To read more content about aviNews International March 2025
To read more content about aviNews International March 2025
With the use of vector vaccines growing across the globe, the topic of plaque-forming units (PFUs) continues to be a misunderstood term in the poultry industry.
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.
PFUs UNIQUE TO EACH VACCINE
The amount of PFUs per dose relates to how adapted the vaccine virus is to cell culture. Even though all commercial HVT vaccines are derived from the FC-126 strain isolated by Witter, 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.
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.
The results demonstrated that by reducing the dose to 1,500 PFU, some vaccines performed better than others (Figure 1).
Figure 1. Replication of rHVT in chickens administered at 1,500 PFU in ovo.
A conventional HVT and three rHVTS were administered in ovo (IO) at a reduced dose of 1,500 PFU. The recommended dose for each vial is indicated above the bars. PI= protective index when challenged with vv+MDV strain 648A at day of age. Different lower case letters above the bars indicate that differences found were significantly different (P < 0.05).
Alternatively, increasing the dose might not be good in some cases. Adding a very high dose of Marek’s disease (MD) vaccine (i.e., CVI988) when mixing with a recombinant HVT (rHVT) has been shown to result in reduced rHVT replication, which emphasizes the relevance of following manufacturer recommendations (Figure 2).
Figure 2. Protocols need to be optimized to maximize rHVT replication ensuring MD protection.
A recombinant HVT (rHVT) at the recommended dose (RD) was administered either in ovo (IO) or at day of age via subcutaneous (SC) alone or in combination with CVI988 at two different doses (SRD=standard recommended dose or DD= double dose). PI= protective index when challenged with vv+MDV strain 648A at day of age. Different lower case letters above the bars indicate that differences found were significantly different(P < 0.05).
PFU VARIABILITY
MD 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.
A study demonstrated that the PFU dose-to-dose variability at the time of reconstitution in each vial is between 10% and 34%. PFU ratings are averages.
Figure 3. Vaccine dose variability.
Dose variability of a MD vaccine at the time of reconstitution. Each bar represents a vaccine dose.
Effect of time and mixing on vaccine dose variability. Blue bars (0) represent titration under optimal conditions; red bars (60-Mixing) represent titration 1 hour postreconstitution with continuous mixing; black bars (60- Not mixing) represent titration 1 hour postreconstitution with no mixing.
Vaccine dose variability can get worse as time goes by. The moment a vaccine is reconstituted, it has the maximum number of PFUs.
Over time, they begin to drop off. Improper handling and mixing as well as the use of antibiotics in combination with an HVT can lead to a very inconsistent PFU levels. In some cases, chickens might not receive enough dosage to confer protection.
Figure 4. Vaccine dose variability
PFU VARIABILITY
Overall, each HVT product – whether conventional or recombinant – is unique in their ability to protect against MD, while the rHVT have the advantage of providing immunity against other viral diseases.
However, storing, handling, and administration of the vaccines are critical, and it is important to follow manufacturer’s instructions. 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.
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