Spatial Distribution of Taenia solium Porcine Cysticercosis within a Rural Area of Mexico

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Abstract
Cysticercosis is caused by Taenia solium, a parasitic disease that affects humans and rurally bred pigs in developing countries. The cysticercus may localize in the central nervous system of the human, causing neurocysticercosis, the most severe and frequent form of the disease. There appears to be an association between the prevalence of porcine cysticercosis and domestic pigs that wander freely and have access to human feces. In order to assess whether the risk of cysticercosis infection is clustered or widely dispersed in a limited rural area, a spatial analysis of rural porcine cysticercosis was applied to 13 villages of the Sierra de Huautla in Central Mexico. Clustering of cases in specific households would indicate tapeworm carriers in the vicinity, whereas their dispersal would suggest that the ambulatory habits of both humans and pigs contribute to the spread of cysticercosis. A total of 562 pigs were included in this study (August–December 2003). A global positioning system was employed in order to plot the geographic distribution of both cysticercotic pigs and risk factors for infection within the villages. Prevalence of pig tongue cysticercosis varied significantly in sampled villages (p = 0.003), ranging from 0% to 33.3% and averaging 13.3%. Pigs were clustered in households, but no differences in the clustering of cysticercotic and healthy pigs were found. In contrast, the presence of pigs roaming freely and drinking stagnant water correlated significantly with porcine cysticercosis (p = 0.07), as did the absence of latrines (p = 0.0008). High prevalence of porcine cysticercosis proves that transmission is still quite common in rural Mexico. The lack of significant differentiation in the geographical clustering of healthy and cysticercotic pigs weakens the argument that focal factors (e.g., household location of putative tapeworm carriers) play an important role in increasing the risk of cysticercosis transmission in pigs. Instead, it would appear that other wide-ranging biological, physical, and cultural factors determine the geographic spread of the disease. Extensive geographic dispersal of the risk of cysticercosis makes it imperative that control measures be applied indiscriminately to all pigs and humans living in this endemic area. Taenia solium cysticercosis is a parasitic disease that severely affects human health in underdeveloped countries and has re-emerged in North America. The adult parasite lives in the intestines of humans, where it thrives and sheds packages (proglottids) loaded with thousands of eggs that are, in turn, expelled upon defecation. Cysticercosis occurs after tapeworm eggs are ingested by an intermediate host (pig or human) and then hatch, migrate, and lodge in the host's tissues, where they develop onto larval cysticerci. Deficient hygiene, inadequate feces disposal, outdoor defecation, freely roaming pigs, and inadequate meat inspection promote transmission. Success in lowering transmission is limited by the complex network of biological and social factors that maintain the endemia. Effective control will require social development and powerful, sustained interventions targeted at the transmission cycle's crucial nodes. Rural pigs are obligate intermediate hosts and thus prime targets for control through vaccination and treatment. It has been proposed that pigs be used as sentinels to monitor environmental T. solium contamination. We conducted a spatial study of cysticercotic pigs among 13 neighboring villages in rural Central Mexico, constituting the first step of an attempt to discover a cost-effective and accurately targeted control intervention. Cysticercotic pigs were not found clustered in specific households, suggesting that the risk of infection is widely dispersed in the area and indicating that extensive and inclusive control measures are needed.