Vaccine Formulations for Oral Immunization of Laboratory Animals and Wildlife Against Rabies

Summary

As part of a program to control rabies in Ontario wildlife (mainly foxes and skunks), we are attempting to develop a vaccine that can be incorporated into baits for dispersal in rabies-endemic areas. Primarily for safety reasons, we have been concentrating on the development of immunity following intestinal placement of inactivated vaccine, although work on enteric delivery of live vaccine is also being undertaken.

Lawson and coworkers (Can. J. Comp. Med. 46 (1982) 382) and we (Babins et al., in preparation) have shown that approximately 30% of foxes given two doses of inactivated ERA vaccine directly into the duodenum, by endoscope, develop specific IgG class serum antibodies. Vaccine fed orally, however, must be protected against the proteolytic activity/acid pH of the stomach. We have developed a rapid and inexpensive method for the preparation of enteric-coated inactivated vaccine in the form of 2–3 mm diameter microspheres (Maharaj et al., J. Pharm. Sci., in press). These preparations are only now being tested in foxes; however, administered orally to guinea pigs, the microspheres elicited neutralizing antibody titres ranging from 0.15–1.4 I.U. in 13/15 animals. After a further oral dose at 14 days, all animals responded with titres ranging from 0.31–3.3 LU.

Mice were found to develop good antibody responses following oral administration of liquid inactivated vaccine alone (i.e., no enteric coating). Animals fed the equivalent of 4 ml crude cell culture fluid (conc. 10 ×), in 2 doses 14 days apart, seroconverted by 28 days. Saponin (1–10 mg/ml), administered orally with the vaccine, increased antibody titres up to 10 fold, occasionally even more. Radiolabelling experiments indicated that the saponin increased the uptake of vaccine into the bloodstream.

A method has been developed to coat # 5 gelatin capsules evenly with cellulose acetate phthalate (CAP; an enteric-coating material also used in microsphere production). Radiographic studies on foxes fed CAP-coated capsules containing barium sulfate have shown that the capsules are completely able to withstand stomach conditions for at least 5 hours, and to disintegrate rapidly upon release into the intestine. Similar CAP-coated capsules have been prepared containing lyophilized, live vaccine (Convac-ERA: Connaught Laboratories, Toronto). About 1 log. of infectious virus is lost in the CAP-coating step: at present, we can prepare capsules containing 10⁴ PFU Convac-ERA, although efforts are being made to increase this titre. Studies on the ability of these capsules, containing live virus, to immunize foxes, are under way.

This work was performed under contract for the Minister of Natural Resources for the Province of Ontario, Canada.