Make the Diagnosis: Malaria
Prior Probability of Malaria
For clinicians who have not traveled to endemic areas, the magnitude of malaria as a health problem is difficult to grasp. There are an estimated 250 million annual cases of malaria infection worldwide, and malaria kills nearly 1 million people per year, of whom most are children.1 The heterogeneous prevalence in existing studies requires that clinicians in endemic areas have an appreciation for the local prevalence. Travelers to potentially endemic areas should get advice about the presence of malaria and the potential need for antimalarial prophylactic medicine from the Centers for Disease Control and Prevention (CDC, http://www.cdc.gov/malaria/travelers/country_table/a.html), but health care workers in endemic areas will need to get information from local health experts to understand the point prevalence. Unless cases are validated with objective tests, clinical diagnosis alone may lead to overdiagnosis.
Similar to the situation for endemic malaria, the probability of malaria depends on the areas visited. While existing studies report prevalence of 12%-35% in patients with fever after travel to endemic areas, the prevalence will vary as a function of local outbreaks of other disease. A more detailed interactive application for local areas is available from the CDC (http://cdc-malaria.ncsa.uiuc.edu/).
Populations in Whom Malaria Should Be Considered
Malaria should always be considered in febrile patients in endemic areas or in those with compatible symptoms who have returned recently from an endemic area. In 2010, imported malaria in the United States was most commonly attributable to P falciparum (58%) or P vivax (19%) and acquired in Africa (65%) or Asia (19%).Of cases with data available, 75% of patients with imported malaria had not taken a chemoprophylactic medication recommended by the CDC,.2 The symptoms of malaria occur during the blood stage, about 7-14 days after the bite of an infected mosquito. Among nearly 30 000 returning travelers, the median number of days to presentation after return from a malaria-endemic area was 13 days (95% CI, 12-14 days) in patients with malaria and 21 days (95% CI, 21-22 days) in patients without malaria. However, this depends on the type of malaria infection with P ovale and P vivax infected patients presenting 42-44 days after travel.3
Assessing the Likelihood of Malaria
No symptom, including the classic findings of fever, chills, rigors, and headaches,is specifically diagnostic for malaria. A diverse range of clinical syndromes results from malaria infections, and the type of syndrome results from interactions between the infecting parasite species, the immune status of the host, and the use and timing of antimalarial drugs.4 Nonimmune patients (such as children in endemic areas and travelers typically) manifest an undifferentiated febrile illness with fluctuating paroxysms of fever and rigors, accompanied by malaise, headache, myalgias, arthralgias, ...