Chad had been free of debilitating guinea worm disease for a decade when ten new cases were uncovered in 2010. Logistical factors have prevented containment and the disease persists into the current transmission season. The Ministry of Health in Chad is currently examining 9 cases of human guinea worm but also 50 cases in dogs and one in a cat.
Donald Hopkins and colleagues recently explained that “Guinea worms in dogs have been reported for almost a century in some areas of Asia, Africa, and North America, including in some recently endemic countries that have interrupted transmission, but disease caused by D. medinensis has never been reported again after human transmission was interrupted.” In this situation where the cases in dogs far outnumber those in humans, one wonders hat the future holds for breaking transmission.
Two key lessons from this experience are to be learned by the malaria community. First is the fact that once eliminated, an infectious disease can return. The context though probably relates to the neighborhood. Post-conflict South Sudan is one of the few remaining countries with continued guinea worm transmission and refugee movement may have contributed to the problem.
The second lesson is the potential for inter-species transmission. Plasmodium knowlesi, a disease of primates, appears about to become entrenched in the human population of Southeast Asia. Cases of monkeys having P. vivax have been documented in South America and Africa. Elimination of human transmission of malaria may be hampered by non-human reservoirs.
We may not be able to say that elimination has occurred until there is no local transmission of any species of Plasmodium that affect humans in any other life form. We have a hard enough time tracking malaria in humans when it falls below levels detectable by microscopes and rapid tests. What of the challenges of tracking it in monkeys? Maybe these complications explain why to date smallpox remains the exception to the eradication rule.