Round 3 Against Porcine Circovirus

By Jennifer Ryan

July 24, 2019

Producers, veterinarians brace themselves for a battle as evidence points to another pathogenic species of porcine circovirus.

Image of pigs being fed

In 2015, a third species of porcine circovirus was identified in the United States: porcine circovirus Type 3 (PCV3). It joined two other recognized species of the virus and multiple strains. Yet, it’s only this year that preliminary research results tied PCV3 to instances of fetal death, lesions of myocarditis, vasculitis of the heart and vasculitis of the spleen, according to the Swine Health Information Center.1

It remains to be seen where PCV3 will fall in the lineup of most costly diseases the swine industry faces. So far, porcine reproductive and respiratory syndrome virus (PRRSV) and Influenza A virus to be more of a threat, notes Bailey Arruda, DVM, Ph.D., assistant professor at the Iowa State University College of Veterinary Medicine’s Veterinary Diagnostic Laboratory.

“PCV3 does appear to be contributing to fetal wastage as defined by the presence of mummified fetuses and stillborns delivered at, or near, term based on cases submitted to the veterinary diagnostic laboratory,” she says. “It is difficult to accurately compare PCV3’s impact on fetal wastage to PRRSV, PCV2 and PPV (porcine parvovirus infection) based on our available data set. Currently, PCV3 does not appear to be contributing to abortions, respiratory disease or enteric disease.”

Pinning down losses

While PCV3 was first reported in the scientific literature in 2016, it has probably been around for decades, Arruda says.

The circovirus strain was first isolated from a sow farm in North Carolina. Incidence of the disease was reported at 12.5 percent, and deaths occurred among fetuses and sows — although exact numbers caused by PCV3 are unknown, according to information from the Swine Health Information Center and the College of Veterinary Medicine at Iowa State University.2

PCV3 has been associated with production losses from:

  • • Porcine dermatitis and nephropathy syndrome (PDNS)
  • • Reproductive failure
  • • Cardiac and multi-organ inflammation.

Sorting out the circoviruses

The PCV3 and PCV2 genomes are different but cause similar syndromes, as reported in a 2019 article published in the Journal of Virological Methods.3

“It has been hard to discern correlation of PCV3 with disease lesions for some veterinarians and pathologist(s), especially when the industry acknowledges that PCV2 has evolved which makes it more difficult to identify,” stated Paul Sundberg, DVM, director of the Swine Health Information Center, in a release. “Today, in the general swine veterinary population, experiences in the field generally influence whether a veterinarian believes that PCV3 causes disease. This ongoing discussion yields a sense of déjà vu for swine veterinarians who lived through similar doubts and conversations with PCV2 back in the early to mid-2000s.”

The similarities make it tough to differentiate clinical symptoms caused by the two circoviruses. Even trying to identify losses caused by PCV2, which is well established as pathogenic, can be difficult.

That’s because circovirus-associated diseases are multi-factorial in nature. In piglets with postweaning multisystemic wasting syndrome (PMWS) — most often associated with PCV2 — morbidity ranges from 4 to 60 percent, while mortality is slightly lower at 4 to 20 percent. Rates of reproductive failure are also varied, with farrowing rates below 60 percent and litters of up to 75 percent mummified fetuses or stillborn piglets reported.

Transmission and disinfection

Researchers don’t know how PCV3 is transmitted but are betting it’s similar to PCV2 simply based on the structure of the virus.

PCV2 transmission occurs by direct contact with the primary route being oronasal. Transmission from sow to piglet can occur in utero and has been seen with both PCV2 and PCV3.

As pork producers know, biosecurity and disinfection protocols go a long way towards preventing disease losses of all kinds. Yet, specific disinfection information for PCV3 is not yet available.

In the laboratory, potassium peroxymonosulfate and sodium chloride (Virkon™ S), sodium hypochlorite (Clorox® Bleach) and sodium hydroxide appear to be the most effective virucidal agents against PCV2. However, chlorhexidine, ethanol, aldehydes and iodine products are generally not effective disinfectants for circovirus.

Vaccination

In terms of vaccines, several PCV2 vaccines are commercially available in the U.S. and have significantly reduced post-weaning mortality. Given the long lead time associated with vaccine development — combined with the unknown severity of PCV3 at this time — pork producers shouldn’t expect a PCV3 vaccine any time soon.

“PCV2 vaccines are not thought to protect against PCV3 infection,” Arruda says. “Further investigation into the impact of PCV3 in growing pigs may be warranted based on observations in the field. Although funding for PCV3 research appears to be rather limited, which makes finding answers and solutions difficult.”

Key Points

  • • PCV1 is considered non-pathogenic.
  • • However, PCV2 is a major swine pathogen.
  • • PCV2 can cause circovirus-associated diseases (PCVAD) including post-weaning multisystemic wasting syndrome (PMWS), porcine dermatitis and nephropathy syndrome (PDNS), porcine respiratory disease complex (PRDC), enteritis and reproductive failure.
  • • PCV2 has been reported worldwide.
  • • Within PCV2, there are four distinct clades identified: PCV2a, PCV2b, PCV2c and PCV2d.
  • • Epidemiological studies suggest that PCV2d (mPCV2b) has displaced PCV2b as the dominant genotype in recent years.
  • • PCV3 has been detected in several other countries including Brazil, China, Thailand, Sweden, Denmark, Italy, Poland, and Spain.

 

References

1. Sundberg P. SHIC-sponsored studies show correlation between PCV3 and lesions and improve diagnostic tools. Swine Health Information Center. April 18, 2019. Available at: https://www.swinehealth.org/shic-sponsored-studies-show-correlation-between-pcv3-lesions/.
2. Swine Health Information Center and the College of Veterinary Medicine at Iowa State University. Porcine Circovirus 3. Available at: http://www.cfsph.iastate.edu/pdf/shic-factsheet-porcine-circovirus-3.
3. Wang Y. A multiplex real-time PCR assay for the detection and differentiation of the newly emerged porcine circovirus type 3 and continuously evolving type 2 strains in the United States. Journal of Virological Methods;269, July 2019:7-12.

In: July Digital Edition 2019Topic: Swine

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