Porcine Epidemic Diarrhoea (PED) virus took the US pig industry by surprise.
The understanding of it is progressing, albeit slowly. Dr Tom Burkgren, executive director of AASV, explains which pieces of the puzzle are yet to be found.
By Vincent ter Beek
Although it was May 2013 when the virus was first found on a farm in Indiana, United States, the strain of Porcine Epidemic Diarrhoea virus (PEDv) that hit the North American pig industry still remains a mystery in many ways. As the virus has also spread to 30 US states and other American countries like e.g. Canada, Mexico, the Dominican Republic, Colombia and Peru and simultaneously has been causing big losses in Asia, predominantly Japan of late, there are still many lessons to be learnt.
In the words of Dr Tom Burkgren, executive director at the American Association of Swine Veterinarians (AASV), at the 2014 World Pork Expo, in Des Moines, IA: "About a year ago, we did not know a lot about PED virus. Now we are one year further, we know a tremendous lot of things. But we still don't know how it got here and all the ways it spreads."
The AASV is arguably the key organisation in the United States when it comes down to continuously gathering, monitoring and sending out the latest information about the virus. "Educating and informing our members – that is our role," Burkgren tells Pig Progress. "We help to propel the development of knowledge. The National Pork Board is the main funding body, providing grants. Recently they made $3 million available for PEDv research."
The last year has been manic, and the challenge of PEDv has proven to be a tough one, Burkgren notes. He has been attached to the AASV since 1994 and has as such been involved with e.g. the eradication of pseudorabies (Aujeszky's Disease) in the US pig industry. It's tempting to compare the outbreaks of PEDv with that of earlier viruses to those of Porcine Circovirus 2 (PCV2) and PRRSv (Porcine Reproductive and Respiratory Syndrome). Burkgren says, "In a way, we can see some of the same effects. The death losses and farms affected – and very severe clinical signs. All three diseases have shown very high mortality especially for the younger piglets."
Just to recap for those outside the Americas and Asia – what is PEDv exactly? In Pig Progress' recent Piglet Health Special Edition, Prof Dr Robert Morrison and Dane Goede, of the University of Minnesota, describe PEDv as follows: "Suckling piglets show vomiting and severe diarrhoea and these symptoms in conjunction with reduced milking lead to fatal dehydration and malnourishment. Most piglets less than one week of age will die in a naïve herd. [...] Growing pigs and breeding animals will generally have watery diarrhoea and vomiting which can cause increased mortality and reduced average daily gain."
This, however, is by far not all there is to know. Burkgren had a clear message at the World Pork Expo, when addressing show delegates at the event: "The truth is that if we don't learn enough of what caused it to come into the country – and why it spread, and if we don't understand what sow immunity is, then next fall and next winter could be as bad. And I must admit that keeps me awake at night."
In this article, Pig Progress and Burkgren dive into the major questions – and try to uncover which are yet to be answered. What do we know – and what needs to be yet unveiled?
Is it one virus or more?
PEDv is classified as being a coronavirus and as far as it is known, there are currently two subtypes of the virus
present in the US, Burkgren says. The 'original' version showed very much resemblance to earlier isolates found in China's Anhui province – hence this country is considered to be the origin of the virus. Burkgren says, "The virus destroys the villi of the intestines and it is very similar to Transmissible Gastro-Enteritis (TGE). In a way it is quite shocking to see what damage it can do to baby pigs."
The virus appears to have mutated as well, as in January 2014, researchers at the Animal Disease Diagnostic Lab of the Ohio Department of Agriculture distinguished a second variety which was later confirmed by Iowa State University and the University of Minnesota, Burkgren says. In this variety sows were known to be infected, and although piglets also did show clinical signs and died, it was in some cases not as severe as the original strain.
To make things a little more complicated, another coronavirus also first appeared in the US in February of this year. Clinical signs of this virus, called Swine Delta Coronavirus (SDCv), has been reported to be similar to PEDv, causing diarrhoea and vomiting and all age groups and mortality in nursing pigs. Mortality rates, however, appear lower than in cases of PEDv. Both diseases (PED and SDCv) are reportable in the US and Canada.
Dr Tom Burkgren, executive director AASV: "I would not wish PEDv on any country."
How did it get into the US?
Burkgren says, "We don't really know the route of entrance into the United States, this is a subject of analysis. We need to establish what have been risk factors and what products might have been infected. This does mean, however, that a lot of products need to be investigated – think of feed, but also e.g. vaccines. And don't forget that
people travel a lot – including Chinese nationals."
He continues, "What has been striking is a certain pattern. The virus showed up in three different places more or less simultaneously – it happened within days of each other. The earliest entrance was backtracked to April 15, 2013 in Ohio. But a retrospective test also showed outbreaks in Indiana, Iowa, Colorado and Minnesota."
How does PEDv transmit?
There are mixed opinions about this, as theories point into several directions including road transport, feed inclusion or air transmission, Burkgren says. "We definitely have to spend time on the matters we haven't fully understood. There have been several room studies in which infected pigs shared air space with naïve pigs – and the naïve pigs did not get infected. At the same time, there have been trials trying to establish whether or not PEDv is an airborne virus, at the University of Minnesota. They found traces of PEDv using PCR air samples ten miles away from an infected herd."
Burkgren also pointed to research in pig dense North Carolina, where the disease spread very rapidly. Research by the University of Minnesota showed that farms within a three-mile radius from PEDv positive farms have a higher risk of infection.
For more information about this topic, see the box PEDv and feed.
How to fight it?
Above all, tight biosecurity is paramount of course – see also the box PEDv and disinfection. Another strategy is that of vaccination – these developments are described in the box PEDv and vaccines.
While waiting for various vaccines to become widely available, maternal immunity has been a widely applied strategy for producers and veterinarians. Burkgren explains, "This comes down to stimulating sow immunity by offering her 'feedback', as what we need to achieve is an increase in immunity combined with a decrease of the viral load. Usually, once PEDv is confirmed, the sows will be aborted or piglets will be euthanised. Part of this will be fed back to sows. They may get a little bit sick or be off feed for a little while, but no mortality is observed. What is more important is that she will also build up immunity. While doing that, a cleaning and disinfection process will start (decreasing viral load in the environment)and pregnant sows are being moved in again. This would mean that the production is hopefully back to normal, about eight to ten weeks after the first onset of clinical signs."
He continues, "Once the sows are immune, she will have immunoglobulin A (IgA) antibodies in her milk. The piglets thus can develop a balance point with lactogenic immunity – they need a lot of this to resist a viral load. Here again, the real concern is that we don't actually know how high the immunity is. It is an area where more research is needed."
It is one of the areas where PEDv
surprised science, as this summer, sow immunity proved not to be a 100% safe option. Burkgren says, "Some of the farms applying sow immunity, the first being in Indiana again, have seen a
re-occurrence of the virus, eight to
nine weeks post-infection. That poses questions about sow immunity."
How much is lost?
Another question which is easier to ask than to formulate an answer. At the World Pork Expo, Dr Steve Meyer from Paragon Economics estimated that roughly seven million piglets had died – by mid-August estimates had grown towards eight million. Burkgren comments, "These figures are not for sure, as nobody is tracking mortality. There are also production losses of between two to five days in finishing pigs. But there are also reproductive problems as sows don't rebreed, causing farrowing rates to get depressed."
Hence, total costs are difficult to estimate too. Burkgren says, "The amount of piglets that have perished can be multiplied with $75, the average price of a weaned pig. And that is just the pig losses. I'm not sure where we stand if we also add production and reproduction losses or the cost of cleaning, disinfecting and increased biosecurity."
Ironically, on the US pig market, the effects of PED virus have not been noted to a very large extent well into summer, Burkgren says, as slaughter numbers are down, but average weights have increased. He adds, "Pigs are being sent to market quite a bit heavier; this could be 5-10 pounds up to even 20 pounds heavier (2.25-4.5 kg and 9 kg respectively, ed.)."
It is feared that stronger effects will be felt next winter, as the virus is known to thrive in colder temperatures. When addressing the audience at World Pork Expo, Burkgren pointed to the declining trend in summer, saying: "We like to see this being related to the virus not being stable in warmer summer months. And we like to think this is because biosecurity has been stepped up. But I'm not going to tell you that it is going to go away."
What has been learnt?
It's not all doom and gloom, Burkgren says as in the wake of all losses, several good developments have also been
happening. He says, "The attention for biosecurity in the US definitely has gone up. Establishing a good cleaning and disinfection programme is really required in order to achieve an exclusion of the virus. This includes people, equipment, feed, pigs – all supplies that are being carried in."
In addition, our understanding of viral diseases has gone up as well. You could also add that the cooperation of the three pig organisations, National Pork Producers Council (NPPC), National Pork Board (NPB) and the AASV has definitely been improved as a result of the disease.
Last but not least, Burkgren has wise words for the pig industry in those countries which so far have been spared from PEDv outbreaks. He says, "I would not wish PEDv on any country. The past 15 months of dealing with PEDv have been extremely difficult. I believe that identification of pathways of entry into a country is paramount. Once identified then vigilance, preparation and increased biosecurity are needed."
PEDv and feed
Can PEDv be transmitted through livestock feed? Yes, many in the US have been believing for some while. But how? Earlier this year plasma was thought to be the ingredient to blame, but in August, a study showed that there is a chance that livestock feed can carry the virus. In the study, researchers collected feed residue from three farms in Iowa and Minnesota that had outbreaks of PEDv and had received feed from the same source. They fed it to five piglets in an experiment at South Dakota State University, and all became infected with the virus. Piglets that were not fed the infected feed did not get sick. The study did not determine how the feed became infected with PEDv.
PEDv and vaccines
How is the progress with regard to PEDv vaccines? In Korea, several vaccines have been developed, but for several reasons of compliance and different subtypes, these cannot be used in the Americas. Several larger animal health companies are known to work on vaccines. Both Harrisvaccines and Zoetis managed to receive a conditional USDA licensure for its sow vaccine. In addition, Merck and Boehringer Ingelheim have all announced to work on a vaccine.
PEDv and disinfection
Properly disinfecting is one important element to eradicate PED virus from a farm. Paul Thomas, Iowa State University, spoke at the recent IPVS Congress in Cancún, Mexico, about his attempts to figure out what could be good procedures to inactivate the virus in the presence of faeces in transportation trucks. He concluded that ten minutes at 71°C (or 160°F) is sufficient; or one week at room temperature (20°C or 68°F). He added, "No other combinations of time and temperature were shown to be effective at inactivating PEDv."
PEDv and the moment of infection
Dr Laura Greiner of Carthage Veterinary Services, Iowa, also spoke at the IPVS Congress in Mexico. She told that, based on on-farm detection, there appears to be a delay of 24 to 48 hours post-introduction before clinical signs of PED virus are detected by caretakers. This means that infection of PEDv could have happened as much as two to four days prior to the onset of disease. For veterinarians and pork producers, this is important knowledge, as this places the quest for a potential source of infection on a farm in a different perspective.
[Source: Pig Progress magazine no. 7- 2014]