Epidemic disease in Double-crested Cormorants in Saskatchewan
An outbreak of a virus infection of the brain and spinal cord (encephalitis and myelitis) occurred in August 2012 at a large colony of Double-crested Cormorants (DCCO) on Doré Lake in the southern boreal forest of Saskatchewan. The cause of the outbreak was avian paramyxovirus-1 (APMV-1), a virus that occurs in DCCO across their range in North America and regularly results in mortality of nearly full grown young-of-the year DCCO close to the end of the nesting season. Acutely-infected cormorants show a range of signs associated with damage to the central nervous system, as shown in this short video:
Many birds die during outbreaks, but some recover with various forms of paralysis. At affected colonies, it is common to see birds attempting to take off and fly with only one wing due to paralysis of the other wing, and birds “walking” on their wings or unable to dive due to leg paralysis. Total mortality appears to reach about 50% of hatch year birds in some outbreaks. The surviving birds with paralysis also will die when winter arrives. Only young cormorants appear to suffer disease from APMV-1. Affected birds generally are 5-8 weeks of age; by 16 weeks of age, they no longer show signs of disease when they become infected.
APMV-1 infection has occurred on this same breeding colony many times since it was first noted in 1995. Although large numbers of American White Pelicans nest here among the cormorants, and several species of gull, ravens, crows and Bald Eagles regularly visit the colony and feed on the dead cormorants, significant mortality and clinical disease due to APMV-1 has only been seen in the cormorants.
Some strains of APMV-1 can cause severe disease in chickens and these strains are then referred to as “Newcastle disease” viruses. It has not yet been determined whether or not the 2012 virus in Saskatchewan DCCO falls into this category.