PRRS virus (porcine reproductive and respiratory syndrome virus), the cause of the Mystery Swine Disease Syndrome, is finally yielding some answers as well as posing yet new questions as researchers, veterinarians, and pork producers work together to fight this problem. The following discussion will cover how it spreads, affects the immune system, causes disease, and maintains itself within infected herds.
Reproductive Disease
Most of us know first hand or through lay magazines that PRRS outbreaks often cause abortions, stillbirths, mummies, and weakborn or thumping piglets. It is less certain whether PRRS causes irregular returns to estrus or reduced litter size by infecting and killing fetuses before 35 days of gestation. Many herds become infected (seroconvert - become positive to antibody tests) without seeing any clinical signs. However, a critical question remains: Will previously infected herds have another outbreak months or years later? Pregnant sows were infected with PRRS at 93 days gestation and farrowed severely affected litters. During the next pregnancy (7 - 8 months later) the same sows were challenged while some PRRS free pregnant sows were first infected. The previously infected sows' litters were normal while the control uninfected sows aborted or had many stillbirths or mummies. Field testimonials from the Midwest veterinarians and producers (PRRS spread rapidly there starting in 1989 whereas North Carolina apparently did not until 1992) indicate most sow herds do not experience second outbreaks. There is at least one report of a 1200 sow herd which receives gilts from an outside source that has had periodic flareups of PRRS reproductive losses during the second year after the initial outbreak. It was discovered that the gilt source was shipping predominantly negative gilts.
Nursery-grower Pig Disease
PRRS seems to persist in herds after the initial reproductive disease outbreak mainly in the nursery grower areas. Many producers, particularly those who do not use all in - all out management, see long term pneumonia or sudden death problems with Strep suis, Haemophilus parasuis, Pasteurella multocida, Actinobacillus pleuropneumoniae, and Salmonella cholerasuis. Atrophic rhinitis also becomes worse if the herd is infected with PRRS. Antibiotics do not work as well as before (if at all) and mortality rates (15%) and feed efficiencies are elevated due to the reoccurring disease problems. PRRS has been isolated from nursery pigs in these herds up to 2.5 years later. In herds that do not see endemic disease problems in the nursery and that keep their own replacement breeding stock, gilts tested as they are added to the herd are seropositive to PRRS even years after the outbreak. This indicates that the virus continues to circulate in onsite nurseries or finishers on most farms. Therefore feces and soiled feed from the nursery and young finishing pigs can be used as feedback to purchased gilts in isolation to try to "immunize" them against PRRS. Older sows on these farms are often seronegative.
Effects on the Immune System
PRRS grows almost exclusively in macrophages, the first line of defense cell in the body's immune system. A large proportion of the cells in the lungs are alveolar macrophages which PRRS easily infects. PRRS may initially stimulate these cells, but after two or three days the virus kills them and leaves the lung open to bacterial infections. This is because the immune system needs the macrophage to process the bacteria before other defense mechanisms can be activated. While the antibody response to PRRS may be slower and temporary, PRRS infection seems to actually increase the antibody titers to OTHER unrelated viruses and bacteria. This may fool people trying to make a diagnosis of what is causing a disease problem based simply upon an increase in antibody titers.
Virus Spread or Transmission
PRRS is very infectious, requiring only 10 to 100 viruses to infect pigs. PRRS has been isolated from boar semen for 35 days after infection by piglet inoculation. Semen from PRRS infected boars was able to infect and cause fevers and antibody production in uninfected gilts after insemination. The gilts failed to conceive and returned to estrus 21 days later. PRRS infection has been reported to decrease semen volume for 14 days after infection with variable reports on whether motility and sperm morphology are adversely affected. While the pig seems to be the only mammal that PRRS virus infects, the virus has replicated in Mallard ducks for 2 to 3 weeks. The significance of this finding and whether other birds can also replicate and possibly transmit the virus must be determined.
Diagnostic Tests: Availability and Limitations
All antibody tests developed and reported to date can detect PRRS antibody for only a year after initial infection. This is a strange characteristic of PRRS and does not necessarily mean that the test negative animals are not still carrying virus. Therefore, if an animal tests positive, it probably is infected. However, if an animal tests negative, it may not be virus free. Before purchasing gilts from new herds, have them test not only their sows but nursery and finishing pigs to establish whether their herd is infected with PRRS and whether the virus is still being spread between animals within the herd. Nursery and finishing pigs are much more likely to be antibody positive since they can only be infected within the last 6 months. Another complication of PRRS testing is the presence of different strains of the virus, namely the Lelystadt (European strain) virus which does not react on many of the current American diagnostic laboratory antibody tests. This type of virus mutates quickly, therefore there may be new antigenic forms needing new tests in the future, much like AIDS. An ELISA is being developed that may detect most or all strains of PRRS antibody responses.
Take-Home Message
