2 Two years later, chloroquine resistance was noted in indigenous semi-immune inhabitants of Kenya infected with P. falciparum . Because of the difficulty and the limited availability of in vitro testing for resistance to antimalarial drugs, in 1967 the World Health Organization developed clinical criteria for the definition of resistance. In vitro tests for chloroquine-resistant P. falciparum correlated well with the clinical criteria, including those tests performed in the field. The latter mechanism is responsible for the majority of new cases in areas not known to be chloroquine-resistant. The low level of the drug, indiscriminate use of chloroquine and noncompliance are the main factors for induction of resistance.
Subtherapeutic doses contribute to selective parasite survival.
In Saudi Arabia, malaria endemicity is localized to the southern and southwestern parts of the country, with the highest number of cases reported from Jizan and Asir regions. Since 1982, the Saudi Ministry of Health has been monitoring the occurrence of locally transmitted chloroquine-resistant P. falciparum parasites and reporting such isolates to the WHO annually. Additionally, sensitivity testing of the vector to identify resistance to the utilized pesticides is done on a regular basis. Seventy-four percent of the cases were acquired locally. In 1992, in vitro testing of P. falciparum isolates from patients who developed malaria in Saudi Arabia documented resistance to 4-aminoquinolines for a small number of locally acquired cases.