From January to March 1996, 476 Plasmodium falciparum malaria cases were detected in Gellwa Sector of Al-Baha, in Saudi Arabia, compared with a range of 28 to 176 cases during the preceding 5 years. This malaria outbreak coincided with a 10-fold increase in rainfall.
Methodology
We identified all slide-confirmed malaria cases that were registered in the malaria center in Gellwa Sector. Three villages were selected, representing high and medium attack rates situated in one valley and enumerated all houses, persons and registered malaria cases. We compared malaria attack rates by water use, distance from Anopheles breeding sites, livestock, and bed net use.
Results
P. falciparum began appearing in different villages during the first week of January 1996. The outbreak peaked 12 weeks later. Attack rates by village (n=23) showed a wide variation from 1% to 28%. Within the three selected villages P. falciparum attack rates were higher (28%, 73% and 75%) than indicated by surveillance and cases clustered by house (p<0.01, chi square test). However, no specific factor related to housing, including distance from Anopheline breeding, livestock, or domestic water management was associated with higher P. falciparum attack rates. Households using bed nets (n=3) had a higher mean attack rate (97%) than other household (59%) (p<0.03, t-test), but bed nets were incorrectly used and had holes.
Conclusion
This outbreak probably resulted from excessive rainfall in an area of unstable malaria. Clustering of cases by house suggests that socioeconomic related risk factors such as sanitation, houses' designs, occupation, poor quality bed nets which trapped the mosquitoes inside, and environmental factors such as presence of Arak trees around the valley contributed to excess transmission.