Etiology And Histopathological Observation Of Muscular Ulcer In Monopterus Albus

In recent years,there had been an outbreak disease characterized as skin broken on one side dorsal fin and muscle ulcer of Monopterus albus in si chuan,hu bei and other regions,which had caused heavy economic losses.In order to figure out the pathogen and pathogenesis of the disease,artificial infection proved that the bacterium was the pathogen of the disease the aetiology,pathology and virulence factors were studied.A pathogenic bacterium(hs-1) was isolated from liver,intestinal tract,kidney and par aff of moribund of the diseased fish.The results showed that I.M.and scuffing immerse caused the similar skin ulcer and death.The strain grew well in nutrient broth,which had flocculent precipitate in the bottom and mycoderm on the surface.On the nutrient agar, bacterium colonies were white to gray,round and convex,and entire margins.The strain was that:Oxidase-positive,Mannitol-positive,Lysine decarboxylase-positive,Arginine double hydrolytic enzyme-positive,Lactose-positive,Gelatin liquefaction-positive,V-P -positive,Aesculin hydrolysis-negative,Salicin-negative,H₂S-negative.A 1425bp 16SRNA complementary fragment of hs-1 was checked by amplification using bacteria general PCR primer.The sequence classifer in RDP Database suggested that it belonged to Aeromonas.A phylogenetic tree was constructed by comparing the 16S rDNA sepuences of the isolated strain(GenBank accession number FJ461353) with other four strains Aeromonas sobria and six strains Aeromonas hydrophila in the GenBank databases.The homology between strain hs-1 and other Aeromonas sobria was 98.3%.According to the results of morphological,physiological and biological characteristics and phylogenetic analysis,the isolated strain(hs-1) was identified as Aeromonas sobria.Based on antibiotic susceptibility studies,the strain was high sensitive to Cefotaxime sodium,Roceplin,Ciprofloxacin,Omefloxacin,Norfloxacin,Spectionmycin,Nalidixie acid,Teracycline,Sulfafurazole,Trimethoprim,Eposerin;the strain was middle sensitive to Cefalexin,Rifampicin,Streptomycin,Kalamycin,Erythromycin,Nitrofurantoin;the strain was no sensitive to Ampicillin,Acetylspituamycin,Novobiocin. Pathological changes of rice field eel infected Aeromonas sobria were observed in this study.Infection outbreak was an impatient process.The clinical symptoms after atifical infection were similar with the natural disease.The peak of death was between 12h to 36h after post infectionem.9h after atifical infection,there had visible symptoms,such as muscular ulcer of the anus and so on.In 12h after infection,the Monopterus albus began to death.In 18h after infection,skin broken on one side dorsal fin and muscle ulcer.The histopathological examination showed 6h to 12h after infection that the arrangement of hepatic cord is chaotic the hepatocytes displays swelling and vacllolar degeneration,the cytolemma of some hepatocyte ruptured;epithelium of renal tubule appeared swelling, severe granular degeneration and some of it appeared slight vacuolus degeneration;9h to 24h after infection villi of small intestine abscission,lamina propria lymphatic nodule hyperplasia nodules,submucosa layer,muscularis,placenta percreta erythrocyte increase;15h to 30h after infection furtherly,epithelium of renal tubule appeared necrosis,nucleuses were dyed slightly even disappeared;21h to 30h after infection muscle cells were necrotic and a lot of inflammatory cells were soaking.