Editor’s note: This is the first of a two-part series of articles. The series is based on information presented at the Fourth International Swine Disease Eradication Symposium held in conjunction with this year’s University of Minnesota Allen D. Le man Swine Conference in St. Paul, Minn.

Eradicating and/or controlling disease continues to be a day-to-day job for swine veterinarians that require dynamic programs to keep up with changes in health status of the production systems, says Paul Yeske, DVM, MS.

“Even though we want herds to stay at a high health level, history has shown that the health status changes over time,” says Yeske, a swine veterinarian with Swine Vet Center, St. Peter, Minn.  “There are many factors that cause these changes and we have to be ready to adapt to the changes.”

 

Detailed cleaning of a site for herd depopulation/repopulation includes washing the entire facility and removing all organic matter such as feed and manure.

Yeske says that disease-free production is “the goal which would allow producers to realize the full production potential of their animals.” At the same time, he remarks, “disease-free does not always have to mean pathogen-free. The main goal for commercial pork production is to keep the pathogen level below the threshold of disease.”

8 eradication protocols
Disease eradication, says Yeske, is “the gold standard for removing pathogens from the herd.” He cites the following eight eradication methods that can be used in today’s production systems, depending on the disease you are trying to eliminate:

Depopulation/repopulation
Depopulation and repopulation is the most straightforward method of disease eradication, according to Yeske. “The benefit to this method is that more than one disease can be eradicated at once.”

Depop-repop has been a successful tool for diseases such as pseudorabies (PRV), porcine reproductive and respiratory syndrome (PRRS),  atrophic rhinitis, Actinobacillus pleuropneumonia, swine dysentery and mange, among other swine diseases, Yeske says.

Besides the obvious disease eradication benefits, depop/repop also provides the means for upgrading genetics or restructuring a herd’s parity distribution problems, Yeske says.

One challenge when depopulating a herd is to find disease-free stock to repopulate the herd with, he says. Additionally, there is always the possibility that a repopulated herd might re-break with a disease, despite heightened levels of biosecurity in the new herd.

Swine Vet Center veterinarians have successfully used the following procedures for depopulating and repopulating swineherds for their clients, Yeske says:

  • Detailed clean up of the entire site. This includes washing the entire facility and removing all organic matter such as feed, manure and saliva. All items that cannot be thoroughly washed should be removed and replaced.
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    Disinfecting of a hog facility during a depopulation/repopulation requires that everything, including water cups or water troughs, be completely dry.

    Maintain a specified down time without pigs on the site.  The time is determined by the cost of failure in the system and the specific pathogens you desire to attempt to eradicate. The most common time frame has been four weeks, but some successful programs have utilized as little as a one-week down time.

  • Disinfecting the entire site. Everything in the facility must be completely dry. This might require the use of a leaf blower or air compressor to blow water from troughs or water cups. Inspect water lines and repair if they are leaking. Any equipment that can be removed from the building should be taken outside to dry in the sum. A double disinfection using two different types of disinfectant is a good idea.
  • Fumigate facility. This should be done two to three days prior to the arrival of the new population.
  • Implement strict biosecurity procedures. Monitor and regulate people, including employees, visitors and service people. Monitor and regulate transportation of animals into and out of farms, including the implementation of internal trucking between sites in a system and the installation of internal truck washes. Ensure that internal trucks never leave the system. Properly remove and compost or incinerate dead animals. Quarantine incoming supplies, including medications and vaccines. Implement strict isolation protocols and test all groups of new pigs prior to entry into the herd. Have good pest control programs in place, including rodent and insect control.

Test and removal
Test and removal as a means of disease eradication requires that the animals that have been exposed to a specific pathogen are no longer shedding the organism, says Yeske. Test and removal proved to be a very effective way to eradicate PRV, he says.

“With PRV there was an effective vaccine that developed good immunity and reduced the shedding in affected animals, as well as having markers that allowed for differentiating infected animals from vaccinated animals.”

Yeske says that test and removal has been used in efforts to eradicate PRRS virus, “but it has been much more difficult to predict the outcome. Most of the herds that have used this method have been small and only had one strain of virus present in the herd and usually were done in combination with a herd closure and after replacement with negative animals. Except by seedstock suppliers and boar studs, this procedure has not been widely used for PRRS eradication because there is a higher rate of failure and the time required to accomplish this is long.

 

Disease-free pork production does not necessarily mean pathogen-free, says Minnesota swine veterinarian Paul Yeske.   

“First, the herd must stabilize and then wait for six to 12 months for titers to go down before testing can begin. There has always been some concern of whether or not the herd is truly negative until all the animals that were infected have been removed. Using direct virus exposure helps to ensure that at the time of the outbreak all animals were infected and allow the herd to stabilize quicker, thus making the procedure a little faster.”

Swiss depopulation
Swiss depopulation is a method developed to eradicate Mycoplasma, says Yeske. “Some Mycoplasma experts would argue that the method doesn’t actually eradicate the disease but gets it to a low enough level to not cause clinical disease.

“However, a number of European countries (Switzerland, Finland and others) have used this method with good success. The premise of this method is utilizing the pig’s immune system to fight off disease and prevent having animals present that can keep infection going in the herd.

Yeske explains that Swiss depopulation protocols require not bringing any positive animals into the herd that are less than 10 months of age. These animals have been previously infected at a young age so that they have had time to develop immunity and stop shedding.

The entire population should be medicated for two weeks to kill the Mycoplasma present in the herd. Farrowing should be stopped for the two-week period to avoid having piglets that could spread the organism. After this time, the herds can then use negative replacements and should stay negative, Yeske says, adding that “this method requires a lot of coordination to not be farrowing and medicating and strict biosecurity protocols.”

While this method has proven successful in some instances, it is not foolproof and requires further evaluation for use in specific types of production systems, says Yeske.

Medication programs
Medication programs have been successful for some of the bacterial diseases such as swine dysentery, says Yeske. To be successful requires treatment of the entire herd at once. “Eradication of swine dysentery has been done with whole herd medication, clean up and sanitation, and rodent control.

“This was done using a number of different antibiotics for a two-week period. Eradication of mange is another example of using a whole-herd medication approach to eradicate a disease, Yeske says.

Segregated early weaning
Yeske believes that segregated early weaning has reshaped the swine industry. This technology has enabled most commercial pork production systems to adapt multiple-site technology, he says.

“Multi-site production that is all in/all out by site essentially allows for a total depopulation and repopulation of the nursery, finisher or wean-to-finish site with every turn of pigs. The principle for this system is to use the herd’s immune status to protect the piglets and wean them at an early age and remove them from the site while they have high levels of maternal immunity and before they can be infected.

“Weaning age is dependent on the pathogens to be eradicated and the rate of decay of the maternal antibodies. In some segregated early weaning programs to eradicate bacterial diseases, antibiotic treatment is added to the sows and piglets to help put one more barrier to infection prior to moving the piglets off‑site.

 

Vaccination can be used for swine disease eradication programs if the vaccine can control the pathogen and reduce shedding of the organism in the herd.

Yeske says that this method has been effective for eradication of PRV, atrophic rhinitis and Actinobacillus pleuropneumonia. It has been used in some cases to attempt eradication of  PRRS but has not always been successful because the virus was still circulating in the herd’s population, according to Yeske.

Herd closure
Herd closure is the practice of not bringing gilt replacements into the sow herd for an extended period of time, allowing the herd’s immunity to stabilize and stop shedding organism because there are no longer any susceptible populations for the pathogen to circulate in.

Yeske says that when herd closure has been used to attempt PRRS eradication, the reported successes have been with a minimum of 200 days following the last recovered animal. “The challenging part is knowing when the last animal is exposed. Using direct virus exposure can help to ensure that all animals are exposed. The next biggest problem is maintaining the breeding herd inventory during this time to keep the system full.

“Using an off-site breeding project is a solution to this problem. This involves maintaining a negative population of gilts at another site where they are bred and introduced back into the herd after the closure and before they are due to farrow. This can dramatically reduce the amount of sow herd inventory erosion and keep the numbers up in the project, but there are additional costs and risks when there is another site involved.

Herd closure may also be effective for other diseases but to date has mainly been used in efforts to eradicate PRRS, says Yeske.

Direct virus exposure
The principle of direct virus exposure is to expose all animals in the herd and not leave the pathogen anywhere to maintain itself within the population.

“This approach is probably best suited for viral diseases that don’t develop chronic shedding,” says Yeske. “The classic example of direct virus exposure has been eradication of TGE from herds. In TGE eradication programs, all animals are infected in the herd with the virus from the farm. It has been successful in both acutely infected herds as well as chronically infected herds.

Direct virus exposure is being attempted in PRRS virus eradication programs to more quickly generate negative pigs for downstream flows, Yeske says. “In some cases it may be effective in herd eradication, especially if used in combination with herd closure and before rolling over the positive population with negative replacements.”

Vaccination
Vaccination can be used for eradication of some diseases if the  vaccine can control the pathogen and reduce shedding of the organism in the herd, says Yeske. PRV eradication is  a good example of where a vaccine has been used to eradicate a virus.

Yeske concludes that some pork production systems use a combination of the above methods to eradicate more than one disease at a time from the herd. “All have to be fit to the unique aspects of each production system.”