By Albert Rovira, Zhen Yang and Fabio Vannucci, University of Minnesota Department of Veterinary Population Medicine, Veterinary Diagnostic Laboratory
Porcine circovirus type 3 was discovered in 2016 in the United States associated with cases of systemic disease and reproductive disorders. At the time of its discovery, the spread of the virus in the United States or in the rest of the world was unknown. However, multiple studies performed during the last two years have shown that this virus is widespread and has been around for quite some time.

PCV3 has been found in all countries where it has been searched for. The long list of countries includes the United States, China, South Korea, Poland, Italy, Brazil, United Kingdom, Thailand, Germany, Spain, Canada, Sweden and Denmark. Some of these studies investigated the presence of the virus in archived samples and were able to prove that PCV3 was present in pig populations many years ago. PCV3 was found in samples from 2002 in the United Kingdom, samples from 1996 in Spain, samples from 1996 in China and samples from 1993 in Sweden.

At the University of Minnesota Veterinary Diagnostic Laboratory, we have been monitoring for the presence of PCV3 on a routine basis during the last two years. During this time, we have been collecting data trying to answer two basic questions: 1) how common is PCV3 in the United States? and 2) how relevant is it?

We tested more than 2,000 samples from more than 700 submissions for PCV3 by polymerase chain reaction. Overall, we found that PCV3 was present in 27% of the samples and 35% of the submissions. Positive cases were obtained from 18 different U.S. states. PCV3 was found in a wide variety of tissues and clinical samples. The rate of positive PCV3 samples was high for oral fluids (89%) and processing fluids (81%), and lower for serum (23%) and tissue homogenate (18%). Samples from suckling, nursery and growing pigs were positive at a rate of 15-18% while samples from adult pigs were positive at a rate of 35%. Positive PCV3 results were found in samples from cases with all types of clinical signs. Forty percent of abortion cases, 25% of diarrhea cases and 25% of respiratory cases were positive for PCV3. In summary, we learned that PCV3 is widespread in the United States. It can be found pretty much everywhere: in multiple tissues and samples, in pigs with multiple clinical conditions and in healthy pigs.

This takes us to the second question: is PCV3 relevant? To answer that question we need to look at the diagnostic interpretation of each case. The vast majority of PCV3-positive cases were interpreted by the pathologist as subclinical infections. This means that, while the pigs were infected with PCV3, there were no reasons to believe that this virus was the cause of disease. However, in a few cases PCV3 was considered significant based on association with microscopic lesions (in some cases confirmed by in situ hybridization), presence of large amounts of PCV3 (low Ct values obtained by PCR) and lack of other common pathogens. Of particular interest are cases of systemic vasculitis and myocarditis (Figure 1), and abortion cases in which PCV3 was the only pathogen detected (16% of all the abortion cases).

Figure 1: Systemic infection with PCV3 in a 1-week-old pig. a) Vasculitis affecting blood vessels in the kidney. b) In situ hybridization showing presence of PCV3 in the affected blood vessels.

In conclusion, during the last two years we learned that PCV3 is widespread in the United States and all over the world. Most of the times this virus appears to be harmless. However, in rare cases it may be responsible for systemic disease or reproductive disorders.