Persistent Pathogens

Excerpt:

Abstract

Background: The flaviviruses causing tick-borne encephalitis (TBE) persist at low but consistent levels in tick populations,

despite short infectious periods in their mammalian hosts and transmission periods constrained by distinctly seasonal tick

life cycles. In addition to systemic and vertical transmission, cofeeding transmission has been proposed as an important

route for the persistence of TBE-causing viruses. Because cofeeding transmission requires ticks to feed simultaneously, the

timing of tick activity may be critical to pathogen persistence. Existing models of tick-borne diseases do not incorporate all

transmission routes and tick seasonality. Our aim is to evaluate the influence of seasonality on the relative importance of

different transmission routes by using a comprehensive mathematical model.

Methodology/Principal Findings: We developed a stage-structured population model that includes tick seasonality and

evaluated the relative importance of the transmission routes for pathogens with short infectious periods, in particular

Powassan virus (POWV) and the related ‘‘deer tick virus,’’ emergent encephalitis-causing flaviviruses in North America. We

used the next generation matrix method to calculate the basic reproductive ratio and performed elasticity analyses. We

confirmed that cofeeding transmission is critically important 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 strongly enhance pathogen prevalence when it operates in combination with cofeeding

transmission.

Conclusions/Significance: Our results demonstrate that the consistent prevalence of POWV observed in tick populations

could be maintained by a combination of low vertical, intermediate cofeeding and high systemic transmission rates. When

vertical transmission is weak, nymphal ticks support integral parts of the transmission cycle that are critical for maintaining

the pathogen. We also extended the model to pathogens that cause chronic infections in hosts and found that cofeeding

transmission could contribute to elevating prevalence even in these systems. Therefore, the common assumption that

cofeeding transmission is not relevant in models of chronic host infection, such as Lyme disease, could lead to

underestimating pathogen prevalence.

Citation: Nonaka E, Ebel GD, Wearing HJ (2010) Persistence of Pathogens with Short Infectious Periods in Seasonal Tick Populations: The Relative Importance of

Three Transmission Routes. PLoS ONE 5(7): e11745. doi:10.1371/journal.pone.0011745