Update in PRRSV vaccinology

NEBRASKA - In 1987 Porcine Reproductive and Respiratory Syndrome (PRRS) was first diagnosed in the U.S. The disease-causing virus (PRRSV) was identified in 1991, and several different strains of the virus have since been isolated from field cases, reports Dr. Marilia Oliveira.
calendar icon 10 November 2006
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Two groups of PRRSV vaccines have been developed: killed vaccines (KV) and modified live vaccines (MLV).

Killed vaccines, also known as inactivated vaccines, are produced through a process of virus inactivation. The inactivated vaccine virus cannot infect other animals and cannot revert to virulence, thus making the vaccine safe to use. The disadvantages of PRRSV killed vaccines are complete inability to provide protection against infection and clinical disease. Currently, there are no PRRSV killed vaccines on the U.S. market, although some autogenous inactivated vaccines have been used in farms. Autogenous vaccines are produced from virus isolated from a swine herd and intended for use only in the herd from which the virus was isolated.

Modified live vaccines (MLV) use live strains of virus. The vaccine virus is passaged several times on cell culture to make it less disease-causing. PRRSV MLV induces a good protection against infection and clinical disease caused by the homologous strain (the same strain of the vaccine virus). However, if the wild-virus is different than the vaccine virus (called a heterologous strain), the level of protection can vary greatly. Thus, vaccination with PRRSV MLV does not necessarily provide full protection. Currently, there are two MLV available in the U.S. market.

It is important to work with a veterinarian to assess the hazards and benefits associated with PRRSV vaccination. Vaccination with PRRSV MLV may not stop transmission of virus, but might protect against losses due to clinical disease. Unfortunately, vaccination is sometimes detrimental. In 1996, outbreaks of an atypical and severe form of PRRSV in Danish herds were linked to the use of a MLV. In fact, MLV can revert to virulence and the results can be devastating.

Also, MLV have other disadvantages, for example, the vaccine virus can be shed from vaccinated to non-vaccinated animals causing potential virus-related disease in the non-vaccinated group. Also, vaccinated animals cannot be differentiated from infected animals, thus, the use of MLV is discouraged in non-infected herds where spread of the vaccine virus could be detrimental to the sale or export of PRRSV negative breeding stock.

New approaches to PRRSV vaccination are currently under investigation. Examples are subunit, recombinant, and/or genetically engineered vaccines. Undoubtedly, as our knowledge of PRRSV immunology improves, development of more efficient vaccines will emerge.

Despite the wide distribution of PRRSV, uninfected herds do exist. Preventing the introduction of the PRRS virus, as well as elimination of virus from previously infected swine herds has been successful. Chile is the first country worldwide that is very close to eradicating the virus from the domestic swine stock. They have done so exclusively by managing infection, applying biosecurity, and without using vaccines. Management strategies to prevent, control, and eradicate PRRSV should be discussed and routinely re-evaluated with your veterinarian.

Further Reading

For further information on PRRS visit our PRRS Disease Section

Reproduced courtesy

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