Porcine circovirus (PCV) is a common virus of swine, found throughout the world. The detection and emergence of a novel porcine circovirus (porcine circovirus type 2; PCV2) in the early 2000’s has led to PCV2 being an endemic pathogen of swine, by definition a pathogen that we expect all pigs to be infected with at some point during their life. PCV associated disease (PCVAD) was originally described as a postweaning multisystemic wasting syndrome (PMWS). Additional disease presentations have been described throughout the years with various disease manifestations present in all ages of swine. Vaccines have played a critical role in PCV control and are considered the gold standard for prevention and control of PCVAD.
PCV2 and associated PCVAD represent endemic pathogens of swine and their herd impact is always difficult to quantify. While we typically associate disease “outbreaks” with epidemic disease-causing agents such as PED, ASF, etc., novel introduction of heterologous endemic pathogens can absolutely manifest itself as a disease outbreak within a susceptible population, even if that population has partial immunity through heterologous PCV vaccination and/or natural exposure.
Several breed-to-wean (BTW) sow herds have observed a high incidence of PCVAD in recently weaned pigs despite the continuous use of industry standard PCV2 vaccination programs. Each of these farms represent a “closed herd” in which BTW herd replacement animals are produced internally with no outside animal introductions following the initial farm stockings. All replacement gilts and teaser boars were vaccinated with commercial PCV2 vaccine at processing, weaning and again at 20 weeks of age. The PCV2 instability resulted in dramatically increased culls and mortality in growing pigs. In most situations, early observations of PCV2 outbreaks at the sow farm level existed only in the post-weaning piglet population. Nursery aged piglets were diagnosed with PCVAD via RT-PCR and IHC from tissues of affected pigs. The majority of PCV2 outbreaks on BTW farms will go unnoticed at the BTW herd level, without obvious clinical signs or production metric impacts. Occasionally, PCV2 outbreaks will manifest clinical signs and performance impacts at the BTW herd level, with increased percentages of low viability piglets, stillborns and mummies at birth as well as the presence of “wasting” piglets in the internal GDU that fail to respond to medication. In my observations, PCV2 outbreaks at the sow farm have been much more frequent in purebred herds produced parent stock females compared to their commercial cross bred herd counterparts, but this observation is limited in scope and scale and should be taken with a heavy grain of salt.
While PCVAD has had a dramatic impact on the global swine industry over the last two decades, very little work has been done to define industry standard active surveillance and herd categorization systems to reflect the status of PCV2 control in BTW farms. We need more robust work in this area. Agreeing upon categorization systems and associated diagnostic monitoring strategies has been important in our improved management of PRRSv in BTW herds. PCV2 management would benefit from similarly adopted strategies. From my perspective, PCV2 PCR of processing fluids (PF) have been a very useful monitoring tool to both evaluate PCV2 control in BTW herds and predict PCVAD cases in the downstream pig flow. Given the endemic nature of this pathogen, results interpretation of PCV2 PF PCR tests is more nuanced than epidemic disease agents such as PRRS. A positive result is not necessarily a sign of uncontrolled PCV2 within the BTW herd, rather you need routine monitoring to look at the frequency of positive tests as well as the CT counts from positive tests to help infer the volume of piglet viremia present at the time of processing. This monitoring process is admittedly as much art as it is science today, but the ability to monitor changing test outcomes over time has been extremely useful information in monitoring both currently stable herds as well as active herds in which you’ve implemented an intervention plan. For all multipliers I personally work with, we monitor PFs for PCV2 weekly. Sporadic positive test results with CT values >30 do not set off alarms, but a shift in the percentage of positive test results and or CT values of <25 do trigger a deeper investigation into PCV2 control at the BTW herd as well as notification to downstream pig recipients of the potential increase in PCVAD activity.
When PCV2 outbreaks have occurred at BTW farms, the standard intervention has been mass vaccination of all adults and growing gilts in the herd. I have used multiple commercial vaccines for this intervention as well as multiple dosing strategies including partial dose and full dose. Regardless of product or dosing strategy, the intervention appears to be associated with improved PCV2 control as defined by both improvements in PF test results as well as an increase in maternal immunity passed from sow to piglet via colostrum. It should be noted that association is not causation, and the improved control could certainly be a function of herd immunity through active PCV2 infection as much as it is through the efficacy of vaccination.
Abortions are always a concern when administering a mass vaccination to pregnant animals, but to date, I have been fortunate to not see any significant unintended consequences from mass vaccination. The mass vaccination events certainly build a high level of immunity within adults, and as such, decrease sow to pig transmission of PCV2, presumably both in utero as well as post-farrowing. It should be noted this increase in maternal immunity may require a shift in your piglet PCV2 vaccination strategy. Opinions vary on the impact of maternal immunity on PCV2 vaccination efficacy and it does appear there is a difference in the impact across the range of commercial PCV2 vaccines. As a general rule of thumb, I would consider shifting your piglet PCV2 vaccination out of the sow farm and into the nursery after the mass vaccination event has occurred at the BTW farm.
PCV2 IFA titers have been another useful tool to monitor the progression of control status post-intervention. Serial sampling of growing pigs starting at weaning and running end point dilution IFAs allows veterinarians and producers to monitor the impact of the mass vaccination event and determine ideal ages of vaccination for growing pigs. In my opinion, this step is necessary following mass vaccination as vaccinating the piglets in the face of high maternal antibodies may result in vaccination failure. Vaccinating the pigs weeks after maternal antibodies decay, on the other hand, may result in PCV2 infection in the growing pig before the vaccine has a chance to build immunity.
Finally, PCV2 vaccination of growing pigs experiencing a PCVAD outbreak has a therapeutic effect. This observation has been surprising to me, but I have experienced this effect in numerous groups of pigs using multiple different commercial PCV2 vaccines. Groups of pigs with extremely severe PCVAD clinical signs have turned around, and the vast majority of the piglets recovered using a “rescue dose” of PCV2 vaccine. For producers feeding pigs from a BTW farm which has experienced a PCV2 outbreak, all hope is not lost for the current pigs on feed. Implement the rescue dose immediately and consider implementing it routinely in the mid-to-late nursery stage until the BTW outbreak is under control and piglet infection and maternal antibody status are such that normal vaccination timing and dose can be efficacious.
Source: Clayton Johnson, who is solely responsible for the information provided, and wholly owns the information. Informa Business Media and all its subsidiaries are not responsible for any of the content contained in this information asset. The opinions of this writer are not necessarily those of Farm Progress/Informa.