23 August 2023

7 minute read

By: The Pig Site

North America

Key takeaways

• Risk factors that may interfere with PCV2 vaccine efficacy are numerous. The domino effect of an inadequate vaccine response can cause clinical PCV2 or PCV-associated disease (PCVAD) to be seen downstream in vaccinated herds.
• Not all vaccination risk factors are created equal. Identifying risk factors will improve vaccination success, as well as help producers avoid issues with PCVAD downstream.
• PCV2 has the highest rate of mutation reported for a single-stranded DNA virus and recombination can occur causing more changes in the PCV2 genotype.
• Monitoring pigs for PCV2 allows a farm to catch problems before they escalate and avoids risking an unknown herd status change on the sow farm.^1-2

Vaccinating for porcine circovirus type 2 (PCV2) has been shown to be effective in most swine herds, so why is it important to monitor for the virus?

Because PCV2 is such a common, resilient virus, it remains a constant threat — even when we think we have it under control. It’s true that vaccination reduces the presence of PCV2 and is an effective tool. But farms don’t exist in a vacuum, and there’s a laundry list of risk factors that can interfere with a vaccine’s efficacy. Knowing the risk factors that could impede a vaccine’s effectiveness helps to strengthen the farm’s vaccination strategy and bolster its success.

What’s the biggest risk factor that comes to mind?

It’s hard to name just one, but a co-infection with porcine reproductive and respiratory syndrome virus (PRRSv) is certainly one of the most common. We know PRRSv is a virus that can be immunosuppressive. Having that instability in a herd can hamper the likelihood of optimal response to any vaccine.

The domino effect of an inadequate vaccine response can cause clinical PCV2 or PCV-associated disease (PCVAD) to be seen downstream in vaccinated herds. Routine monitoring for PCV2 therefore gives farms a measure of where they’re at and potentially catches a problem before it becomes one.

What other risk factors need to be considered when vaccinating for PCV2?

Before you can manage risk factors, you need to learn how to identify them. A risk factor is anything that can affect a vaccine’s efficacy — something that will make it vulnerable. But just as important, identifying a risk factor is sort of a heads-up that things are happening within the herd or operationally on the farm that need investigation.

Do the same risk factors exist on all farms?

No, they vary from farm to farm, even barn to barn. (See accompanying list.) It’s also important to realize that not all vaccination risk factors are created equal.

Risk Factors that affect vaccination

High risk factors

• Past PCV2 challenge in herd
• Unvaccinated gilts
• Lack of PCV2 vaccine booster prior to sow farm entry
• Sows that have never seen PCV2 vaccine
• Viremia at processing
• Co-infection with PRRS, influenza A virus-swine, Mycoplasma hyopneumoniae
• Partial dosing of vaccine
• Crowding pigs instead of picking up for vaccination
• Vaccinating at <15 days of age
• Failure to vaccinate/vaccine compliance
• Skill level of vaccination crew
• Suboptimal environment (double stocking, ventilation, etc.)
• Poor cleaning/sanitation procedures (higher environmental load)
• High Viral challenge in colostrum
• Malnutrition
• Source-herd disease status
• Developing-gilt viremia/shedding dynamics

Medium risk factors

• Timing of vaccination in replacement gilts
• High maternal antibodies (measured by IFA)
• Poor management practices (Delta (1) – piperideine-2-carboxylate, colostrum intake, etc.)
• One-dose vaccination regimen
• Vaccine shipped during extreme temperatures
• Improper vaccine handling/storage
• Extra-label vaccine mixing
• Mixing pigs (different PCV2 statuses, genotypes, etc.)
• Maternal barrows
• Employees with misinformation on PCV2 vaccination
• New genotype introductions (can lead to recombination events)
• Maternal age/parity (lactogenic factors)

Low risk factors

• PCV2-vaccinated sow herd
• Poor biosecurity (rodent control, etc.)
• Unknown feed-sourcing/mitigation strategies

For example, some risk factors — like the co-infection of PRRSv I mentioned earlier — most likely would be indicative of other health problems. But then there are other risk factors that aren’t necessarily a reflection of the health of the herd but can still influence a vaccine’s efficacy — the timing of the vaccination of replacement gilts, for instance, or poor management practices for newborn piglets.

When we vaccinate sows, we increase their neutralizing antibodies — and that’s a positive for shutting down circovirus or reducing shedding. But vaccinating sows is not going to help the piglets unless they get that colostrum into them ASAP. So, it’s very important in that first 24 to 36 hours to get colostrum into the pigs, get them dried off and turned over to mom to start nursing. Poor first-day pig care can have implications for immunity down the road. This step is an important one that veterinarians and caregivers sometimes tend to lose focus on.

What about human error or lapses in management? Do they affect the success of PCV2 vaccination?

Most definitely. Many vaccination risk factors are created unintentionally. Pigs might receive only a partial dose or get missed altogether simply because you’re vaccinating so many of them.

Insufficient cleaning of facilities — farrowing houses, nurseries, wean-to-finish barns — also comes into play. We know the PCV2 organism is ubiquitous in swine herds. Maybe we can’t completely get rid of it, but once it starts accumulating in the environment, we add to the disease pressure and the risk of mutations and recombination leading to new virus strains. PCV2 has the highest rate of mutation reported for a single-stranded DNA virus.¹ Additionally, recombination is known to occur with this virus, allowing for more changes in the PCV2 genotype.²

Identifying risks like these and dealing with them will improve vaccination success, as well as help producers avoid issues with PCVAD. This is where some routine field surveillance can help.

What situations might warrant a farm using field surveillance? Could you give some examples?

Sure, I can give examples. But in practice, it will depend a lot on the farm’s situation and the problems it’s encountering.

For instance, maybe a farm wants to know what the PCV2 status is for piglets at birth. That status is directly related to how well a vaccine will protect those pigs. If a piglet is PCV2-positive at birth and you give a vaccine, the vaccine can help, but it’s not going to be as effective as if you had vaccinated a negative piglet for PCV2 and then that piglet became exposed later down the road when immunity has been established.

Is PCV2 surveillance difficult?

It doesn’t have to be. Collecting placental umbilical cord samples can be tricky because contamination can happen if the placenta falls on the floor. Bleeding baby pigs — the 3-pounders — isn’t simple, either. So, a good option here for sample collection is processing fluids. Most likely you’re already using them for PRRSv. You’re already castrating and docking tails, so it’s easy to collect those fluids weekly and use those samples to get a read on herd status. But keep in mind that the diagnostics have to be interpreted appropriately. Detection doesn’t always mean disease.

Can the amount of PCV2 virus in the bloodstream, known as viremia, also affect vaccine performance?

Yes, it’s important to know the viremia level in weaned pigs about to be vaccinated. The goal is to vaccinate healthy animals for the best outcome, and this is an area where routine monitoring could prove useful. For instance, knowing the viremia level can tell us if a farm’s vaccine program should be adjusted to two doses.

To keep costs down, a farm could collect serum from 30 due-to-wean piglets once every 90 days. The results of polymerase chain reaction (PCR) and indirect immunofluorescence assay (IFA) titer testing can help determine how “vaccinatable” the population is as well as establish if there might be any maternally derived antibody impact on vaccination efficacy.

Is it also important to know the viremia status of gilts?

In some cases, yes. Maybe the farms want to find out if gilts that have already been vaccinated could use an extra dose because there is some intermittent shedding happening. What if this shedding goes on during farrowing? What’s getting passed on to the piglets? And are new genotypes being brought onto the sow farm?

Monitoring could be done on gilt entry to isolation. The farm could collect serum from 15 gilts for viremia profiling using PCR. Gilts of varying ages should be selected in a cross-sectional manner for the profiling.

Should veterinarians undertake all of these methods of surveillance if they decide to begin monitoring for PCV2?

They might want to start with monitoring replacement gilts and processing fluids. Like I said earlier, a lot of this will depend on the dynamics of a specific farm. But I could envision farms over time implementing more than one of these practices to stay aligned with PCV2 changes on a farm.

Monitoring pigs for PCV2 allows a farm to catch problems before they escalate and avoids risking an unknown herd status change on the sow farm.

Doing routine surveillance also allows you to observe what’s happening over time. For example, using processing fluids to monitor and chart cycle threshold levels. Some farms may already be monitoring PCV2 Ct levels, but are we charting those results in a way that values can be viewed over time? This is a key step that’s maybe missed.

Also, become familiar with the risk factors that can interfere with a vaccine’s effectiveness. Understanding all the variables that can impact how well your vaccination strategy will or won’t perform is eye-opening.