Differential virulence of West Nile strains for American crows

Abstract

Crow deaths were observed after West Nile virus (WNV) was introduced into North America, and this phenomenon has subsequently been used to monitor the spread of the virus. To investigate potential differences in the crow virulence of different WNV strains, American Crows were inoculated with Old World strains of WNV from Kenya and Australia (Kunjin) and a North American (NY99) WNV genotype. Infection of crows with NY99 genotype resulted in high serum viremia levels and death; the Kenyan and Kunjin genotypes elicited low viremia levels and minimal deaths but resulted in the generation of neutralizing antibodies capable of providing 100% protection from infection with the NY99 strain. These results suggest that genetic alterations in NY99 WNV are responsible for the crow-virulent phenotype and that increased replication of this strain in crows could spread WNV in North America.

Figure 1

Viremia profiles for West Nile virus (WNV)–infected American Crows after injection of 1,500 PFU of KUN or KEN/NY99 WNV. Viral titers were determined by plaque formation on Vero cells and represented as geometric means. A detection limit of >1.7 log₁₀ PFU/mL crow serum was determined. Bars represent standard deviations (SD) of the mean. hd, high dose.

Figure 2

Survivorship of eight American Crows, each injected with 3.2 log₁₀ PFU of NY99, KEN, or KUN virus. An additional eight crows were injected with a high dose (hd) of the KEN virus (3.8 log₁₀ PFU). Crows were monitored daily for signs of disease through 14 dpi. No deaths were found within the control group (data not shown).

Figure 3

Survivorship of American Crows previously immunized with West Nile virus (WNV)-KUN or WNV-KEN viruses after injection with 1,500 PFU of NY99 WNV. hd, high dose.

Figure 4

Viremia production of American Crows previously immunized with West Nile virus (WNV)-KUN or WNV-KEN viruses after injection with 3.2 log₁₀ PFU of NY99 WNV. No detectable levels of viremia (>1.7 log₁₀ PFU/mL crow serum) developed in the KUN virus–immunized crows (0/8). Data points for the naïve (unexposed to WNV) crows challenged with the NY99 virus represent the mean of three samples chosen randomly. Bars represent standard deviations (SD) of the mean. hd, high dose; PRNT, plaque reduction neutralization assay.