Arness M K, Bradshaw R D, Biomndo K, Shanks G D
Disease Epidemiology and Surveillance, Walter Reed Army Medical Center, Washington, DC, USA.
East Afr Med J. 2003 May;80(5):253-9. doi: 10.4314/eamj.v80i5.8696.
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.
A cross-sectional study with rolling, random subject enrollment from April 1998 through October 1999.
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.
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).
Tea estate workers do not receive malaria chemoprophylaxis, but were given easily available free treatment for any symptomatic infections.
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.
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)
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.
调查肯尼亚西部高地茶园工人中恶性疟的流行病学情况，该地区疟疾传播季节性非常有限，以确定哪些因素与肯尼亚高地疟疾传播风险增加相关。
一项横断面研究，于1998年4月至1999年10月进行滚动随机受试者招募。
位于肯尼亚西部裂谷高地南纬0度22分、东经35度17分的高地茶园，该地区疟疾传播季节性有限。
研究数据来自一家大型茶叶公司医疗系统的门诊患者登记，该公司有18个种植园，22000名工人，约50000人有资格享受医疗保健。在研究期间评估的2796例患者中，798例疟疾通过外周血涂片阳性确诊；1998例涂片阴性患者被判定未感染，作为对照（比例：2.52:1）。
茶园工人未接受疟疾化学预防，但对任何有症状感染给予易于获得的免费治疗。
将涂片阳性病例与涂片阴性患者在多个人口统计学和疾病变量方面进行比较，包括性别、年龄、旅行史、种族、家乡地区传播风险指数和居住时间。描述了疾病特征，包括寄生虫类型、计数和临床症状，以及所给予的治疗。
疟疾主要为恶性疟（>99%）；无性寄生虫计数范围为每立方毫米1 - 10440个，平均为803.6（95%置信区间：695.2, 912.0）。配子体血症在7.5%的涂片阳性疟疾病例中存在，但在无血液无性形式时罕见（0.5%）。之前使用多种抗疟药物极为常见，且对前来临床治疗的患者的寄生虫血症具有负预测性（Pearson卡方检验50.81，p < 0.001），过去一年有疟疾感染主观病史的情况也是如此（F = 26.65，14自由度，p < 0.001；单因素方差分析）。阿莫地喹是治疗涂片确诊或临床疑似疟疾病例最常用的药物，占治疗的56%；乙胺嘧啶/磺胺多辛用于治疗27%，青蒿素8%，氯喹仅用于3%，联合治疗用于5%的病例，仅0.1%的病例记录使用奎宁。有疟疾治疗史的受试者再次感染的可能性在统计学上较低（Pearson卡方检验50.81，p < 0.001）。出现提示疟疾的症状在统计学上与寄生虫血症相关，特别是发热、头痛和头晕（所有p < 0.001，单因素方差分析），但总体而言，临床症状不是疟疾疾病的有效鉴别指标。种族预测疟疾感染，传统上来自维多利亚湖低地地区的群体寄生虫血症患病率更高（F = 2.04，4自由度，p = 0.002，单因素方差分析）。寄生虫血症与年龄小于十岁显著相关（Pearson卡方检验145.99，p < 0.001），与六周内离开驻地超过二十公里的旅行史相关（Pearson卡方检验58.28，p < 0.001），与到达种植园一年或更短时间相关（Pearson卡方检验185.12，p < 0.001）
有既往感染史的人感染率较低意味着有保护作用；证实了疟疾对年轻和免疫未成熟受害者的偏好。一些种族群体前往高疟流行地区旅行的倾向也解释了近期旅行、低地种族群体与感染之间的强关联。综合这些发现表明，疟疾传入高地以及离开高地旅行是在那里生活和工作的人新感染的重要来源。
