When More Transmission Equals Less Disease: Reconciling the Disconnect between Disease Hotspots and Parasite Transmission

Abstract

The assumed straightforward connection between transmission intensity and disease occurrence impacts surveillance and control efforts along with statistical methodology, including parameter inference and niche modeling. Many infectious disease systems have the potential for this connection to be more complicated–although demonstrating this in any given disease system has remained elusive. Hemorrhagic disease (HD) is one of the most important diseases of white-tailed deer and is caused by viruses in the Orbivirus genus. Like many infectious diseases, the probability or severity of disease increases with age (after loss of maternal antibodies) and the probability of disease is lower upon re-infection compared to first infection (based on cross-immunity between virus strains). These broad criteria generate a prediction that disease occurrence is maximized at intermediate levels of transmission intensity. Using published US field data, we first fit a statistical model to predict disease occurrence as a function of seroprevalence (a proxy for transmission intensity), demonstrating that states with intermediate seroprevalence have the highest level of case reporting. We subsequently introduce an independently parameterized mechanistic model supporting the theory that high case reporting should come from areas with intermediate levels of transmission. This is the first rigorous demonstration of this phenomenon and illustrates that variation in transmission rate (e.g. along an ecologically-controlled transmission gradient) can create cryptic refuges for infectious diseases.

Vector species richness increases haemorrhagic disease prevalence through functional diversity modulating the duration of seasonal transmission

“…We also make use a disease data set collated at the University of Georgia which records occurrence of white-tailed deer morbidity and mortality associated with HD at the county level from 1980-present (detailed in Park et al 2013). Here, we restrict our analysis to 2008-2012, in line with data on the Culicoides species identified from light trap surveys.…”

Detection in and circulation of Bluetongue virus among domestic ruminants in Madagascar

“…The lack of clinical manifestation may be due to this high seroprevalence. Indeed, Park et al, have demonstrated that areas with intermediate seroprevalence have high number of cases reporting whereas low case reporting was associated with a high seroprevalence (Park et al, 2013). Only one unpublished study has evaluated seroprevalence of BTV infection in small ruminants of Madagascar (Rasamoelina-Andriamanivo, Dakar: UCAR, 2005).…”

Apparent Increase of Reported Hemorrhagic Disease in the Midwestern and Northeastern Usa

“…In white-tailed deer (Odocoileus virginianus), which is the species most affected by BTV and EHDV in North America, clinical disease and potential population impacts are highly variable throughout their range (Howerth et al 2001). In general, the disease is most severe in more northern latitudes, where HD occurs sporadically and where population immunity is minimal (Howerth et al 2001;Park et al 2013). In contrast, areas of enzootic stability exist in portions of the southwestern US, where high infection rates occur annually, but clinical disease is rarely reported Flacke et al 2004).…”