Two genetically distinct infectious recombinant virus clones (pMLV, constructed from Ingelvac PRRS MLV and pMN184, constructed from virulent strain MN184) were developed to study attenuation of contemporary porcine reproductive and respiratory syndrome virus (PRRSV) strain MN184. Two reciprocal chimeric clones (pMLVORF1/MN184 and pMN184ORF1/MLV) were then constructed, such that the 5'UTR/ORF1 of one genotype was linked to ORF2-7/3'UTR from the other genotype. In vitro studies demonstrated that the rescued chimeric viruses possessed intermediate growth properties compared to recombinant rMLV and rMN184. Swine inoculation with rMN184 and rMLV verified that these viruses fully mimicked the respective parent virus. In addition, earlier and higher antibody responses were detected in animals infected with rMN184 in contrast to those infected with rMLV. Chimeric virus treatment groups showed similar antibody responses as seen with these parent viruses, but much less severe pathogenesis when compared to the rMN184 group. These data suggested that genetic aspects of Ingelvac PRRS MLV 5'UTR/ORF1 replicase region and/or the structural proteins/3'UTR can serve to attenuate virulent strain MN184. The data also indicated that designed PRRSV vaccines could be developed, keeping the known 5'UTR/replicase region of an early vaccine strain such as Ingelvac PRRS MLV intact, but replacing the structural protein/3'UTR domain with that of an emerging virulent virus.

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Immunization with an avirulent live S choleraesuis vaccine has demonstrated consistent clinical and bacteriologic control of S choleraesuis infections in swine. Clinical signs, including febrile response, diarrhea and mortality, along with growth rate, are significantly improved (p<0.05). Pigs from one day of age and older may be safety vaccinated intranasally, or orally, via drinking water, against disease associated with S choleraesuis, and to reduce the culture prevalence and shedding of other serovars of Salmonellae (Nolan et al., 2000). Additionally, levels of the organism are effectively reduced in vaccinated, challenged pigs. Fecal shedding of S choleraesuis is significantly lower in vaccinated challenged pigs as compared to challenged control pigs. Organ culture prevalence is likewise dramatically lowered in vaccinated pigs. Vaccination at one day of age is at least as efficacious, and may offer even greater advantage over non vaccinated controls.

To evaluate immune responses in Alaska Native infants at high risk for invasive Haemophilus influenzae type b (Hib) disease, we studied PRP-pertussis and PRP-D conjugate vaccines in this population relative to responses in white infants in California and New York. Infants were immunized at two, four, and six months of age (both vaccines). In the PRP-pertussis trial, there were no significant differences in antibody levels at any age between Alaska Native infants and infants from California. Only 50% of the infants had a twofold or greater antibody rise after three doses. In the PRP-D trial, antibody levels at two months of age (presumably maternally acquired) were significantly higher for Alaska Native infants compared with infants from New York (P = .002). There were no significant differences in antibody levels after any of the three doses. Among Alaska Native infants there was no significant difference in antibody response based upon degree of ethnic purity.

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