Studies on the live-virus vaccine against infectious laryngotracheitis of chickens. II. Evaluation of the tissue-culture-modified strain C7 in laboratory and field trials.

Laboratory and field tests were conducted to evaluate the immunogenicity of a live-virus vaccine against infectious laryngotracheitis (ILT) of chickens that was prepared using tissue-culture-modified strain C7. Eighty-three percent or more of chickens vaccinated by the ocular (OC) or intranasal (IN) route with 10(4.75) TCID50 of the attenuated strain C7 at 50 days of age were protected against challenge with a virulent strain of ILT virus without showing any clinical signs for 4 weeks post-vaccination (PV). When vaccine was administered by aerosol, however, only 65% of vaccinated chickens were protected against challenge. Fifty-seven percent of chickens vaccinated at 70 days of age maintained immunity for 6 months PV. Immune response in younger chickens was inferior to that in older ones. In the field trials, clinical observation revealed no adverse reactions caused by the vaccination, and 60% or more of broilers and 80% or more of layers vaccinated by the OC route were protected against challenge at 4 weeks PV. These results confirmed the safety and efficacy of the vaccine.

Studies on a live virus vaccine against infectious laryngotracheitis of chickens. I. Biological properties of attenuated strain C7.

The biological properties of tissue-culture-modified strain C7 of infectious laryngotracheitis (ILT) virus were studied. Strain C7 formed well-defined small white pocks that were distinguishable from those formed by the original strain when it was inoculated onto the chorioallantoic membrane of embryonating chicken eggs (pock marker). Strain C7 grew in chicken kidney cell cultures and embryonating chicken eggs better than the original strain. When inoculated intratracheally into chickens, strain C7 propagated primarily in the larynx, trachea, and lung, but the virus was unrecoverable after 6 days postinoculation. No contact infection was demonstrated among chickens. These biological characteristics of strain C7 were stable through five consecutive back-passages in susceptible chickens. In chickens vaccinated ocularly at 14 days of age or older, strain C7 gave good protection against challenge with the virulent strain of ILT virus. These results indicate that strain C7 may be an adequate candidate for a live virus vaccine against ILT.

Pathogenicity of infectious laryngotracheitis virus as measured by chicken embryo inoculation.

A comparison was made between pathogenicities for chicken embryos of unattenuated and attenuated strains or isolates of infectious laryngotracheitis (ILT) virus. All 11-day-old chicken embryos inoculated with 10(3.0) or 10(4.0) TCID50 of unattenuated strain NS175 via allantoic cavity died within 6 days. On the contrary, no chicken embryos of the same age died when inoculated with the same amount of cell-culture-attenuated isolate C7 in a like manner. The mortality index for chicken embryos (MICE) was obtained by dividing the cumulative number of embryos dying within 7 days by the cumulative number of embryos surviving 7 days. The reliability of the MICE test was confirmed by duplicate and triplicate experiments with strain NS175 and isolate C7. MICE obtained in the experiments with 9 different strains or isolates of ILT virus ranged from 0 to 1, and the values were well correlated with the pathogenicities for chickens. The results from the present work suggest that strains or isolates with MICE less than 0.16 would have low or no pathogenicity for chickens, and those with MICE more than 0.27 would be highly pathogenic. Further studies are needed using additional isolates of ILT virus with varied pathogenicities.