|The following article is a special report from the 2014 Allen D. Leman Swine Conference|
The Allen D. Leman Swine Conference was held recently in St. Paul, Minn. and many of the presentations were sharply focused on Porcine Epidemic Diarrhea virus (PEDv) and Porcine Deltacorona virus (PDCov). We are coming into our second winter of dealing with PEDv, and we still don’t know how to protect herds against the virus, even though we have new tools to help us minimize its effects.
After listening to the speakers, it’s easy to see why this is the case.
Dick Hesse, a virologist with the Kansas State University Diagnostic Laboratory, presented the Pijoan Lecture at the conference and discussed the recent emergence of swine coronaviruses in North America.
“Somehow, we have to figure out a surveillance program on an international basis, because this is a global community,” he states. “We have to recognize what’s going on in our nation, and also in our hemisphere.”
Hesse says there are a variety of effective surveillance tools, but they could be used more effectively. “All we needed to do was pay attention to what the literature was telling us.”
What We Know
We do know some things, however, and he provided a review of the “knowns” surrounding PEDv and PDCov:
- They are pig-only disease diseases that do not affect other species or humans and are not a food safety concern.
- PED has been in Europe since 1971; Asian variants are more virulent than European strains and traditional vaccines don’t work anymore.
- PDCo virus is a new diarrheal disease of swine and we don’t know much about it.
- PED incubation time is typically less than 36 hours and virus is shed in feces for 42 days or more.
- PED is age-dependent in disease severity; approaching 100 percent mortality in nursing pigs.
- PDCo virus also is age-dependent, but mortality in nursing pigs ranges from 20-60 percent.
- PDCo virus will cause vomiting and occasional diarrhea in sows and gilts and mild to moderate diarrhea and vomiting in recently weaned pigs.
Hesse notes that for both PEDv and PDCov, laboratory diagnosis is required for definitive confirmation, especially in older pigs. “Laboratories are sharing information – and good reporting is taking place.”
“Clinical signs of PEDv and PDCov are indistinguishable from the epidemic form of the disease caused by a different porcine coronavirus, Transmissible Gastroenteritis Virus (TGE),” says Hesse, “And there is no cross-protection between these coronaviruses.”
In addition, no one knows the duration of immunity for PEDv, says Hesse. When asked why is PEDv is harder to eliminate than TGE, Hesse believes it’s because more virus is being spewed out and it’s a virus contamination issue.
Hesse shared preliminary results of a 42-day, pathogenesis study on PDCo virus that was done at the Life Science Annex of the University of Nebraska. The purpose was to “determine the sites of tissue localization, routes of viral shedding, duration of virus carriage, kinetics of antibody response, and potential of aerosol transmission of Porcine Deltacoronavirus (PDCoV) following inoculation of nursing pigs and their dams.”
Animals were housed in five isolation rooms and were challenged with a pool of fecal derived virus that was passaged in 2-4 day-old pigs. Nursing pigs and their dams where challenged when pigs were 2 days of age via intranasal and oral routes with 5 ml of inocula per route. Hesse says they also collected a spectrum of tissues.
Clinical observation of the sows following challenge showed that all sows were clinically normal after day 8 post-inoculation.
Aerosol contact sows developed soft to diarrheic feces on day 5 of the study and returned to normal after day 8. The non-infected control sow remained clinically normal throughout the study.
Clinical observation of the pigs showed morbidity of 100 percent. Hesse reports that mortality was variable among the litters but especially high in two of the six inoculated litters. All piglets from those two litters were either euthanized due to severe dehydration or had died by day 8 post-inoculation, he says. Inoculated piglets developed soft to profuse diarrheic feces on day 2 post-inoculation, as did the contact controls, and clinical disease was over at 12-days post inoculation. Aerosol contact litters developed diarrhea on days 3 and 4. All surviving piglets had returned to normal by day 12, and the non-infected negative control piglets remained clinically normal.
“We never could pick up viremia in any of the sows,” he adds. “We could still pick up virus in their GI tracts and in the lymph nodes of most of them 35 days post inoculation.”
What We Don’t Know
“We had viremic animals five days post-inoculation,” says Hesse. “But 42 days out, we were still finding it in the lymph nodes and the intestinal tract. Is it infectious? We don’t know.”
The biggest uncertainty surrounding PEDv and PDCov is that nobody knows what will happen this fall and winter, says Hesse. “I’d like to be optimistic and say we have things under control, but we don’t know. I hope we don’t see it, but it won’t surprise me if we do.”
Regarding tissue localization, Hesse and other virologists have only been able to find virus in the GI tract, and mainly in the small intestine, but “they do generate a nice antibody response,” he says.
The Nebraska study was 42 days long, and the researchers were able to isolate virus in the lymph nodes and intestinal tract, as mentioned.
“Is the virus there longer than 42 days?” asks Hesse. “I would bet a year’s paycheck that it probably is.”