The serogroup of EHDVs, which includes EHDV-1 and EHDV-2 in the United States, is made up of double-stranded RNA viruses in the family Reoviridae, genus Orbivirus.1 These viruses infect some species of wild and domestic ruminants and are transmitted between infectious and susceptible individuals via the bite of midges in the Culicoides genus. Infection can lead to epizootic hemorrhagic disease, characterized by a febrile illness with oral and hoof lesions. Infection with EHDV can be severe and cause large die-offs, particularly in wild deer populations. It is one of the most important diseases of white-tailed deer (Odocoileus virgineanus) that occurs in the United States.2
The EHDV serogroup is biologically and morphologically similar to the serogroup of BTVs, of which 5 are known to circulate in the United States.1 Bluetongue viruses cause severe disease in naïve sheep populations. Concern over their potential spread has led BTV-free countries to impose restrictions on the movement of live animals and germplasm from countries where BTV is endemic.3 The antigenic similarity between EHDV and BTV can result in cross-reactivity in the AGID test, one of the prescribed serologic tests used to certify animals as BTV free.1
Understanding of EHDV distribution in the United States has come primarily from wildlife surveillance.4 Most reports of EHDV-related illness have come from the southeastern United States. Infection is thought to be extremely low or absent in New England and the northern states bordering the Great Lakes. This distribution roughly matches that of BTV in the United States.5
In naturally infected cattle, clinical signs of epizootic hemorrhagic disease are rare. However, sporadic cases of disease have been reported.5,6 Epizootic hemorrhagic disease virus was the suspected cause of ≥ 3 outbreaks in cattle of the United States as follows: in Oregon in 1969, in Tennessee in 1972, and in Colorado in 1972.7 In 1993, clinical illness was observed in cattle in West Virginia that were seropositive for EHDV-2 but seronegative for BTV. Additionally, EHDV-2 was isolated from a die-off of 200 deer that occurred at that time in the same area.8 Several studies have evaluated cattle that were experimentally infected with EHDV-1 or EHDV-2.8–11 In each of these studies, none of the infected cattle developed clinical illness. However, all cattle developed viremias of variable duration (maximum 44 days), suggesting that cattle might serve as reservoir hosts and play a role in the transmission cycle of EHDV.
Little is known about the epidemiology of EHDV in cattle. Most information has come from clinical specimens submitted to the National Veterinary Services Laboratory in Ames, Iowa.5–7 These data suggest a similar geographic distribution to that seen in susceptible wildlife species. At least 2 attempts have been made to describe the prevalence of EHDV infection in cattle populations. A 1980 serosurvey of 1,068 cattle in Georgia, by use of the AGID test, found evidence of EHDV infection in 42% of samples.12 A serosurvey of 4,610 cattle from British Columbia and Alberta, Canada, in 1987, by use of the AGID and serum neutralization tests found antibodies against EHDV in 3% of samples.13 Nearly all of the samples with positive test results were traced to cattle that originated from the Okanagan Valley of southern British Columbia, adjacent to the state of Washington.
The epidemiology of EHDV in cattle in the lower Midwest, including Illinois and Indiana, has not been described. Results of a study in cattle in this region found that seroprevalence of antibodies against BTV increased in a north to south gradient, reflecting the transitional nature of this region as part of the zone between areas of high transmission in the southern United States and little or no transmission in the northern United States.14 The objectives of the study reported here were to estimate seroprevalence of antibodies against EHDV in cattle and to describe the spatial distribution of antibodies against EHDV in cattle herds located in Illinois and western Indiana. We hypothesized that seroprevalence would be higher in the southern parts of the study area. A serologic survey of cattle was conducted with a sample collection strategy that was designed to test this hypothesis, and statistical modeling and geostatistical methods were used to investigate local variation in the distribution of seroprevalence of antibodies against EHDV.
Agar gel immunodiffusion
Epizootic hemorrhagic disease virus
Local indicator of spatial autocorrelation
Trimble GeoExplorer, Trimble Navigation Ltd, Sunnyvale, Calif.
Stata Statistical Software Release 8.0, StataCorp LP, College Station, Tex.
SPSS, version 13.0 for Windows, SPSS Inc, Chicago, Ill.
GeoDa, version 0.9.5-i, University of Illinois, Urbana-Champaign, Ill.
ArcGIS, version 9.0, Environmental Systems Research Institute, Redlands, Calif.
VarioWin, version 2.21, Yvan Pannatier, Laussane, Switzerland.
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