Arboviruses in North Dakota, 2003-2006

Abstract

To investigate arbovirus transmission in North Dakota, we collected and screened mosquitoes for viral infection by Vero cell culture assay. Seven viruses were isolated from 13 mosquito species. Spatial and temporal distributions of the important vectors of West Nile virus (WNV), Cache Valley virus, Jamestown Canyon virus (JCV), and trivittatus virus are reported. Snowshoe hare virus, Potosi virus, and western equine encephalomyelitis virus were also isolated. The risks of Culex tarsalis and Aedes vexans transmitting WNV to humans were 61.4% and 34.0% in 2003-2006, respectively, but in 2003 when the largest epidemic was reported, risks for Ae. vexans and Cx. tarsalis in Cass County were 73.6% and 23.9%, respectively. Risk of humans acquiring an infectious bite was greatest from about the second week of July through most of August. West Nile virus sequences were of the WN02 genotype. Most JCV strains belonged to a single clade of genetically related strains. Cache Valley virus and JCV were prevalent during August and early September and during July and August, respectively.

Figure 1.

North Dakota Counties where mosquito traps were placed.

Figure 2.

(A) Weekly number of West Nile virus isolations and infection rates (IR) relative to mean numbers of Culex tarsalis per trap-night, North Dakota, 2003–2006. (B) Weekly number of West Nile virus isolations and infection rates relative to mean numbers of Aedes vexans per trap-night, North Dakota, 2003–2006.

Figure 3.

Relation between onset of human disease and total number of West Nile virus isolates from all mosquito species, North Dakota, 2003–2006.

Figure 4.

Stacked area graphs showing weekly vector index for Culex tarsalis, Aedes vexans, and seven other species relative to weekly numbers of human cases diagnosed with West Nile virus infection 2003, 2004, 2005, 2006, and 2003–2006. The other species were Coquillettidia perturbans, Culiseta inornata, Ochlerotatus dorsalis, Ochlerotatus flavescens, Ochlerotatus spencerii, Ochlerotatus triseriatus, and Ochlerotatus trivittatus.

Figure 5.

Phylogenetic tree depicting relationships of WNV isolates from North Dakota based on ML analysis of NS3 sequences. Branch lengths are proportional to the number of nucleotide substitutions. Numbers at node indicate bootstrap P values ≥ 50%.

Figure 6.

Weekly numbers of Cache Valley virus isolations and infection rates (IR) relative to the mean number per trap-night of Aedes vexans, Aedes cinereus, Anopheles earlei, Culex tarsalis, Culiseta inornata, Ochlerotatus dorsalis, Ochlerotatus flavescens, Ochlerotatus melanimon, and Ochlerotatus trivittatus, North Dakota, 2005.

Figure 7.

Weekly numbers of Jamestown Canyon virus isolations and infection rates (IR) relative to the mean number per trap-night of Aedes vexans, Culex tarsalis, Culiseta inornata, Ochlerotatus dorsalis, Ochlerotatus flavescens, Ochlerotatus melanimon, Ochlerotatus sticticus, and Ochlerotatus trivittatus, North Dakota, 2003–2006.

Figure 8.

Phylogenetic tree showing relationships of Jamestown Canyon, snowshoe hare, and trivittatus viruses from North Dakota based on ML analysis of S-segment nucleotide sequences. Branch lengths are proportional to the number of nucleotide substitutions. Bootstrap P values ≥ 50% are given only at major nodes for clarity.

Figure 9.

Weekly numbers of trivittatus virus isolations and infections rates (IR) relative to the mean number per trap-night of Ochlerotatus trivittatus, North Dakota, 2003–2006.