Ante-mortem vs. post-mortem sampling procedure comparison for the detection of Mycoplasma hyopneumoniae by PCR and the influence of pooling on results

Ante-mortem detection of Mycoplasma hyopnuemoniae (Mhp) can be difficult during the mid stages of an infection and in vaccinated populations. Commonly used methods to determine herd status such as nasal swabs, oral fluids, and oropharyngeal swabs, have shown low sensitivity and antibody testing is inconclusive in vaccinated populations. A more sensitive ante-mortem procedure is needed. The objective of this study was to compare ante-mortem tracheo-bronchial (TB) catheter and laryngeal sampling (LS) procedures to post-mortem bronchial swabbing (BS) (the reference standard) by individual and pooled samples for detection of Mhp by polymerase chain reaction (PCR).

Three groups of 35 non select gilts approximately 180 days of age were sampled from 3 separate gilt development units with known Mhp exposure beginning at 60 days of age. Each gilt was tagged to be identified at marketing; TB, LS, and serum samples were collected from each gilt. TB samples were collected using a 60 cm catheter passed orally. Immediately after, LS were taken with a smooth, long handled spoon and transposed onto a swab. The next day gilts were followed to the harvest facility where post-mortem bronchial swabs were collected individually. All samples taken were frozen at -80oC. Following all collections, samples were simultaneously submitted for Mhp PCR testing. Blood samples were tested for Mhp antibodies by IDEXX ELISA. Pools of 2, 3 and 5 samples were also tested for Mhp by PCR in submision order. A stochastic model was used to estimate the case detection rate with differing prevalence and sample size for LS.

TB and LS had similar individual sensitivity (42.4% and 47.8%). BS had a significantly lower Mhp PCR cycle quantity values (29.2) when compared to TB (32.1) and LS (33.2). TB and LS had similar sensitivity results for pools of 2:1 (41.6% and 43.3%), 3:1 (64.1% and 53.8%), and 5:1 (66.7% and 70.8%) and were less sensitive than BS pools of 2:1 (91.1%), 3:1 (93.5%), and 5:1 (100%). Serum samples were 95% percent ELISA positive.

As expected, TB and LS were less sensitive compared to BS; however, these are currently the most sensitive ante-mortem sampling procedures available. As ante-mortem samples, TB and LS allow for an increased sample size compared to post-mortem BS and an alternative to ELISAs for vaccinated animals. Utilizing pooling and increasing sample size allows for a higher herd detection rate while pursuing the most economical approach. LS were easier to perform and train in the field than TB and would be recommended by the authors for Mhp detection in the early and mid stages of an infection and in vaccinated populations.

Disclosure of Interest: None Declared.