Processing fluids’ effectiveness in monitoring PCV2 in sow herds

Porcine circovirus type 2 (PCV2) is a hardy virus that, today, can be found throughout US pig production systems. It commonly impacts nursery and growing pigs, causing a failure to thrive, wasting and death. Co-infections are common and the clinical manifestation is known as porcine circovirus associated disease (PCVAD).

Vaccines have provided effective control options, but vertical transmission remains a challenge for some farms. Monitoring methods are limited and typically involve sampling placental umbilical cords, colostrum and serum from pre-suckling pigs, all of which are labor intensive and difficult to collect. As piglet processing fluids (PF) gain traction for other pathogens, it raises the question whether they would be an easy alternative for PCV2

With that in mind, Kayla Castevens, third-year veterinary student at North Carolina State University, set out to determine if PF could serve as a monitoring tool to establish sow vaccination protocols and whether parity influenced PCV2 detection.¹

Sow farms enrolled

Seven sow farms were selected for the study and individually identified as Farm A through G. Here are the farm background details at the start of the study:

• Farm A — A start-up farm; elevated downstream mortality, showing clinical signs of PCVAD, virus confirmed within associated lesions. Negative for porcine reproductive and respiratory syndrome virus (PRRSV) and porcine epidemic diarrhea virus (PEDV).
• Farms B-G — Established farms; no PCVAD signs in downstream pig flows. Historically these herds were PRRS- and PEDV-positive but were currently stable and showed no signs of either disease. Farms increased biosecurity measures for piglet processing.

PCV2-vaccination protocols on the farms included:

• All farms vaccinated piglets with a two-dose PCV2 vaccine at processing and weaning.
• Farms B-G vaccinated gilts with a full-dose PCV2 vaccine at both 10 and 23 weeks of age.
• Farm A vaccinated gilts with a single, full-dose PCV2 vaccine at 20 weeks of age. However, after PCVAD was confirmed, the farm added a full-dose PCV2 vaccine administered at 10 weeks of age.
• None of the farms had a PCV2-vaccination protocol for sows.

PF collections and results  

Personnel at each of the farms began collecting tails and testicles from daily piglet processing, which were extracted and combined to submit one pooled sample per farm for the week. Castevens pointed out that a collection adjustment was made a month or so into the study for Farm A, due to PCVAD, and Farms B and C, due to intermittent PCV2 polymerase chain reaction (PCR)-positive test results. For those farms, samples were collected by individual litter for 5 days, for a total of 655 litter-specific samples.

The results shown in the accompanying table (Table 1), outline that for Farm A 100% of the samples tested positive, with Ct values less than 30. Farms B-G had varying results with 75% of the PCR-positive tests recording Ct values > 30 and 100% had a Ct  25.

Table 1. Weekly PCV testing of processing fluids

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When the samples were collected and pooled by day, Castevens found that Farm A tested positive for all 5 days, Farm B was PCV2-positive 4 of 5 days and Farm C tested positive 2 of 5 days. The daily samples were pooled for a weekly result for which Farms A and B tested positive and Farm C tested negative (Table 2).

Table 2. Pooled processing fluid PCV2 PCR results by day collected

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Castevens reviewed the pooled-PF results by sow parity (Table 3). Farm A, which was a start-up and only had parity 0 and parity 1 sows, recorded Ct values from 18.4 to 28.1. Those were notably lower than the Ct values for the positive test results for Farms B and C.

It is also noted that parity does not appear to be influential in viral shedding as the parity with PCR positive Ct values varies between farms.

Table 3. Pooled processing fluid PCV2 PCR Ct values by parity at each farm

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Take-home messages

The overall conclusions, according to Castevens, are that PF could be used as a monitoring tool for PCV2, that PCV2 viral shedding varies and parity is not influential.

“We need to go beyond just looking at positive or negative results,” she said, “and consider the degree of positivity.” Therefore, she proposed the following “Positivity Scale” to help determine the interventions needed, as shown here.

Ct < 25 = Intervention: Clinically relevant; there is likely active PCV2 infection; more diagnostics are needed; consider implementing a vaccination program.

Ct 25-30 = Alerted: Monitor the animals closely by increasing PCV2-testing frequency and checking downstream pig flows for developing PCVAD symptoms.

Ct > 30 = No Action: Not clinically significant.

She acknowledged that the Positivity Scale is a start but that it needs to be tested further and validated.

While her study supports the practice of using PF to detect PCV2, Castevens added that additional investigation is needed to determine the relationship to other monitoring methods, environmental contamination, co-morbidities, dilution factors from pooled samples and farrowing-house biosecurity to further validate PF as an accurate PCV2-monitoring option in sow herds.

¹ Castevens K, et al. Use of processing fluids to monitor sows for porcine circovirus type 2 and determine parity influence on viral detection. Student Research, 50^th Am Assoc of Swine Vet Annual Meeting. 2019;86.