Evaluating the Role of Corrals and Insects in the Transmission of Porcine Cysticercosis: A Cohort Study

Abstract

The widespread dispersion of pigs infected with cysticercosis across endemic villages, low cyst burden among infected pigs, and low prevalence of taeniasis all suggest that pig ingestion of human feces is not the only mode of transmission for Taenia solium. Our objective was to evaluate the risk of porcine cysticercosis associated with exposure to human feces, dung beetles, and flies in an endemic community setting. We used a cluster-randomized cohort design to compare the risk of developing antibodies and infection among 120 piglets raised in either free-roaming (FR), standard corral (SC), or netted corral environments (NC). We collected monthly blood samples to detect serum antibodies and necropsied all pigs after 10 months to identify cysts. A total of 66 piglets developed antibodies with the relative risk of seropositivity in FR vs. all corralled pigs increasing significantly after 18 weeks. Of 108 necropsied pigs, 15 had T. solium cysts, all belonging to the FR group. Corrals were protective against infection but less so against seropositivity. NC, which did not completely exclude insects, did not provide added protection against seropositivity as compared to SC. The results of this study suggest that dung beetles and flies do not play an important role in infection.

Keywords: cohort; corrals; free roaming; insects; porcine cysticercosis; seroincidence.

Figure 1

Representative photos of the three exposure environments. (a): Pigs in free-roaming environments were allowed to roam the village unrestrained. This image also shows placement of a typical fly and dung beetle trap. (b): Pigs in standard corrals were restricted to the corral space but were otherwise exposed to the open air. (c): Pigs in netted corrals were restricted to the corral space and were also protected from external flying and crawling insects. with double door to control the entrance of insects. Netted corrals were completely enclosed with dual layer polypropylene mesh netting, incorporated a separate double-door entry area, and staff were required to follow strict entry procedures including changing boots, examining the interior of the entry room for insects, and killing any insects found with insecticide spray before entering the protected corral space.

Figure 2

Individual survival time for each pig evaluated in the three cohorts. Vertical segments cover dry (yellow) and rainy (blue) seasons. The symbols at the end of each line are pigs that seroconverted (positive to LLGP-EITB), empty circles only seroconverted, filled circle also had cysts at necropsy and triangles had other cestode at necropsy. Lines without a shape at the end contributed to the model as right censored.

Figure 3

Kaplan–Meier survival curves representing time to development of a positive result on EITB among pigs in the three exposure environments. The dashed line at 18 weeks highlights the point at which the hazard for pigs in the Free Roaming environment increases and diverges with respect hazard in the other groups.