Litter level agreement of PRRS PCR results in testicles, tails and udder wipes samples

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This recent publication by former MSHMP post-doc Dr. Carles Vilalta compares various sample types used to assess PRRS status of piglets at the litter level. The paper is available in the Preventive Veterinary Medicine journal.

Methods

Samples were collected from a commercial 5,000 sow farm, starting 10-weeks after the beginning of a PRRS outbreak. Twenty-four litters, between 3 and 5 days of age were enrolled in this study based on convenience. Individual serum samples from each piglet, processing fluids ( i.e. serosanguinous exudate from tails and testicles) from each litter, and sows udder wipes were collected and tested for PRRS by RT-PCR.

Results

23.6 % of piglets were PRRSv positive by serum so that more than half of the litters had at least one positive. No differences were seen between male and female piglets at the litter level or based on sow’s parity.

Summary of the PRRSv RT-PCR Ct values of the litters that have at least 1 positive animal grouped by the number of positive (Ct value <35) animals of each litter (top grey strip) that contributed to the aggregated sample of testicles (only males) (A), tails (B) and udder skin wipes (C).

Overall, the sensitivity of the tail fluid was lower than that of the testicles. The fluid collected from the testicles and the udder wipes showed the highest and the lowest sensitivity of all three types of aggregated samples, respectively. As expected, the probability of a positive PRRS results from a population-based sample increased with the number of individual PRRS positive piglets and with the lowering of their Ct-value.

Read the entire publication on the journal website.

Abstract

Porcine reproductive and respiratory syndrome (PRRS) is currently the most detrimental disease in the U.S swine industry. Clinical signs of PRRS virus (PRRSv) infection in breeding herds include reproductive failure with abortions, stillbirths, premature farrowings and increased pre-weaning mortality. Serum from due-to-wean piglets is considered the most suitable specimen to monitor PRRSv infection and stability in breeding herds. However, processing fluids (PF – the serosanguinous exudate resultant of the collection of tails and testicles during processing) are a new specimen proposed to monitor piglets at processing (3–5 days of age) and udder wipes (UW) of lactating sows is yet another specimen to monitor infection status of suckling piglets indirectly. Here, we assessed which specimen type (e.g. sera, testicles, tails or UW) should be used to accurately establish the PRRSv status of a litter. Twenty-four litters were conveniently selected on a farm at 10 weeks post PRRSv outbreak. Blood samples, tails and testicles from every piglet in a litter, and an udder skin wipe from the sow were collected at processing (3–5 days). Individual litter testicles and tails as well as the udder wipe were placed each in a reclosable bag to prevent cross-contamination. Sensitivity (Se), specificity (Sp), negative predictive value (NPV), positive predictive value (PPV) and global agreement at the litter level were calculated using the sera results of the litter as the gold standard. The optimum cycle threshold (Ct) value to classify a sample as negative was ≥35 for serum and ≥36 for the aggregated samples (testicles, tails, and UW) based on the ROC curve analysis. Using those thresholds, the fluid collected from the testicles showed the best overall performance (Se = 92 % [62–100]; Sp = 82 % [48–98], NPV = 90 % [55–100], PPV = 85 % [55–98], global agreement = 87 %) compared to tail fluid and UW. Sensitivity of the tail fluid was 62 % (32–86) and the UW was 23 % (5–54), both of which yielded a 100 % specificity and PPV. This study provides information on the contribution of each of the tissues collected at processing on the detection of PRRSv, which becomes relevant in countries were castration and/or tail docking is banned.