Elsevier

Veterinary Microbiology

Volume 154, Issues 3–4, 27 January 2012, Pages 376-383
Veterinary Microbiology

Development of an experimentally induced Streptococcus uberis subclinical mastitis in goats

https://doi.org/10.1016/j.vetmic.2011.07.031Get rights and content

Abstract

Streptococcus uberis is a major environmental mastitis-causing pathogen. The infections are predominantly subclinical and are frequently undetected and untreated for extended periods of time. More information about the pathogenesis of S. uberis mastitis would be useful. To our knowledge, no experimental studies into the mastitis pathogenesis caused by S. uberis have been described in lactating goats. The aim of this study was to reproduce an experimentally induced S. uberis subclinical mastitis in lactating goats aimed to evaluate the inflammatory response, dynamics of infection and the pathological findings within the first hours of intramammary inoculation with S. uberis. Six Saanen goats in mid-lactation were inoculated with 1.7 × 108 cfu of S. uberis. Bacterial growth peaked in milk from challenged right mammary halves (RMH) at 4 h PI. Shedding of viable bacteria showed a marked decrease at 20 h PI. Mean somatic cell counts in milk from the RMH peaked at 20 h PI. Inoculation with S. uberis was followed by a decrease in the mean total number of leukocytes. Signs and systemic symptoms were not evoked by intramammary inoculation. S. uberis could be isolated in tissue from all RMH. Histological examination of specimens of the RMH and lymph nodes of the goats showed an increased inflammatory response throughout the experiment. The histological findings correlated with the immunohistochemical detection of S. uberis in RMH. In conclusion, the experimental inoculation of S. uberis in lactating goats is capable of eliciting an inflammatory response and causing pathological changes, resulting in a subclinical mastitis. This investigation shows that goat might to represent a valuable model for the study of the mastitis pathogenesis caused by S. uberis.

Introduction

Mastitis is the most expensive disease affecting dairy cows throughout the world. Streptococcus uberis is a major environmental mastitis-causing pathogen. In spite of its importance the pathogenesis of mastitis has not been well understood yet. Development of experimental intramammary infections (IMI) models would facilitate the study of the mastitis pathogenesis caused by this pathogen. The pathogenesis in S. uberis experimental bovine IMI has been studied by different authors (Almeida et al., 2004, Pedersen et al., 2003, Rambeaud et al., 2003). The costs associated with experimental IMI in cows are prohibitive in developing countries when a minimal number of animals are required to get valid statistics. In mastitis research, mice and small ruminants have been used as experimental animals (Brouillette and Malouin, 2005). A mouse model has been used to asses the physiopathology of IMI caused by different genera of pathogens (Diker et al., 1992, Honkanen-Buzalski et al., 1985, Reid et al., 1976). However, the mammary gland of a modern dairy cow differs considerably from that of small experimental animals. Studies on experimental goat models using less common mastitis pathogens were also published (Bassam and Hasso, 1997, Castro-Alonso et al., 2009, Koul et al., 1993), considering that structural genomic studies have shown that goats are relatively closely related to bovine species (Schibler et al., 1998). Obviously, the key characteristic for any host to qualify as a model for bacterial pathogenesis is to respond to the infectious agent in the same way as cows. To our knowledge, no experimental studies into the mastitis pathogenesis caused by S. uberis have been described in lactating goats. The objective of the present study was to reproduce an experimentally induced S. uberis subclinical mastitis in lactating goats aimed to evaluate the inflammatory response, dynamics of infection and the pathological findings within the first hours of intramammary inoculation with S. uberis.

Section snippets

Animals

Six Saanen goats in mid-lactation were chosen according to the following criteria: first parity, absence of IMI infection and somatic cell count (SCC) in their milk (<1,000,000 cells/ml). Goats were monitored for IMI with bacteriological analysis of milk samples at 7 and 14 days prior to infection. Bacteriological analysis was performed as previously described by Moroni et al. (2005). According to what will be described afterwards.

Mammary glands of all goats had a low SCC in their milk

Bacteriological results

Selected goats were free from IMI with S. uberis and other mammary pathogens throughout the first weeks of lactation. To ensure that the dairy goats used in the experiment were free from bacterial infection at the moment of challenge, milk samples were taken from both udder halves 24, 48, 72 and 96 h prior to intramammary S. uberis infusions and tested for bacterial growth. No bacterial growth was present in the milk from any of the udder halves.

Bacterial growth peaked (3.29 × 107 cfu/ml) in milk

Discussion

To our knowledge, this is the first report on experimentally induced S. uberis IMI in lactating dairy goats. In the present study all midlactation goats developed mild subclinical mastitis in the right mammary halves in response to intramammary bacterial infection with 1.7 × 108 cfu/ml of the strain IR47of S. uberis. The infection dose used was based on preliminary studies in two goats by testing with different doses of the same S. uberis strain. The first preliminary testing with a dose of 8.0 × 102

Conflict of interest

None.

Acknowledgements

This work was financially supported by SECyT (Secretaría de Ciencia y Técnica, Universidad Nacional de Río Cuarto) and FONCyT (Agencia Nacional de Promoción Científica y Tecnológica). The authors are very thankful to Ph.D. C. Perfumo (Instituto de Patología, Facultad de Ciencias Veterinarias, Universidad Nacional de La Plata) for their technical assistance on the performing histological and immunohistochemical sections. We also thank Silvana A. Dieser, Pablo A. Gutiérrez and María L. Gambero

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