Inoculation of lactating ewes by the intramammary route with Mycoplasma agalactiae: comparative pathogenicity of six field strains

Abstract

-Contagious agalactia affects goats and sheep. In most infected sheep, the causal agent, Mycoplasma agalactiae, induces mastitis and/or agalactia, keratoconjunctivitis and arthritis. However, a few strains of M. agalactiae were isolated from tank milk from flocks without any clinical signs. The present study was undertaken to compare these apparently "asymptomatic" strains to classical virulent strains in order to assess the pathogenicity of four "asymptomatic" strains. Six groups of lactating ewes were inoculated by the intramammary route with 10 8 viable mycoplasmas of each strain. The clinical signs were regularly evaluated; the excretion of bacteria in milk and the serological response were measured. Ewes were necropsied 7 weeks after inoculation and the level of infection in retromammary lymph nodes was determined. Among the 4 apparently "asymptomatic" strains, 2 were fully virulent as were the strains isolated from diseased animals, and the other 2 induced somewhat less severe clinical symptoms. The other parameters, in particular the level of excretion in milk and the level of infection of regional lymph nodes following necropsy were similar for all strains. Mean antibody response was also comparable between the apparently "asymptomatic" and virulent strains, in spite of great individual variability. This observation shows that flocks without any clinical sign from which M. agalactiae is isolated in bulk milk, must be kept under strict control since mycoplasmas may induce severe outbreaks later with changing conditions of breeding.contagious agalactia / Mycoplasma agalactiae / virulence / experimental infection / sheep disease Résumé -Inoculation de Mycoplasma agalactiae par voie intramammaire chez des brebis en lactation : virulence comparée de 6 souches de terrain. L'agalactie contagieuse affecte les chèvres et les moutons. Chez la plupart des moutons infectés, l'agent causal, Mycoplasma agalactiae, induit des mammites et/ou de l'agalactie, des kératoconjonctivites et des arthrites. Cependant, quelques Vet. Res. 31 (2000) 329-337 329

Simultaneous Identification of Potential Pathogenicity Factors of Mycoplasma agalactiae in the Natural Ovine Host by Negative Selection

“…Previously, we had studied the colonization of M. agalactiae wild-type strain PG2 and PLMs (Vpma phase-locked mutants) using this model (27). Besides the colonization and multiplication of M. agalactiae in the udder and simultaneous shedding in milk, it also leads to clinical signs and systemic spreading to various body sites (10,27,55,56). The latter would be an important added advantage for an infection model while performing a challenge infection experiment with single selected mutants to verify their attenuation.…”

“…Studying a similar approach in M. agalactiae and other ruminant mycoplasmas has been hindered due to the lack of small-animal models. However, general experimental infections of natural ruminant hosts have demonstrated adequate antibody titers, host colonization, systemic spreading, and visible clinical signs (8,26), as also has been observed during M. agalactiae experimental intramammary infections of sheep (10,27). To understand M. agalactiae factors mediating colonization of the host udder, the current study exploited the use of a heterogeneous pool of Tn mutants that were simultaneously screened in a sheep intramammary infection model.…”

“…Earlier studies in this regard were based on in vitro analysis of genes involved in host-cell interaction (5), and a recent report has focused on a single factor in an experimental infection study (6). In general, the previously reported in vivo studies dealt with either characterizing of field strains or deciphering events occurring in mammary epithelial cells during M. agalactiae infection (7)(8)(9)(10). However, there is a complete lack of studies on the systematic analysis of M. agalactiae genetic factors involved in the colonization of the udder and subsequent systemic spread and persistence leading to chronic infections.…”

Detection of Mycoplasma agalactiae Antigen in Sheep and Goats by Monoclonal Antibody-Based Sandwich ELISA

“…This Mabs-based sandwich ELISA involves preenrichment, a short-term incubation of the sample directly on an ELISA plate, and the use of monoclonal antibodies; the former renders the method more sensitive and the latter facilitates its better reproducibility and increases its specificity. The use of this ELISA method improves laboratory diagnosis of infectious agalactia also in terms of reliability, accuracy and time, which is very important because some infected sheep and goats have shown atypical forms of the disease (Nicolet 1994b); in addition, asymptomatic disease carriers have been reported (Bergonier 1996ab;Lillini et al 1996;Sanchis et al 2000) which are difficult to detect by less sensitive laboratory methods.…”

Validation of the suppressive subtractive hybridization method in Mycoplasma agalactiae species by the comparison of a field strain with the type strain PG2

“…Antigenic and genomic analysis of diverse M. agalactiae strains or field isolates has shown a strong heterogeneity between virulent isolates [4,11,14,19]; experimental and field studies suggest that certain M. agalactiae strains have a lower virulence compared to the fully pathogenic strains [23]. Comparison of selected M. agalactiae strains should help to understand the molecular basis of these differences.…”