"Animal reservoirs" refers to animals that can carry and spread diseases without getting sick themselves. They act as a source of infection and may pass on these diseases to humans or other animals. Full definition
The outbreak started from a single transmission event from an unknown animal reservoir into the human population. (science.sciencemag.org)
Factors affecting the level of risk of heartworm infection include the climate (temperature, humidity), the species of mosquitoes in the area, presence of mosquito breeding areas, and presence of animal reservoirs (infected dogs, foxes, coyotes). (vippetcare.com)
It is also important to improve understanding of the spread of the disease among key decision makers, so effective control policies can be implemented in animal reservoirs.» (sciencedaily.com)