to local transmission by mosquitoes in focal areas. Hence, traveler-imported cases remain a concern for malaria.

What mosquito species can transmit the malaria protozoan in the U.S.?

In the U.S., depending on region, Anopheles quadrimaculatus, Anopheles freeborni and Anopheles pseudopunctipennis can transmit malaria protozoans.

What are the symptoms of malaria?

Common symptoms of a malaria infection include fever, chills, headache, nausea/vomiting, abdominal pain, etc. In cases where severe malaria is experienced (primarily due to Plasmodium falciparum), more severe symptoms may be observed (e.g., coma, convulsions, abnormal bleeding, renal failure).

Is there a malaria vaccine?

From 2009-2014, the RTS,S vaccine (against Plasmodium falciparum) entered phase three trials in several countries in sub-Saharan Africa. This vaccine candidate is designed to require four doses (three monthly doses and a final dose after 18 months) and is being administered to 15,000 children. Preliminary results indicate an efficacy of 18-39%; however, this depends on the age of the child when the vaccine is administered. The administration of the fourth dose appear to not be an essential component of protection. Expanded (phase four) trials must be undertaken and the World Health Organization indicates that this vaccine is being considered as a component for malaria prevention, to be used in conjunction with other existing preventative measures.

How is malaria treated?

According to the CDC, treatment for malaria depends on the species of the infecting malaria protozoan and the region in which the victim was infected. In some regions, protozoans have developed resistance to some of the anti-malaria drugs, so knowledge of the infecting parasite is helpful when designing a treatment plan.

Most drugs (e.g., choroquine, quinine) are effective in combatting the protozoans in the blood stream. In more advanced infections, when the protozoans have entered the liver, the drug called primaquine can be used to treat malaria patients; however, this drug should not be taken by pregnant women or by those who have certain medical issues (physician consultation is required).

What should travelers do before going to a malaria-endemic country?

Before traveling to a malaria-endemic country, there are precautions that can be taken for protection. According to the CDC, travelers can take oral drugs to prevent malaria infection. However, no drug is guaranteed to work and people should still protect themselves from mosquito bites by wearing insect repellent, covering their bodies with long sleeves and pants and using a bed net if screened and/or air conditioned sleeping quarters are not available.

The Centers for Disease Control and Prevention provide information about regional resistance to certain prophylactic drugs to help travelers plan their trip. Some medications should not be taken by pregnant women or individuals with certain medical conditions, hence consultation with a medical provider is essential.