Porcine Reproductive and
Respiratory Syndrome (PRRS)

by Bill Epperson, DVM, Extension veterinarian;
David Benfield, Ph.D., professor of veterinary science; and
Kurt Rossow, Ph.D., DVM, assistant professor of veterinary science

History

A previously unrecognized disease of swine was first described in the United States in 1987. The condition was characterized by abortion, premature farrowing, stillborn and mummified pigs, and respiratory disease with death loss and chronic poor performance of nursing and weaned pigs, This disease was initially referred to as Mystery Swine Disease (MSD).

As a result of research performed at South Dakota State University, the University of Minnesota, and the Central Veterinary Institute in the Netherlands, the condition became better characterized and was renamed Porcine Reproductive and Respiratory Syndrome (PRRS). PRRS continues to be a major cause of reproductive and respiratory disease of swine today.

The Cause of PRRS

PRRS is caused by a virus. The virus, a member of the genus artervirus, is easily inactivated by heat, drying, and common disinfectants. At freezing temperatures, PRRS virus can persist for longer periods of time. Like other RNA viruses, PRRS viruses are regarded as genetically diverse, so not all PRRS viruses infections may be exactly the same.

Clinical Signs

The clinical signs of PRRS virus infection are different in breeding animals when compared to young pigs. The severity of clinical signs in a herd depends on many factors including stress, concurrent disease, herd management, and virulence of the specific PRRS strain. Some herds experience severe disease, while others apparently observe no clinical problems following introduction of the PRRS virus.

Breeding Females

PRRS causes fever, depression, and decreased appetite in sows and gilts. Initially, a "rolling inappetence" is seen as the virus spreads through a herd. Reproductive problems will follow and affect primarily females in late gestation. A sudden, drastic increase in stillborns (20% or more of pigs born), followed by an increase in the number of weak, liveborn pigs is seen. A large increase in pre-weaning mortality results (20-50% is not uncommon). Up to 10% of sows may abort or farrow early. The reproductive signs tend to appear suddenly and regress so that in 2-6 months normal reproductive parameters are again observed. Generally, very few sows die during a PRRS outbreak.

Boars

Boars infected with PRRS virus may exhibit depression with fever. Often, PRRS infections are sub-clinical. PRRS can infect the reproductive organs, and the virus is shed in semen for up to 92 days (range 6-92), with an average shedding time after infection of about 35 days.

Nursery Pigs

PRRS predominately affects the respiratory system in young pigs. Abnormally rapid, labored breathing or "thumping" is observed. The severity of problems from other bacterial and viral pathogens often will seem to intensify, with death losses and treatment costs ensuing. Poor performance may follow, with pigs taking an extra 3 or more weeks to finish.

Finishing Pigs

Signs in finishing pigs are similar to signs seen in mature animals. Some pigs may go off feed- with fever, depression, and coughing. A "rolling inappetence" is observed. Secondary bacterial or viral infections with other pathogens may follow, and some death loss may result. PRRS virus infection in finishing pigs usually is less severe when compared to PRRS infection in nursery pigs.

Diagnosis of PRRS in a herd

Aborted and mummified fetuses generally are poor samples to submit for diagnosis of PRRS infection, but they are useful to diagnose other common pathogens. Weak, liveborn, 3-4 day old pigs generally are better diagnostic specimens. PRRS virus does not survive well in dead pigs or decomposed tissue. Blood samples from littermates of stillborn or weakborn pigs can sometimes be used to isolate the PRRS virus.

In mature animals, PRRS antibody can be detected in blood. The presence of antibody indicates that the animal has, at some time, been exposed to the PRRS virus. Currently, PRRS antibody tests offered by diagnostic laboratories do not differentiate between animals previously infected with a field strain of PRRS and those receiving one of the commercial vaccines.

Transmission of PRRS

The primary method of PRRS virus transmission is from animal to animal. Pigs infected with PRRS virus can shed the virus in oral-nasal secretions, urine, and feces. When infected, boars can shed PRRS virus in semen and transmit PRRS to susceptible females.

Transmission of PRRS virus through air has been thought to occur in the field, but controlled experiments have not documented airborne transmission. There is little field evidence to indicate that wild animals (birds, rodents, etc.) are important in PRRS transmission.

Since PRRS virus can be shed in urine and feces, pit slurry is a potential source of infection, but PRRS virus is not considered hardy outside the pig. PRRS virus is stable at freezing temperatures, so transmission from contaminated facilities may occur more easily in the winter, However, introduction of swine shedding PRRS virus is by far the most common way the disease is introduced and transmitted in a herd.

Treatment of PRRS

During PRRS outbreaks, antibiotics may be administered to help decrease death loss from secondary bacterial infections. Antibiotics have no direct effect on viral agents, such as PRRS.

Prevention and Control

Prevention and control of PRRS employ similar concepts as other infectious diseases of swine. Prevention strategies are intended to keep PRRS virus out of a herd. Control strategies contain and minimize the effects of disease, if introduced. Actual strategies employed for prevention and control depend on:

Several central concepts of bio-security are particularly applicable to PRRS. If you do not have PRRS, do everything possible to keep PRRS out.

Quarantine incoming pigs. Hold replacements in strict quarantine for 60 days and observe them. Test for PRRS antibodies on entry to quarantine, If PRRS vaccination is part of the unit's PRRS control program, the initial vaccination may be administered within the first week. Test replacements again for PRRS before release from quarantine, especially if PRRS vaccination is not given. To interpret test results, it is important to know the vaccination history of pigs from the source farm.

For replacements, know the PRRS status of the source herd. Knowing only that "they don't have PRRS" is not enough. If the source herd is vaccinating for PRRS, ask why they are vaccinating. It is imperative not to purchase replacements from herds experiencing clinical signs of acute PRRS.

If your herd is truly PRRS negative (has never had PRRS and is PRRS antibody negative), be very careful about additions. Purchase only from PRRS negative herds and check to make sure all additions are PRRS negative both at arrival and at quarantine release. For herds that are PRRS positive (have had clinical PRRS or animals have antibodies to PRRS), make sure that all replacements have had PRRS exposure before leaving quarantine, by documented natural exposure or vaccination early in the quarantine period.

Match the status of the boars or semen purchased with your herd. If your herd is PRRS negative, purchase only PRRS negative boars (or semen) from PRRS negative herds. For herds seropositive for PRRS, make sure boars are seropositive for PRRS before leaving quarantine, just like the replacement females.

Use strict All-in-All-out management (AIAO). AIAO is a useful management strategy to control many diseases and enhance performance. All herds should work toward AIAO management. This is especially important in the nursery and grow/finish areas.

Know why you're vaccinating. Vaccination does not prevent the pig from being infected with PRRS virus; it only lessens the clinical signs following infection. Vaccination may be useful in herds with ongoing PRRS problems or in herds at high risk of PRRS infection. Vaccination does not confer 100% protection, and vaccinated herds can still be affected with PRRS. It is important to not place total reliance on PRRS vaccination for PRRS control. Common uses of the vaccines are in young pigs to control PRRS signs in situations where future exposure is likely to occur and in breeding animals prior to breeding. Work closely with your veterinarian to design a vaccination program.

Implement partial depopulation/nursery depopulation. Partial depopulation can be a useful strategy for herds with ongoing PRRS problems limited to the nursery area. Work closely with your veterinarian before attempting nursery depopulation. The farm must be AIAO from the nursery and on, and there must be no circulation of PRRS virus in the breeding herd. Nursery depopulation can be used to eliminate PRRS virus from a unit, but a strict set of conditions must be met for success.

Control PRRS virus circulation in the breeding herd. This is imperative if you wish to control the chronic effects of PRRS virus on pig performance. Failure to control virus circulation in the breeding herd will not permit complete control in the nursery through grow-finish areas.

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