The ecology of parasites: temporal dynamic of strongyles populations in donkeys

Aim of the work. The aim of this work was to develop a method for calculating the temporal dynamics parameters of gastrointestinal strongyles infection in a donkey population. In particular the force of infection, defined as the changement rate of the number of parasites, was estimated on the basis of quantitative coprological results. Materials and methods. The research was conducted in an organic farm of donkeys in Salvarano di Quattro Castella (Reggio-Emilia) which is the largest donkeys farm in Europe housing a population of about 600 individuals. This population can be considered just like a free-living population primarily because of the animals are never treated with antihelminthic and because it is essentially an outdoor and closed-cycle farming. During the month of July 2009, 66 individual samples of faeces were collected from animals identified by microchip. A stratified sampling by age was performed in order to study the temporal dynamics of intestinal strongyles infection. Each specimen was examined using quali-quantitative coprological analysis and individual coprocoltures were performed for the strongyle eggs development up to the third larval stage (L3). The number of L3 belonging to different genera or species was compared to the number of strongyle EPG so as to estimate the number of L3/g both for small strongyles (Cyathostominae) and Strongylus vulgaris. In order to assess the progress of infections over time, the age of the donkeys was used as a proxy of the time of infection itself; therefore, the OLS regression model (linear or not) that best fits the observed data, so as to explain the relationship between the age of the donkeys and the quantity of larvae of parasites, was chosen. For the statistical analysis the software STATA 9.1 was used. For the study of the estimated functions and in particular for the calculation of the derivatives, the software Derive 6 was used. Results. The dynamic of the infection over time (age of animals), both for small strongyles and S. vulgaris, fitted a logistic growth curve, whose study made it possible to estimate the force of infection of parasites and the maximum L3/g density reachable (carrying capacity K). The force of infection for small strongyles gets to a peak, equal to 3.3 L3/g month, at 5.3 months. The force of infection then decreases and becomes zero in 47 months, therefore the L3s remain stable after this time. In the case of S. vulgaris the force of infection was approximately equal to 0.22 L3/g month during the whole temporal range. The carrying capacity of the population of S. vulgaris is never reached, continuing to grow during the whole lifetime of the host. Conclusions. This work has allowed to develop a method to calculate the force of infection and therefore to study the temporal dynamic of the major parasites of the donkey. It represents an important basis for models building, estimation of R0 and the realization of parasite control based on ecological data. Furthermore, the difference between the dynamic of small and large strongyles infections has been highlighted, showing that while the population of small strongyles attains its maximum level, the population of S. vulgaris never reaches its carrying potential. We want to point up that the true dynamic paramethers usually refers to the adult parasite population; nevertheless the force of infection estimated from coprological data represents a valid indicator of parasite dynamic, obtainable also in living animals and with a quite simple method.