The objective was to compare the efficacy of two experimental Staphylococcus aureus mastitis bacterins and a currently marketed five-isolate-based Staph. aureus bacterin (Lysigin, Boehringer Ingelheim Vetmedica, Inc.) with unvaccinated controls. Forty-seven Holstein-Friesian heifers were randomly assigned to one of four groups such that Group 1 (n=11) received a three-isolate experimental bacterin, Group 2 (n=11) received a five-isolate experimental bacterin, Group 3 (n=14) received Lysigin, and Group 4 (n=11) served as unvaccinated controls. Vaccinations were administered twice 28 d apart in late gestation. All groups were challenged with a heterologous strain of Staph. aureus (ATCC 29740) on days 6, 7, and 8 of lactation. Mastitis score, somatic cell count (SCC), milk culture yield, and total daily milk yield data were collected before and after challenge. All 47 cattle developed a Staph. aureus IMI post-challenge with three animals in Group 1 and one animal in Group 3 clearing their Staph. aureus IMI by the end of the study. However, there was no evidence of a difference between vaccinates and control with regard to Staph. aureus clearance rates post-challenge (P> or =0.214). Cattle vaccinated with Lysigin had a lower mean duration of clinical mastitis and lower total mastitis score post-challenge than controls (P=0.045 and P=0.046, respectively). Overall, there was no evidence that any of the vaccinated groups had a lower mean SCC than control (P> or =0.148) for the tested study days. Likewise there was no evidence that vaccinates had greater milk yield than controls post-challenge (P=0.617). Hence, there was no evidence that the vaccines reliably prevented Staph. aureus IMI, but Lysigin showed benefit in reducing the clinical severity and duration of clinical disease post-challenge. Neither of the experimental bacterins appeared to perform better than Lysigin.
The leukotoxin of Pasteurella (Mannheimia) haemolytica is believed to play a significant role in pathogenesis, causing cell lysis and apoptosis that lead to the lung pathology characteristic of bovine shipping fever. Using a system for Cre-lox recombination, a nonpolar mutation within the lktC transacylase gene of the leukotoxin operon was created. The lktC locus was insertionally inactivated using a loxP-aph3-loxP cassette, and then the aph3 marker was excised from the chromosome by Cre recombinase expressed from a P. haemolytica plasmid. The resulting lktC strain (SH2099) secretes inactive leukotoxin and carries no known antibiotic resistance genes. Strain SH2099 was tested for virulence in a calf challenge model. We inoculated 3 ؋ 10 8 or 3 ؋ 10 9 CFU of wild-type or mutant bacteria into the lungs of healthy, colostrum-deprived calves via transthoracic injection. Animals were observed for clinical signs and for nasal colonization for 4 days, after which they were euthanized and necropsied. The lower inoculum (3 ؋ 10 8 CFU) caused significantly fewer deaths and allowed lung pathology to be scored and compared, while the 3 ؋ 10 9 CFU dose of either the wild-type or mutant was lethal to >50% of the calves. The estimated 50% lethal dose of SH2099 was four times higher than that of the wild-type strain. Lung lesion scores were reduced twofold in animals inoculated with the mutant, while clinical scores were nearly equivalent for both strains. The wild-type and mutant strains were equally capable of colonizing the upper respiratory tracts of the calves. In this study, the P. haemolytica lktC mutant was shown to be less virulent than the parent strain.
A commercial MLV combination vaccine containing type 1 and type 2 BVDV given to the dam prior to breeding protected 100% of fetuses against type 1 BVDV infection and 95% of fetuses against type 2 BVDV infection. Use of a bivalent MLV vaccine in combination with a comprehensive BVDV control program should result in decreased incidence of persistent infection in calves and therefore minimize the risk of BVDV infection in the herd.
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