Plasmodium malariae in Ahsa region, Saudi Arabia, 1994-1995

From January 1994 through May 1995, 45 cases of Plasmodium malariae were reported from Ahsa region. Ahsa was malarious before 1970 and since the vector, Anopheles stephensi is still present in the area, an epidemiologic investigation was begun. Initially, 17 smears were reviewed by a reference laboratory and the parasitologic diagnosis was reconfirmed on all smears. Nine cases were excluded from the investigation because eight were asymptomatic and were detected through residence permit screening immediately after arrival in Saudi Arabia, and one was from Gizan and could not be found for an interview.

The 36 symptomatic P. malariae cases occurred without seasonal pattern. They were scattered throughout the population centers of the Ahsa region. No cases occurred among permanent Saudi, Ahsa residents, or in nationals of non-malarious countries. All but two were nonprofessional workers.

Twenty-two cases (61%) had onset of fever in the first 70 days after arrival in Saudi Arabia, with 13 cases occurring before the first 20 days (minimum incubation period for P. malariae). Of these 22 cases, 19 (86 %) were Indian. For each case, five controls were selected from the residence permit list of the malaria center and were matched with case-persons by nationality, sex and age.

According to their passports, all Indian-cases had passed through Bombay in comparison to 73% of Indian control-workers selected at random from the resident permit list (p=<0.05,0R=16). Case-patients reported median staying from one to 90 days (median seven days) in Bombay compared to one to 14 days (median two days) for the controls who passed through Bombay (p<0.001, Kruskal Wallis test). These case-persons were not associated with any particular home state in India. The remaining 14 cases had onset after 70 days of arrival. We were able to contact and interview seven cases and 70 matched controls. P. malariae was associated with sleeping in an open field (odds ratio [OR]=16, confidence interval [CI]=1.2-222), and a preference for injection for medical treatment (OR=undefined, CI=3.7-infinity).

The course of P. malariae is not unduly severe but its long incubation period and persistence in a human host is notorious. Because recrudescence may occur as long as 52 years after exposure, it is difficult to determine if individual cases are acquired locally. Several findings in this investigation suggest that P. malariae with onset more than 70 days after arrival was recrudescent and not locally acquired in Ahsa. The cases did not cluster in time or location. There were no cases among permanent Ahsa residents or in nationals of non malarious countries. The time of exposure did not coincide with the expected season of transmission of malaria in Ahsa. Although two indirect indicators of vector borne or accidental malaria local transmission (sleeping outdoors and preference for injections) were suggested by the case control study, interviews were done up to 16 months after the illness onset, and results were subject to recall bias.

The association of imported cases (onset under 70 days after arrival) with Bombay, suggest foci of transmission in Bombay, India. The continuing importation and possible local transmission of P. malariae will require improved surveillance with prompt epidemiologic and case investigations to identify and control introduced malaria transmission in Ahsa.