FALSE-POSITIVE ASCARIS-SUUM EGG COUNTS IN PIGS

False-positive Ascaris suum egg counts in pig faeces are frequently ob served under both experimental and natural conditions. Data from 12 ex periments with A. suum infections in pigs were summarized and showed t hat the percentage of false-positive faecal samples ranged from 4 to 3 6%. False-positive egg count values varied greatly between pigs and ex periments (range 20-1060 eggs per gram faeces). Indoor experiments wit h pigs housed groupwise in pens generally produced more and higher fal se-positive egg counts, which may reflect differences in surface area and hence exposure to infective eggs, compared with pasture experiment s. The positive predictive value (the number of pigs diagnosed positiv e by faecal sample that actually harboured worms) was low for indoor e xperiments (45%) compared with pasture experiments (89%). Differences in design far indoor experiments, such as poor type and use of bedding material, did not influence the positive predictive value (44-47%). A positive correlation was found (r = 0.56, P < 0.05) between faecal eg g counts of true-positive and false-positive pigs that were penned tog ether. The results of this survey strongly support previous suggestion s that false-positive. A. suum egg counts in pigs are the result of co prophagia in indoor experiments and coprophagia/geophagia in pasture e xperiments. False-positive A. suum egg counts in pig faeces may vary g reatly in prevalence and magnitude, and depend in part on management/h ousing factors. (C) 1997 Australian Society for Parasitology. Publishe d by Elsevier Science Ltd.