Porcine Epidemic Diarrhea - a new disease in the Americas

Monday, August 5, 2013

While PED is emphatically not present in Canada, this diarrheal disease has been found in a number of Midwest U.S. States. Intensified biosecurity measures are the best protection against it  

by ERNEST SANFORD

In May 2013, outbreaks of Porcine Epidemic Diarrhea (PED) erupted in a number of Midwest U.S. states, including Iowa, Minnesota, Indiana, Colorado, Illinois, Missouri, Oklahoma, South Dakota, Ohio, Michigan and Nebraska at time of writing.

Of note, over the first few months of the outbreak no cases were reported from North Carolina – the state, second only to Iowa, with most pigs in the United States. Just before going to press, however, two cases were reported from North Carolina, bringing the total number of cases reported to 265 from 14 states.  

After doing some backtracking, U.S. diagnosticians are now able to confirm that the initial case occurred in Ohio on April 16, 2013. It took some time to confirm as it was thought to have been TGE initially. The first thing I immediately have to say is, in spite of numerous early reports that identified Canada as one of the countries in which PED is present, this is not true. PED is NOT present anywhere in Canada and has NEVER been in Canada.   

With the disease in our neighbour's herds right next door, however, we need to pay careful attention, learn more about it and, above all, increase our biosecurity to ensure it doesn't get into any of our herds.

PED is a diarrheal disease remarkably similar to transmissible gastroenteritis (TGE). It is caused by a coronavirus similar to TGE virus (TGEV). It is not a new disease, having been first described in the United Kingdom in 1971 and the PED virus (PEDV) was isolated by Dr. Maurice Pensaert and his colleagues at University of Ghent in Belgium in 1978.

Clinical Signs. Similar to TGE in a naïve population, PEDV causes acute, explosive outbreaks of vomiting and diarrhea in pigs of all ages, but most especially in neonatal, suckling pigs. Mortality in suckling pigs below seven days of age approaches 100 per cent. Older pigs are likely to survive but can become stunted and unthrifty. The disease eventually becomes endemic, whereupon new outbreaks become sporadic with diarrhea and vomiting limited to suckling and recently weaned pigs. Again, similar to TGE, treatment is limited to providing water and electrolytes as supportive therapy to counteract dehydration.  

Diagnosis. A presumptive diagnosis of PED can be made on clinical signs of TGE-like disease where laboratory samples consistently test negative for TGEV. PCR analyses are available for quick identification of PEDV in fecal samples or intestines of infected pigs and for differentiation from TGEV. Isolation of the virus is more difficult and is seldom attempted, preference being for the faster and more reliable PCR tests. Atrophic enteritis, again similar to that seen in TGE, is found on histological examination of the small intestines. Immunohistochemical (IHC) stains are used to identify PEDV and differentiate from TGEV. Serological tests are available to demonstrate seroconversion to the virus in pigs two to three weeks after they have recovered from an acute outbreak.

Epidemiology. PEDV is transmitted by the fecal-oral route. Virus introduction occurs mainly through infected pigs, but indirect transmission can occur via virus-carrying fomites, such as trucks, boots, clothing and farm supplies. The virus survives well under cold and wet conditions; hence outbreaks are more likely in the winter and other colder periods of the year. It is destroyed by heat and dry conditions. Regular cleaning agents and disinfectants will also destroy the virus.

Prevention. Biosecurity! Biosecurity! Biosecurity! Intensifying biosecurity measures is a must. Biosecurity, however, has to extend beyond the farm to include washing and drying of trucks returning from delivering pigs to U.S. farms, assembly yards, slaughter plants and the like.

Outbreaks in the United States have occurred simultaneously in totally different, unrelated herds in several widely separated states with no indications so far of where the virus originated and how it appeared suddenly in so many different locations.

The nature of the outbreaks suggests it's a PEDV strain currently circulating in Asia. Because of the almost simultaneous eruptions in different types of herds in multiple states, speculations are that the virus might have been introduced via imported feed or supplements from Asia, possibly vitamin/mineral premixes, amino acids or some other feed ingredients. However, tests on all feed ingredients have been negative. Since heat destroys this virus, it is expected that heat for the pelleting process will destroy the virus in pelleted feed. Vaccines are available in Asia, but their efficacy has not been unequivocally established.

Public health and trade concerns. PEDV infects pigs only. It does not infect humans. There are no food safety concerns. Likewise, there are no trade implications and no pig movement restrictions.

Some reports have erroneously identified Canada as a country in which PEDV is present. This probably stems from a paper in the Canadian Veterinary Journal in 1980 in which a non-TGE coronavirus caused outbreaks of diarrhea in pig herds in Quebec.

At the time, the authors speculated that the coronavirus might have been PEDV. Subsequent tests later in the 1980s ruled out PEDV as the cause of the outbreaks, although a definitive identification of the virus was never established. The outbreaks subsided without identification of which coronavirus had caused them.

I've mentioned TGE repeatedly throughout this article because of the many similarities between TGE and PED. I do realize, however, that most of our younger producers have probably never encountered TGE. It was a common disease from the 1960s through to the 1990s. It is much less common today following a mutation of the TGE virus in the 1990s in Europe that produced an entirely different virus called porcine respiratory coronavirus (PRCV).

PRCV can cause a mild to moderate respiratory disease in growing pigs. Much more importantly, however, once PRCV emerged, it spread rapidly throughout the pig populations in Europe and then North America. PRCV creates cross-protection against TGEV and this has provided cross-protection against TGE, resulting in very few TGE outbreaks over the last 15-20 years.

These PED outbreaks in the United States represent the very first time that PEDV has been identified anywhere in North, Central or South America. Herds throughout the Americas, including Canada of course, are totally naïve to PEDV and, if infected, severe diarrhea and vomiting with high mortality in suckling pigs can be expected. BP

S. Ernest Sanford, DVM, Dip. Path., Diplomate ACVP, is a swine specialist with Boehringer Ingelheim Vetmedica (Canada) in Burlington. Email:ernest.sanford@boehringer-ingelheim.com