The vector vaccine Poulvac® Procerta™ HVT-ND can be a valuable addition to vaccination strategies against Newcastle disease virus (NDV) genotype VII — a virulent form of NDV (vNDV) which can cause heavy losses in poultry production from the Middle East and North Africa to Southeast Asia.

NDV’s genetic diversity, bringing increased virulence, has posed difficulties in providing satisfactory vaccine coverage for the industry’s programs using conventional live and inactivated ND vaccines. But a new class of vaccines using the herpesvirus of turkey (HVT) to act as a viral vector to express NDV antigen F has raised hopes of improved performance.

Independent studies in Thailand and Egypt, using local isolates as a challenge virus, highlight the improved efficacy of vaccination programs when Poulvac Procerta HVT-ND is used in combination with live and inactivated vaccines — an approach likely to be used by producers facing high-risk and complex vNDV field challenges.

Strong protection using vector vaccine

In research carried out at Chulalongkorn University in Bangkok,[i] 210 broilers were divided into three groups: one where 70 birds received Poulvac Procerta HVT-ND with a live ND vaccine, the second with 70 birds receiving both a live and inactivated ND vaccine in addition to Poulvac Procerta HVT-ND. The third group was unvaccinated. All vaccines were administered at 1 day of age.

Twenty birds in each group were challenged with vNDV genotype VIId at each timepoint of 21 or 28 days of age, with post-challenge clinical signs and mortality monitored for 14 days.

Between 21 and 35 days of age, no mortality was seen in the group receiving the combined vaccination of the vector, live and inactivated ND vaccines, while there was 5% mortality in the group receiving just the vector and live ND vaccines. There was 30% mortality among unvaccinated, challenged birds.

“When we compare with real vaccination programs on farms in Thailand, on top of day of age vaccination, they still vaccinate with a live ND vaccine as a booster between day 10 and 14,” explained Nida Sirikobkul, PhD, senior poultry technical manager, Zoetis.

“Here, the results show that Poulvac Procerta HVT-ND in combination with a live vaccine or both live and killed ND vaccines means that producers only need to vaccinate on day of age to still have 95% to 100% protection of commercial broilers against field challenge of vNDV genotype VIId.”

Rigorous sampling shows enduring performance

To check for the presence of vNDV, oropharyngeal and cloacal swabs were taken at 14 days post-challenge of each timepoint, with detection by one-step, real-time polymerase chain reaction (PCR) tests and a molecular approach known as restriction endonuclease analysis.[ii]

At day 35, vNDV was detected in 60% of the sampled birds that had received vector and live ND vaccines. At day 42, this had dropped to 40%. For the group receiving three vaccines, the results were 50% and 30%, respectively, while the virus was found in 100% of unvaccinated, challenged birds sampled at both days 35 and 42.

Spleens were collected from five birds per group at 7 and 14 days post-challenge to check for presence of the HVT vector.

At both days 7 and 14, four out of five birds that had received Poulvac Procerta HVT-ND with a live ND vaccine tested positive, while in the group receiving three vaccines, four out of five tested positive at day 7 and all five at day 14. This indicated that the recombinant HVT vaccine undergoes swift replication within a short period and persists in birds. The HVT genome was absent in spleen samples of control birds sampled at both time points.

Antibodies against the F protein of NDV after vaccination, before and after the challenge at both timepoints, were tested by a commercial NDV-F ELISA test kit.

The group receiving all three vaccines showed the highest titers against the F protein of NDV, significantly different from the group receiving only vector and live vaccines. Both vaccinated groups had significantly higher titers against NDV F protein than unvaccinated birds. There was no significant difference in titer levels of NDV F protein between the vaccinated groups by day 42; however, both groups had significantly higher levels than unvaccinated birds.

“The vaccine virus was detected until almost the slaughter age, which is consistent with the concept of this vaccine. Vaccinating using the vector vaccine should ensure it stays a long time in the birds,” Sirikobkul said.

Bodyweight gain was also measured, with the group receiving the vector vaccine in combination with live and inactivated ND vaccines seeing the greatest gains by both days 35 and 42. By day 42, unvaccinated, challenged birds had, on average, a significant weight loss.

“The strong performance using the vector vaccine alongside live and inactivated ND products fits well with what’s happening in broiler operations right now, where almost all of the broiler producers use what could be called a ‘maximum protection’ program,” Sirikobkul said.

“Now, some customers in Thailand have taken the inactivated vaccine out of their ND vaccination program. I think this is not more than 5% to 8%; however, it’s still growing in percentage, depending on the area and disease situation,” she added.

Protection against Egypt’s vNDV genotype VII.1.1 put to test

Research in Egypt[iii] was based on a similar premise: testing Poulvac Procerta HVT-ND both alone and in combination with other conventional live and inactivated vaccines.

While sampling methods were broadly similar to the work in Thailand, here, a greater number of vaccines were administered, including at days 7 and 14. Five different vaccination programs simulating the actual farm practices were tested on groups of commercial broilers, with two unvaccinated control groups. An isolate specific to the Middle East, known as velogenic NDV genotype VII.1.1, was used, with birds challenged at 28 days of age.

While all the vaccination programs were able to induce antibody levels before challenge and reduce post-challenge clinical signs, mortality, lesions and challenge-virus shedding, the best performance was observed when Poulvac Procerta HVT-ND was used alongside a live vaccine on days 1 and 14 of age and a killed NDV vaccine on day 7 of age.

This combined approach showed complete protection (100%) against mortality, an absence of clinical signs, higher antibody levels pre- and post-challenge, reduced virus shedding and lower gross and microscopic lesion scores after the challenge.

Reducing mortality, boosting antibodies

After vaccination, blood samples were taken randomly at 1 and 2 weeks post-challenge for antibody testing. Brain, trachea, spleen and bursa were collected from dead birds after challenge and from surviving birds in each group at 7 days post-challenge.

Tracheal and cloacal swabs were collected from 10 birds per group at 28, 30, 32 and 35 days of age for detection of challenge-virus shedding by real-time reverse transcription PCR, while birds in each group were weighed pre-challenge at 21 and 28 days old and at 1 and 2 weeks post-challenge.

At 28 days of age, the highest level of mortality was seen in the group which received a boosted, live ND vaccine without vector vaccine (33.3% at 7 days post-challenge). In contrast, there was no mortality in the group receiving Poulvac Procerta HVT-ND plus the boosted, live vaccines and killed vaccine. Of the unvaccinated, challenged birds, 90% died.

Birds receiving a live vaccine followed by live booster, without vector vaccine, had the highest proportion with respiratory and digestive disorders, while the lowest numbers were again seen in the group receiving vector, live and inactivated vaccines.

Vector product brings added benefits

In vaccinated groups, the birds receiving just Poulvac Procerta HVT-ND had the lowest antibody titers before challenge. Groups receiving the vector vaccine plus either a live vaccine plus live booster or a live vaccine, booster and inactivated vaccine had the highest. Fourteen days post-challenge, birds which received a vaccination program including the vector vaccine had higher titers than those without.

“Impressively, the group that was challenged after only receiving Poulvac Procerta HVT-ND showed a significant decrease in shedding of the challenge virus from 7 days post-challenge. Only 20% of the birds were shedding, with very low titers,” said Dr. Khaled Mahgoub, PhD, Zoetis Egypt poultry technical manager.

Notably, the vector vaccine has demonstrated effectiveness in the presence of maternally derived antibodies in commercial broilers, he said, with birds vaccinated using the vector product expressing high antibody titers against the NDV F protein at 4 weeks of age.

“The velogenic nature of NDV genotype VII.1.1 means that, for many producers, the use of one type of vaccine is not considered sufficient to provide optimum levels of protection against the high virulence form of Newcastle disease,” he added.

“Even with both conventional live and killed vaccines on the market, this study clearly shows that Poulvac Procerta HVT-ND adds beneficial effect to ND prevention and control programs.”