Escherichia coli TK18-A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

鶏呼吸器性マイコプラズマ病に関する基礎的検討の一環として,本病の一実験感染系確立の為に,さきに著者ら(1980)21)はM. gallisepticum (MG) 1RF株を用いて,今回と同様な実験を行った成績について報告した。今回は鶏大腸菌症の原因ならびにMG感染症の混合感染菌として重要な,E.coliの気嚢内人工感染による初生ヒナに対するvirulenceの程度を知る目的で本実験を行った。

E. coli TK18-A株(O群2)液体培養の10倍段階希釈(5.4×10^0-5/0.4ml)を,ブロイラー初生ヒナ(雄)の右後胸気嚢内に接種し,4週間観察した。各実験群20羽宛のヒナにつき,臨床症状,増体量,飼料要求率,病理変状,各臓器からのE. coli分離等の項目につき検討し,下記の成績が得られた。

1) 全群呼吸器症状はみられず,各群ヒナの死亡は主に感染後7日以内にみられ,例外的に10及び14日後1例ずつの死亡がみられた。菌接種0～10^5 CFU各群の死亡率は,それぞれ0,5,5,40,60,100,100%であった。供試菌のLD50は5.4×10^2.4 CFUであり,接種菌10CFU以下では死亡率は極めて低く,また接種菌数が減少するに従い,死亡時期がやや遅延する傾向がみられた。
2) 増体量・飼料要求率については,早期死亡例が多く,各群間の比較は困難であった。40%のヒナが生残した10^3 CFU菌接種群とそれ以下の菌接種群での成長曲線では,10^3 CFU菌接種群が最低を示した。
3) 肉眼病変出現率と接種菌数とはほぼ平行する傾向がみられ,LD50以上の菌接種群で病変陽性の傾向がみられた。気嚢病変は菌接種側で高率にみられた。10^3 CFU菌接種死亡例で顕著な病変が多数みられた。気嚢病変は,10^2-5 CFU菌接種群にみられた。
4) 以上の病変は,病理組織学的にも,全体的に滲出性の炎症病相が主体で,従来記載の大腸菌性敗血症病変とよく一致した。
5) E. coli分離率は接種菌数とほぼ平行し,10^4-5 CFU菌接種群死亡例では100%で大部分のヒナは典型的な急性敗血症死とみられた。
6) 本菌株のAID50とLD50の菌数はほぼ同一とみられた。

The present experiments were performed to investigate the virulence of E. coli strain TK18－A40) (O-group 2) induced artificially into the air sac of day-old chicks. This was done with the further purpose to establish an experimental system using chicks for fundamental experiments on the efficacy in vivo of the antibacterial agents that control the avian colibacillosis and the avian respiratory mycoplasmosis complicated with E. coli.

The inocula of 0.4 ml /chick of ten-fold serial dilutions of broth culture were artificially induced into the right posterior thoracic air sac of day-old male broiler chicks, and these chicks were observed during 4 weeks. The experimental groups were designed as groups of 20 chicks each, 6 groups included a non-infected control and 5 infected groups. The inoculum sizes of the organisms of the chicks ranged from 5.4×1 to 10^5 CFU* in 5 dilutions of the broth culture. The following results were obtained.

1) No clinical symptoms were observed except that some chicks died. Death of the chicks was mostly observed within 7 days post inoculation, exceptionally 2 chicks only died 10 and 14 days post inoculation. Mortalities in the group of chicks inoculated with 0－10^5 CFU were 0, 5, 5, 40, 60, 100 and 100%, respectively. The LD50 of the organisms was determined as being 5.4×10^2.4 CFU. A remarkably low mortality was observed in the chicks inoculated with organisms less than 10 CFU. A delaying tendency of the lethal period of the chicks was noticed in accordance to the decrease of the number of organisms inoculated.
2) As a large number of chicks died in the early stage of the experiments, it was difficult to determine a comparison of weight gain and feed conversion for the chicks of each group. In the groups of chicks inoculated with 10^3 or less CFU of the organisms, 40% or more survived ratios were observed. In comparison of the growth curve of these groups, the group inoculated with 10^3 CFU of the organisms showed the lowest curve.
3) An almost parallel tendency was observed between the gross lesion score and the inoculum size of the organisms. The positive tendency of the gross lesion score was investigated in the group of chicks inoculated with a number larger than LD50 of the organisms. A higher gross lesion score was observed on the inoculated side of the air sac of chicks. A large number of severe lesions was observed in chicks that had died after inoculation of 10^3 CFU of the organisms. Air sac lesions were observed in the group of chicks inoculated with 10^2-10^5 CFU of the organisms.
4) The major finding observed in the histopathological examination of chicks was generally an exudative inflammatory manifestation. These findings were similar to those of the septicemic avian colibacillosis described by many workers.
5) The recovery ratio of the organisms ran mostly parallel with the number of the inoculated organisms. One hundred percent ratios were observed in 2 groups of chicks inoculated with 10^4－10^5 CFU of the organisms. It was considered that most cases of the dead chicks were due to coli-septicemia.
6) The AID50 of the organisms in the present experimental conditions was estimated as almost the same number (CFU) of the organisms as that of the LD50.