A new viral disease in grey squirrels?

Last November a cluster of mortalities of grey squirrels (Sciurus carolinensis) was reported in the City of Montreal. A concerned citizen, who suspected malicious poisoning, submitted one of the squirrels’ bodies found in her backyard to the CWHC – Quebec regional centre.

The squirrel submitted was an adult female with an average body condition (reduced body fat reserves). The only significant finding was the presence of congested areas covering more than 50% of the lung volume. These areas, which appeared to be somewhat consolidated, were suggestive of pneumonia. The first thing a pathologist will do in order to confirm a suspected disease is to sample tissue for histological analysis. The microscopic examination of the lung confirmed the lesions of pneumonia. These were characterized by a thickening of the walls of the alveoli due to an infiltration of these walls by inflammatory cells and an increase in number of a cell call type II pneumocytes. These type II pneumocytes are cells that are lining the walls of the pulmonary alveoli and are known to proliferate in case of infection. Very unusual syncytial cells (cells that are fused together) were also regularly observed in the lung.

This is a microscopic image of the lung of the affected squirrel showing a marked thickening of the alveolar walls associated with an infiltration by inflammatory cells (the black dots). Multinucleated cells (arrow) are commonly observed. AL: Alveolar lumens.

Because these lesions (interstitial pneumonia with syncytial cells and type II pneumocytes hyperplasia) are suggestive of a viral infection, a frozen sample of lung tissue was sent to the virology lab to be screened for the presence of viruses. Interestingly, a previously undescribed coronavirus was detected in the lung of this animal. Even if this remains speculative at this time, the presence of this virus associated with pulmonary lesions characteristic of viral infection make us believe that this squirrel died of a fatal infection by a coronavirus.

Different species of coronavirus are known to cause pneumonia in different species of hosts such as rats and pigs and people. To our knowledge pulmonary infection of grey squirrels by coronaviruses has never been reported. We are currently trying to better characterise this virus. This case shows that even if we have been studying wildlife diseases for several decades we still have a lot to discover. If there is any wildlife pathologists that have seen similar cases, please contact me.

Stéphane Lair, CWHC – Quebec