Porcine Respiratory Disease Complex (PRDC) is a common syndrome with the disease interaction of Porcine Reproductive and Respiratory Syndrome (PRRS) and Mycoplasmal pneumonia playing key roles.

Vaccinating for these two PRDC diseases is producing some interesting results, according to Eileen Thacker, DVM, Iowa State University.

In her research on the effectiveness of mycoplasma vaccines, she found that protective immunity induced by mycoplasma vaccination has been shown experimentally but protection against infection is incomplete. Work in her lab at the Veterinary Medical Research Institute showed that seroconversion rates between the vaccines varied between 30% and 100% after vaccination.

One reason for the potential failure of mycoplasma vaccines is that maternal antibodies may, in fact, decrease the piglet's ability to respond to vaccines against mycoplasma, postulates Thacker.

Mix PRRS with mycoplasma and a complex interaction is created which raises more questions about vaccine efficacy.

>From previous work, Thacker demonstrated that mycoplasma, when combined with PRRS, appears to be integral in inducing PRDC-related pneumonia problems. A second, recent study shows that vaccination or infection with PRRS during or following mycoplasma vaccination appears to reduce the effectiveness of the mycoplasma vaccine. How this vaccine interaction works is unknown as researchers still found protective antibodies against mycoplasma present in pigs.

Mycoplasma vaccination did reduce the level of PRRS-induced pneumonia in the dually infected pigs, points out Thacker. But that benefit was lost when pigs were also vaccinated for PRRS. Vaccination for PRRS decreased but did not eliminate PRRS respiratory problems.

In fact, no decrease in PRRS-induced pneumonia was seen with the modified-live-virus (MLV) PRRS vaccine against the strain of PRRS used in their challenge. Nor did the PRRS vaccine provide any protection against the mycoplasma.

In defense of the MLV PRRS vaccine, Thacker observes that the strain of PRRS used to infect the pigs in this study appears to be different from the parent strain of the PRRS vaccine. "Vaccine strain differences may be important in a PRRS vaccine's ability to protect against the various strains of PRRS," she says.

Vaccination with an MLV PRRS vaccine before mycoplasma vaccination, followed by a mycoplasma disease challenge, didn't reduce efficacy of the mycoplasma vaccine, a third study suggests.

"These studies suggest that timing of the PRRS infection may affect the development of a protective immune response to mycoplasma," comments Thacker. "These results raise questions as to PRRS' effect on the efficacy of other vaccines."

In short, this research suggests that producers and their veterinarians need to determine the level of maternal antibodies and when seroconversion to PRRS occurs in their herds. Determining these parameters can ensure timing of mycoplasma vaccines is optimized.

Researchers: Eileen Thacker, DVM; Brad Thacker, DVM; and Pat Halbur, DVM; Iowa State University. Phone Thacker at (515) 294-5097 or e-mail ethacker@iastate.edu.