Basáñez M G, Rodríguez-Pérez M A, Reyes-Villanueva F, Collins R C, Rodríguez M H
El Colegio de la Frontera Sur, San Cristóbal de las Casas, Chiapas, Mexico.
J Med Entomol. 1998 Sep;35(5):745-57. doi: 10.1093/jmedent/35.5.745.
Monthly samples of biting Simulium ochraceum s.l. Walker were collected before and after ivermectin treatment in southern Mexico and analyzed for Onchocerca volvulus Leuckart infection rates, infection intensity, and the characteristics of larval distribution among parous flies. The variance over mean ratio (VMR) indicated that in all cases this distribution departed from Poisson and was strongly aggregated (VMR > 1). The negative binomial was found to be an adequate model with a small value of the aggregation parameter k, but the degree of larval overdispersion increased as the mean larval load decreased, invalidating the use of a common kc value. A linear relationship between k and the mean (mu) was established, k(mu) = k1 mu, which permitted exploration of the relationship between the observed proportion of infected flies, p, and the estimated mean larval burden per fly, m (all larval stages in parous flies). This would allow mean numbers of larvae per parous fly to be predicted from presence-absence data (e.g., from infection rates provided by polymerase chain reaction methods applied to pools of flies), assuming that k1 is a known parameter. Given that both p and m are naturally low in S. ochraceum, their relationship was practically linear within the range of observed values. Predictions were tested with the Mexican data from which the clumping parameter was estimated as well as for Guatemalan data for which this information was not available. Results showed a highly satisfactory degree of agreement between predictions and observations. The sample sizes required to estimate mean larval loads from prevalence data for fixed levels of precision (defined as the ratio between SE[m] and m) were calculated for realistic S. ochraceum infection rates (those found in published pre- and postcontrol field surveys as well as in this work). For the special case in which the relationship between k and the mean is linear and goes through the origin, k(mu) = k1 mu, the number of flies to be examined for O. volvulus infections does not explicitly depend on the aggregation parameter, but rather on the unknown proportion of infected flies. Practical recommendations for the calculation of sample sizes are discussed. For infection levels < 0.2%, a minimum number between 6,000 and 13,000 parous flies would have to be examined to estimate the mean larval load with a precision between 0.20 and 0.30. The linearity between onchocercal infection rate and infection intensity in the fly population indicates that relationships between the former and onchocerciasis patterns in the human population should be further explored for the purposes of monitoring the impact of ivermectin control programs through entomological evaluations.
在墨西哥南部，于伊维菌素治疗前后采集了按月收集的嗜人按蚊复合体（Simulium ochraceum s.l. Walker）叮咬样本，分析了盘尾丝虫（Onchocerca volvulus Leuckart）的感染率、感染强度以及已产卵雌蝇中幼虫分布的特征。方差均值比（VMR）表明，在所有情况下这种分布均偏离泊松分布且呈强聚集性（VMR > 1）。发现负二项分布是一个合适的模型，聚集参数k值较小，但随着平均幼虫负荷降低，幼虫的过度分散程度增加，使得使用共同的kc值无效。建立了k与均值（μ）之间的线性关系，即k(μ) = k1μ，这使得能够探究观察到的感染雌蝇比例p与估计的每只雌蝇平均幼虫负荷m（已产卵雌蝇中的所有幼虫阶段）之间的关系。假设k1是已知参数，这将允许根据存在 - 不存在数据（例如，应用于蝇类样本池的聚合酶链反应方法提供的感染率）预测每只已产卵雌蝇的平均幼虫数量。鉴于在嗜人按蚊中p和m自然都很低，它们在观察值范围内的关系实际上是线性的。用墨西哥的数据对预测进行了检验，从中估计了聚集参数，同时也对没有该信息的危地马拉数据进行了检验。结果表明预测与观察之间的一致性程度非常令人满意。针对实际的嗜人按蚊感染率（已发表的控制前后现场调查以及本研究中发现的感染率），计算了在固定精度水平（定义为SE[m]与m的比值）下从患病率数据估计平均幼虫负荷所需的样本量。对于k与均值之间的关系是线性且经过原点的特殊情况，即k(μ) = k1μ，为检测盘尾丝虫感染而要检查的蝇类数量并不明确取决于聚集参数，而是取决于未知的感染蝇类比例。讨论了计算样本量的实际建议。对于感染水平< 0.2%，必须检查6000至13000只已产卵雌蝇的最小数量，以便以介于0.20和0.30之间的精度估计平均幼虫负荷。蝇类群体中盘尾丝虫感染率与感染强度之间的线性关系表明，为了通过昆虫学评估监测伊维菌素控制项目的影响，应进一步探索前者与人群中盘尾丝虫病模式之间的关系。
