Modification and further evaluation of a fluorescein-labeled peanut agglutinin test for identification of Haemonchus contortus eggs

Gastrointestinal nematodes of the family Trichostrongylidae are the most important parasites of sheep, goats and other ruminants worldwide. Of this group, Haemonchus contortus is usually the most damaging species, particularly in warmer climates or during the summer. It is therefore useful to be able to rapidly differentiate infections with this nematode from other, less pathogenic, species. However, aside from Nematodirus spp., there are only subtle differences between the egg morphology within the trichostrongyles, making it very difficult to identify eggs to the species level. It has been shown previously that peanut agglutinin specifically binds to Haemonchus eggs and not those of other trichostrongyle species. By using this lectin conjugated to fluorescein isothiocyanate, binding to Haemonchus eggs can be visualized under ultraviolet illumination. We adapted this test using eggs purified by routine sugar centrifugation methods and evaluated 26 diagnostic samples from ruminants submitted to our laboratories in Oregon and Georgia. Very good correlations were seen between this test and larval culture (adjusted R2 = 0.72132; F1,25 = 65.7; p < 0.001). There was little variability between two different diagnosticians reading the same sample, suggesting that the test is robust and not subject to reader bias. An additional benefit is that eggs can be examined following preservation; fixation of H. contortus eggs in formalin at 0.5–5% for to up to 4 wk did not affect their staining.