“Porcine reproductive and respiratory syndrome – or PRRS – has long been a four-letter word. But in late 1996 a new, more virulent form of the virus popped up in herds, mostly in southeast Iowa.
At first observers called it “hot” PRRS, SAMS (sow abortion and mortality syndrome), atypical PRRS and other names – some unprintable. The industry finally settled on acute PRRS.
It was more severe than other PRRS infections. Abortion storms hit breeding herds, affecting more than 10 percent of the sows. Fevers in sows reached 104° F to 106° F, higher than in other PRRS outbreaks. Some sows and boars died.
After the initial flareup, this form of the disease dissipated almost as suddenly as it came. Now, like some creature from a horror show, it’s back. Cases are being reported almost everywhere.
“We’re finding it all over the United States,” reports Pat Halbur, pathologist at Iowa State University’s College of Veterinary Medicine. It’s appeared in Iowa, Minnesota, Nebraska, North Carolina and South Dakota, just to name a few states. “There doesn’t seem to be one particular hot spot.”
“It’s picked up since fall,” adds Kelly Lager, veterinarian at USDA’s National Animal Disease Center in Ames, Iowa.
No one is sure why this harsh sister of PRRS has returned, whether it’s related to weather, coincidence or what. But it’s more devastating than normal PRRS.
“It seems we’re working with some PRRS strains that are more virulent than what we’ve seen in the past,” says Bill Mengeling, research leader for the Swine Virology unit at NADC.
An alarming aspect of the new strain (or strains) is that pigs appear to die from exposure to PRRS alone. Classic PRRS seems to weaken the immune system. Pigs get sick from secondary infections such as swine influenza or mycoplasmal pneumonia. If they die, it’s due to the secondary infection.
“In experimental conditions, adult and young pigs have become ill following exposure to some recent PRRS isolates despite an absence of other pathogens,” Mengeling notes. “This may explain why sows and boars have died in some of the acute PRRS outbreaks. However, there’s a chance another agent is tagging along, that we’re not producing just a PRRS strain in these cultures.”
Morris, Minn., veterinarian and PRRS expert Scott Dee doesn’t believe acute PRRS is caused by a PRRS virus strain alone. But Mengeling, Lager and Halbur all suggest the odds of another pathogen being involved appear small.
Part of the challenge is in the definition, Lager says. The strictest definition of acute PRRS includes acute onset, clinical signs over a two- to four-week period, more than 5 percent breeding herd mortality and abortion rates exceeding 10 percent for all parities during all gestation stages.
By that strict definition, Lager says, you could argue only a handful of cases of acute PRRS have been reported.
Jeff Zimmerman, Iowa State University veterinarian, surveyed more than 1,200 U.S. and Canadian veterinarians, to find cases using the strict definition. The 379 respondents reported 138 cases, but it’s unclear how many involved all or only some of the criteria.
Loosen the definition by ignoring breeding herd deaths, and the numbers grow dramatically. Lager says it’s impossible to offer precise case numbers to date.
But one thing is certain: The disease is rearing its head again. This fall’s outbreak doesn’t include remarkably high breeding herd death losses, says Halbur. But it does include severe abortion rates and respiratory problems in neonatal and nursery pigs.
Some producers who discover acute PRRS may not report it for fear of the negative perception it carries.
Generally, veterinarians are seeing it most in large operations, 500 to 1,000 sows or more. Most of these operations are expanding or bringing in replacement stock from several sources, Halbur notes. The real train wrecks, Lager adds, are in operations with high breeding-herd turnover and no gilt isolation facilities.
Smaller operations, especially single-site farms that retain their own breeding gilts, seem less susceptible.
But no operation is immune. And the question of immunity has researchers scratching their heads.
“A few years ago, we thought we were close to controlling PRRS,” Mengeling says. “We had vaccines that appeared effective. But those vaccines may be less effective against some current strains.”
The modified-live vaccines may not work in every case, but they still represent an important route to control PRRS. “Modified-live vaccines can be effective,” Lager says. “Sometimes the best strategy is to vaccinate all breeding animals, hoping it will attenuate their next contact with the field virus. Then you back off and vaccinate only incoming gilts. Eventually you even try to back that off.”
One question about vaccines concerns cross protection against different PRRS strains. Lager says there’s no guarantee you can protect against a new strain that hits unexpectedly.
“We have a lot to learn, but there are a lot of good people working on the problem,” Halbur says. Mengeling contends the pork industry needs to step up its research efforts.
“It’s costing millions of dollars,” Mengeling says. Even though the reproductive aspect gets the most attention, dollar losses due to the respiratory facet may be higher, contends Halbur. That’s because of increased preweaning mortality, nursery pig respiratory disease and severe growth lags in market hogs due to the eventual development of the porcine respiratory disease complex.
PRRS shows no signs of going away. So take care when buying breeding stock. Always isolate incoming pigs and keep biosecurity a priority. Then cross your fingers and hope you don’t have to deal with the more virulent form of this costly disease.
How To Limit Exposure And Control Prrs
There are no guarantees that you will never face a PRRS outbreak. But these tips can reduce the odds that porcine reproductive and respiratory syndrome will hurt your operation.
- Work with your veterinarian to establish a breeding-animal flow plan that keeps PRRS and other diseases from entering your herd.
- If you’re concerned that PRRS exists, you first need a proper diagnosis of the farm’s pattern of viral infection, says Morris, Minn., veterinarian Scott Dee.
- Gilt management, including isolation and acclimatization of PRRS-negative incoming breeding stock, is critical, veterinarians warn. The isolation period should be at least 60 days. Acclimatize incoming gilts for the specific pathogens that exist on your operation.
To do that, you’ll need to find out your breeding herd’s health status. If it’s negative for PRRS, do all you can to keep infected animals out of your operation. If it’s PRRS-positive, consider vaccinating incoming stock to match the status of your existing sows.
Dee proposes classifying operations according to PRRS activity. His four categories are:
2. PRRS-positive with stable/inactive infection.
3. PRRS-positive with stable/active infection.
4. Unstable herds.
- Buy only breeding stock that match your herd’s PRRS status.
- Minimize the number of sources from which you buy breeding stock. When you have crates to fill, it’s tempting to buy from whomever has gilts available. Avoid that temptation.
- Be aware that purchased boar semen may carry PRRS virus. See that your semen supplier runs a clean operation that poses little risk. Suppliers should test the boars’ PRRS status.
- If your sows contract PRRS and you want to repopulate with new genetics, Kelly Lager of USDA’s National Animal Disease Center suggests being wary of how new animals may fare in a unit that may still harbor the PRRS virus. However, Dee contends this is no problem.
- Consider bringing PRRS-negative replacement gilts to your operation early – say at about 50 pounds – raising them and controlling their exposure to all pathogens in your facilities.
Dee also advises stabilizing the breeding herd by strategically using vaccines and gilt management.
To control PRRS in the nursery, consider depopulating that area. Use all-in/all-out pig flow and ensure proper ventilation with no cross contamination.
Bill Mengeling of the NADC says it’s easy to offer general advice. It’s tougher to know exactly what to do when an acute PRRS outbreak confronts today’s commercial operations, where ideal disease prevention practices can be compromised by other decisions. Yet Dee insists producers and veterinarians can solve and control PRRS on farms. The previous tips can help.