The ability of four strains of Streptococcus uberis to induce clinical mastitis after intramammary inoculation in lactating cows

Abstract

AIM: 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.

MATERIALS AND 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 10⁴ cfu and in the contralateral quarter with approximately 10⁶ 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.

CLINICAL RELEVANCE: 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.

KEY WORDS:

• Mastitis
• Streptococcus uberis
• challenge model
• bovine
• bacterial strains

Acknowledgment

We thank Massey University Dairy No1 for use of its facilities and the collaboration of its staff. We also greatly thank IVABS laboratory technicians, Dr. Scott McDougall and the scientific team of Dairy NZ (Newstead, Hamilton), MilkTestNZ, and the director and staff of JL Vet Services for their collaboration in this work.

This research work was financially supported by Bayer Animal Health Ltd.

ORCID

S Notcovich http://orcid.org/0000-0003-3953-8476

G deNicolo http://orcid.org/0000-0002-5021-0386

NB Williamson http://orcid.org/0000-0001-8286-2250

N Lopez-Villalobos http://orcid.org/0000-0001-6611-907X

Notes

*Non-peer-reviewed