Abstract:

Effect of porcine reproductive and respiratory syndrome virus (PRRSV) exposure dose on fetal infection in vaccinated and nonvaccinated swine

James E. Benson, DVM, MS; Michael J. Yaeger, DVM, PhD; Kelly M. Lager, DVM, PhD

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Objective: To evaluate the relative susceptibility of vaccinated and nonvaccinated pregnant swine to varied challenge doses of porcine reproductive and respiratory syndrome virus (PRRSV) and the potential for increased challenge doses of PRRSV to overcome vaccine-induced immunity

Method: Fifteen nonpregnant gilts obtained from a PRRS-free herd were vaccinated twice with a modified-live PRRSV vaccine prior to artificial insemination. At 90 days of gestation, these VACC-CHAL gilts and 16 pregnant, nonvaccinated CHAL sows were randomly allotted to one of four experimental groups: a control group that received a sham inoculation, or to groups that received a "low" (10²CCID₅₀), "middle" (10⁴CCID₅₀), or "high" (10⁶CCID₅₀) dose of an intramuscular challenge of the NADC-8 PRRSV strain.

Results: The number of infected litters in all dosage groups was significantly higher (P<.001) among CHAL females compared to VACC-CHAL females. Dead fetuses and viremia were observed in all litters in the low- and middle-dose groups, and in three of four litters in the high-dose group in the CHAL females; and in no low-dose litters, one of two middle-dose litters, and one of four high-dose litters in the VACC-CHAL females. No fetal death or viremia was identified in control groups. Among infected litters, no significant difference in the percentage of infected fetuses per litter was observed regardless of vaccination status or challenge virus dose. The number of litters with fetal death and infection was significantly lower in the low-dose VACC-CHAL group when compared to the low-dose CHAL group (P<.01), but no significant difference was demonstrated between the two medium or two high dose groups.

Implications: Vaccine-induced protective immunity appeared to protect eight of 10 litters from reproductive failure, but may be overcome with increased (>=10⁴ CCID₅₀) doses of challenge virus. The lowest PRRSV exposure dose (10² CCID₅₀) tested in this study caused reproductive failure in naïve, unvaccinated animals. The percentage of infected fetuses per litter observed suggests that multiple fetuses/weakborn pigs should be sampled to ensure that infected animals are represented. Sampling dead or autolyzed fetuses is generally diagnostically unrewarding for PRRSV infection.

Keywords: PRRSV, vaccine, exposure dose, reproduction, breeding herd

Cite as: Benson JE, Yager MJ, Lager KM. Effect of porcine reproductive and respiratory syndrome virus (PRRSV) exposure dose on fetal infection in vaccinated and nonvaccinated swine. J Swine Health Prod 2000;8(4):155-160.

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