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==Aetiology==

==Aetiology==

The virus that causes PRRS is an arterivirus, within the [[:Category:Arteriviridae|Arteriviridae]] family. The arteriviruses are the only genus contained within the Arterivirdae family, and other members include the agent responsible for [[Equine Viral Arteritis|equine viral arteritis]] and lactate dehydrogenase-elevating virus of mice. The viruses are host-specific and antigenically distinct, and establish persistent infections after invasion. The PRRS virus is only moderately resistant to environmental degradation and is easily inactivated by phenol, formaldehyde, and most common disinfectants.

The virus that causes PRRS is an arterivirus, within the [[:Category:Arteriviridae|Arteriviridae]] family. The arteriviruses are the only genus contained within the Arterivirdae family, and other members include the agent responsible for [[Equine Viral Arteritis|equine viral arteritis]] and lactate dehydrogenase-elevating virus of mice. The viruses are host-specific and antigenically distinct, and establish persistent infections after invasion.

==Transmission and Epidemiology==

==Transmission and Epidemiology==

The virus is spread by contact with infected pigs, and infection also be transmitted venereally via infected semen. Blood, saliva, urine, milk and semen from infected animals have all been shown to contain PRRS virus. Fomites, such as contaminated needles, boots, overalls and transport vehicles can also transmit PRRS, as can certain species of insects including [[Musca spp.|house flies]]. Aerosol transmission is thought to occur, particularly under conditions of high humidity, low temperatures, and low wind speeds, but this has been difficult to reproduce in the field and experimentally. Once infected, adult animals shed PRRS virus for up to 86 days, and younger, weaned pigs for 157 days. Virus excretion in semen can persist for up to 93 days after infection and can transmit disease in female pigs during breeding. This lengthy persistence of infection is an important factor in the maintenance of infection within a herd. Infection of sows occurs through natural breeding or artificial insemination. The advent of artificial insemination and boar studs has created a need for strict biosecurity and monitoring of PRRS virus in these facilities.

The virus is spread by contact with infected pigs, and infection also be transmitted venereally via infected semen. Blood, saliva, urine, milk and semen from infected animals have all been shown to contain PRRS virus. Fomites, such as contaminated needles, boots, overalls and transport vehicles can also transmit PRRS, as can certain species of insects including [[Musca spp.|house flies]]. Aerosol transmission is thought to occur, particularly under conditions of high humidity, low temperatures, and low wind speeds, but this has been difficult to reproduce in the field and experimentally. Once infected, adult animals shed PRRS virus for up to 86 days, and younger, weaned pigs for 157 days. Virus excretion in semen can persist for up to 93 days after infection. This lengthy persistence of infection is an important factor in the maintenance of infection within a herd.

==Pathogenesis==

==Pathogenesis==

==Treatment and Control==

==Treatment and Control==

There is currently no known effective treatment for PRRS. Non-steoidal anti-inflammatory drugs have been used in an attempt to reduce fever, and appetite stimulants to counteract inappetance and poor weight gain. However, these appear to have minimal benefit. Antibiotics may be used to counteract secondary bacterial infections.

There is currently no known effective treatment for PRRS. Non-steoidal anti-inflammatory drugs have been used in an attempt to reduce fever, and appetite stimulants to counteract inappetance and poor weight gain. However, these appear to have minimal benefit. Antibiotics may be used to counteract secondary bacterial infections. As treatment for PRRS is ineffective, prevention is very important. Successful control of the disease is difficult, because of virus variation, large swine populations, and unresolved issues of transmission. In some smaller herds, immunity may be sufficient so that infection is not causing significant economic losses, in which case no intervention is necessary. Often, there are sufficient losses to consider some or all of the following points for control. A control program should be tailored to fit the individual farm situation. In herds where PRRS is not present, bought-in stock and semen should be PRRS-free, and any animals joining the herd should be quarantined on-site before being introduced to the cohort. Pigs should be tested on arrival to the unit, and again 45-60 days later before they join the main herd. In herds where PRRS is endemic, breeding gilts may be exposed to adult sows before they become pregnant in order to induce immunity. Existing infection can also be eliminated by multisite production and segregated early weaning, but the long-term risk of reinfection is high. Complete depopulation, disinfection and repopulation can also be successful, as can test-and-removal strategies.

 

As treatment for PRRS is ineffective, prevention is very important. In herds where PRRS is not present, bought-in stock and semen should be PRRS-free, and any animals joining the herd should be quarantined on-site before being introduced to the cohort. Pigs should be tested on arrival to the unit, and again 45-60 days later before they join the main herd. In herds where PRRS is endemic, breeding gilts may be exposed to adult sows before they become pregnant in order to induce immunity. Existing infection can also be eliminated by multisite production and segregated early weaning, but the long-term risk of reinfection is high. Complete depopulation, disinfection and repopulation can also be successful, as can test-and-removal strategies.  

Both modified live and killed vaccines are available. These are effective in controlling outbreaks and preventing economic losses.

Both modified live and killed vaccines are available. These are effective in controlling outbreaks and preventing economic losses.