In today’s Science Page, Drs. Laurie Pfleiderer and Robert Desrosiers from Quebec share the findings of their case report looking at false positive results during Mycoplasma hyopneumoniae testing.
Key point
- Under the condition of this case, vaccine residues may produce suspicious or positive MHP PCR results
Early detection of Mycoplasma hyopneumoniae (MHP) infection at the multiplication level is of critical importance and relies on routine diagnostic testing, with methods such as enzyme-linked immunosorbent assays (ELISA) and polymerase chain reactions (PCR) performed respectively on sera and tracheobronchial swabs (TBS). False positive results can cause costly flow disruptions.
A gilt developer unit (GDU) was populated during October 2023 from a newly repopulated high-health multiplier. Gilts were received at weaning every 4 weeks and were housed up to 23 weeks of age before being transferred into commercial sow herds. The gilts received a single dose of a MHP vaccine at around 20 weeks of age. Twenty serum samples collected from gilts between 9 and 19 weeks of age were individually tested for MHP ELISA each week prior to transfer. On March 4 of 2024, 3 of 20 sera tested either suspicious or positive to an MHP ELISA test, in the absence of any clinical signs. The 3 suspicious or positive samples were tested using a different ELISA test, and 2 gave a negative result while the third was strongly positive. On March 19, 20 TBS were collected among all the rooms housing gilts over 60kg and analyzed in pools of 2 by PCR. Four of the 10 pools gave suspicious Ct values (min 36.2-max 37.7). The 4 suspicious pools were tested using a different PCR test, and 3 were negative, but the fourth was positive. Immediately, all animal movements were cancelled, and gilts recently introduced in commercial sow herd quarantines remained in place pending further testing and investigation. All epidemiologically linked premises, which included the multiplier, the GDU, and 3 commercial sow herds were tested. Across the 5 premises, 180 TBS samples were collected and analyzed in pools of 3 and 155 MHP ELISA tests were conducted on March 25. All samples gave negative results, except for 5 suspicious and 1 positive ELISA test result, all of which came from MHP-vaccinated gilts from the commercial sow barns quarantines. The GDU was retested one month later by ELISA and TBS. All results obtained were negative and have remained negative ever since.
An investigation was conducted to determine if a possible connection could be found between the use of the MHP vaccine on the farm and the results obtained. Upon log entry verification, the last MHP vaccination prior to the suspicious PCR results was administered 5 days before the TBS samples were collected. The farm manager used a tote to transport vaccines and needles to the group of gilts to be vaccinated. The tote had not been washed or disinfected after previous vaccination events. On March 19, that same tote was used to carry the TBS sampling equipment but was not used during the March 25 sampling. To verify if the tote might have been a source of cross contamination, the vaccination process was recreated using the same conditions. The manager was asked to vaccinate a group of gilts with the MHP vaccine on May 8. Fourteen TBS samples were collected on May 13 and the testing equipment used was transported in the vaccine tote, as it had been on March 19. Of the 7 PCR sample pools, 2 produced suspicious results with Ct values of 36.42 and 35.42
Sequencing of suspicious or positive PCR samples to see if they matched the vaccine used would have added to the strength of our hypothesis. However, the high Ct values would have likely resulted in inconclusive results. With no other identifiable possible source of contamination, this case report suggests that vaccine residues may have been the source of positive or suspicious PCR results, even days after the vaccine was used.