Simplified Malaria Case Definitions

Suspected

• Detection of Plasmodium species by rapid diagnostic antigen testing only - BinaxNOW™ Malaria rapid diagnostic test (RDT).

Confirmed

• Detection of Plasmodium species by blood smear microscopy or polymerase chain reaction (PCR).

Classifying disease acquisition

Based on the case investigation, CDC classifies each malaria case according to the following definitions. An understanding of these distinctions may be helpful for health departments and investigators to guide follow-up and in conducting any enhanced case investigations.

• Congenital malaria: Malaria infection transmitted directly from mother to child during pregnancy or childbirth.
• Cryptic malaria: An isolated case of malaria that cannot be epidemiologically linked to additional cases, and for which epidemiologic investigation does not identify the mode of acquisition.
• Imported malaria: Malaria acquired outside the U.S. The patient must have a recent (within ~2 years) travel history to a country or territory with ongoing malaria transmission.
• Locally acquired malaria: In the U.S., a non-endemic setting without indigenous malaria transmission, locally acquired malaria cases are typically classified into two categories:
  • Induced malaria: Malaria transmission through a blood transfusion, tissue or organ transplantation, or another parenteral route, not mosquito-borne or congenital transmission.
  • Introduced: Malaria likely acquired by mosquito transmission from an imported case in an area where malaria is not a regular occurrence.
• Recrudescent malaria: A repeated attack of malaria due to the survival of malaria parasites in red blood cells can occur for any Plasmodium species and typically occurs in the first four weeks after an initial illness due to failure of the antimalarial treatment to clear all parasites. Some explanations for recrudescence include: (i) incomplete adherence to an appropriate antimalarial regimen, (ii) inappropriate use of oral antimalarials for severe illness (especially if there is hyperparasitemia, where ≥5% of red blood cells are infected), and (iii) antimalarial drug resistance.
• Relapsing malaria: P. vivax and P. ovale species can reactivate dormant liver-stage parasites (hypnozoites), resulting in a malaria relapse, typically 3 months to 3 years after the initial infection. P. falciparum and P. malariae species do not have liver hypnozoites that can be reactivated, so illnesses caused by these species do not result in relapses.