Grey squirrels host numerous pathogens of both human health and conservation concern. One pathogen of particular interest is squirrel poxvirus, a disease to which grey squirrels show little pathology, but which is causing the extirpation of red squirrels in the British Isles. Squirrel poxvirus originated in North America, but was introduced by humans concomitantly with grey squirrels to the Ireland and the UK.

While squirrel poxvirus is not zoonotic (transmissible to humans), eastern grey squirrels can contract many pathogens that are, including Borellia burgdorferi (the causative agent of lyme disease) and West Nile Virus. However, this shared susceptibility does not necessarily make them a disease risk since:

1. Grey squirrels do not host the reproductive stage of many of these pathogens, and are therefore not infectious. If anything, they may even act as “sponges”, soaking up pathogenic particles in the environment that would otherwise infect people.
2. Many other pathogens require transmission via an insect/arthropod vector species, again meaning that squirrels are not directly infectious. Furthermore, squirrels are efficient groomers and so serve to attract and destroy disease vectors in the environment.
3. There is evidence to suggest that squirrels are particularly poor hosts for some of these pathogens, like West Nile Virus or B. burgdorferi for example. Thus, grey squirrels are unlikely to contribute to the prevalence of infected disease vectors on the landscape.

All of this together indicates that eastern grey squirrels may be providing a substantial ecosystem service by buffering transmission of some zoonotic diseases on the urban landscape.

CitiSci(urid) is interested in understanding how life in the urban environment influences wildlife disease dynamics, using the eastern grey squirrel as a model organism. Increased population densities, coupled with immune defenses compromised by the stress of constant human exposure could increase disease prevalence in cities. Conversely, reduced predation pressure, and immune bolstering by heavy anthropogenic food subsidies may decrease rates of disease transmission in urban populations. Regardless, given their prevalence across Eastern North America, and their shared disease susceptibilities, eastern grey squirrels may be valuable as disease sentinels.