title: The ability of four strains of Streptococcus uberis to induce clinical mastitis after intramammary inoculation in lactating cows.
abstract: OBJECTIVE
  To compare the ability of four strains of Streptococcus uberis at two doses to induce clinical mastitis in lactating dairy cows after intramammary inoculation in order to evaluate their usefulness for future experimental infection models.
  
  
  METHODS
  Four field strains of S. uberis (26LB, S418, and S523 and SR115) were obtained from cows with clinical mastitis in the Wairarapa and Waikato regions of New Zealand. Twenty-four crossbred lactating cows, with no history of mastitis and absence of major pathogens following culture of milk samples, were randomly allocated to four groups (one per strain) of six cows. Each cow was infused (Day 0) in one quarter with approximately 10(4) cfu and in the contralateral quarter with approximately 10(6) cfu of the same strain. The other two quarters remained unchallenged. All four quarters were then inspected for signs of clinical mastitis, by palpation and observation of the foremilk, twice daily from Days 0-9, and composite milk samples were collected from Days 0-8 for analysis of somatic cell counts (SCC). Quarters were treated with penicillin when clinical mastitis was observed. Duplicate milk samples were collected and cultured on presentation of each clinical case and on Day 4 from challenged quarters with no clinical signs.
  
  
  RESULTS
  Clinical mastitis was diagnosed in 26/48 (54%) challenged quarters. Challenge with strain S418 resulted in more cases of mastitis (12/12 quarters) than strains SR115 (7/12), 26LB (6/12) or S523 (1/12), and the mean interval from challenge to first diagnosis of mastitis was shorter for S418 than the other strains (p<0.001). The proportion of quarters from which S. uberis could be isolated after challenge was less for strain 26LB (1/6) than SR115 (6/7) (p<0.05), and SCC following challenge was lower for strain S523 than the other strains (p<0.05).
  
  
  CONCLUSIONS
  There were significant differences between the strains in the proportion of quarters developing clinical mastitis, the interval to mastitis onset, SCC following challenge and the proportion of clinical cases from which S. uberis could be isolated. These results illustrate the difference in the ability of S. uberis strains to cause mastitis and the severity of the infections caused.
  
  
  CONCLUSIONS
  Experimental challenge models can be used to compare infectivity and pathogenicity of different strains of mastitis-causing bacteria, the efficacy of pharmaceutical products and host-responses in a cost-effective manner.
authors.affiliation: a Institute of Veterinary, Animal and Biomedical Sciences , Massey University , Palmerston North 4442 , New Zealand.
authors.forename: S
authors.initials: S
authors.lastname: Notcovich
authors.affiliation: a Institute of Veterinary, Animal and Biomedical Sciences , Massey University , Palmerston North 4442 , New Zealand.|b Current address: AgResearch, Private Bag 4749, Christchurch 8140, New Zealand.
authors.forename: G
authors.initials: G
authors.lastname: deNicolo
authors.affiliation: a Institute of Veterinary, Animal and Biomedical Sciences , Massey University , Palmerston North 4442 , New Zealand.
authors.forename: N B
authors.initials: NB
authors.lastname: Williamson
authors.affiliation: a Institute of Veterinary, Animal and Biomedical Sciences , Massey University , Palmerston North 4442 , New Zealand.
authors.forename: A
authors.initials: A
authors.lastname: Grinberg
authors.affiliation: a Institute of Veterinary, Animal and Biomedical Sciences , Massey University , Palmerston North 4442 , New Zealand.
authors.forename: N
authors.initials: N
authors.lastname: Lopez-Villalobos
authors.affiliation: a Institute of Veterinary, Animal and Biomedical Sciences , Massey University , Palmerston North 4442 , New Zealand.|c Current address: School of Animal and Veterinary Science, University of Adelaide, Roseworthy, SA 5371, Australia.
authors.forename: K R
authors.initials: KR
authors.lastname: Petrovski
mesh_terms: D000818:Animals; D002417:Cattle; D005260:Female; D007774:Lactation; D008414:Mastitis, Bovine / Q000382:microbiology*; D008892:Milk / Q000166:cytology; D013290:Streptococcal Infections / Q000382:microbiology / Q000662:veterinary*; D013291:Streptococcus / Q000145:classification* / Q000472:pathogenicity
keywords: Mastitis; Streptococcus uberis; bacterial strains; bovine; challenge model