Changes

==Transmission and Epidemiology==

==Transmission and Epidemiology==

ollowing infection of a naive herd, exposure of all members of the breeding population is inconsistent, leading to the development of naive, exposed, and persistently infected subpopulations of sows. This situation is exacerbated over time through the addition of improperly acclimated replacement gilts and leads to shedding of the virus from carrier animals to those that have not been previously exposed.

 

Following infection of a naive herd, exposure of all members of the breeding population is inconsistent, leading to the development of naive, exposed, and persistently infected subpopulations of sows. This situation is exacerbated over time through the addition of improperly acclimated replacement gilts and leads to shedding of the virus from carrier animals to those that have not been previously exposed.

 

The primary vector for transmission of the virus is the infected pig. Contact transmission has been demonstrated experimentally, and the spread of virus from infected seedstock originating from a single source has been described. Introduction of infected seedstock can lead to the introduction and coexistence of genetically diverse isolates of PRRS virus on the same farm. Controlled studies have indicated that infected swine may be longterm carriers, with adults able to shed PRRS virus for up to 86 days after infection, while weaned pigs may harbor virus for 157 days. Experimentally infected boars can shed virus in the semen up to 93 days after infection.

The primary vector for transmission of the virus is the infected pig. Contact transmission has been demonstrated experimentally, and the spread of virus from infected seedstock originating from a single source has been described. Introduction of infected seedstock can lead to the introduction and coexistence of genetically diverse isolates of PRRS virus on the same farm. Controlled studies have indicated that infected swine may be longterm carriers, with adults able to shed PRRS virus for up to 86 days after infection, while weaned pigs may harbor virus for 157 days. Experimentally infected boars can shed virus in the semen up to 93 days after infection.

Aerosol transmission of the virus has been considered to be a potential route of transmission, particularly under conditions of high humidity, low temperatures, and low wind speeds; however, this has been difficult to consistently reproduce under controlled field conditions and in the laboratory. PRRS virus can also be transmitted by fomites, such as contaminated needles, boots, coveralls, transport vehicles, and shipping containers. Farm personnel are not a risk, unless hands are contaminated with blood from viremic pigs. Finally, transmission via certain species of insects (mosquitos [ Aedes vexans ] and house flies [ Musca domestica ]) has been reported. The role of migratory waterfowl has not been determined. While biologic transmission of PRRS virus has been documented in immature Mallard ducks, results have not been reproducible experimentally using adult Mallards, nor have infected pigs been able to transmit virus to adult Mallards housed under field conditions.

Aerosol transmission of the virus has been considered to be a potential route of transmission, particularly under conditions of high humidity, low temperatures, and low wind speeds; however, this has been difficult to consistently reproduce under controlled field conditions and in the laboratory. PRRS virus can also be transmitted by fomites, such as contaminated needles, boots, coveralls, transport vehicles, and shipping containers. Farm personnel are not a risk, unless hands are contaminated with blood from viremic pigs. Finally, transmission via certain species of insects (mosquitos [ Aedes vexans ] and house flies [ Musca domestica ]) has been reported. The role of migratory waterfowl has not been determined. While biologic transmission of PRRS virus has been documented in immature Mallard ducks, results have not been reproducible experimentally using adult Mallards, nor have infected pigs been able to transmit virus to adult Mallards housed under field conditions.