SHIC/AASV Porcine Circovirus-Focused Webinar Provides Timely Updates

February 21, 2024 — Megan Niederwerder

On February 6, 2024, the Swine Health Information Center along with the American Association of Swine Veterinarians offered a webinar focused on emerging trends of porcine circovirus. Expert presenters provided the latest information on porcine circovirus including domestic and global distribution, new research updates, diagnostic trends, sample types submitted for surveillance, interpretation of test results, and practitioner perspectives for disease management strategies.

There were 311 people registered for the PCV webinar, conducted by the Swine Medicine Education Center at Iowa State University, which is now available online for review. The first presenter, Dr. Tanja Opriessnig, ISU, provided a general disease overview including domestic and global distribution, recent research outcomes, and an update on PCV4. She said although PCVs were first identified approximately 27 years ago, an improved understanding of circoviruses and their impact on swine health has continued to evolve over time.

PCV2 has been a significant health challenge for global pig producers for many decades. Dr. Opriessnig said PCV2-specific vaccines have worked remarkably well to prevent viral health impacts since global introduction in 2006. Modern PCV2 vaccines provide cross-protection against currently known PCV2 genotypes and are among the most widely used vaccine in growing pigs. Dr. Opriessnig shared evidence of the global distribution of PCV3 as well as the recent identification of PCV4 in Asia and Spain, saying a more field-based approach is needed for prevention, preparedness, and response to emerging PCVs. Dr. Opriessnig remarked that initial evidence shows that PCV3 vaccines are helpful for control and that monitoring for emerging strains, including porcine circovirus-like viruses, is ongoing.

Dr. Darin Madson, ISU Veterinary Diagnostic Lab, shared background information for PCV2, stating it is a globally distributed and ubiquitous virus with very few negative swine herds in the US. PCV2 is an economically important disease, ranking as one of the top three infectious agents in the swine industry. Dr. Madson remarked that infection generally occurs after waning of maternal antibodies in pigs 7- to 15-weeks of age. Transmission of PCV2 is possible both horizontally (pig to pig) and vertically (dam to offspring) through nasal-oral and fecal-oral routes. He pointed out a major route of transmission occurs through virus secreted and/or excreted in urine, ocular discharge, nasal discharge, saliva, and feces.

Fomites capable of transmission include needles, insects, and rodents. Aerosol transmission of PCV2 is possible as the virus has been detected in exhaust air and dust outside barns; however, infectivity is unknown. The virus is environmentally hardy and difficult to inactivate with disinfectants.

During his presentation, Dr. Madson shared an AASV Board-approved porcine circovirus position statement (sunset) to aid in accurate diagnosis of PCV.

PCVAD can be subclinical or include one or more of the following clinical manifestations concurrently:

1. High mortality: Doubling of historical mortality rate
2. Respiratory signs including pneumonia
3. Porcine dermatitis and nephropathy syndrome (PDNS)
4. Enteric signs including diarrhea and weight loss
5. Reproductive disorders including abortions, stillbirths, and fetal mummification

For diagnosis, the following signs are triggers:

1. Clinical signs of disease
2. Lymphoid depletion +/- histiocytic replacement
   
3. PCV2 antigen or nucleic acid associated with lesions

Dr. Madson discussed the continued challenge of determining PCV2 infection vs disease. He stated the diagnosticians' dilemma is interpreting positive test results without clinical disease or lesions. His presentation also included a review of PCV2 immunity and reviewed information about possible causes of disease control lapses and immunity failures.

Dr. Pablo Pineyro, ISU VDL, provided further information regarding PCV2 vaccination and impact on reducing viral load in serum and tissues, albeit not completely stopping PCV2 antigen expression. Dr. Pineyro shared knowledge gained from VDL submissions, stating the amount of PCV2 DNA and antigen detected by PCR and IHC strongly correlates with vaccination status. Genetic changes of the virus over time can affect vaccine protection and vaccination challenges should be evaluated in association with genetic changes and biological relevance on the farm. While PCV2 vaccine has reduced PCVAD clinical prevalence, it has not changed the challenges associated with PCVAD diagnosis. For PCVAD diagnosis, a correct interpretation of the result depends on the clinical question and the diagnostic tool.

When considering PCV3, the challenge is to determine clinical relevance, including causation vs detection. Currently, there is a lack of specific lesions and direct detection tools for PCV3. Clinical cases associated with infection and multi-systemic inflammation (grower-finishers) are all being considered, Dr. Pineyro noted.

Dr. Mark Ladd, Smithfield, shared his experience with PCV detection, treatment, and management from 2014-2023. During this time, he oversaw 65,000 sows and all related downstream nurseries and grow/finish units. In 2014, he started using sequencing to identify PCV2 genotype and evaluate vaccination status. If PCV2b was detected, Dr. Ladd concluded either animals were not vaccinated, if seen in multiple ages with a specific flow, or there was a vaccine failure which was usually an individual farm problem. If PCV2d was detected, Dr. Ladd would not look at vaccination status as the primary cause. Calling it the PCV2d puzzle, Dr. Ladd noted vaccinated animals can still be viremic but may or may not have clinical disease as the virus can be found in healthy pigs. These characteristics make diagnosing reproductive failure by PCV2d very difficult, per Dr. Ladd.

Sharing his experience with growing pigs, Dr. Ladd said elevated mortality and a lack of response to antibiotics were factors initiating tissue collection for a diagnostic workup. Suspect PCV cases with either lesions consistent with PCVAD or PCR Ct values less than 25 raised concern. Dr. Ladd said 6% of the PCV2 cases he saw were from nursery pigs (3- to 10-weeks of age) and 94% of the cases were from finishing pigs (11- to 31-weeks of age). Among the PCV2 cases, 88% were also PRRSV positive and 25% were IAV positive, with only 5% having neither PRRSV nor IAV. Dr. Ladd said flows returned to normal when PRRS and/or IAV were addressed. Methods to stabilize the herds included depop/repop if it was a continuous flow growing site or stopping PRRSV circulation upstream at sow farms. IAV vaccine was used after determining strains found in the herd. His experience leads Dr. Ladd to say it appears PCV2d needs co-infection to be clinically relevant. And he asks, does co-infection with PRRS and/or IAV increase the possibility of PCV2 vaccine failure in pigs?

Dr. Chelsea Stewart, Christensen Farms, began by asking if the detection of PCV2 is clinical, subclinical or irrelevant? In her experience, PCV2 was occasionally detected in certain flows without PCVAD clinical signs. She noted significant stressors such as PRRSV, poor ventilation, weather changes, and delayed vaccination appear to be risk factors. When PCVAD was present, signs included CT values in the low teens, lymphoid depletion, wasting, and fallout.

In response to PCVAD in one commingled flow, Dr. Stewart initiated field exposure surveillance in cooperation with ISU VDL and Boehringer Ingelheim. A cross-sectional surveillance of downstream flows was to determine the timing of exposure and if the existing vaccination timing made sense. Sow farm surveillance included collecting processing fluids in one room per week for 12 weeks. Dr. Stewart said this was designed for long-term, system-wide surveillance. From the processing fluids, the VDL sequenced several positives with most being PCV2a or PCV2d, with a few PCV2e. As a result, Dr. Stewart's overall question was, can we correlate PF PCR CT values to sow immune status to gauge efficacy of the current vaccine protocol?

Another step in their process was to determine individual processing fluid correlation to sow immune status. To measure this, two sow farms with PCR positive results and a high rate of fetal mummification were chosen for further investigation. Processing fluids were collected from litters and serum from sows. Sows were equally divided with 10 at parity one, 10 in parities two through five, and 10 at parity five or more. Dr. Stewart said no cross fostering was allowed until after processing fluids were collected. Processing fluids were evaluated for PCV2 by PCR and serum was used for PCV2 antibody testing. Goals of the study were to evaluate if CT value correlated to individual sow status or parity.

Dr. Stewart concluded by saying they did not detect a parity difference but did identify a gap in gilt immunity that may have resulted in the PCVAD cases. As a result, they modified their vaccination program to include booster vaccines at the sow farms and a rescue dose to growing pigs between 5- and 9-weeks of age. Flow has continued to perform around target since discontinuing the rescue dose and sows are a full gestation past whole herd booster. Vaccination compliance is critical, per Dr. Stewart who also recommends not chasing positive CT results without a clinical diagnosis.

PCV presents a significant swine disease challenge and is included in SHIC's 2024 Plan of Work as a priority pathogen.

The Swine Health Information Center, launched in 2015 with Pork Checkoff funding, protects and enhances the health of the US swine herd by minimizing the impact of emerging disease threats through preparedness, coordinated communications, global disease monitoring, analysis of swine health data, and targeted research investments. As a conduit of information and research, SHIC encourages sharing of its publications and research. Forward, reprint, and quote SHIC material freely. For more information, visit http://www.swinehealth.org or contact Dr. Megan Niederwerder at mniederwerder@swinehealth.org or Dr. Lisa Becton at lbecton@swinehealth.org.

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Megan Niederwerder, Executive Director, Swine Health Information Center