The pig industry was in a state of chaos and despair when PCV2 first appeared on the scene, just over a
decade ago. That time was characterised by increased mortality, severe losses and little knowledge as to the disease's origin. Ten years later, the key word is increased productivity. This series aims to shed a light on how the road towards control of PCV2 vaccination helped improve the understanding of pig health in general.
Part 1. How can PCV2 vaccine efficacy be measured in the field?
Vaccination to control Porcine Circovirus 2 (PCV2) has sometimes been described as working miraculously. Science, however, is the basis behind successful vaccination. Understanding the background of the process helps shed a light onto what can and cannot be expected when turning to vaccination.
By Prof John C.S. Harding, Western College of Veterinary Medicine, University of Saskatchewan, Canada
PCV2 piglet vaccination has shown to be highly efficacious in preventing Porcine Circovirus Disease (PCVD) around the globe. To judge the efficacy of PCV2 vaccination in the field, reduced mortality and treatments, reduced number of culls, improved growth performance, and in some farms, reduced medication usage are the most relevant parameters to measure. Piglet vaccines do not prevent PCV2 infection, nor do they consistently result in high antibody levels, yet they are still very effective when used according to label instructions in protecting pigs against PCV2a and PCV2b challenge until market.
There are currently up to three Porcine Circovirus type 2 (PCV2) piglet vaccines licensed (full or restricted) in various countries throughout the globe. These vaccines are produced using slightly different manufacturing technologies, but are all recommended for administration at or around weaning either as a one- or two-dose administrations. Peer-reviewed research demonstrating the efficacy of these vaccines is expanding and clearly demonstrates that, when administered according to the manufacturer's instructions; reduce PCV2-associated mortality, lesions, viraemia and viral load, in addition to improving growth rates. While piglet vaccination is reported to be 'near miraculous' in some cases, PCV2 vaccines are not silver bullets and expectations need to be managed. This article aims to answer three important questions: What should you expect from PCV2 vaccines? What should you not expect from PCV2 vaccines? How can you measure vaccine efficacy in the field?
Vaccination of piglets at or around weaning undoubtedly decreases PCV2-associated mortality occurring after the onset of viraemia. Results may obviously vary according to the farm's health status and management, but the results of 11 studies published since 2007 demonstrate mortality improvements between 50% and 90% (Figure 1), using Ingelvac CircoFlex by Boehringer Ingelheim. Since the onset of viraemia typically occurs between four and 13 weeks of age, most of the mortality improvement would be expected in the grower and finisher stages.
A growing body of literature indicates that improvements in growth performance would also be expected following piglet vaccination. The results of eight field studies reviewed indicate that average daily gain increased on average by about 9.5% or about 60 g/day following vaccination. Some of these studies also reported less variability in finisher weights (Figure 2), reduced numbers of 'light weight' pigs, and improvements in feed conversion ratio and carcass quality.
Importantly, vaccination in the presence of maternally derived antibodies (MDA) is effective. This is based on a small number of peer-reviewed studies investigating vaccine response in piglets with high and low levels of MDA (Table 1). Although most of these studies were conducted using piglets of unvaccinated sows, one non-peer-reviewed study recently evaluated the performance of vaccinated piglets derived from vaccinated or non-vaccinated sows.
In this study, vaccinated piglets from vaccinated sows performed as well as vaccinated piglets from non-vaccinated sows, indicating that sow vaccination did not have a negative influence on piglet vaccination. While promising results, more research is needed in this area.
What not to expect?
While most accept PCV2 vaccines as a valuable tool in a herd health programme, they are not a substitute for poor production practices. The Madec 20 principles that were at least partially effective in controlling PMWS in many European herds prior to the use of PCV2 vaccine, are therefore also useful in reducing the impact of PCVD.
Since PCV2 is an immunosuppressive virus, it may increase the risk of other co-infections. Thus, successful disease prevention strategies, particularly in grow-finish herds, must target PCV2 as well as all other significant pathogens. Importantly, PCV2 vaccination does not always prevent viraemia, although we should expect that pigs vaccinated at or around weaning will not develop high levels of viral load, and therefore will be at lower risk of becoming ill.
Thus, if unacceptable levels of mortality continue after a PCV2 vaccination programme is implemented additional diagnostics to identify significant co-infections should be undertaken. In such investigations, it was recently found that selective PCV2 testing of tissues and/or sera from affected and non-affected animals using immunohistochemistry and/or quantitative PCR was useful in ruling out the involvement of PCV2. These investigations helped these producers address other health issues and get mortality back on target.
Interestingly, the level of virus in sera of vaccinated animals does not appear to affect growth performance whereas in non-vaccinated pigs high viral load in serum may be associated with decreases in average daily gain (Figure 3). More research is needed in this area however, since many other factors also influence average daily gain (ADG) on commercial farms.
Vaccine efficacy in the field?
When administered at or around weaning (about three weeks of age) PCV2 vaccination should protect until slaughter. Because PCV2 exposure typically coincides with the waning of passive immunity, it is not necessary to determine the time course of PCV2 infection using serology (testing for antibodies) or PCR (testing for PCV2 DNA) prior to implementing a successful vaccination program. Serology is not an effective tool for testing the efficacy of PCV2 vaccines because one-dose vaccines do not always induce seroconversion, sooner or later all pigs seroconvert to PCV2, and the level of antibody is not a good indicator of protective immunity. Nevertheless piglet vaccines are very successful in stimulating protective immunity, and though all based on genotype PCV2a appear to protect against genotype PCV2b as well.
To judge the efficacy of PCV2 vaccination in the field, reduced mortality and treatments, reduced number of culls, improved growth performance, and in some farms, reduced medication usage are the most relevant parameters to measure. They are all easily measured on commercial farms and each has a direct impact on the financial situation. While the economic benefits will obviously vary with each situation, one peer-reviewed study reported the economic benefit of PCV2 piglet vaccination was as high as US$9.85 per pig in a non-complicated case of PCVD.
References available on request
Source: Pig Progress Volume 25 nr 7