Comparison of pepsin-digestion and enzyme-linked immunosorbent assay for the diagnosis of trichinosis in swine

Abstract

Comparison of parasitological and serological diagnosis of trichinosis in swine was carried out on 36 pigs given 15,400 infective larvae each by gavage. Circulating eosinophil levels were determined and sera were examined by enzyme-linked immunosorbent assay for anti-Trichinella antibodies. Two pigs were killed per day from days 15 to 29 postinfection. Muscle was examined by pepsin-digestion and comparable tissue was fed to a rat. Eosinophil counts increased at about day 6 and reached peak levels about day 25 postinfection and returned to approximate preinfection levels about two months postinfection in those pigs still in the study. Infective larvae were recovered from all pigs killed at greater than or equal to 18 days postinfection. Using the criterion of 5 x mean optical density readings of negative sera as positive, seroconversion occurred between days 19 and 26 postinfection. Use of a lower criterion of 3 x mean optical density readings of negative sera resulted in only three of 30 pigs killed greater than or equal to 18 days postinfection seroconverting less than or equal to 18 days postinfection, when infective larvae were first recovered in the musculature. In pigs, even in those heavily infected, there is a lag between the period that trichinae in musculature become infective and development of antibodies as detected by enzyme-linked immunosorbent assay which results in false negative reactions in many animals. This study demonstrated that the enzyme-linked immunosorbent assay using an excretory-secretory antigen should not be used to certify pork or pork products free of infective Trichinella larvae or safe for human consumption.