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.
