How do invasive species alter native parasite transmission?
Invasive rats transmit human disease, destroy crops and decimate native fauna. The introduced black rat, Rattus rattus, occurs throughout the native range of the raccoon roundworm, Baylisascaris procyonis, and may be incorporated into the life cycle of this zoonotic nematode if rats consume parasite eggs, acquire viable infections, and are subsequently eaten by raccoons. I tested the potential for this introduced species to amplify parasite transmission by first monitoring animal behavior at raccoon latrines to quantify differences in species contact rates and infection risk. I then surveyed both rodents and raccoons for parasites and examined the trophic links between these intermediate and definitive hosts by conducting scavenger trials using wildlife cameras.
Compared to native mice, rats occupy more contaminated habitat and have greater contact rates with latrines. This exposure leads to viable infections with rats harboring mean parasite intensities 100 times greater than that of the most abundant co-occurring native rodents. Ninety percent of rodent carcasses are picked up by a vertebrate scavenger within the six day lifespan of larval worms in decomposing carrion; however the majority of these are eaten by the introduced Virginia opossum. Although competition with one introduced species may reduce transmission, even when raccons rarely consume rodents, the exceptionally high worm burdens in invasive rats may significantly increase the total parasite population.