Fox Squirrels (Sciurus niger) Develop West Nile Virus Viremias Sufficient for Infecting Select Mosquito Species

Abstract

The West Nile virus (WNV) viremia and shedding profiles of 11 adult fox squirrels (Sciurus niger) infected by needle inoculation or mosquito bite were characterized. Daily mean titers (95% confidence intervals) for all squirrels on days 1 through 6 postexposure (p.e.) were: 10(1.7 (1.32.1)), 10(4.4 (4.04.8)), 10(5.3 (5.05.6)), 10(4.4 (3.94.9)), 10(2.7 (2.03.4)), and 10(1.1 (0.52.1)) plaque-forming units (PFU)/mL. The highest WNV serum titers of individual squirrels infected by needle inoculation or mosquito bite ranged from 10(4.5) to 10(6.1) and from 10(5.1) to 10(5.3) PFU/mL, respectively. Nine (82%) squirrels, including all 4 squirrels infected by mosquito bite, had WNV serum titers > or =10(5.1) PFU/mL that persisted on average for 1.6 +/-0.3 days. Infection and dissemination rates of Culex pipiens (L.) that fed on squirrels with serum titers of 10(4.4 +/-0.1) PFU/mL were 56% and 13%, respectively. Both of these rates increased to over 80% when fed on squirrels with a mean WNV titer of 10(5.5 +/-0.1) PFU/mL. Infection and dissemination also occurred in Aedes triseriatus (Say) but at a much lower rate. WNV was isolated from the oral and rectal cavities of all squirrels and from urine that was opportunistically collected from 5 squirrels. The largest quantity of WNV recovered from swabs of the oral cavity and urine was 10(3.1) PFU. The longest periods after exposure that WNV was isolated from the oral cavity and urine from a squirrel were 22 and 17 days p.e., respectively. WNV RNA was also detected in kidney tissue in 1 squirrel 29 days p.e., suggesting that fox squirrels can be persistently infected. Collectively these observations provide further evidence that squirrels can contribute to the natural history and epidemiology of WNV, especially in peridomestic environments. 1 PFU/mL were 56% and 13%, respectively. Both of these rates increased to over 80% when fed on squirrels with a mean WNV titer of 10 5.5 ؎ 0.1 PFU/mL. Infection and dissemination also occurred in Aedes triseriatus (Say) but at a much lower rate. WNV was isolated from the oral and rectal cavities of all squirrels and from urine that was opportunistically collected from 5 squirrels. The largest quantity of WNV recovered from swabs of the oral cavity and urine was 10 3.1 PFU. The longest periods after exposure that WNV was isolated from the oral cavity and urine from a squirrel were 22 and 17 days p.e., respectively. WNV RNA was also detected in kidney tissue in 1 squirrel 29 days p.e., suggesting that fox squirrels can be persistently infected. Collectively these observations provide further evidence that squirrels can contribute to the natural history and epidemiology of WNV, especially in peridomestic environments.

Arboviruses in North Dakota, 2003–2006

“…It increased to 35.5% and 60.0% in early September. Aedes vexans may become infected by feeding on small or medium-sized mammals, which have been reported to have specific antibodies to WNV, 47 shown to produce viremias sufficient to infect mosquitoes, [48][49][50][51] and found naturally infected in the field. 52 The risk that humans will acquire an infectious bite was greatest from the middle of July through the end of August with transmission still occurring in early September (Supplemental Table 4).…”

Ecology of West Nile Virus Transmission and Its Impact on Birds in the Western Hemisphere

“…Laboratory infection studies to estimate host competence have been published for 44 species of nondomesticated birds in 23 families and 11 orders Reisen et al 2005aReisen et al , b, 2006Reisen et al , 2007aClark et al 2006;Nemeth et al 2006;Owen et al 2006;Reisen and Hahn 2007;Platt et al 2008), 3 species of wild mammals (Tiawsirisup et al 2005b, Root et al 2006, Platt et al 2007, and 5 species of reptiles and one amphibian Komar 2003, Klenk et al 2004). In these experiments, animals are infected by either allowing infectious mosquitoes to feed on them or by an intramuscular or subcutaneous injection of virus.…”

Experimental Infection of Raccoons (Procyon lotor) with West Nile Virus

“…For example, viremia titers of ≥ 10 5.0 plaque forming units (PFU)/mL serum were observed in some experimentally infected golden hamsters ( Mesocricetus auratus ). 11 In addition, fox squirrels ( Sciurus niger ), [12][13][14] eastern gray squirrels ( Sciurus carolinensis ), 15 eastern chipmunks ( Tamias striatus ), 16 and eastern cottontail rabbits ( Sylvilagus floridanus ) 17 developed viremia titers considered sufficient to infect some mosquitoes. 18 Raccoons ( Procyon lotor ) are common throughout many urban and suburban communities in the United States.…”

“…[12][13][14][15][16][17] The potential importance of these animals should not be a priori discounted, as the apparent insignificance of wild mammals in WNV ecology may be from lack of scrutiny rather than from lack of significance. 24 We conducted experimental infections of raccoons with WNV.…”