Malaria outbreak in Gellwa, Al-Baha, Saudi Arabia, January to March 1996

From January 1 to March 31, 1996, 476 confirmed Plasmodium falciparum malaria cases had been reported from in and around Eliab Valley in Al-Baha region. This was an increase from cases reported for each year from 1990 to '1995 (between 32 and 176) (Figure). In 1996, following heavy rainfall in Tihama, many valleys were flooded. The most affected valleys were Eliab, Rumadah, Muzera, Summa, and Ahseba. The total population of the villages around Eliab valley is 7946, around Rumadah valley, 3103, and around Ahseba valley, 3100. The people are casual laborers, fanners, and shepherds. The malaria center in Gellwa informed the malaria department of the Ministry of Health (MOH) and the MOH asked the Field Epidemiology Training Program to work with the malaria department to investigate the outbreak, assess the malaria problem, and make recommendations.

A case of malaria was defined as an acute elevation of body temperature (>37°C) with any Plasmodium species identified on thick or thin blood film from January 1 to March 31, 1996 in a resident of any village in the Eliab, Rumadah, Nawan, or Ahseba valleys. Malaria case data was obtained from the registry in the Malaria center in Gellwa city.

Of the 478 cases of malaria, 476 were identified as P. falciparum and two were identified as P. vivax. Cerebral malaria occurred in 14 patients, two died and one developed permanent brain damage. All age groups were heavily affected with malaria except for infants (0 to 11 months). The mean age for a malaria case person was 14.3 years (range 6 months to 70 years). The majority of cases (98%) were in Saudis (466). Male to female ratio was 1.2:1.

Of 478 cases, 268 (56%) were reported from the Eliab Valley [Attack rate(AR)=3.6%)], 110 (23%) from the Rumadah valley (AR=3.6%), 71 from the Nawan sector (AR=2.3%), and 27 from the Mekhwa sector (AR=0.18%). Attack rates in individual villages within heavily affected Eliab and Rumadah valleys varied greatly. In the Eliab valley, three out of the ten villages (Dalafa, Hareeja, Agsan) had malaria attack rates above 10% and accounted for 68% of the malaria cases in that valley. Similarly, three villages out of 13 in the Rumadah valley had malaria attack rates above 10% which accounted for 44% of malaria cases in that valley.

Entomological and ecological investigation

The vector identified in Gellwa sector was Anopheles arabiensis. Villages with high malaria attack rates were usually within one Km of a watercourse which had Arak trees growing on both banks. Anophelines appeared to favor this plant as a natural resting site. Most people slept outdoors, clustered in adjacent beds near their houses. The indoor anopheline resting density detected in three villages (Hareejah, Agsan, Dalafa) from 3 March to 5 April 1996 was 3.7 mosquitoes per room. The anopheline density in the same villages after applying adulticide was 5.5 mosquitoes per room. The malaria control center then changed the insecticide and intensified spraying. These measures were effective.

Epidemiological study

Three villages from this valley (Dalafa, Agsan, and Abagaraf) were selected as representative of high and medium attack rates. Two teams of interviewers enumerated persons and houses, linked reported malaria cases to this census, and determined characteristics of each household. Although information, including surveillance reports, at the malaria center showed 160 malaria cases from these villages, enumeration revealed that 217 of the registered cases were actually living in these three villages (Table). Accordingly, attack rates computed from the house to house census were much higher than indicated by surveillance. Within these villages the method of water storage, distance to breeding site, or method of keeping livestock were not associated with malaria attack rates. However houses which used bed nets had higher attack rates than houses that did not (P-value <0.05). Bed nets had holes and were improperly hung.

Al-Baha is located in the southwestern area of Saudi Arabia where malaria is endemic. Anopheles arabiensis mosquito is a vector for malaria. Plasmodium falciparum is the most common parasite. Our findings were consistent with Mazoub's 1980 study[1]: that the seasonal peak for malaria incidence is from January to March 1996. The pattern of malaria seen here with wide fluctuations from year to year and a major epidemic characterizes unstable malaria.

Although using bed nets was associated with higher malaria rates bed nets were of poor quality and improperly affixed. This problem has been recognized elsewhere, and as a solution, highly effective insecticide-impregnated bed nets have been developed.

Good surveillance information is needed to monitor the malaria diseases progress in the community and evaluate the outcome of the control measures' effectiveness.

This investigation demonstrated that malaria transmission can be highly localized. Only a few villages were responsible for most of the malaria transmission, when misclassification of cases to village was corrected this localization became even more extreme. Traditionally, malaria surveillance has been used to evaluate the outcome of control measures over large geographic areas. However, surveillance data if reported quickly and accurately can be of great assistance to control through identification of localized transmission foci and early detection of outbreaks in areas of unstable malaria.