Communities of practice
Impact of long-term treatment with ivermectin on the prevalence and intensity of soil-transmitted helminth infections
8 Oct 2008
Citation: Moncayo AL, Vaca M, Amorim L, Rodriguez A, Erazo S, et al. (2008). Impact of Long-Term Treatment with Ivermectin on the Prevalence and Intensity of Soil-Transmitted Helminth Infections. PLoS Negl Trop Dis 2(9): e293.
Helminths are a class of arthropods that can cause several diseases in humans. In developed countries these multicellular parasites have been controlled and no longer represent a threat to the population. However, much work still needs to be done in developing countries where these organisms still affect millions, mostly children.
Helminths can be found in contaminated food or can be passed through insect vectors or directly through the skin from contaminated soil. This last category, known as soil-transmitted helminths (STH), includes Ascaris lumbricoides, Trichuris trichiura, hookworm and Strongyloides stercolaris. These organisms represent a major threat in developing countries, where infected children suffer from impaired nutrition and limited growth.
Mass treatment with a combination of four drugs is the recommendation for treating and controlling STHs. The drugs against STHs currently recommended by the World Health Organization are albendazole, mebendazole, levamisole and pyrantel.
STH infections are mostly transmitted by faecal contamination. Therefore, if the environment is not cleaned and these organisms not removed, re-infection is a constant risk. For this reason, the current recommendation is that individuals living in at-risk areas take the recommended drugs one or twice a year, for many years. This strategy means that treatments are repeated frequently whereas, in a clean environment with no risks of re-infection, one or two dosages of medication would normally be enough to treat patients and keep them protected.
In Ecuador another drug, ivermectin has been used for 17 years in a number of communities for the control of onchocerciasias (river blindness), which is a filarial infection. Ivermectin is known to be highly effective against some STH infections, namely A. lumbricoides and S. stercolaris, but its efficacy against other STHs is only moderate or lower. The long-term treatment for onchocerciasis has provided a basis for evaluating the long-term effects of ivermectin on the prevalence of infection and transmission of STHs parasites, which is an assessment that has not been done before.
To investigate the effects on STHs parasites of the 17-year administration of ivermectin, the researchers collected faecal samples from 3705 school-age children from 58 communities. Of these, 31 communities were ivermectin-treated whereas 27 were not.
The researchers found an association of long-term use of ivermectin with a significant reduction in the prevalence of infection with some STHs parasites, although no reduction in the prevalence of hookworm was observed. The most affected parasite was T. trichiura, whereas little effect was observed in A. lumbricoides. This finding came as a surprise, as ivermectin is known to be very effective against A. lumbricoides. For the low effect on A. lumbricoides, the researchers offer three possible explanations: (i) a high incidence of A. lumbricoides re-infection, as this parasite is very resistant to environmental changes and may have increased across the years, (ii) the development of drug resistance by A. lumbricoides during the 17-year period, and (iii) a natural high incidence of A. lumbricoides in these communities.
As for the effects on hookworm, the researchers observed that, curiously, the incidence of hookworm infections was actually higher in treated communities. This fact the authors were not able to explain, as no baseline data were available. In any case, it is possible that the incidence of this parasite was unusually high in these communities before the mass treatment started.
The authors claim that the study was the first to assess the effects of mass drug administration against SHTs, although the drug which has been evaluated is not one of those currently recommended for treating infections of these parasites. Nevertheless, the study suggests that the additional use of ivermectin may result in additional benefits towards the eradication of T. trichiura and should be considered to be used in combination with a second anthelmintic drug such as albendazole for additional, and successful, treatment of A. lumbricoides.
2008 Moncayo et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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