THE PROGRAMME► Research ► Disease Control

Oral vaccination of wolves: bait trails
Prepared by Darryn Knobel, April 2005

The 2002/2003 denning season saw work begin on the development of a technique to protect wolves against rabies using new oral vaccines. If successful, such technology would allow wolves to be vaccinated safely and cheaply against rabies. In the past, control of the killer disease has relied on the immunisation of domestic dogs in and around wolf habitat. These dogs are responsible for maintaining rabies and other diseases such as canine distemper, which are then passed on to the wolves. Successful control of rabies relies on vaccinating at least 70% of the dog population - a difficult task when dealing with local people who are not compliant. Although this target has been met through the hard work of the EWCP Vet Team, difficulties in catching highly aggressive dogs for vaccination has lead researchers to seek unique alternatives to disease control. One such solution could be the use of oral vaccines, both in domestic dogs and in the Ethiopian wolves themselves.

Trials thus began in December 2002 to devise a bait capable of delivering the oral rabies vaccine to the wolves. Such a bait would need to be acceptable to both adult wolves and to pups, and would need to be thoroughly chewed rather than being gulped down whole. This is because the vaccine needs to be absorbed through the mucous membranes of the mouth - if it is swallowed whole it is destroyed by the acid in the stomach.

Trials were conducted using six wolf packs in Web Valley, BMNP from January-March 2003. Four bait types (two commercially produced and two locally available) were tested in a series of preference trials. Rodents (the wolves' natural prey) were found to be the most preferred bait and all subsequent trials were therefore conducted using this bait type. Rodents were also shown to be capable of delivering a placebo oral vaccine: 6/9 (66.7%) adult wolves and 4/6 (66.7%) pups showed staining of the oral cavity and tongue following ingestion of a rodent containing a 2-ml sachet of methylene blue dye as a vaccine placebo.

Loss of rodent baits to diurnal non-target species proved problematic. In 130 hours of observation, 36 rodents were taken by non-target species. All bait losses were to raptors or corvids (black kite Milvus migrans 17, augur buzzard Buteo augur 4, tawny eagle Aquila rapex 4, Montagu's harrier Circus pygargus 4, fan-tailed raven Corvus rhipidurus 3, Eurasian marsh harrier Circus aeruginosus 3, lanner falcon Falco biarmicus 1). More than 50% of bait losses occurred between 09h00 and 13h00. Placement of baits around dens in the mid-afternoon should therefore reduce bait losses to raptors.

Rodents therefore provide a potential means for delivering oral rabies vaccine to Ethiopian wolves. They fulfil the criteria of an ideal bait, being acceptable to both adults and pups, and being inexpensively obtained through snap-trapping within the wolf ranges. Vaccination of wolves as young as ten weeks is achievable through this method. Although younger pups also set upon them with vigour, they often do not chew them sufficiently to rupture the vaccine capsule. Older pups, however, develop the adult habit of biting the rat on the neck to `kill' it. Trials using a dye as a vaccine placebo showed that more than two-thirds of pups aged 10-12 weeks could be vaccinated by placing the capsule under the skin of the rodent's neck. This is an ideal age, as the pups are still at the den and are therefore easy to find. These results are a promising step towards protecting wolf populations from the threat of rabies.

This work forms the basis to Darryn's PhD which he is carrying out in the Centre for Tropical Veterinary Medicine in the University of Edinburgh.