A modified-live-virus (MLV) vaccine, propagated in an innovative porcine alveolar macrophage cell line, designated ZMAC, was effective in protecting pigs from porcine reproductive and respiratory syndrome (PRRS) virus.

Use of the ZMAC-grown, MLV vaccine prevented the weight loss observed in non-immunized animals within seven days after exposure to a highly virulent strain of PRRS virus.

“Remarkably, analyses of the virus load in serum and lung samples from PRRS virus-immunized and challenged animals revealed that the vaccine virus grown in ZMAC cells was significantly more effective at reducing the extent of viremia (the presence of virus in the blood) at seven days post-challenge, and also at eliminating virulent virus from their lungs by 10 days post-challenge,” says Federico A. Zuckermann, professor of immunology at the University of Illinois College of Veterinary Medicine.

The researcher says the degree of protection afforded by this vaccine “against a genetically divergent and highly virulent PRRS virus has important implications for the prospect of developing an effective vaccine against this pathogen.

“Namely, the results of this study suggest that the effectiveness of a PRRS MLV vaccine can be improved, and that it is not, as it is commonly believed, only determined by its genetic similarity to the challenge virus, but is also influenced by how it is produced.

“The results of this study provide great hope that an effective MLV vaccine against PRRS virus can be developed,” says Zuckermann.

The goals of this project, funded by the National Pork Board, were to use an innovative porcine cell line to produce a PRRS MLV vaccine, and to compare this virus' efficacy to that of vaccine made traditionally in the simian MARC-145 cell line, the only other type of cell line known to support the growth of PRRS virus.

To evaluate the vaccine potential of the ZMAC-grown virus, a standard immunization test was conducted. Six, 8-week-old pigs were injected with the Prime Pac commercial vaccine (Schering-Plough Animal Health) propagated in either ZMAC or MARC-145 cells, while two groups of three animals were not immunized and served as controls.

Four weeks later, all vaccinated pigs and one of the PRRS naïve groups were challenged with an “atypical PRRS abortion storm” virus.

The result was that the Prime Pac vaccine grown in either cell line proved equally effective at preventing weight loss by pigs exposed to virulent virus seven days earlier. (See Figure 1, where weight changes of non-immunized and vaccinated pigs at seven days after challenge with atypical PRRS virus are depicted.)

The vaccine virus grown in ZMAC cells, however, was much more effective than the one generated in MARC-145 cells at reducing the extent of infection and also at eliminating virus from the lungs at 7 or 10 days post-challenge.

Researcher: Federico Zuckermann, University of Illinois. Contact Zuckermann by phone (217) 333-7767, fax (217) 244-7421 or e-mail fazaaa@illinois.edu.