West Nile virus outbreak in North American Owls, Ontario, 2002

Emerging Infectious Diseases, Dec, 2004 by Ady Y. Gancz, Ian K. Barker, Robbin Lindsay, Antonia Dibernardo, Katherine McKeever, Bruce Hunter

[FIGURE 1 OMITTED]

A total of 91 outbreak survivors, which were kept outdoors during the entire outbreak period and not vaccinated against WNV, were tested by ELISA for anti-WNV IgG. Of these, 69 (75.8%) were seropositive. Agreement between ELISA and PRNT results was good with [kappa] = 0.857 (0.58-1.13). The two tests produced conflicting results for 1 of 20 samples; ELISA results are shown in Table 2. Species-specific exposure rates are shown in Table 3. The overall exposure rate was 84.3%.

Being kept outdoors during the outbreak period was found to be a highly significant risk factor (p < 0.0001) for WNV-related death. None of the 10 birds kept indoors died during the outbreak period, despite the fact that 8 of 10 belonged to species that otherwise had very high death rates (four Northern Hawk Owls, three Boreal Owls, and one Northern Saw-whet Owl). These birds were excluded from further analysis.

Species' northern native breeding range and large-to-medium body size were significant risk factors for exposure to WNV (p < 0.05), with OR = 52.56 (95% confidence interval [CI] 3.13-881.84) and OR = 16.82 (95% CI 3.79-74.67), respectively. Age and taxonomy at the subfamily level were not significant risk factors for exposure. Among exposed birds, northern native breeding range was a highly significant risk factor for WNV-related death (p < 0.0001), with OR = 1,507 (95% CI 85.51-26,557). Birds >1 year of age were also more likely to die of WNV (p < 0.05) with OR = 4.87 (95% CI 2.46-9.62). Size and subfamily were not found to be significant risk factors for WNV-related death.

Native breeding range was significantly associated with the date of death (p < 0.01). Northern species died earlier during the outbreak period (mean and standard deviation 23.8 [ or -] 9.8 days, n = 93) compared to other species (35.0 [ or -] 12.9 days, n = 8). Large or medium birds also died earlier during the outbreak period (21.9 [ or -] 8.5 days, n = 77) compared to small species (33.9 [ or -] 11.2 days, n = 24) (p < 0.05). Subfamily and age were not significantly associated with the date of death.

The six louse flies examined were identified as Icosta americana (order Diptera, family Hippoboscidae). WNV RNA was detected by real-time RT-PCR in 16 (88.9%) of 18 flies collected from dead or sick owls during the outbreak period. Five adult flies collected from birds that appeared healthy were WNV negative.

Discussion

WNV RNA was present in most owls that died at the Owl Foundation during the outbreak period, and antibodies against WNV were detected in most outbreak survivors (overall exposure rates 84.3%). The similarity between the epidemiologic curve of the outbreak at the Owl Foundation and that of dead corvid sightings in the Niagara region (Figure 1) and the fact that the initial cases occurred in several cages scattered throughout the Owl Foundation property, suggest that the outbreak was part of a regional WNV activity rather than a point-source introduction (e.g., by admitting a viremic bird). As none of the birds kept indoors were affected, the major route of WNV transmission likely was vector-borne.