PCV3 is ‘absolutely’ a pathogen but proceed with caution
Porcine circovirus type 3 (PCV3) is “absolutely” a pathogen but merely finding the virus isn’t enough to conclude it’s the cause of disease, Emily Byers, DVM, Prestage Farms, told Pig Health Today.
PCV3 is already widespread among US herds. Whether it causes disease, however, has been a topic of debate. Some researchers consider PCV3 to be a primarily harmless virus, pointing out it can be found in healthy pigs.1 Based on her hands-on experience, however, Byers is convinced PCV3 can cause disease.
“I believe it’s underlying and that there’s potential for it to cause disease but I still caution and urge people that you’ve got to take the clinical picture in context with a positive PCR (polymerase chain reaction) result,” she said. If PCV3 is the only pathogen present and there is clinical evidence consistent with circovirus infection, “in my mind PCV3 is absolutely a pathogen.”
Clinical findings can vary
Clinical findings Byers has seen that she attributes to PCV3 can vary widely from farm to farm, in keeping with reports in the literature. Variation in clinical presentation is why coming up with a true case definition is challenging for the swine industry, she continued.
However, at the sow farm level, Byers said she “fairly consistently” sees inconsistent farrowing performance characterized by mummified fetuses and stillborns. Extreme increases in mummy rates are generally associated with individual sows, which can have as many as three, four or even five mummified fetuses.
“Clinically, those present very much like porcine parvovirus or porcine circovirus type 2 (PCV2) in that you’ve got fetal death that occurs at multiple states during gestation…That’s one of the hallmarks that I see with PCV3,” she said.
Downstream in herds with PCV3, pigs don’t perform the way they should. They don’t start on feed. They hang their heads when they go to the nursery and generally just don’t look good. “It’s hard to measure, it’s hard to document,” Byers said.
Processing fluids helpful
She has found testing of aggregate processing fluids an excellent way to monitor sow farm status for PCV3. “Sow blood has not historically, for me, been a great sample,” Byers noted. If she’s investigating sow mortality, she submits various tissue samples but especially skin samples if there are skin lesions. And “mummified fetuses — I can’t stress that enough.”
If Ct levels based on processing fluids are lower than 25, she is confident she needs to “chase those mummies and prove that [PCV3] is there.”*
Upon investigation, Byers considers whether the farm is reaching its true potential and rules out other potential causes of clinical findings such as porcine reproductive and respiratory syndrome (PRRS).
“Oftentimes when I see PCV3…those farms are endemically infected with PRRS…I do believe PRRS is an important co-factor for PCV3, but I will note that I have often seen PCV3 as a sole agent,” she said.
Byers noted that she almost never sees PCV2a or PCV2b anymore but does find PCV2d, still the most prevalent of the PCV2 strains in US herds.
Byers is convinced PCV3 is a completely different virus from PCV2. “They’re still both circular DNA viruses but when you look at them on a phylogenetic tree, they’re very different. They’re very distant. And I still think we can extrapolate from PCV2 somewhat, but as we learn more about PCV3, I think we’re finding that it is a bit of its own virus.”
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*Editor’s note: Using real-time PCR technology, the DNA of a virus is identified with a fluorescent signal. It can take multiple cycles to identify a virus, and the number of cycles it takes is the Cycle threshold (Ct) value. When more virus is present, it takes fewer cycles to be identified with the test. When less virus is identified, it takes more cycles.
1 Rovira A, et al. New insights into spread and relevance of porcine circovirus type 3. National Hog Farmer. June 28, 2018.