Bluetongue Virus Serotypes 1 and 4 in Red Deer, Spain

We studied the potential of red deer as bluetongue maintenance hosts and sentinels. Deer maintained detectable bluetongue virus (BTV) serotype 4 RNA for 1 year after the virus was cleared from livestock. However, the virus was not transmitted to yearlings. BTV serotype 1 RNA was detected in red deer immediately after its first detection in cattle.

ing the mating period (August-September in Spain), which is also the period of maximal activity for Culicoides imicola mosquitoes. Therefore, all of these facts, together with the capability of wild ruminants to overcome BT infection and their free-range life, make deer suitable for BTV maintenance. We hypothesize that 1) BTV RNA would be detectable in red deer even after its control in livestock by vaccination, and 2) the virus or specifi c antibodies would be detected in red deer early after its detection in livestock.

The Study
The study site was a deer farm with ≈900 hinds, including 550 adult hinds and 350 yearling hinds. This farm is located in the Los Alcornocales Natural Park in the Cádiz Province (Andalucía, southern Spain; 36°17′N, 5°47′W), an area near the sea that is <500 m above sea level. Abundant wild red deer and moderate densities of roe deer (Capreolus capreolus) are present in the area.
Blood samples were collected by cervical puncture from 510 living farmed red deer, placed in sterile tubes containing EDTA, and frozen at -20°C. Samples from adult deer hinds (n = 160) were obtained on July 12 and 13, 2007; yearling stags (n = 350) were sampled on August 28, 2007.
We tested 200 serum samples by using a competitive viral protein 7 (VP7) ELISA (Institute Pourquier, Montpellier, France). The samples were analyzed in duplicate according to the manufacturer's instructions.
Of the analyzed serum samples, 57.60% showed positive results in the ELISA. The prevalence of BTV antibodies was high; 92.45% of the adults were positive. All yearling deer were ELISA negative except for 3 doubtful samples; all of them had negative results in the BTV, BTV-1 and BTV-4 RT-PCR assays (Figure 1).
Of the adult deer, 25% showed positive results in the BTV group-specifi c PCR. Positive samples were sequenced to confi rm the presence of BTV nucleic acid and further analyzed for the identifi cation of the serotype. Six RNA samples from adult deer were positive for the BTV-4-specifi c RT-PCR, and their sequences were confi rmed by using BLAST software (http://blast.ncbi.nlm.nih.gov/Blast.cgi). None of the samples from adult deer were positive either for BTV-1-specifi c or EHD-specifi c RT-PCRs. Yearlings, however, showed a different pattern of results: 16.33% animals showed positive results in the group-specifi c and the BTV-1-specifi c RT-PCRs. No yearling samples were positive by the BTV-4 specifi c RT-PCR.
No visible clinical signs were noticed, and no deaths occurred. This result suggests that, although adult deer maintained circulation of BTV-4 RNA, this serotype did not infect the yearlings despite the presence of the vector and the optimal conditions for infection in the study area. Surprisingly, several animals were positive to the EHDspecifi c assay. However, when the PCR products were purifi ed and sequenced, none of the obtained sequences showed homology with published EHD sequences. These results support those found by Agüero et al. (11), in which BTV-1-positive samples cross-reacted with the available EHD primers. The amplifi ed PCR product obtained had approximately the same size as the PCR product expected for EHD, thus giving a false-positive result.

Conclusions
Our results agree with what was found in livestock during surveillance programs: adult animals had probably been in contact with BTV-4 during the outbreak that started in southern Spain in 2004. In contrast to the vaccinated domestic ruminants, deer were able to maintain BTV-4 RNA, thus confi rming our initial hypothesis. However, detection of BTV RNA without concurrent virus isolation does not mean that deer are a long term reservoir host of BTV (12). Simultaneous evaluation of adjacent cohorts of domestic and wild ruminants by using the same virus detection assays will be required to unambiguously defi ne the precise role of wildlife in the epidemiology of BTV infection.
Yearling deer were apparently infected with BTV-1, which has been present in Spain since 2007. When epidemiologic information about the study area was compared with the information for the deer samples analyzed, evidence was found supporting our results: adult deer were sampled on July 12, 2007, and yearlings were sampled August 20, 2007, i.e., 26 days after BTV-1 presence was confi rmed at 60 km distance from the deer farm (www.oie.int/ wahis/reports/en_imm_0000005799_20070726_123322. pdf) (Figure 2). Thus, adult deer had been sampled when BTV-1 was not present in the country yet. In contrast, yearlings were already positive to BTV-1 only 26 days after this serotype was fi rst reported in livestock in the same area. There are 2 explanations for this fi nding: 1) BTV-1 is a highly pathogenic serotype (13), causing high death rates in sheep, that may also cause high death rates in deer; and 2) deer and other wild ruminants may be highly susceptible to BTV infection, thus, making them good sentinels for this disease. However, BTV-1 was detected earlier among sentinel cattle than among deer.
Regarding EHD, despite the negative results obtained, lack of robust molecular tools for its detection is noteworthy. All available RT-PCRs are based on the sequences of EHD strains that have never been detected in the Mediterranean area. Results from yearlings were negative; results from adults showed an age-increasing trend of contact with BTV. Bars represent 95% confi dence intervals for prevalence (binomial exact, Clopper-Pearson).