SHIC-Funded Study Examines Mammalian Orthoreovirus, Adenovirus, Novel Pathogens in US Swine Herd

The Swine Health Information Center funded a study to examine whether mammalian orthoreovirus (MRV) and porcine adenovirus (PAdV) are present in US swine herds and whether they pose health risks. Led by Dr. Wenjun Ma, University of Missouri, the study suggests that MRV and PAdV can infect pigs and contribute to general enteric disease but that these viruses are not associated with severe or unique clinical signs. These findings suggest that both viruses circulate in swine populations, with evidence pointing toward intestinal shedding or prior exposure rather than widespread active disease in routine tissue submissions. For producers and their veterinarians, this study helps interpret diagnostic findings on MRV or PAdV, enabling more informed herd health decisions.

Read the industry summary of study #24-022 here.

Emerging and re-emerging viral pathogens remain an ongoing threat to the US swine industry, and undiagnosed disease outbreaks continue to highlight important gaps in pathogen surveillance and characterization. Although MRV and PAdV have been reported in pigs in other countries globally, contemporary data for US swine populations are limited. Previous research suggests that MRV and PAdV may contribute to clinically significant disease in swine, yet their prevalence, epidemiology, pathogenic potential, and impact on production remain poorly defined. This project was designed to address two objectives: 1) define the prevalence and epidemiology of MRV and PAdV in US swine herds and 2) determine the pathogenicity and transmissibility of MRV and PAdV in weaned pigs, including the potential contribution of co-infection to clinical disease.

To provide a clearer picture of how common these viruses may be and whether they pose a health risk, lung, intestinal, blood, and fecal samples were tested for evidence of MRV and PAdV. A total of 794 swine tissue samples collected through diagnostic laboratories were screened, including eight intestinal samples and 783 lung samples from 18 US states (North Carolina, Oklahoma, Minnesota, Indiana, Illinois, Missouri, Montana, Arkansas, California, South Dakota, and Kansas), as well as three lung samples from Mexico. RT-qPCR results showed that 29 of 786 (3.69%) swine lung tissue samples were positive for MRV and that one of eight (12.5%) intestinal samples were positive for MRV. However, only one virus was isolated from one RT-qPCR -positive lung sample, and not from other RT-qPCR-positive samples.

Additionally, investigators established an MRV ELISA and tested 481 serum samples collected from pigs in the US, of which 60% were seropositive. Together, these findings suggest that active MRV infection was infrequently detected in field tissue samples, whereas prior exposure to MRV was common in the sampled swine population.

To determine the prevalence and epidemiology of PAdV in US swine herds, researchers screened 576 swine samples by PCR, including 516 lung samples and eight intestinal samples collected for MRV screening, as well as 52 fecal samples collected from a Missouri swine farm. All eight intestinal samples were negative for PAdV, whereas one of 516 (0.02%) lung samples and six of 52 (11.5%) fecal samples were PCR-positive. Virus isolation was attempted for the seven PCR-positive samples but was unsuccessful, indicating that no infectious virus was recovered. These findings indicate that PAdV was not detected in the lung or intestinal tissue samples examined and that the positive fecal and lung PCR results did not provide confirmed evidence of live PAdV infection.

To determine the pathogenicity and transmissibility of MRV and PAdV, including the potential contribution of co-infection to clinical disease, controlled pig studies were conducted to determine whether MRV and PAdV individually or in combination could cause clinical disease and spread to other pigs. Groups of 3- to 4-week-old weaned pigs confirmed to be negative for swine influenza virus, PRRSV, PEDV, TGEV, porcine group A rotavirus, MRVs, and PAdVs were used in the study. Pigs infected with MRV, PAdV, or both developed mild clinical signs, including diarrhea and fever, and viral genetic material was found in intestinal tissues of infected pigs and contact pigs. Overall, these findings indicate that both viruses can infect pigs, but that co-infection did not produce significantly more severe clinical disease under these conditions.

For pork producers, these results suggest that MRV and PAdV are both capable of infecting pigs and can contribute to enteric and respiratory disease. However, neither virus was associated with severe or clearly distinct disease in this study. These results demonstrate that MRV exposure is common and that both MRV and PAdV can infect the intestinal and respiratory tracts of pigs, potentially contributing to diarrheal and respiratory disease. The detection rate of virus in field samples suggests low rates of active infection but may be due to sample limitations including the small number of intestinal and fecal samples included in this study.

Overall, continued monitoring and surveillance are necessary to determine when these viruses may contribute to disease and pork production losses in the field, such as when they are combined with other infections or management stressors. Investigating emerging diseases such as MRV and PAdV in US swine helps SHIC fulfill its mission to monitor potential threats to the US swine industry and develop information to protect the US herd.

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].