Iowa State University swine veterinarians provide answers to some of the most-asked questions about porcine epidemic diarrhea virus.
By now, swine producers should be well aware of the emergence of porcine epidemic diarrhea virus in the United States. Do not underestimate the likelihood of transmission to your herd; this is a sneaky virus.
Some answers to common questions asked at the diagnostic laboratory follow, as well as tidbits on how to understand disease impact and decreases losses of suckling pigs.
What is PED virus?
Porcine epidemic diarrhea virus is a disease occurring only in pigs, caused by a coronavirus that does exactly what the disease name implies: produces acute and severe outbreaks of diarrhea that rapidly transmits among all ages of pigs (epidemic).
Where did PED virus come from?
Sequence data from U.S. strains thus far suggest all to be similar to a strain deposited in GenBank from China in 2012. PED virus is present in many countries in Asia and has been present in Europe since the 1970s.
How common is PED virus in the United States?
Data provided by veterinary diagnostic laboratories to the National Animal Health Laboratory Network suggests that there are 40 to 50 new cases of PED virus diagnosed each week, with the disease now reported in 14 states. The epidemic form of the disease is easy to recognize clinically so samples may not always be submitted to a laboratory, hence currently reported data likely underestimates true prevalence. An accurate estimate of U.S. prevalence is difficult to achieve and will eventually need to rely on serologic surveys, particularly if the disease becomes endemic.
How did PED virus get into the United States?
There is no confirmation of particular source or location of entry of the virus. Speculation abounds, but it is unlikely that we will ever know how PED virus entered the United States with certainty. However, this does provide the opportunity to scrutinize the possibilities to prevent future events.
How is PED virus spread?
Huge numbers of virus particles are shed in feces. One thimble-full of feces could contain enough virus to infect all the pigs in the United States. The PED virus is being detected in samples collected from pig collection points, slaughter facilities, transportation vehicles and innumerable fomites illustrating the vast potential for transmission. It is expected that survivability and transmission of virus will be enhanced in cold weather. Farm biosecurity efficacy is likely to be tested aggressively in the coming months.
How do I get an accurate diagnosis of PED virus?
If PED virus is suspected, consult your veterinarian. Sometimes, 10 ml of feces from acutely affected pigs tested by polymerase chain reaction is sufficient to confirm the presence of virus. However, submission of tissues from acutely affected pig(s) for complete diagnostic testing (bacteria, viruses, parasites) will allow for diagnosis of PED virus as well as other diseases. The value of histopathology and immunohistochemistry testing is illustrated in Figures 1-4 below. Sampling tips are found at http://vetmed.iastate.edu/vdpam/disease-topics/porcine-epidemic-diarrhea-ped-diagnostic-testing as well as the addendum below.
What is the impact of PED virus in weaned pigs?
Once pigs are weaned, the mortality rate from PED virus plummets. When not complicated by other diseases, pigs in nursery-finisher stages generally recover in about a week. The rumors of severe disease or high mortality in weaned pigs usually involve co-infections with salmonella or hemolytic E. coli or other risk factors associated with the environment or feeding practices.
1. PED virus acting alone should not be a large contributor to mortality in weaned pigs; PED virus would be expected to rob three to seven days in growth and performance measures.
2. Consult a veterinarian and get an accurate diagnosis if disease impact is greater than expected. There likely are other factors contributing.
What is the impact of an acute outbreak of PED virus in sow farms?
PED virus has very high mortality – approaching 100% – in suckling piglets. Herd immunity will mitigate losses starting about three weeks after herd infection. Thus, it is likely that three to five weeks of production will be lost in continuous farrowing operations, whereas batch farrowing operation losses can range from a few to as much as 100%.
Why is there a difference in mortality and impact between sucking piglets and weaned piglets?
Suckling piglets have greater risk for severe disease for several very important reasons:
1. More susceptible cells - The villi of the small intestine of neonates are very long with many more enterocytes available to be infected by virus (Figure 1).
Figure 1 shows the intestinal villi from a normal neonatal pig. Note that the villi are very long (indicated by the black arrow), with a large surface area to absorb nutrients and water. The crypts are where new cells are produced, which will eventually slide up and replace the cells on the villi.
2. Heal slower - The regeneration time for new epithelial cells (enterocytes) produced in the crypts is longer in neonates than in older pigs (Figure 1).
3. Immature homeostasis - The colon of neonates is functionally immature and less able to compensate for acid-base and electrolyte imbalances or to resorb water.
4. Osmotic pressures from undigested milk - Damage to epithelium reduces enzyme activity and disrupts digestion of milk, which increases the osmotic pressures within the intestine, effectively sucking water out of the piglet’s body and into the intestine and feces (Figures 2, 3, 4).
Figure 2 shows early PED virus infection in a neonatal pig. The brown staining is infected cells lining the villi in a neonatal piglet roughly eight hours after infection. All of the brown cells will die, releasing billions of virus particles to infect more cells and more pigs.
Figure 3 shows severe decrease in the height of villi (villus atrophy--see the black arrow) and loss of absorptive epithelium approximately 36 hours after infection of a neonatal pig. There is much less surface area to absorb nutrients or water. The crypts are increasing in length as they produce more cells to rebuild the absorptive cells of the villi.
Figure 4 shows just a few brown-stained PED virus-infected cells remaining in the gut of a neonatal piglet roughly 36 hours after infection. Note that there is very little absorptive surface remaining since most of those cells have been destroyed by PED virus.
What can a sow farm do to decrease impact of PED virus?
Always consult a veterinarian for current advice since the “best practices” for PED virus control are still evolving. Useful concepts derived from control of a similar disease, transmissible gastroenteritis, suggest:
1. Expose all sows to the virus as rapidly and uniformly as possible to establish “herd immunity” as quickly as possible.
2. Wean piglets early/as soon as possible: Maldigestion and osmotic impact of milk diet will exacerbate diarrhea and dehydration.
3. Provide water, electrolytes, alternative nutrition and supportive care.
Take home messages:
1. No one knows what the impact of PED virus will be this fall, winter or in the coming year(s). Stay informed and communicate with your herd veterinarian.
2. Scrupulously executed biosecurity practices at sow farms will be critical to prevent introduction of PED virus. Anything that has been near a pig (feces) is a potential source of introduction of virus. Biosecurity practices likely will need to be modified; PED virus is not the same as porcine reproductive and respiratory syndrome virus.
3. If PED virus strikes a sow farm, exposure of all sows for rapid herd immunity and the early weaning of suckling piglets may save lives.
4. Mortality due to PED virus in postweaned pigs is not severe unless there are co-infections or other risk factors at play. Accurate diagnosis of bacterial co-infections and risk factors will aid in implementation of necessary and timely interventions.
PED virus diagnosis:
At least 10 ml. of feces or intestinal contents on ice from acutely-affected pigs within the first 24 hours of the onset of diarrhea bested by PCR is sufficient to confirm a diagnosis of PED virus. However, a complete diagnostic workup is often warranted, using the guidelines in the table below.
Sampling tips can be found at http://vetmed.iastate.edu/vdpam/disease-topics/porcine-epidemic-diarrhea-ped-diagnostic-testing.
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