Mycoplasma gallisepticum 1RF株の気嚢内人工感染による初生ヒナに対するVirulenceについて

in vivoでの抗菌剤効果の基礎的検討の為に,初生ヒナを用いた一実験感染系を確立する目的で,以下の実験を試みた。

供試菌Mycoplasma gallisepticum (MG)1RF株の新鮮液体培養10倍段階希釈の0.4ml/ヒナを,ブロイラー初生ヒナ,雄の右後胸部気嚢内に接種し,4週間観察し,接種菌のvirulenceの程度を調べた。実験群は20羽/群とし,非感染対照を含む6群を設定した。接種菌数は4×10^1-5 CCUの5段階とした。各実験群のヒナについては,臨床症状を観察し,増体量,飼料要求率を調べて,毎週各群5羽ずつを殺して剖検し,MG分離,MG血清凝集素価の推移,肉眼ならびに組織病変等について経時的観察を試みた結果,下記の成績が得られた。

1) 全期間,全実験群において呼吸器症状はみられなかった。しかし,接種菌数の多い群で頸部捻転と脚麻痺を主徴とする死亡ヒナがみられ,これらのヒナの脳病変は病理組織学的検査の結果,脳軟化のそれに一致した。それらヒナの一部の脳からはMGが分離された。
2) 接種菌数の多い群では.ヒナの増体量は少なく飼料要求率は悪かった。
3) 接種菌数の少ない群での死亡率は極めて低く,接種菌数の多い群で14日令まで高く,その後28日令までは死亡ヒナはみられなかった。
4) 気嚢および気管の病変出現率は,MG接種菌数の多い群で高く,また病変の程度も強いものが多かった。なお低率ながら各群のヒナに肝の点状出血や,線維素性肝包膜炎,壊死巣などが,MG接種菌数の多少にかかわらず認められた。
5) MG分離率は,気嚢よりも気管において高く,またMG非接種側気嚢よりもMG接種側気嚢において高かった。
6) MG血清凝集素価は,1週令殺時のヒナ群では低く,2週令時以後の殺ヒナ群で上昇がみられた。MG接種群間では接種菌数の多少にかかわらず凝集素価に大きな差は認められなかった。
7) 供試菌株のAID50は10CCU前後とみられた。

結論として,供試1RF株はM. gallisepticumとしては,ヒナに対し比較的強毒な菌株である事が知られた。

In order to establish an experimental system using chicks for the fundamental observation on efficacy in vivo of the antibacterial agents that control the avian respiratory mycoplasmosis, the present experiments were conducted to investigate the virulence of Mycoplasma gallisepticum strain 1RF26) induced artificially into the air sac of day-old chicks.

The inocula of 0.4 ml/chick of ten-fold serial dilutions of broth culture of the organisms were artificially induced into the right posterior thoracic air sac of day-old male broiler chicks, and the chicks were observed during 4 weeks. The experimental groups were designed as group of 20 chicks each, 6 groups included a non-infected control and 5 infected groups. The inoculum sizes of the organisms to the chicks ranged from 4×10 to 10^5 CCU* in 5 dilutions of the broth culture. The following results were obtained.
1) Clinical symptoms were observed only death with characteristic findings of limberneck and leg weakness in the chicks inoculated with a large number of the organisms. According to the results of the histopathological examinations, the cerebral lesions appearing in these chicks were similar to those of the encephalomalacia. The inoculated organisms were recovered from the brain of some of those chicks.
2) Low weight gains and high feed conversions were observed in the group of chicks inoculated with a large number of the organisms.
3) The mortality rate was very low in the group of chicks inoculated with a small number of the organisms. In the group of chicks inoculated with a large number of the organisms, the mortality of chicks was comparatively high up to 14 days post inoculation, but no mortality was observed during the 15 to 28 days post inoculation.
4) Incidence and severity of macroscopic lesions in the air sac and the trachea of the chicks inoculated with a large number of the organisms were comparatively high and severe. Under low incidence of the macroscopic lesions including petechiae, fibrinous perihepatitis and necrotic foci etc. in the liver of chicks were observed to be not depending on the inoculum size of the organisms.
5) Recovery ratio of the organisms from the chicks was found to be higher in the trachea than that in the air sac, and furthermore was higher on the inoculated side than on the other side of air sac.
6) Serum agglutinins against M. gallisepticum were observed to be at a low titer in the chicks killed 1 week post inoculation, and increased titers were observed in the chicks killed after 2 weeks post inoculation. No remarkable difference of the serum agglutinin titers depending on inoculum size of the organisms was observed in the chicks.
7) The AID50 of the organisms was estimated as ca. 10 CCU.

Consequently, it was proved that the strain 1RF examined is comparatively high virulent against chicks, among the strains of M. gallisepticum.