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Korean Journal of Epidemiology 1991;13(2): 169-184.
A study on leptospiral infection status and biological characteristics of the strains isolated from wild rats in some areas of Korea.
Seon Il Park, Joung Soon Kim
Abstract
In order to study Leptospiral infection status and biological characteristics of the strains isolated from wild rats, a total of 34 wild rats were captured alive from three locations including Hwasung-gun, Yangju-gun and Seoul city during the period of four months from the December 1990 to the March 1991. Rodents were anesthetisized with ether, blood was taken from heart puncture and then autopsied. Blood and kidney specimens were inoculated into tubes of EMJH semisolid medium and incubated at 30°C for six weeks. These cultures were examined 6 times by dark field microscopy at weekly intervals. Serogroup and serovar of isolates were identified by micro-agglutination test and cross-agglutination absorption test Lung, liver and kidney were sectioned with a sharp scalpel and touch printed on slide glasses and then silver-stained. Blood specimens were allowed to clot and sera were collected for serologic test. These procedures were necessary to see the distribution of Leptospira by tissues and antibody titer as the length of infection gets longer. Pathogenicity test and horizontal infection test were also carried out on Guinea pigs and mice with the strains isolated. The summarized results are as followings : 1. Kidney tissue and blood sample from 34 rats were cultured for Leptospira and four(11.8%) strains were isolated. All strains belonged to Leptospira interrogans Icterohemorrhagiae serogroutp 1 out of 1 Apodemus agrarius, 2 out of 10 Rattus rattus, 1 out of 16 Rattus norvegicus, none out of 7 Mus musculus. Of 4 isolates, cultured H-9 was identified as Serogroup Icterohemorrhagiae serovar lai. 2. Leptospiras were seen in 8 of 34 (23.5%) silver-stained kidney sections, 5 in lung sections (14.7%), and 4 in liver sections (11.8%) 3. All strains isolated were Gram negative, resistant to 5-FU, sensitive to 8-Azaguanine, spherical conversion in 1 M NaCl ; typical Leptospiras with hook in E.M. were observed These strains did not grow in 13℃ culture temperature. Thus all strains demonstrated Leptospim interrogans characteristics. 4. The overall Leptospira seropositive rate was 20.6% (7 samples/ in 34 blood of wild rats. Of 7 seropositive rat sera, 3 (8.8%) were positive for L. Icterohemorrhagiae, 2 (5.9%) for 87M-67 (local lai), one showed cross reaction between L. Sejroe and L. Bataviae. 5. As Guinea pigs and mice were inoculated intraperitoneally with 1 ml of 7 day old culture containing approximately 2 x 108org/ml of Y-2 isolated strain, massive hemorrhage and inflammatory cell infiltration (lymphocyte, plasma cell,) were major pathological findings in heimatoxylin-eosin stained sections. LD50 by probit analysis was 1.87 x 1011 org/ml [y(probit)=-13.530+1.644 Log x] and horizontal infection between mice was confirmed. 6. Leptospires were seen in blood of experimental animals at 24 hours after experimental inoculation and they disappeared around 288 hours. Leptospires were observed in liver first and then in all organs by 4 days after experimental infection. 7. Antibody was detectable in 2 days after the experimental infection and then increased gradually. Although the titer dropped incidentally to 1 : 20 at 6 days after infection, relatively high titer was maintained from 12 days of infection. Key words : Leptospira, wild rat isolation, serology, histopathology.


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