SHIC-Funded Study Examines Piglet Postmortem Sampling for PRRS Detection

With a goal to provide welfare-friendly and cost-effective methods for active PRRSV surveillance, Drs. Cesar Corzo, Mariana Kikuti and colleagues from the University of Minnesota led a study to evaluate the accuracy of different postmortem specimens collected from piglets in breeding herds for disease detection. Funded by the Swine Health Information Center, the study focused on the sensitivity of PRRSV detection by PCR across six sample types, including nasal, oral, and rectal swabs, tongue-tip fluids, superficial inguinal lymph nodes, and intracardiac blood. Overall, investigators concluded that oral swabs and lymph nodes showed the best diagnostic performance. Tongue-tip fluids had high sensitivity (92.2%) but low specificity (53.9%) due to likely environmental contamination and may be a less suitable sample type for individual pig diagnosis.

Published by mdpi.com, you can find the entire piglet postmortem sampling study here, including citations for content included in this summary.

In the US, PRRS continues to be the primary health challenge faced by swine herds. PRRS outbreaks afflict a significant portion of US breeding and growing herds, causing a major economic impact and production losses across the industry. The estimated weekly PRRS virus prevalence was 20%–40% between 2019 and 2023 in US breeding herds, according to the SHIC-funded Morrison Swine Health Monitoring Project.

Traditionally, PRRS surveillance efforts in breeding herds have heavily relied upon live animal sampling methods, including individual pig-level serum, oropharyngeal and nasal swabs, along with group-level oral fluids and environmental samples. Live animal specimens collected directly from individual pigs tend to have higher analytical sensitivity, the ability to detect a true positive sample, when compared to the group or environmental specimens. However, individual pig sampling poses labor and logistical challenges including concerns for staff safety and animal welfare.

Monitoring the dead piglet population through sampling tongue-tips for PRRS diagnosis has been proposed by European researchers (Baliellas et al., 2021) as an alternative to processing-fluids surveillance for PRRS and was quickly adopted in the US as a complementary specimen when monitoring breeding herds. In the study described herein, the aim was to determine the sensitivity of PRRSV detection across six postmortem sample types collected from dead piglets in US breeding herds.

Three Midwestern US farrow-to-wean farms undergoing PRRS elimination after an outbreak were utilized as the study population. Farms that utilized PRRS-modified live virus vaccination protocols for sows or piglets were excluded. Farm size ranged from 2500 to 5000 sows and all farms were representative of modern pig production and have year-round negative-pressure air-filtration systems.

For each farm, 30 and 60 samples were collected at eight and 20 weeks, respectively, after the PRRS outbreak. Postmortem samples included nasal swabs, oral swabs, rectal swabs, tongue-tip fluids, superficial inguinal lymph nodes, and intracardiac blood. All samples were tested for PRRSV RNA using RT-PCR. Intracardiac serum served as the gold standard for calculating the sensitivity, specificity, and predictive values of other post-mortem specimens.

To establish PRRS prevalence within each farm, blood samples were collected from live piglets via jugular venipuncture in the same rooms as the ones used for post-mortem sampling. Sera from live piglets were tested for PRRS by RT-PCR in pools of five; any sera from positive pools were then tested individually to estimate the within-herd PRRS prevalence. All specimens were submitted to the University of Minnesota Veterinary Diagnostic Laboratory for individual PRRS RT-PCR testing.

PRRSV was detected in all specimen types and at all sampling points, except for two sampling points, where swabs (oral, nasal, rectal) and intracardiac sera did not detect the virus, respectively. Oral swabs and lymph nodes showed the best overall diagnostic performance with sensitivity ranging from 94.6% to 100% and specificity ranging from 83.9% to 85.1%. Tongue-tip fluids had high sensitivity (92.2%) but low specificity (53.9%), indicating a potential for environmental contamination affecting results. Nasal swabs and rectal swabs had moderate sensitivity and specificity, indicating some diagnostic utility, but had less accuracy compared to oral swabs and lymph nodes. The agreement between each specimen and intracardiac sera, as well as agreement between tongue-tip fluids and oral swabs, were calculated. The agreement between postmortem specimens and intracardiac sera was highest for superficial inguinal lymph nodes (88.89%), followed by oral swabs (87.10%) and nasal swabs (85.48%).

Three specimens were selected to investigate the success rate in obtaining a PRRSV ORF5 sequence based on their routine use in the field (tongue-tip fluids), ease of collection (oral swabs), and lowest probability of environmental contamination (intracardiac serum). The ORF5 sequencing was successful in 29 out of the 31 tongue-tip samples, 23 out of the 28 oral swabs, and 21 out of the 23 serum samples. Even though the sequencing success rate was higher in tongue-tip fluids, all three specimens presented a good sequencing success rate (82.1% to 93.5%), indicating that sequencing was not a limitation for sample type tested.

While there were challenges in meeting sampling targets due to variable pre-weaning mortality, PRRS was detected in all postmortem specimens. Tongue tips are easy and inexpensive specimens to collect but require some manipulation of the piglets with sharps, which can result in longer personnel time requirements when compared to swabs. Swabs (whether oral, nasal, or rectal) are quick, safe, and easy to collect, requiring minimal training. However, the costs associated with investment in individual swabs are high. Oral and nasal swabs showed promising results for disease monitoring. Although tongue tip fluids had high sensitivity, they also had lower specificity, making them less suitable for individual pig diagnostics due to environmental contamination.

Overall, this study provides insight into postmortem sample types as a welfare-friendly alternative for disease monitoring in breeding herds.

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 [email protected] or Dr. Lisa Becton at [email protected].