Serological evidence of infection with dengue and Zika viruses in horses on French Pacific Islands.

New Caledonia and French Polynesia are areas in which arboviruses circulate extensively. A large serological survey among horses from New Caledonia and French Polynesia was carried out to investigate the seroprevalence of flaviviruses in the horse population. Here, 293 equine sera samples were screened for flaviviruses using a competitive enzyme-linked immunosorbent assay (cELISA). The positive sera were then confirmed using a flavivirus-specific microsphere immunoassay (MIA) and seroneutralization tests. This serosurvey showed that 16.6% (27/163) and 30.8% (40/130) of horses were positive for cELISA tests in New Caledonia and French Polynesia, respectively, but the MIA technique, targeting only flaviviruses causing neuro-invasive infections in humans and horses (i.e. West Nile virus [WNV], Japanese encephalitis virus [JEV] and tick-borne encephalitis virus [TBEV]), showed negative results for more than 85% (57/67) of the cELISA-positive animals. Seroneutralization tests with the main flaviviruses circulating in the South Pacific revealed that 6.1% (10/163; confidence interval [95% CI] 3.0%-11.0%) of sera in New Caledonia and 7.7% (10/130; 95% CI 3.8%-13.7%) in French Polynesia were positive for dengue virus serotype 1 (DENV1) and 4.3% (7/163; 95% CI 1.7%-8.6%) in New Caledonia and 15.4% (20/130, 95% CI 9.7%-22.8%) in French Polynesia were found positive for Zika virus (ZIKV). Seroprevalence of the JEV and WNV flaviviruses on the 293 samples from both island groups were comparatively much lower (less than 2%). This seroprevalence study in the horse population shows that horses can be infected with dengue and Zika viruses and that these infections lead to seroconversions in horses. The consequences of these infections in horses and their role in ZIKV and DENV epidemiological cycles are two issues that deserve further investigation.

Resistance of Black-lip learl oyster, Pinctada margaritifera, to infection by Ostreid herpes virus 1µvar under experimental challenge may be mediated by humoral antiviral activity.

Ostreid herpesvirus 1 (OsHV-1) has induced mass mortalities of the larvae and spat of Pacific oysters, Crassostrea gigas, in Europe and, more recently, in Oceania. The production of pearls from the Black-lip pearl oyster, Pinctada margaritifera, represents the second largest source of income to the economies of French Polynesia and many Pacific Island nations that could be severely compromised in the event of a disease outbreak. Coincidentally with the occurrence of OsHV-1 in the southern hemisphere, C. gigas imported from New Zealand and France into French Polynesia tested positive for OsHV-1. Although interspecies viral transmission has been demonstrated, the transmissibility of OsHV-1 to P. margaritifera is unknown. We investigated the susceptibility of juvenile P. margaritifera to OsHV-1 µvar that were injected with tissue homogenates sourced from either naturally infected or healthy C. gigas. The infection challenge lasted 14 days post-injection (dpi) with sampling at 0, 1, 2, 3, 5, 7 and 14 days. Mortality rate, viral prevalence, and cellular immune responses in experimental animals were determined. Tissues were screened by light microscopy and TEM. Pacific oysters were also challenged and used as a positive control to validate the efficiency of OsHV-1 µvar infection. Viral particles and features such as marginated chromatin and highly electron dense nuclei were observed in C. gigas but not in P. margaritifera. Mortality rates and hemocyte immune parameters, including phagocytosis and respiratory burst, were similar between challenged and control P. margaritifera. Herpesvirus-inhibiting activity was demonstrated in the acellular fraction of the hemolymph from P. margaritifera, suggesting that the humoral immunity is critical in the defence against herpesvirus in pearl oysters. Overall, these results suggest that under the conditions of the experimental challenge, P. margaritifera was not sensitive to OsHV-1 µvar and was not an effective host/carrier. The nature and spectrum of activity of the humoral antiviral activity is worthy of further investigation.