Infectivity periods of different tick-borne pathogens in host species can vary widely, and the variation affects how the pathogens are maintained in tick populations. In addition to systemic and vertical transmission, cofeeding transmission has been proposed as an important route for the persistence of pathogens with short infectivity such as viruses that cause tick-borne encephalitis (TBE). Because cofeeding transmission requires ticks to feed simultaneously, the temporal dynamics of tick populations become important. Existing models of tick-borne diseases do not fully incorporate all three transmission pathways and tick seasonality. We developed a comprehensive stage-structured population model that includes seasonality and evaluated the relative importance of the three transmission pathways for pathogens with short infectivity. We used the next generation matrix method to calculate R0 and performed elasticity analyses for the complex disease systems. We used the ecology of the transmission cycle of TBE-causing viruses involving black-legged or deer tick (Ixodes scapularis) in upper Midwest to the northeast regions of the United States as a model system, although the model is intended to inform broader situations.
Results/Conclusions
We found that cofeeding transmission is a critically important route for such pathogens to persist in seasonal tick populations over the reasonable range of parameter values. At higher but still plausible rates of vertical transmission, our model suggests that vertical transmission can be a strong enhancer of pathogen prevalence when it operates in combination with cofeeding transmission. Overlap between larvae and nymphs can have a large impact on prevalence when vertical transmission is low. The basic reproductive number, R0, calculated from the next generation matrix, indicates that the pathogen would spread quite easily upon new invasion when both vertical and cofeeding transmission pathways take place. Elasticity analysis on the next generation matrix shows that cofeeding transmission has the highest relative contribution to R0 over a large region of the transmission parameter space. Compared to existing models without seasonal structure, pathogen persistence in seasonally-structured tick populations relies more on vertical transmission, which can be explained by reduced larva-nymph cofeeding opportunities. We conclude that cofeeding transmission is integral for pathogen persistence in tick populations when infective period is short and that vertical transmission can become important, depending on seasonal structure of ticks and the transmission rate.
