Epidemiology of Malaria

Malaria is a potentially fatal but ultimately preventable and treatable vector-borne parasitic disease that is characterized by cycling fever and chills, and generalized flu-like symptoms. Significant findings on physical exam are frequently anemia, jaundice, and hepatosplenomegaly.

According to 2008 estimates from the World Health Organization, there are 350 to 500 million cases of malaria worldwide each year, and malaria claims an estimated one million lives annually, with the majority of deaths occurring in children under 5 years of age. Spread by Anopheles mosquitoes, malaria thrives in tropical zones with warm temperatures and humid conditions. It is endemic in 100 countries in Africa, Latin America, Asia and the Middle East, and Sub-Saharan Africa is disproportionately affected with 90% of the total deaths. Malaria is strongly linked to poverty. Inadequate housing, poor water and sanitation systems, displacement, and lack of access to adequate medical care are significant risk factors for morbidity and mortality.

There are four main species of the malaria parasite (Plasmodium falciparum, P. vivax, P. ovale, and P. malariae), each with a distinct global distribution. For the sake of simplicity, I will concentrate on P. falciparum which is responsible for the majority of deaths worldwide. P. falciparium is sometimes referred to as "the malignant malaria," although the other species are certainly not benign, despite their historical classification. Travelers to endemic areas must be provided with specific counseling and chemoprophylaxis depending on the prevalent species.

Individuals at greatest risk include:

• Children under the age of 5 years
• Travellers to malarial zones from non-endemic areas who have little or no immunity
• Non-immune and semi-immune pregnant women
• People living with HIV/AIDS (PLWHA)

Malaria in pregnancy results in high rates of miscarriage and WHO estimates that it is responsible for 10% of maternal deaths worldwide. Even in the case of subclinical disease, severe anemia and impaired fetal growth can result. Additionally, the sequestration of malaria parasites in the placenta increases the risk of maternal to child transmission of HIV. Other groups at risk include individuals with sickle cell disease, although sickle cell trait provides some protection against malaria.

Due to targeted vector-control programs which relied largely on DDT, malaria has been eradicated from the US and Europe. (See map of previously malarious areas of the US. In 1914 there were 600,000 cases in the US!) While the cycle of transmission within the US has been broken, the CDC reported 1,505 cases of malaria in the US in 2007. This was not significantly different from the number of cases reported in 2006. Of these, the vast majority occurred among persons who had contracted malaria while travelling to endemic areas. However, in one case, transmission occurred through blood transfusion in a patient with transfusion-dependent sickle cell disease. (The implicated donor was identified, and while he confirmed that he had a history of malarial infection, he declined treatment.) In previous years, there have also been confirmed cases of congenital transmission. One death occurred after infection with P. vivax.

In the US, malaria is classified as a notifiable disease, and confirmed (positive blood film, rapid diagnostic test, or PCR) cases must be reported to local and state health departments which conduct the case investigation. Findings are then submitted to the CDC through the National Malaria Surveillance System, and the National Notifiable Diseases Surveillance System. My article for this week is from the CDC's MMWR series:

Centers for Disease Control and Prevention. "Malaria Surveillance - United States, 2007." MMWR 2009;58(2).

In 2007, P. falciparum was identified in a majority of cases (43.4%), while the number of cases in which the infecting species was unreported or undetermined was surprisingly high (30.2%). Of the cases with known residential status, 73.6% occurred among U.S. residents, and 26.4% occurred among residents of other countries. The majority of patients (both US residents and non-residents) reported that their reason for travel was visiting friends and relatives (VFR), highlighting the importance of this group. The highest estimated relative case rates (using estimated number of US travelers to endemic countries) appeared among travelers from West Africa. In 80.1% of cases, clinical malaria appeared within 30 days of arrival in the US.

Strikingly, it was found that among the cases of US residents (and for whom chemoprophylaxis information was available), 62.9% had not followed a chemoprophylactic drug regimen recommended by CDC for the area to which they had traveled. However, it was not clear in this report whether they had initially been prescribed an incorrect regimen, or had been non-adherent to an appropriately prescribed regimen. Of the total number of cases in women, 4.5% of those occurred in pregnant women, none of whom had adhered to a complete chemoprophylactic regimen. Problems with adherence make it difficult to identify areas of emerging drug resistance (by identifying cases that occurred in spite of chemoprophylaxis) as well as increasing the likelihood that resistance will develop.

This article did not discuss delays in treatment due to missed diagnosis, or the percentage of cases who were asked to report a travel history in their first contact with a healthcare provider. Nor was there an analysis of the appropriateness of clinical therapies once the diagnosis was made.

However, the article stressed the importance of prompt treatment, and the potentially life-threatening complications that can develop within a short period of time from the onset of symptoms. While malaria should always be considered in febrile patients with a travel history to malaria endemic areas, the CDC recommends that malaria should also be included in the differential diagnosis for all cases of FUO, regardless of the travel history. The article also drives home the importance of correct chemophrophylaxis (especially in at-risk individuals) and preventive measures to avoid contracting malaria in the first place!


From the CDC Website:

Health care providers needing assistance with diagnosis or management of suspected cases of malaria should call the CDC Malaria Hotline: 770-488-7788 (M-F, 9 am - 5 pm, eastern time). Emergency consultation after hours, call: 770-488-7100 and request to speak with a CDC Malaria Branch clinician.