Host-to-host transmission studies help decipher infection risk of large-scale contact patterns in populations of threatened Gopherus agassizii

For directly transmitted infectious diseases, contact networks present an opportunity to predict pathogen transmission though wildlife populations using natural social patterns. Care must be taken, though, to properly... [ view full abstract ]

For directly transmitted infectious diseases, contact networks present an opportunity to predict pathogen transmission though wildlife populations using natural social patterns. Care must be taken, though, to properly represent transmission networks. Certain hosts or contacts may disproportionately cause new infections and incorporating this variation in transmission risk may be critical to accurately model some infectious diseases. With data on transmission efficiency, contact networks can be made to reflect both natural variation in contact patterns and transmission probability per contact to produce more realistic models of host-to-host transmission. We documented transmission and contact variation in the threatened desert tortoise Gopherus agassizii. We first created heterogeneity in Mycoplasma agassizii exposure (a bacteria causing respiratory disease) by varying the duration of interactions between infected and uninfected captive desert tortoises. Using qPCR, we identified new infections and compared models of transmission probability as a function of contact duration and pathogen load of transmitting hosts. Models predicted low transmission probability for short interactions, unless the infectious host had a high load of M. agassizii: such hosts were predicted to transmit infection at higher rates with any amount of contact. We then examined the contact patterns of a wild tortoise population using proximity loggers to identify the distribution of contacts with high predicted transmission risk. We observed predominantly short-lived interactions and thus, expect transmission patterns in this population to vary considerably with the frequency and duration of high infection levels. Mean field models may misrepresent natural transmission patterns in this and other populations depending on the distribution of high-risk contact and shedding events. Rapid outbreaks in generally solitary species may result from changes to their naturally low-risk contact patterns or due to increases in the frequency of severe infections or super shedding events– population characteristics that should be further investigated to develop effective management strategies.