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EPIDEMIOLOGY OF HIGHLAND MALARIA IN WESTERN KENYA
Abstract
ABSTRACT
Objectives: We investigated 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.
Design & Setting: This was a cross-sectional study with rolling, random subject
enrollment from April 108 through October 1999. Falciparuin malaria was investigated
in workers and their families at a highland tea plantation located at 0' 22' south and
35' 1 T cast in the Rift Valley highlands of western Kenya, an area with seasonally
limited malaria transmission.
Subjects: 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 2796 patients evaluated
during the study period, 798 cases of malaria were confirmed by positive peripheral
Hood srnear; 1998 smear-negative patients were pressured to be non-infected and served
as controls (Ratio: 2.52: 1). Interventions: Tea estate workers do riot receive malaria
chemoprophylaxis, but were given easily available free treatment for any symptomatic
infectious.
Main-Outcome Measures: Stricar-positive cases were compared with smear-negative
patients for multiple demographic and disease variables, including sex, age, travel
history, ethnic origin, borne district transmission risk index and length of residence.
Disease characteristics, including parasite types, counts and clinical symptoms, and
treatments administered were described.
Results and Conclusions:: Malaria was predominantly P.(>99%); asexual parasite counts
ranged from 1-10,440 per mm3, with a mean of 903.6 (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 < .001), as was a subjective
history of previous malaria infection in the past year (F = 26.65, 14 df, p <.001; univariate
ANOVA). Amodiaquine was the most commonly used drug to treat cases of either smearproven
or clinically suspected malaria, accounting for 56% of therapy; pyrimethamine/
sulfadoxine was used to treat 27%, artemisinin 8% and chloroquine was administered
to only three percent, while combination therapy was used in five percent 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 < 001). This implies a protective effect of prior infections. Presenting
with symptoms suggestive of malaria was statistically associated with parasitennia,
particularly fever, headache and dizziness, (p <.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 parasiternia (F - 2.04, 4 elf,
p = 0.002, univariate ANOVA). This is likely related to a proclivity in these groups
for travel to these holoeridemic areas, which also accounts for the strong associations
between recent travel, lowland ethnic group and infection. Parasitemia was significantly
March 2003 EAST AFRICAN MEDICAL JOURNAL 33
associated with age less than ten years (Pcarson Chi-Square 145.99, p < .001), confirming
the predilection of the disease for these young and immunologically naive victim with
a history of travel more than twenty kilometers from site within six weeks (Pcarson
Chi-Square 58.28, p < .001) and with time since arrival on the plantation of 1 year
or less (Pearson Chi-Square 185.12, p <.001). 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.
Objectives: We investigated 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.
Design & Setting: This was a cross-sectional study with rolling, random subject
enrollment from April 108 through October 1999. Falciparuin malaria was investigated
in workers and their families at a highland tea plantation located at 0' 22' south and
35' 1 T cast in the Rift Valley highlands of western Kenya, an area with seasonally
limited malaria transmission.
Subjects: 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 2796 patients evaluated
during the study period, 798 cases of malaria were confirmed by positive peripheral
Hood srnear; 1998 smear-negative patients were pressured to be non-infected and served
as controls (Ratio: 2.52: 1). Interventions: Tea estate workers do riot receive malaria
chemoprophylaxis, but were given easily available free treatment for any symptomatic
infectious.
Main-Outcome Measures: Stricar-positive cases were compared with smear-negative
patients for multiple demographic and disease variables, including sex, age, travel
history, ethnic origin, borne district transmission risk index and length of residence.
Disease characteristics, including parasite types, counts and clinical symptoms, and
treatments administered were described.
Results and Conclusions:: Malaria was predominantly P.(>99%); asexual parasite counts
ranged from 1-10,440 per mm3, with a mean of 903.6 (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 < .001), as was a subjective
history of previous malaria infection in the past year (F = 26.65, 14 df, p <.001; univariate
ANOVA). Amodiaquine was the most commonly used drug to treat cases of either smearproven
or clinically suspected malaria, accounting for 56% of therapy; pyrimethamine/
sulfadoxine was used to treat 27%, artemisinin 8% and chloroquine was administered
to only three percent, while combination therapy was used in five percent 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 < 001). This implies a protective effect of prior infections. Presenting
with symptoms suggestive of malaria was statistically associated with parasitennia,
particularly fever, headache and dizziness, (p <.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 parasiternia (F - 2.04, 4 elf,
p = 0.002, univariate ANOVA). This is likely related to a proclivity in these groups
for travel to these holoeridemic areas, which also accounts for the strong associations
between recent travel, lowland ethnic group and infection. Parasitemia was significantly
March 2003 EAST AFRICAN MEDICAL JOURNAL 33
associated with age less than ten years (Pcarson Chi-Square 145.99, p < .001), confirming
the predilection of the disease for these young and immunologically naive victim with
a history of travel more than twenty kilometers from site within six weeks (Pcarson
Chi-Square 58.28, p < .001) and with time since arrival on the plantation of 1 year
or less (Pearson Chi-Square 185.12, p <.001). 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.
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