Department of Food Animal & Equine Medicine

College of Veterinary Medicine

North Carolina State University

Raleigh, NC 27606

Disease Syndrome

The causative agent for "Mystery Pig Disease" has been identified. The disease syndrome is now called Swine Infertility and Respiratory Syndrome (SIRS) or Porcine Reproductive and Respiratory Syndrome (PRRS). A new virus has been isolated in both the Netherlands and at South Dakota State University, which can cause devastating reproductive losses and nursery-grower pig respiratory disease. PRRS has spread extensively in Germany and England also. Most of the sows in the herd may go off feed in a one-week period of time, however some North Carolina outbreaks have seen no sows off feed. Nursery to finisher-aged pigs may also go off feed and show various signs of pneumonia. Within one to two weeks sows between 105 and 112 days gestation abort. Abortions do not occur at earlier stages of gestation and the abortion ‘storm’ lasts for only two weeks. Other litters born at this time and later may have piglets that are weak at birth which often die or become infected with secondary diseases and then die. Preweaning mortality may reach 40% at this time. Weak born piglets do not typically show the nervous system signs like convulsions, paddling, or drunken walk typically seen during pseudorabies outbreaks. There is a sharp increase in the number of stillborn pigs in litters farrowed one to three weeks after the PRRS abortions start. Approximately three weeks up to 14-16 weeks after abortions begin there is a large number of mummified pigs born. The mummies are almost full grown size at 3-5 weeks after the abortions. The mummies get smaller (shorter head to tail) as the time since the abortions increases. Typically the mummies born to all litters during a given week are the same size. This is different than a parvovirus outbreak where mummies within a single litter can be different sizes and ones from different litters born at the same time are usually different sizes. Increased returns to estrus, lowered farrowing rates, and lower litter size have been reported occasionally. These observations may not be due to PRRS virus infection since fetuses were susceptible to experimental infection only in the last half of gestation. Observations made by practicing veterinarians and Dr. Erickson at Rollins Diagnostic Laboratory indicate that PRRS infection may make other reproductive diseases occur more often such as leptospirosis, campylobacter abortion, and possibly parvovirus. Once all breeding groups have farrowed, reproductive performance usually returns to normal. In North Carolina this year reproductive losses often occurred for only 4 to 6 weeks.

Many herds report increased disease problems with Strep suis, Hemophilus parasuis, atrophic rhinitis, enzootic pneumonia (Mycoplasma spp.), and Salmonella cholerasuis in the nursery and grower which can continue for long periods of time or go away only to flare up again at a later time. Increased problems in the sow herd such as leptospirosis or parvovirus, may also be seen. Currently many herds only see an increase in disease rates in nursery-aged pigs after the herd becomes infected. A lot of herds apparently have become infected without any clinical signs of disease since their sows are found carrying antibodies to PRRS but the herd has never seen any of the typical signs. The virus seems to suppress the immune system, which allows other disease problems to become worse. Grow-finish feed efficiency and condemnation rates rise and average daily gains usually fall after the outbreak.

History

The disease syndrome was first recognized in the United States in North Carolina in 1987. A cause was not identified and the outbreaks stopped in 1988. Testing of stored serum in Midwest serum banks indicates some pig herds were infected with PRRS as early as 1985. This suggests that the virus was not causing disease in infected pigs there until 1988 when it may have changed or a new virulent strain was brought in. Currently it is projected that 40% of the herds in Iowa are infected.

PRRS Virus Infection

The PRRS virus comes from a new group of viruses known for their ability to partially hide from the immune system and typically cause symptomless infection. Each of these viruses usually infects only one species of animal. The cells PRRS reproduces in are called alveolar (lung) macrophages, front line cells in the body's immune system. Without macrophages the body is unable to process foreign viruses or bacteria for presentation to the rest of the immune system. Macrophages are the major cells found in the lung. Lung macrophages are damaged or destroyed during a PRRS infection leaving the animal open for many other types of infectious diseases. The PRRS virus can be isolated from the blood of infected pigs over a month after infection. This means the pig has not controlled the infection even though it has already produced antibodies against it. Infected sows may be able to infect new penates over 100 days after the sows were first given the virus. PRRS virus therefore seems to act like the other viruses in its group by causing a persistent infection (not controlled or rapidly eliminated by the body) in the pig, often without causing clinical signs of disease. Therefore, pigs may remain lifetime carriers of the disease, however we do not have scientific evidence for this occurring in the pig yet. The infection spreads to other pigs through saliva, blood, semen, and, under unique conditions, aerosol. The major source of infection will therefore be infected pigs, needles, AI semen collected during outbreaks, and contaminated equipment or personnel.

Diagnosis

Diagnosis of PRRS is still slow, if not difficult, because the virus has been recently isolated and because it only grows well on porcine lung macrophages that have to be collected fresh from pigs and cannot be continually grown in cell culture. Antibody tests depend upon infected cultures of lung macrophages in an assay that is slow to prepare and results are read by technicians, not automatic machines. Therefore laboratories are accepting only 10 to 20 blood samples per herd to determine if it is infected. The virus can be easily isolated from blood or serum samples, lung, spleen, or fetal tissues. However the isolation process is also slow and is available only in a few laboratories which can harvest and culture lung macrophages. Blood samples from ten or more poor doing nursery pigs from a herd should be cultured in order to isolate the virus. Virus has been isolated from infected pigs' blood up to 41 days after infection. It is unknown whether the virus survives in chilled pig meat, although proper rendering procedures probably inactivate the enveloped virus easily.

Control/Management Suggestions

Definite control recommendations or vaccines are not available yet because automated serology is unavailable to identify symptomless infected herds, infected animals may become seronegative within 6 -9 months after infection with current serologic tests, the virus is moderately difficult to grow in large amounts, and there is a lack of understanding of the immune response necessary to control the virus.

Strict herd biosecurity is needed to prevent entry of the virus into your herd. Purchase animals that have come from herds with no history of PRRS in the sows or endemic disease in the nursery pigs. Once serology becomes easily available, purchase animals from PRRS seronegative herds only if your herd is also negative. Isolate newly purchased animals and retest them at least 21 days after arrival since current blood tests are not always able to detect early infection. Make sure any commercial trucks or your pig hauling vehicles are clean and disinfected before loading on your hogs. Strictly limit visitors entering the hog buildings and require them to shower in or at least wear clean boots and coveralls. Do not employ workers who have any pigs of their own. There are no known carriers other than swine, but control rodents and prevent access to pigs by dogs, cats, and wild animals.

PRRS seems to increase ‘endemic’ or ‘chronic’ disease problems (as discussed above) most frequently in continuous flow nurseries and grow-finish operations. Therefore, if your buildings or pigs rooms are not being filled at one time and totally emptied before new, young, susceptible pigs are added, strongly consider remodeling or changing pig flow to this all in – all out system. Veterinary experience gained from PRRS infected herds with chronic disease problems in the nursery or grow-finish pigs indicates the problem was frequently eliminated or greatly reduced after the facilities were remodeled for all in – all out pig flow. If your facilities are built for all in -all out production, make sure that no poor doing pigs are held back or added to younger groups in the middle of the nursery production cycle as these pigs have been the easiest to isolate virus from. If possible identify only one worker to treat sick pigs and have them move from young to old, working as late in the day a~ possible to minimize the amount of virus carried to young susceptible pigs. If you are considering building new facilities design them for all in -all out flow and consider multiple site production with offsite nurseries with only one or two age groups per site per manager/operator. This should minimize the spread and continued effects of PRRS virus infection should it enter your operation.