Epidemiology of highland malaria in western Kenya.

OBJECTIVE

To investigate the epidemiology of falciparum malaria in workers from a highland tea plantation in western Kenya with very seasonally limited malaria transmission to determine what factors are associated with increased risk of malaria transmission in the Kenyan highlands.

METHODS

A cross-sectional study with rolling, random subject enrollment from April 1998 through October 1999.

METHODS

Highland tea plantation located at 0 degrees 22' south and 35' 17' east in the Rift Valley highlands of western Kenya, an area with seasonally limited malaria transmission.

METHODS

The data for the study were obtained from enrollment of outpatients from the healthcare system of a major tea company, which has 18 estates with 22,000 workers and approximately 50,000 persons eligible for health care. Of the 2796 patients evaluated during the study period, 798 cases of malaria were confirmed by positive peripheral blood smear; 1998 smear-negative patients were pressured to be non-infected and served as controls (Ratio: 2.52: 1).

METHODS

Tea estate workers do not receive malaria chemoprophylaxis, but were given easily available free treatment for any symptomatic infections.

METHODS

Smear-positive cases were compared with smear-negative patients for multiple demographic and disease variables, including sex, age, travel history, ethnic origin, home district transmission risk index and length of residence. Disease characteristics, including parasite types, counts and clinical symptoms, and treatments administered were described.

RESULTS

Malaria was predominantly P. falciparum (>99%); asexual parasite counts ranged from 1-10,440 per mm3, with a mean of 803.6 (95% confidence interval: 695.2, 912.0). Gametocytemia was present in 7.5% of smear-positive malaria cases, but was rare in the absence of blood asexual forms (0.5%). Prior use of a variety of antimalarial drugs was extremely common and negatively predictive of parasitemia in patients presenting for clinical treatment (Pearson Chi-square 50.81, p < 0.001), as was a subjective history of previous malaria infection in the past year (F = 26.65, 14 df, p < 0.001; univariate ANOVA). Amodiaquine was the most commonly used drug to treat cases of either smear-proven or clinically suspected malaria, accounting for 56% of therapy; pyrimethamine/sulfadoxine was used to treat 27%, artemesinin 8% and chloroquine was administered to only 3%, while combination therapy was used in 5% of cases, and only a single treatment (0.1 %) was recorded using quinine. Subjects with a prior history of treatment for malaria were statistically less likely to be infected again (Pearson Chi-square 50.81, p < 0.001). Presenting with symptoms suggestive of malaria was statistically associated with parasitemia, particularly fever, headache and dizziness, (p <0.001 for all, univariate ANOVA), but in general, clinical symptoms were not an effective discriminator of malarial disease. Ethnic group predicted malaria infection with groups traditionally from the Lake Victoria lowland regions having a greater prevalence of parasitemia (F = 2.04, 4. df, p = 0.002, univariate ANOVA). Parasitemia was significantly associated with age less than ten years (Pearson Chi-Square 145.99, p < 0.001), with a history of travel more than twenty kilometers from site within six weeks (Pearson Chi-square 58.28, p < 0.001) and with time since arrival on the plantation of one year or less (Pearson Chi-square 185.12, p <0.001)

CONCLUSIONS

Lower infection rates in persons with a history of prior infection implies a protective effect; the predilection of malaria for young and immunologically naive victims was confirmed. The proclivity in some ethnic groups for travel to holoendemic areas also accounts for the strong associations between recent travel, lowland ethnic group and infection. These findings taken together suggest that importation of malaria to the highlands, as well as travel away from the highlands, are important sources of new infections among persons living and working there.