Bagaza virus is pathogenic and transmitted by direct contact in experimentally infected partridges, but is not infectious in house sparrows and adult mice

Abstract

Bagaza virus (BAGV) is a mosquito-borne flavivirus belonging to the Ntaya serocomplex. In 2010, a disease outbreak was reported in Cádiz (Southern Spain) affecting game birds (red-legged partridges and common pheasants). In this work, red-legged partridges were inoculated experimentally with infectious BAGV isolated from this outbreak in order to make a complete clinical and analytical assessment of the disease caused by the pathogen in this species. Viral load (by real-time RT-PCR) in blood, oral and cloacal swabs, and feathers, and neutralizing antibody titres (by VNT) were measured. In order to determine direct contact transmission, non-inoculated partridges were caged together with the inoculated ones. To assess infectiousness in other species, house sparrows and mice were also inoculated with the virus. All the inoculated partridges were clinically affected, and 30% of them died. All the infected individuals lost weight, with larger losses being recorded in females. Conversely, no mortality or disease symptoms were observed in the sparrows or mice. Remarkably, all the contact partridges acquired the infection by direct (non-vectored) transmission. This study confirms that the red-legged partridge is a susceptible host for BAGV infection, and that this pathogen is transmitted by direct contact. Long-lasting viral loads detected in calami of immature feathers demonstrate that feather sampling could be a useful strategy in active surveillance programs for early detection of BAGV.

Figure 1

Mortality after inoculation of BAGV in red-legged partridges. Percentage of survivors observed at different days post-inoculation. Closed circles represent BAGV-inoculated partridges. Open squares represent sham-inoculated partridges.

Figure 2

Weight loss of red-legged partridges after inoculation with BAGV. The course of the variation in weight in the different groups of red-legged partridges, expressed as the percentage of initial weight (i.e. weight measured on the day of inoculation), measured at different days post-inoculation. Circles indicate male and squares female. Closed symbols represent BAGV-inoculated partridges, while open symbols represent sham-inoculated partridges. Error bars represent the standard error of the mean.

Figure 3

Mean daily blood viral genome load and viremia titres for BAGV-inoculated red-legged partridges. The course of A blood viral genome load and B viremia titres during the experiment of BAGV inoculation in red-legged partridges, determined at different days post-inoculation. Each point represents the mean obtained for the surviving individuals at different times post-inoculation. Error bars represent the standard error of the mean.

Figure 4

Antibody response to BAGV in serum from BAGV-inoculated red-legged partridges. The course of the antibody response in the individuals inoculated with BAGV, measured on different days post-inoculation, is plotted as a solid line with open squares representing the mean log titres of BAGV-neutralizing antibodies measured in the virus-neutralization test (VNT). Error bars represent the standard deviation of the data.

Figure 5

Viral genome load in swabs (oral, cloacal) and feathers from BAGV-inoculated red-legged partridges. The course of the viral RNA load in each type of sample, measured by real-time RT-PCR specific for BAGV, is represented on different days post-inoculation. Open squares indicate viral load in oral swabs; closed squares viral load in cloacal swabs and closed circles viral load in feathers. Error bars represent standard error of the mean.