Prevalence of the novel orthoreovirus 3 (MRV3) in diarrhea cases in US pigs in 2015

A novel mammalian orthoreovirus 3 (MRV3) has been discovered in cases of severe diarrhea at the beginning of the PEDV epidemic in 2013 in US field cases. It has been suggested that not PEDV alone but the combination of MRV3 and PEDV has caused the severe clinical signs seen in North American pigs.

In order to investigate the prevalence of MRV3, 277 randomly collected fecal samples from pigs with a history of diarrhea submitted to the Iowa State University Veterinary Diagnostic Laboratory from April to May 2015 were used. The samples originated from 41 farms and 1-to-34 pigs on each farm with an age range from neonatal to adult. All samples were tested with a real-time RT-PCR designed to target the S1 gene of MRV3. As an internal control, a portion of the samples were tested with a conventional MRV3 RT-PCR targeting also the S1 gene but resulting in a larger PCR product compared to the real-time PCR. A positive control, courtesy of Dr Subbiah Elankumaran, Virginia Tech, Blacksburg, USA, was included in all runs. In addition to MRV3 testing, all samples were also tested for PEDV RNA and PDCoV RNA by real-time RT-PCRs.

Low levels of MRV3 RNA were detected in 5.7% (16/277) of the investigated samples from 19.5% (8/41) of the farms corresponding to 1 to 7 samples on each farm by real-time RT-PCR and two samples were confirmed by the conventional RT-PCR assay. Interestingly, in 1/8 MRV positive farms PEDV was detected but the remaining 7/8 MRV3 positive farms were negative for PEDV and PDCoV. The overall prevalence of PEDV in the investigated sample set was 10.5% (29/277) and corresponded to 22% (9/41) of the farms. The overall prevalence of PDCoV was 0.4% (1/277) and the positive sample was identified in a PEDV positive farm.

Our results indicate that the prevalence of MRV3 in the Midwest during 2015 is comparable with that of other enteric pathogens such as PEDV. Since 2015, the prevalence of PEDV is continuously decreasing indicating that herds are becoming immune to this virus. Our findings suggest that a similar scenario may also apply to MRV3, however, prevalence studies for MRV3 during 2013, when the virus was first recognized, and serological studies during 2013-2015 need to be conducted to further confirm this.

Disclosure of Interest: None Declared.