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[Effect of physicochemical factors on infectivity of Spirometra mansoni plerocercoid].

OBJECTIVE
To observe the effect of different physicochemical factors on the infectivity of Spirometra mansoni plerocercoids.


METHODS
The muscle samples with plerocercoids taken from Rana nigromaculata (about 1 cm3 each piece) were treated with different temperature (-20 degrees C, 4 degrees C, 37 degrees C and 56 degrees C) or different concentrations of ethanol (20%, 30%, 40%, 50% and 60%) for 1, 2 or 3 h, or soaked in ginger juice, vinegar (total acid concentration of 4.5%, pH 3.05) or soy sauce (containing 19.3% NaCl) for 3, 6, 12 or 24 h. The muscle with plerocercoids treated with normal saline under 20 degrees C served as control. 30 plerocercoids were used under each condition and fed to 10 mice averagely (3 larvae/mouse). Another 20 plerocercoids with frog meat were comminuted for 3 min then fed to 10 mice. One week later, the mice were sacrificed to collect the parasitic plerocercoids and the number of positive mice and plerocercoids was recorded.


RESULTS
None of the mice fed with plerocercoids treated under -20 degrees C for 2 h was infected. All the mice fed with plerocercoids treated under 56 degrees C for 2 h and 3 h were infected. The percentage of infective plerocercoids was 60% (18/30) and 43% (13/30), respectively, considerably lower than those of the control (90%, 27/30) (P < 0.05). None of the mice fed with plerocercoids soaked in 60% ethanol for 2 h was infected. All the mice fed with plerocercoids soaked in 60% ethanol for 1 h, or in 50% ethanol for 2 h or 3 h were infected. The percentage of infective plerocercoids was 60% (18/30), 57% (17/30), and 50% (15/30), respectively, considerably lower than those of the control (93%, 28/30) (P<0.05). None of the mice fed with plerocercoids soaked in vinegar for 24 h, or soy sauce for 6 h was infected. The infectivity of the plerocercoids treated by ginger juice for 24 h was similar to the control (P>0.05). Among the ten mice fed with comminuted frog meat with plerocercoids, five were infected with 6 plerocercoids.


CONCLUSION
Treatment with -20 degrees C or 60% ethanol for 2 h, soy sauce for 6 h, or vinegar for 24 h can destroy the infectivity of plerocercoids in 1 cm3 frog muscle.