Gastrointestinal helminths increase Bordetella bronchiseptica shedding and host variation in supershedding

Co-infected hosts, individuals that carry more than one infectious agent
at any one time, have been suggested to facilitate pathogen transmission,
including the emergence of supershedding events. However, how the host
immune response mediates the interactions between co-infecting pathogens
and how these affect the dynamics of shedding remains largely unclear. We
used laboratory experiments and a modeling approach to examine temporal
changes in the shedding of the respiratory bacterium Bordetella
bronchiseptica in rabbits with one or two gastrointestinal
helminth species. Experimental data showed that rabbits co-infected with
one or both helminths shed significantly more B. bronchiseptica,
by direct contact with an agar petri dish than rabbits with bacteria
alone. Co-infected hosts generated supershedding events of higher
intensity and more frequently than hosts with no helminths. To explain
this variation in shedding an infection-immune model was developed and
fitted to rabbits of each group. Simulations suggested that differences in
the magnitude and duration of shedding could be explained by the effect of
the two helminths on the relative contribution of neutrophils and specific
IgA and IgG to B. bronchiseptica neutralization in the respiratory tract.
However, the interactions between infection and immune response at the
scale of analysis that we used could not capture the rapid variation in
the intensity of shedding of every rabbit. We suggest that fast and local
changes at the level of respiratory tissue probably played a more
important role. This study indicates that co-infected hosts are an
important source of variation in shedding, and provides a quantitative
explanation of the role of helminths in the dynamics of respiratory
bacterial infections.