Bundy D A, Hall A, Medley G F, Savioli L
WHO Collaborating Centre for the Epidemiology of Intestinal Parasitic Infections, Imperial College, London, United Kingdom.
World Health Stat Q. 1992;45(2-3):168-79.
Intestinal parasitic infections are among the most common infections of humans in developing countries, but the resources available for their control are severely limited. Careful evaluation of control measures is essential to ensure that they are cost-effective. The evaluation of the effects of control on intestinal helminths and intestinal protozoa requires an understanding of the different epidemiological patterns of these two groups of parasites. The transmission dynamics and morbidity associated with the major helminth infections (Ascaris lumbricoides, Trichuris trichiura and the hookworms) are dependent on the size of the worm burdens. Thus the important parameter for evaluating the impact of control on morbidity and transmission is the intensity of infection, which can be assessed by determining the mean density of parasite eggs in faecal specimens. Estimation of intensity is subject to systematic errors, however, due to the complex pattern of worm burden distributions. The frequency distribution of burdens is highly overdispersed, and individuals exhibit predisposition to particular levels of infection. Furthermore, mean intensity is age-dependent, in a species-specific manner, and is clustered spatially and within families. These complex patterns imply that the estimation of intensity is exceptionally sensitive to the size and demographic structure of the population sample selected for assessment. They also have the effect that prevalence estimates, the most commonly used measures of infection in communities, can seriously mislead. Paradoxically, prevalence is least useful where infection is most common because the relationship between prevalence and intensity is most markedly non-linear when the prevalence is high. Thus in areas where control is most needed, evaluation using prevalence might suggest that control had failed while evaluation by intensity would, correctly, show the measure of success. With the major protozoan infections (Entamoeba histolytica, Giardia duodenalis and Cryptosporidium parvum) an estimate of intensity is of little value and the central parameter for evaluation is prevalence. Prevalence does exhibit age and spatial heterogeneity, which may be species-specific, so there remains a need to ensure a consistent sample structure, although this is less critical than for the helminths. The major constraint on evaluating the control of protozoan infections is the need to identify pathogenic species and, in some cases, pathogenic strains. Harmless commensal protozoans are ubiquitous and often morphologically very similar to pathogens, but their control is both unnecessary and impracticable. Species such a E. histolytica appear to exist as strains with differing pathogenicity, thus control will be cost-effective only if the focus is on pathogenic strains. Effective diagnosis is therefore central to the evaluation of the control of protozoan infections.(ABSTRACT TRUNCATED AT 400 WORDS)
肠道寄生虫感染是发展中国家人类最常见的感染之一，但用于控制这些感染的资源严重有限。仔细评估控制措施对于确保其成本效益至关重要。评估控制措施对肠道蠕虫和肠道原生动物的影响需要了解这两类寄生虫不同的流行病学模式。与主要蠕虫感染（蛔虫、鞭虫和钩虫）相关的传播动态和发病率取决于虫负荷的大小。因此，评估控制措施对发病率和传播影响的重要参数是感染强度，可通过测定粪便标本中寄生虫卵的平均密度来评估。然而，由于蠕虫负荷分布的复杂模式，感染强度的估计存在系统误差。负荷的频率分布高度离散，个体表现出对特定感染水平的易感性。此外，平均感染强度以物种特异性的方式依赖于年龄，并且在空间上和家庭内部聚集。这些复杂模式意味着感染强度的估计对为评估所选人群样本的大小和人口结构异常敏感。它们还会导致流行率估计值产生严重误导，而流行率估计值是社区中最常用的感染测量指标。矛盾的是，在感染最常见的地方，流行率最无用，因为当流行率高时，流行率与感染强度之间的关系最明显是非线性的。因此，在最需要控制的地区，使用流行率进行评估可能表明控制失败，而通过感染强度进行评估则会正确显示成功程度。对于主要的原生动物感染（溶组织内阿米巴、十二指肠贾第虫和微小隐孢子虫），感染强度的估计价值不大，评估的核心参数是流行率。流行率确实表现出年龄和空间异质性，这可能是物种特异性的，因此仍然需要确保样本结构一致，尽管这比对蠕虫感染的情况不那么关键。评估原生动物感染控制的主要限制是需要识别致病物种，在某些情况下还需要识别致病菌株。无害的共生原生动物无处不在，而且在形态上往往与病原体非常相似，但对它们进行控制既不必要也不切实可行。像溶组织内阿米巴这样的物种似乎以具有不同致病性的菌株形式存在，因此只有将重点放在致病菌株上，控制措施才具有成本效益。因此，有效的诊断对于评估原生动物感染的控制至关重要。（摘要截选至400字）
