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. 2011 Dec;105(12):683-93.
doi: 10.1016/j.trstmh.2011年08月01日3. Epub 2011 Oct 29.

Preventive chemotherapy in human helminthiasis: theoretical and operational aspects

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Preventive chemotherapy in human helminthiasis: theoretical and operational aspects

A-F Gabrielli et al. Trans R Soc Trop Med Hyg. 2011 Dec.

Abstract

Preventive chemotherapy (PC), the large-scale distribution of anthelminthic drugs to population groups at risk, is the core intervention recommended by the WHO for reducing morbidity and transmission of the four main helminth infections, namely lymphatic filariasis, onchocerciasis, schistosomiasis and soil-transmitted helminthiasis. The strategy is widely implemented worldwide but its general theoretical foundations have not been described so far in a comprehensive and cohesive manner. Starting from the information available on the biological and epidemiological characteristics of helminth infections, as well as from the experience generated by disease control and elimination interventions across the world, we extrapolate the fundamentals and synthesise the principles that regulate PC and justify its implementation as a sound and essential public health intervention. The outline of the theoretical aspects of PC contributes to a thorough understanding of the different facets of this strategy and helps comprehend opportunities and limits of control and elimination interventions directed against helminth infections.

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Conflict of interest statement

Conflicts of interest: None declared.

Figures

Figure 1
Figure 1
Graphical representation of the evolution of morbidity associated with helminth infections (continuous line), at individual level, in the absence of preventive chemotherapy (PC) interventions (A) and of the three possible scenarios showing the impact of PC on morbidity (B–D). In each of the four figure parts, the grey area represents irreversible morbidity, the dotted line represents the threshold of morbidity (threshold of high intensity) and the arrows represent the treatment rounds. (A) PC is not implemented: acute inflammation progresses into chronic stages associated with irreversible sequelae; morbidity is not controlled. (B) PC targets a chronically infected patient who has already developed irreversible sequelae. In this case, no reversion of such chronic morbidity can be achieved. However, as in endemic areas this individual is likely to concomitantly suffer from different time stages of morbidity, treatment will revert the early-stage morbidity consequent to more recent re-infection episodes. (C) PC targets an individual who has developed reversible morbidity but not yet its irreversible stages. In this case, all the existing morbidity will be reverted and its burden reduced. (D) PC targets an individual who has not yet developed any morbidity and is implemented throughout the period of exposure to the infectious agents, with an interval of re-treatment that is shorter than the period of time needed for re-infection episodes to increase the number of worms above the morbidity threshold. In this case, PC will fully prevent morbidity and the infected subject will never develop any of the pathological consequences of high-intensity infections.
Figure 2
Figure 2
The transmission breakpoint (TBP) of a given infection corresponds to the quantity of worms circulating in the target area below which they are unable to perpetuate themselves, with the consequence that their population will eventually die out. The TBP can be expressed as a prevalence of infection in the human population, whose level depends on the pre-intervention efficiency of transmission (EoT) of the infection. Preventive chemotherapy (PC) interventions (arrows) reduce the number of circulating worms and decrease the target infection’s EoT, thus moving the prevalence of infection to a level that is lower than the pre-intervention level. (A) If the post-intervention number of circulating worms and the corresponding EoT and prevalence of infection are still above the TBP, after discontinuation of such an intervention the pre-intervention levels of all indicators will be re-established as a consequence of continuing re-infection episodes. (B) If the post-intervention number of circulating worms and the corresponding EoT and prevalence of infection reach the level associated with the TBP, the worm population will eventually die out, thus leading to interruption of transmission of the target infection.

Comment in

References

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