Modified live-virus vaccines for oral immunization of wildlife
Several types of modified live-virus vaccines have been proposed for the oral immunization of animals in the past 20 years; however, only five have proved suitable for use in the field for vaccination of foxes (Canada and Europe) and racoon dogs (Finland). All these vaccines are derivatives of the original SAD virus (for safety considerations).
Four SAD-related vaccines (ERA, SAD-Bern, SAD-B19, and Vnukovo-32) are pathogenic for adult mice (by the intracerebral, intramuscular and oral routes), and for many other rodent species. They do not appear to be pathogenic for North American and European carnivores and other large mammals when they are given by the oral route, except in the case of skunks.
SAG vaccine is a deletion mutant of SAD developed using selected monoclonal antibodies. SAG vaccine is pathogenic neither for adult mice nor for any wild rodents tested by the oral, intramuscular or intracerebral routes; however, it is pathogenic for suckling mice when given by the intracerebral and oral routes.
The SAD-Bern vaccine was used in plastic capsules stapled to chicken head baits in the first large scale field trial ever carried out. Between October 1978 and October 1990, 1.3 million such baits were distributed in Switzerland. Continual surveillance led to the detection of three cases of vaccine-induced rabies. No other vaccine-related deaths were noted in over 900 animals examined.
The SAD-B19 vaccine has been widely used in the field. Since 1983 millions of baits containing this virus have been distributed in Europe (including Belgium, France, Germany, Italy, Luxembourg and a number of Eastern Europen countries) with no reported deaths among non-target species.
Other vaccine strains (SAG 1, SAG 2, ERA, Vnukovo-32) have been distributed in certain Western European countries, Canada and the Russian Federation.
Safety assessments for target and non-target species. The candidate vaccine strain should be characterized according to procedures recommended for rabies vaccines for veterinary use.
The vaccine chosen should not produce any disease in 10 young (3-6 months old) animals belonging to the target species when administered orally at 10 times the dose recommended for field use.
The possibility of excretion of vaccine virus in the saliva of the animals described above should also be examined. Following immunization, swabs should be taken daily. No virus should be present after 3-4 days. Any virus recovered should be characterized using monoclonal antibodies.
In addition, where feasible, at least 10 and if possibly 50 of each of the most common local rodent species should be fiven the field dose of vaccine (i.e. the dose which is contained in a bait) orally and intramuscularly (this may require use of different virus concentrations and volumes for different species, depending on their weight and size). No more than 10% of the animals so vaccinated should exhibit sickness or die from rabies.
Relevant local wild or domestic animal species that may take baits should be examined using monoclonal antibodies to ensure that no vaccine-induced rabies has occurred.