No abstract

The influence of vaccine strain and antigen mass on the ability of inactivated avian influenza (AI) viruses to protect chicks from a lethal, highly pathogenic (HP) AI virus challenge was studied. Groups of 4-week-old chickens were immunized with inactivated vaccines containing one of 10 haemagglutinin subtype H5 AI viruses, one heterologous H7 AI virus or normal allantoic fluid (sham), and challenged 3 weeks later by intra-nasal inoculation with a HP H5 chicken-origin AI virus. All 10 H5 vaccines provided good protection from clinical signs and death, and produced positive serological reactions on agar gel immunodiffusion and haemagglutination inhibition tests. In experiment 1, challenge virus was recovered from the oropharynx of 80% of chickens in the H5 vaccine group. In five H5 vaccine groups, challenge virus was not recovered from the cloaca of chickens. In the other five H5 vaccine groups, the number of chickens with detection of challenge virus from the cloaca was lower than in the sham group (P < 0.05). Reductions in the quantity of challenge virus shed from the cloaca and oropharynx were also evident in some H5 vaccinate groups when compared to the sham group. However, there was no positive correlation between the sequence identity of the haemagglutinin gene from the vaccine strain and challenge virus, and the ability to reduce the quantity of challenge virus shed from the cloaca or oropharynx. As the quantity of AI antigen in the vaccines increased, all parameters of protection improved and were virus strain dependent. A/turkey/Wisconsin/68 (H5N9) was the best vaccine candidate of the H5 strains tested (PD50= 0.006 μg AI antigen). These data demonstrate that chickens vaccinated with inactivated H5 whole virus AI vaccines were protected from clinical signs and death, but usage of vaccine generally did not prevent infection by the challenge virus, as indicated by recovery of virus from the oropharynx. Vaccine use reduced cloacal detection rates, and quantity of virus shed from the cloaca and oropharynx in some vaccine groups, which would potentially reduce environmental contamination and disease transmission in the field.

Two replicate experiments were conducted to test the efficacy of two different Salmonella enteritidis oil-emulsion bacterins (an experimentally prepared acetone-killed vaccine and a commercially available vaccine) for protecting laying hens against intestinal colonization following oral exposure to S. enteritidis. Each vaccine was administered twice (4 weeks apart), and all hens were challenged with 10(8) cells of a nalidixic-acid-resistant S. enteritidis strain 2 weeks after the second vaccination. Fecal samples from vaccinated and unvaccinated control hens were cultured at three weekly intervals post-challenge to determine the incidence of intestinal colonization and the numbers of S. enteritidis shed into the environment. Both vaccines significantly reduced the incidence of intestinal colonization (P < 0.05) and the mean number of S. enteritidis cells shed in the feces (P < 0.01) at 1 week post-challenge. However, the degree of protection afforded by vaccination was only partial, as more than half of the vaccinated hens still shed substantial numbers of S. enteritidis. If used in conjunction with other flock sanitation and infection-monitoring strategies, vaccination with bacterins could potentially reduce the overall level of environmental contamination and thereby also reduce the horizontal transmission of S. enteritidis within and between laying flocks.

Newly hatched chicks lack immunological maturity, which could compromise their ability to respond to infection by pathogens such as Salmonella enterica serovar enteritidis (S. enteritidis; SE). A study was conducted in which chicks were infected with a sublethal dose of SE at 1 d posthatch, and the systemic and intestinal immune responses to the challenge were followed over time. Birds infected at this age experienced difficulty in clearing the infection, and 50% of the individual birds remained persistently infected until 23 wk of age. These birds exhibited only a marginal systemic and mucosal humoral immune response to the infection. No response or little response was observed 1 wk postchallenge; responses increased somewhat over time. On many of the sampling times, 50% or more of the culture-positive birds lacked a detectable plasma or intestinal response. Levels of 10(3) to 10(5) SE/g of feces could be found in the intestines of birds eliciting a good IgA response, indicating that, when these birds did respond mucosally, the IgA produced was incapable of clearing the organism once the infection was established. Birds infected during this time also experienced reduced ability to respond to vaccination. Compared with uninfected controls, depressed responsiveness to an S. enteritidis bacterin was observed in infected birds 1 and 2 wk after administration, whereas those individuals receiving an inactivated Newcastle disease vaccine (NDV) experienced a reduced response 4 and 6 wk postvaccination, indicating that the persistent infection affected the ability of the immune system to respond to homologous and heterologous antigens. These results demonstrate that exposure of chickens to SE early in life interferes with the ability of these individuals to respond humorally to the infection and to other antigenic stimuli; such effects can be observed for at least 23 wk.

To assess the potential protective efficacy of a Salmonella enteritidis bacterin, an acetone-killed oil-emulsion vaccine was prepared from a phage type 13a S. enteritidis strain and administered subcutaneously to hens in two experiments. Hens were housed individually, and every other hen was vaccinated (at 23 weeks of age in one experiment and at 45 weeks in the other). A second (booster) bacterin injection was administered 6 weeks later in both experiments. Three weeks after the second vaccination, all hens were challenged with an oral dose of approximately 10(9) cells of a heterologous (phage type 14b) S. enteritidis strain. In both trials, S. enteritidis was isolated from fewer internal organs (spleens, ovaries, and oviducts) and pools of egg contents from vaccinated hens than from unvaccinated control hens. Vaccination did not, however, affect the percentage of hens that shed S. enteritidis in feces in either experiment.