Evaluation of 793/B-like and Mass-like vaccine strain kinetics in experimental and field conditions by real-time RT-PCR quantification

Abstract

Infectious bronchitis virus (IBV) is a great economic burden both for productive losses and costs of the control strategies. Many different vaccination protocols are applied in the same region and even in consecutive cycles on the same farm in order to find the perfect balance between costs and benefits. In Northern Italy, the usual second vaccination is more and more often moved up to the chick's first d of life. The second strain administration together with the common Mass priming by spray at the hatchery allows saving money and time and reducing animal stress. The present work compared the different vaccine strains (Mass-like or B48, and 1/96) kinetics both in field conditions and in a 21-day-long experimental trial in broilers, monitoring the viral replication by upper respiratory tract swabbing and vaccine specific real time reverse transcription PCR (RT-PCR) quantification. In both field and experimental conditions, titers for all the vaccines showed an increasing trend in the first 2 wk and then a decrease, though still remaining detectable during the whole monitored period. IBV field strain and avian Metapneumovirus (aMPV) presence also was also investigated by RT-PCR and sequencing, and by multiplex real-time RT-PCR, respectively, revealing a consistency in the pathogen introduction timing at around 30 d, in correspondence with the vaccine titer's main decrease. These findings suggest the need for an accurate knowledge of live vaccine kinetics, whose replication can compete with the other pathogen one, providing additional protection to be added to what is conferred by the adaptive immune response.

Infectious Bronchitis Virus Evolution, Diagnosis and Control

“…A multitude of RT-PCR and qRT-PCR-based methods have been validated, either generic and targeting virtually all IBV subtypes, or genotype- or strain-specific [ 64 , 65 , 66 , 67 , 68 ]. The most commonly targeted region is the S1 gene, where the genetic variability featuring IBV variants is concentrated [ 3 ].…”

“…Field strains quantification may help to discriminate an incidental detection from a disease primarily caused by IBV, while vaccine strain titres can be used to assess vaccine coverage, replication and persistence. The description of vaccine kinetics in longitudinal studies may be of great aid in characterizing different vaccination protocols, replication dynamics of the various vaccines and potential interactions when co-administered [ 68 ].…”

“…Different strains may have a different tropism [ 76 ], thus samples collected from different districts may lead to different results. Pooled samples are typically considered an acceptable cost/benefit trade-off to obtain information about the epidemiological status of a group of birds as an epidemiological unit, while individual samples should be analyzed to assess the vaccine coverage or prevalence of a field virus within a flock [ 68 ].…”

“…In a typical field situation, the common adoption of multiple live vaccines may complicate the diagnostic process and mask the simultaneous presence of field strains [ 68 , 74 , 82 ]. For this reason, the implemented vaccination protocol should be taken into account while interpreting the results.…”

“…Alternatively, an accessory evaluation of vaccine kinetics [ 68 ] and coverage [ 124 , 125 ] can be carried out by molecular assays (i.e., qRT-PCR), which quantify the vaccine titres replicating in the bird respiratory tract along the productive cycle. Because of the common persistence of the vaccines and the expected competition between vaccine and field strains for cellular receptors [ 68 , 82 ], the vaccine yield could be considered as a proxy of vaccination quality and protection, in addition to classical immunity assessment. Nevertheless, vaccination strategies in the field are often far from coherent, standardized or constant [ 74 , 126 ], even within the same country where the local epidemiological scenario is known.…”