Diagnostic experts: PCV3 is common in US swine herds

Porcine circovirus type 3 (PCV3) was first reported in 2016, but appears to have been present in swine populations decades prior to that time, said Bailey Arruda, DVM, a diagnostic pathologist at Iowa State University’s Veterinary Diagnostic Laboratory. She noted the majority of infections are subclinical and that PCV3 strains are similar to one another but are very different from PCV2.

“It’s like comparing pigs with cows,” she said. In addition, PCV2 vaccination does not appear to protect against PCV3 infection.

“Everywhere we looked for the virus, we found it,” said Albert Rovira, DVM, diagnostician at the University of Minnesota Diagnostic Laboratory. He and Arruda both spoke at the 2019 Allen D. Leman Swine Conference. Rovira said PCV3 was found in swine herds all over the US and was identified in tissue, serum, blood, oral fluids and processing fluids.

“You can find it in any age pig, and surprisingly, it was also commonly found in fetuses,” he said.

On average, cases of PCV3 may not appear to cause major problems in a herd, but averages can be deceiving. “If you are looking at averages, you are probably missing some things. You have to look at the individual sow and litter to get an accurate diagnosis. It’s important to increase your diagnostic sensitivity by sending in four to six fetuses per affected litter,” Arruda said.

“PCV3 is strongly associated with mummies and stillborns, particularly among gilts and/or low-parity sows, which tells me we need to do a better job of acclimating our gilts,” she added. “It’s commonly detected in multiple litters/groups that are submitted together, and at PCR Ct values that indicate a significant amount of PCV3 is present in fetal tissues.”

PCV3 in growing pigs

Between February 2018 and October 2018, 12 PCV3 cases were submitted to the Iowa State lab from 10 different sites (five from the same sow source). The pigs were commonly 4- to 6-weeks old but ranged from 3- to 10-weeks old. Clinical signs varied but included porcine dermatitis and nephropathy syndrome (PDNS), acute death, lameness, enteric and respiratory illness and loss of condition. Such a variation in reported clinical signs could suggest that these pigs are selected for a diagnostic investigation as they are not performing as well as cohorts. Lesions in these pigs were evident at the cell level, but there were no noticeable gross lesions.

“PCV3 appears to have a proclivity for the cardiovascular system,” Arruda said.

Arruda recommended leaving plenty of time between PCV3 exposure and breeding, noting that viremia appears to be greater than a month, based on inoculation studies in cesarean-derived, colostrum-deprived pigs. In summary, Arruda said PCV3 is a putative cause of clinical disease in a subset of pigs it infects, but based on the data set and research currently available, it does not appear to cause primary enteric disease, primary respiratory disease or loss of condition like PCV2 does.

Rovira has observed PCV3 associated with vasculitis/myocarditis and fetal death. However, some pigs exhibit no clinical signs.

While Arruda has not seen abortions associated with PCV3, Rovira’s experience is different. Of 67 fetal death cases at the Minnesota lab, PCV3 was found in 40% of the cases, along with other diseases. Of those cases, 16% had PCV3 by itself.

“While most infections are subclinical, PCV3 appears to be relevant in occasional cases of mummies/abortion, myocarditis and vasculitis,” Rovira said. “If you looked hard enough, you would likely find many more pigs positive to PCV3.”

Researchers hope additional funding will help further their study of PCV3.