Formalin performs better than ethanol for recovery of strongyle-type eggs using established and novel McMaster techniques

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Abstract Gastrointestinal parasites (GI), including soil-transmitted helminths (STHs), continue to impose a considerable health burden in endemic regions, particularly in low- and middle-income countries. Reliable and cost-effective diagnostic tools are essential for parasite surveillance and control, yet preservation methods can influence diagnostic accuracy. Coproscopy remains the gold standard for morphological identification of GI parasites, however time constraints often require samples to be preserved for later analysis. Faecal samples are conventionally stored in 10% buffered formalin for microscopy, and a counterpart stored in ethanol if molecular analysis is required, however recent studies have suggested that ethanol may serve as a suitable preservative for both. This controlled study evaluated the suitability of 10% buffered formalin and 70% ethanol for preserving strongyle-type eggs over a three-month period and compared parasite recovery between two McMaster quantification techniques: the modified McMaster (McMaster1) and McMaster2 methods. Recovery of strongyle- type eggs was 8.5-fold higher in formalin-preserved samples than in ethanol-preserved counterparts using the McMaster1 method (95% CI 6.1–11.7), and 5.0-fold higher using McMaster2 (95% CI 3.6–6.9). In addition, the morphological integrity of eggs was better maintained in formalin, with reduced damage and clearer visualisation of diagnostic features. These findings demonstrate that 10% buffered formalin performs better than 70% ethanol for both structural integrity and recovery of strongyle-type eggs for coproscopic quantification and identification. The McMaster2 method yielded significantly more strongyle-type eggs than the McMaster1 method in both mediums, but the improvement was negligible in formalin (4.7%) compared with ethanol (69.4%).

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