Diagnosis of intestinal parasites through examination of fresh faecal samples is hampered by its unpleasantness and the urgent need to detect all parasitic forms. In this paper, we compared the standard Kato-Katz (KK) technique with a traditional fixation method, the merthiolate-iodine-formalin (MIF) method. Two hundred and twenty-seven faecal samples from individuals living in a rural setting in Venezuela with high to moderate prevalences of Ascaris lumbricoides (Al), Trichuris trichiura (Tt) and hookworm infections were examined. The 'gold standard' used here was derived from the combination of the outcomes from both methods. KK performed better at detecting Tt, and showed higher sensitivity and negative predictive value for both Tt and Al, probably due to a higher capacity of KK to detect low parasite loads. Both methods showed an almost perfect agreement using the Kappa index. MIF provided a higher median of parasitic loads for low and total egg counts for the three helminths. Differentiating fertile from infertile eggs of Al did not affect the results; infertile eggs were present only at low and intermediate parasitic loads, but absent at high loads. KK was not able to detect high loads of any of the helminths. MIF allowed for the detection of other helminths, such as Strongyloides stercoralis, and protozoan infections, for which KK is not specific. In conclusion, MIF is a simple and inexpensive technique that performs competitively with KK in both laboratory and field work on intestinal helminths, particularly in resource-limited settings.

Soil-transmitted helminths, such as Ascaris lumbricoides, are the most prevalent parasites globally. Optimal anthelmintic treatment for A. lumbricoides in endemically infected communities is challenged by several host-related and environmental factors influencing infection acquisition. We assessed the risk of A. lumbricoides (re)infection after treatment in a Venezuelan rural community. Individual merthiolate-iodine-formaldehyde-fixed faecal samples were collected from 224 persons before a single-dose pyrantel treatment and at 1, 3, 6, 9 and 15 months after treatment. Effects of age, sex and socioeconomic status (SES) on A. lumbricoides prevalence, eggs/gram faeces (EPG) and infection (re)acquisition were assessed using both generalised linear mixed-effects models and survival analysis. Pre-treatment A. lumbricoides prevalence was 39.7%. Higher prevalence was associated with younger age and lower SES. Higher EPG values were observed among young children. Median time to A. lumbricoides infection was six months after treatment: at 1, 3, 6, 9 and 15 months post-treatment, cumulative incidence was 6.7%, 18.9%, 34.6%, 42.2%, and 52.6%, respectively. Younger age, lower SES, and pre-treatment A. lumbricoides infection status showed significantly elevated hazard ratios. Mass drug administration protocols would benefit from considering these factors in selective treatment strategies and possibly more than just annual or biannual treatments in the target population.