Control of nematodes at different life stages, which stage should we target?
Résumé
Gastro-Intestinal nematodes are responsible for severe yield loss in grazing herbivores farming and are even responsible for the loss of animals in some cases. Nematodes life-cycle is composed of a free living stage, where the nematodes evolved from eggs to infective L3 larvae within the pasture. Once ingested by the grazing animals, the L3 can continue their growth to adult reproductive larvae inside their host, this is the parasitic stage. During this stage, the nematodes population is naturally controlled by the host immune system, with more or less success depending on the host’s resistance. Most commonly, the action of the immune system has to be completed in order to maintain the flock to a healthy state. There exists a range of different control actions, such that anthelminthic treatment, genetic selection, optimized nutrition or rotational grazing. The different control methods all focus their action at particular stages of the nematodes life-cycle. To date, there exist no fully efficient method in the long term and research aims at improving the current method, as well as proposing new one.
Beside the inherent efficacy of control methods, nematodes control at different stages can potentially impact the adult larvae population in different proportion. In this work, we consider several key stages of the nematode life cycles (eggs, infective L3 on pasture, established L4 and adult worms), as well as two key parameters of their life cycle (establishment rate and adult fecundity). We ask the following question: if nematodes can be control at different removal rates at each of these stages, which stage will have the largest impact on the flock’s adult nematodes population? This question is particularly interesting in order to discuss the most promising research direction in terms of nematode control.
We propose a simulation framework, based on a matrix population model, particularly common in Ecology. In a matrix population model, the population is separated into stages (here egg, L3, L4 and adult) and the number of individuals in each stage is studied. At a daily time step, the individuals transition to the next stage, until reaching an upper stage (adult). At each transition, some individuals are removed due to the natural population mortality. This type of model suits well to the nematode life cycle and allows a compact representation of the nematode population dynamic. We use [1] to parameterize this model for Teladorsagia circumcincta and [2] to model the action of the host immune system. The model is sufficiently general to be adapted to other nematodes population.
We use this model to simulate a flock of 35 individuals grazing on a rotation of 5 pastures. We use the model to estimate the average worm burden of the flock over a period of 200 days, when the nematode population is controlled at different life stages, with different removal rates. Each control scenario is then compared based on the estimated worm burden of the flock. With our particular parameterization, we show that focusing control inside the host (i.e., L4 or adult) allows the most significant impact. We challenged this conclusion by performing a sensitivity analysis.