As the use of recombinant or vector vaccines for poultry continue to grow around the world, misperceptions around a vaccine’s plaque-forming unit (PFU) level are leading to inaccurate conclusions about vaccine protection.
Plaques form when virus infects cells. Marek’s disease virus is able to spread to adjacent cells without killing them. However, this infection leads to changes in the morphology of the infected cells forming what is known as plaques. Counting plaques in cell culture is the gold standard method to establish Marek’s disease vaccine titers (plaque-forming units or PFUs).
Frequently, high PFU levels of herpesvirus of turkey (HVT) vaccines are inaccurately associated with greater protection; and worse, PFU levels are used to determine a vaccine’s possible dilution rate, said Isabel M. Gimeno, DVM, PhD, professor, North Carolina State University.
However, PFUs are not a measure of vaccine efficacy and there is no magic PFU number that guarantees protection. The number of PFUs necessary to achieve the greatest efficacy depends on the product, said Gimeno, an expert in vector vaccine research.
PFUs unique to each vaccine
The amount of PFUs per dose relates to how adapted the vaccine virus is to cell culture, Gimeno said. Even though all commercial HVT vaccines are derived from the FC-126 strain isolated by Witter,1 each vaccine manufacturer has its own seed virus and will do several passages to get to commercial levels. Depending on several factors – cell type use, days between passages, number of passages and culture media, among others – viruses adapt differently to grow in cell culture.
“The more adapted the virus is, the higher PFUs per dose can be obtained. On the other hand, better growth in cell culture might lead to less replication in vivo,” she said.
It is critical to follow the manufacturer’s recommended dosage for an HVT vaccine and not dilute it based on its PFU level. Any change in the recommended dose could result in subpar results, as shown in a 2016 study.2 In this study, different HVT products, including one conventional and three recombinant HVTs, were administered in ovo at 1,500 PFUs regardless of what the recommended dose was for those vaccines. Some vaccines performed better than others when reducing the dose to 1,500 PFUs.2
Alternatively, increasing the dose might not be good in some cases, she said. Adding a very high dose of a Marek’s disease vaccine such as CVI988 when mixing with a recombinant HVT (rHVT) was shown to result in reduced rHVT replication, which emphasizes the relevance of following manufacturer recommendations.3
PFU variability
Marek’s disease vaccines are cell associated, meaning that there is a mix of infected (5%-20%) and non-infected (80%-95%) cells in the vial instead of free infectious viral particles. Hence, they are extremely fragile (the virus dies if the cell dies) and there is great variability on the vaccine dose, Gimeno explained.
She pointed to a 2019 study that demonstrated the PFU dose-to-dose variability at the time of reconstitution in each vial is between 10% and 34%.4
“PFU ratings are averages. Don’t think that because a vaccine has a particular PFU that every single dose has this PFU level,” she said.
Over time, the number of PFUs per dose can get more variable. The moment a vaccine is reconstituted, it has the maximum number of PFUs, which start to drop off. Improper handling and mixing, as well as the use of antibiotics in combination with an HVT, can lead to very inconsistent PFU levels, Gimeno said. In some cases, chickens might not receive enough dosage to confer protection.
Getting the most from vector vaccines
Overall, rHVT vaccines have been shown to be as efficacious as conventional HVT vaccines with the advantage of providing immunity against other viral diseases, such as infectious bursal disease or Newcastle disease, Gimeno concluded. To get the most out of a vector vaccine program, the focus should be on vaccine handling, preparation and timely administration – factors under the control of poultry veterinarians.
References:
1 Witter RL, Nazerian K, Purchase HG, Burgoyne GH. Isolation from turkeys of a cell-associated herpesvirus antigenically related to Marek’s disease virus. Amer J Vet Res. 1970;31:525-538.
2 Gimeno IM, Cortes AL, Fais N, et al. Efficacy of Various HVT Vaccines (Conventional and Recombinant) Against Marek’s Disease in Broiler Chickens: Effect of Dose and Age of Vaccination. Avian Dis. 2016;60(3):662-668.
3 Gimeno IM, Cortes AL, Reilley A, Barbosa T, Alvarado I, Koopman R, Martinez A. Study of Efficacy and Replication of Recombinant Vector Vaccines by Using Turkey Herpesvirus Combined with Other Marek’s Disease Vaccines. Avian Dis. 2019;63(2):335-341.
4 López de Juan Abad B, Cortes AL, Correa M, et al. Evaluation of Factors That Influence Dose Variability of Marek’s Disease Vaccines. Avian Dis. 2019;63(4):591-598.
About Zoetis
As the world’s leading animal health company, Zoetis is driven by a singular purpose: to nurture our world and humankind by advancing care for animals. After innovating ways to predict, prevent, detect, and treat animal illness for more than 70 years, Zoetis continues to stand by those raising and caring for animals worldwide – from veterinarians and pet owners to livestock farmers and ranchers. The company’s leading portfolio and pipeline of medicines, vaccines, diagnostics and technologies make a difference in over 100 countries. A Fortune 500 company, Zoetis generated revenue of $8.5 billion in 2023 with approximately 14,100 employees. For more information, visit www.zoetis.com.
