Canine distemper-immunization with avianised virus

Abstract

1. In a brief review of the literature, methods which have been advocated for the immunisation of dogs against distemper and possible reasons for failures in immunity are discussed. 2. The propagation of Green's distemperoid virus in developing chick embryos is described. Conditions found most favourable for multiplication, were injection of eight-day-old embryos by the chorio-allantoic membrane route and incubation at 35° C. An inoculum with a high virus titre was most satisfactory for maintaining the derived Onderstepoort strain in embryonated eggs. 3. Continued serial passage of the Onderstepoort virus in developing chick embryos, resulted in a loss of contagiousness for ferrets by the 25th passage. 4. By the 130th egg-passage the degree of attenuation was such that the Onderstepoort virus could safely be used for the immunisation of both dogs and ferrets. 5. Immunity tests in ferrets and dogs vaccinated with the Onderstepoort virus, showed close immunological similarity between this strain and those obtained from encephalitic forms of distemper. 6. The keeping qualities of the Onderstepoort virus were examined. (a) Macerated infected chorio-allantoic membranes showed considerable decrease in potency after storage at 32°C. for 24 hours, and possibly complete loss after 48 hours. (b) Freeze-dried preparations of infected membranes showed very little loss of potency after storage at 37°C. for seven days provided buffered lactose-peptone solution was employed for making suspensions prior to freeze-drying. (c) Freeze-dried preparations retained potency for at least 81 days when stored at -15° C. 7. Titration of viral activity made in eggs, ferrets and dogs indicated that approximately 500 egg infective doses are required for the immunisation of both ferrets and dogs. 8. Immunisation of dogs with Onderstepoort virus has been undertaken on a large scale and to date approximately 40,000 doses of vaccine have been used in Southern Africa. The results have proved satisfactory. 9. It has become apparent that some artificially or naturally immunised dogs are liable to develop the encephalitic form of distemper. As the pathogenesis of this clinical manifestation is obscure, the necessity for determining the significance of the duration of immunity, the neurotropic affinities of various virus strains, concurrent infections and the general state of health of dogs prior to infection, is stressed.