Determination of the specific gravity of eggs of equine strongylids, Parascaris spp., and Anoplocephala perfoliata

Given the ever-increasing levels of anthelmintic resistance in livestock parasites globally, it is recommended to use parasite fecal egg counts to make treatment decisions and to evaluate treatment efficacy. The consensus in equine parasitology is to use a flotation medium with a specific gravity (SG) of ≥ 1.20 to float the main parasite egg types of interest in egg counting techniques. However, the density of common equine endoparasite eggs has been sparsely investigated. Equine tapeworm eggs are known to be particularly difficult to determine and count in fecal samples. It is unknown whether this could be because of differences in egg density. The aim of this study was to provide estimates of relative densities for equine ascarid, strongyle, and tapeworm eggs. Six aqueous glucose-salt solutions with specific gravities ranging from 1.06 to 1.16 were made and placed from most to least dense into thirteen 15 mL centrifuge tubes. Concentrated aqueous suspensions of the three types of endoparasite eggs were placed on top of each tube. These tubes were then centrifuged at 800 g for 20 min and each layer of flotation solution was carefully pipetted and transferred to a McMaster egg counting slide. Egg type and count were recorded for each specific gravity layer. Each egg was assigned a specific gravity based on the specific gravity layer it was observed in. In a second trial of this study, five similar flotation media were made ranging from 1.02 to 1.10 and were used in four subsequent replicates. In total between the two trials, the mean egg SGs of Anoplocephala perfoliata (n = 3811), Parascaris spp. (n = 3478), and strongylid type eggs (n = 9291) were 1.0636 (95% confidence interval (CI): 1.0629–1.0642), 1.0903 (95% CI: 1.0897–1.0909), and 1.0453 (95% CI: 1.0448–1.0458), respectively. The three egg types were statistically different from each other (p < 0.0001). This is the first time that the specific gravity of equine strongylid and Anoplocephala perfoliata eggs has been determined. With a tapeworm egg density demonstrated to be between that of strongylids and Parascaris spp., the poor recovery of tapeworm eggs in equine fecal samples must have other explanations.