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.

A dominant feature of host-parasite interactions is the large variation in infection and infectiousness. Individuals infected with HIV, for example, may either rapidly develop acquired immune deficiency syndrome, or take many years before showing overt symptoms [1] . In a similar manner, individuals exhibit large variation in parasite infection with some remaining chronic while others are rapidly cleared [2] . The significant variation in the establishment and growth of parasites among individuals is such that few individuals are responsible for a large proportion of the transmission events [3, 4] . One striking example was illustrated by the severe acute respiratory syndrome epidemic where 103 of the first 201 cases were infected by just five source cases-individuals termed super-spreaders, i.e. those that infect an unusually large number of secondary cases [5] [6] [7] [8] . Typically, super-spreaders are those with higher than average contact rates although increased host infectiousness is also implicated in super-spreading events [5, 8] . Individuals with the potential to be highly infectious can be referred to as a 'super-shedders'; individuals that for a period of time yield many more infective stages than most other infected individuals of the same host species [5] . A major challenge in disease biology is to identify some of the mechanisms that may generate super-shedders. One of the drivers of variation in parasite load and host infectiousness is proposed to be underlying secondary infection, or co-infection, the simultaneous infection of an individual with two or more parasitic species [9] [10] [11] . We investigate the hypothesis that co-infection alters the likelihood of parasite establishment, growth and shedding of both parasites and may generate super-shedders. Here, we use the term parasite to include both macroparasites (helminths, protozoa) and microparasites (viruses and bacteria).