Leukocyte response to eastern equine encephalomyelitis virus in a wild passerine bird

Leukocyte counts are frequently used to assess the immunologic status of animals; however, few studies have directly looked at the predictive value of leukocyte counts and an animal's ability to respond to an infection with a pathogen. Understanding how an animal's leukocyte profile is altered by an active infection can assist with interpretation of leukocyte profiles in animals for which infection status is not known. In this study we examine the leukocyte counts of gray catbirds (Dumetella carolinensis) infected with eastern equine encephalomyelitis virus (EEEV). Blood smears were collected from infected catbirds on -4, 2, 5, and 14 days postinoculation (dpi) with EEEV, and from a corresponding uninfected control group, to monitor leukocyte counts. Although we found that preinfection leukocyte counts were not a reliable predictive of a catbird's viremia, we did find that infected catbirds exhibited significant hematologic changes in response to EEEV infection. We observed a significant drop in all subpopulations of leukocytes (i.e., lymphocytes, monocytes, and granulocytes) following infection. Lymphocytes and granulocytes still had not recovered to preinfection levels at 14 dpi. Uninfected catbirds also exhibited statistically significant changes in leukocyte counts, but this was due to a slight increase at 14 dpi and was not considered biologically relevant. Studies such as this can provide important information for field ecoimmunologists that use leukocyte counts to assess immunocompetence in free-living animals.