"Assessing the effects of the diagnostic methods on the schistosomiasis dynamics"
Schistosomiasis is a neglected tropical disease that affects around 200 million people worldwide. It is a macroparasite infection, caused by trematode from genus Schistosoma, whose intermediate host is a snail from genus Biomphalaria that is caracteristic of places with fresh water. Moreover, it is estimated that, every year, 100 thousand individuals die due to schistosomiasis-related causes. In Brazil, schistosomiasis is endemic in 13 states and affects around 25 million people that live in risk areas, with 6 million infected individuals (estimated).
Its biological cycle is very complex, having five different life stages: egg, miracidium, sporocyst, cercaria and schistosomula, which makes its control even more difficult. The recomended diagnostic method, named Kato-Katz, seeks for eggs in the individual's feces and does not have a high sensitivity. In 2007, a group of researchers developed a new method, named Helmintex, that uses paramagnetic markers to find the eggs in feces and showed to be three times more sensitive than Kato-Katz.
We sought to investigate the effects on the schistosomiasis dynamics of applying a mass diagnostic strategy, the idea is that infected individuals, who are diagnosed, are treated. In order to do that, we build an ordinary differential equations model that considers: a human population divided in susceptible individuals and three classes of infected people that represent different levels of worm burden; a snail population, susceptible and infected, and a miracidium reservoir, that is important in order to take in to account the reinfection effects of the highest level of worm burden. The cercaria dynamics is implicitly considered through the human infection parameters.
We performed the equilibrium and local stability analysis for different scenarios in order to compare the results. (i) First, considering that a more sensitive diagnostic method, the Helmitex, is applyed, setting to zero the human population in the two highest levels of worm burden; (ii) considering that a less sensitive method, the Kato-Katz, is applyed, setting to zero only the highest level of worm burden; and the last case, (iii) the complete model that represents the scenario in which there is no treatment/diagnostic strategy.
Our results, besides the conditions for existence and local stability of the endemic equilibrium points, suggest that the low sensitivity of the classic method can explain the why it is so difficult to control the infections and why, usually, after stop the treatment on a population, or precaution strategies, the infection prevalences returns to a high level.