Experimental screening studies on rabies virus transmission and oral rabies vaccination of the Greater Kudu (Tragelaphus strepsiceros)

Abstract

Rabies in the Greater Kudu (Tragelaphus strepsiceros) in Namibia is unique and found in such magnitude as has not been reported elsewhere in southern Africa. Reasons as to why Kudus appear to be exceptionally susceptible to rabies still remain speculative at best. Because the current severe rabies endemic in Kudus continues to have an enormous negative impact on the Namibian agricultural sector, we set out to question existing dogmas regarding the epidemiology of the disease in a unique experimental setting. In addition, we explored effective measures to protect these antelopes. Although we were able to confirm high susceptibly of kudus for rabies and sporadic horizontal rabies virus transmission to contact animals, we contend that these observations cannot plausibly explain the rapid spread of the disease in Kudus over large territories. Since parenteral vaccination of free-roaming Kudus is virtually impossible, oral rabies vaccination using modified life virus vaccines with a high safety profile would be the ultimate solution to the problem. In a proof-of-concept study using a 3rd generation oral rabies virus vaccine construct (SPBN GASGAS) we found evidence that Kudus can be vaccinated by the oral route and protected against a subsequent rabies infection. In a second phase, more targeted studies need to be initiated by focusing on optimizing oral vaccine uptake and delivery.

Figure 1

To the right: Map depicting the location of Namibia in Africa (right, dark grey). To the left: Enlargement of the middle regions of Namibia showing the locations of the three conservancies the Kudus were captured and of the experimental holding facility; 265 km northwest of Windhoek. Names of the Namibian districts are indicated.

Figure 2

Aerial photography of the experimental holding facility at the Okosongoro Safari Ranch showing the individual pens as well as the supply buildings and staff quarters (A). To prevent contact with free-roaming wildlife from adjacent areas, the entire experimental holding facility and surrounding area was secured by a single 3.2 m high wire mesh game proofed fence. Overhead catwalks enabled staff to better observe and manipulate animals (B). Individual pen (“boma”) (7 m × 14 m) consisting of a covered - and an open area (C). The sliding door could be used to connect the pen with an adjacent pen for separating single animals. The height of the indoor part of the pen was 3 m and the outdoor section was surrounded by a 3 m high wall.

Figure 3

Figure displaying the experimental design for both the transmission (A) and the vaccination studies (B) as a timeline of events and planned target. The blue dots represent individual time points of interventions in terms of capturing, vaccination, challenge and blood sampling (BS).

Figure 4

Survival curves of Kudu infected with a high dose (black solid line) and low dose (black dashed line), contact animals from the transmission group (red line) and the Kudu vaccinated by direct oral application (DOA, blue line). The median incubation period for high and low dose infected Kudus was 13.5 days and 233 days, respectively. Among animals from the DOA group that succumbed to rabies the median incubation period was 15 days. The difference in the survival of vaccinated vs. high dose infected animals from the transmission study was below the statistical level of significance (Log-rank/Mantel-Cox test, p = 0.0786).

Figure 5

Graphical display of individual serological data from parenterally (full grey circles) and orally (blue triangles) vaccinated Kudus using ELISA (A) and RFFIT (B) over the course of the experimental study. B0 = blood sample at day of capture; B1 = day 28 p.v., B2 = day 56 p.v.; B3 = day 183 post infection (challenge). The interpretation of the values as positive, indeterminate and negative are indicated.