Generating super-shedders: co-infection increases bacterial load and egg production of a gastrointestinal helminth

Lass, Sandra; Hudson, Peter J.; Thakar, Juilee; Saric, Jasmina; Harvill, Eric; Albert, Réka; Perkins, Sarah E.

Helminth eggs were counted from host faecal samples, collected at approximately the same time of the day, from day 5 postinoculation, i.e. just before the time point when helminth larvae moult into adults [36] . We monitored helminth egg shedding every 3 days until day 44, thereafter weekly sampling occurred until no eggs were found in an individual for three consecutive time points (last time point collected was day 365). Prior to collecting faeces for egg counts, mouse cages were cleaned at 16.00 + 2 h, a subsample of faeces was collected approximately 18 h later, and helminth eggs quantified using standard McMaster techniques. To determine whether co-infected mice shed significantly more helminth eggs than single-infected mice, over time, we carried out a GLMM with random terms using ASReml in R (after [34] and [35] ). A unique identification number for each animal was included in the model as a random term to remove the variation caused by individuals in order to gain insights into any underlying relationships in the fixed model. For the fixed model, we fitted a spline to the parasitic load, over time, of the single and co-infected groups and used this as the response variable with treatment group as the explanatory variable. To quantify whether co-infection generated super-shedders, we used the distribution of eggs shed from the helminth-only infected animals as a baseline distribution. We defined super-shedders as individuals (from the co-infected treatment group) that were 2 s.d.s above the mean of single-infected individuals.