Evaluation of Streck tissue fixative, a nonformalin fixative for preservation of stool samples and subsequent parasitologic examination

We undertook a study to evaluate Streck tissue fixative (STF) as a substitu te for formalin and polyvinyl alcohol (PVA) in fecal preservation. A compar ison of formalin, PVA, (mercuric chloride based), and STF was done by aliqu oting fecal samples into each fixative, Stool specimens were collected in H aiti, and parasites included Cyclospora cayetanensis, Giardia intestinalis, Entamoeba coli, lodamoeba butschlii, Endolimax nana, Ascaris lumbricoides, Trichuris trichiura, Strongyloides stercoralis, and Necator americanus. Pr eserved stools were examined at various predetermined times (1 week, 1 mont h, and 3 months) to establish the quality of the initial preservation as we ll as the suitability of the fixative for long-term storage. At each time p oint, stool samples in fixatives were examined microscopically as follows: (i) in wet mounts (with bright-field and epifluorescence microscopy), (ii) in modified acid-fast-, trichrome-, and safranin-stained smears, and (iii) with two commercial test kits. At the time points examined, morphologic fea tures remained comparable for samples fixed with 10% formalin and STF, For comparisons of STF- and 10% formalin-fixed samples, specific findings showe d that Cyclospora oocysts retained full fluorescence, modified acid-fast- a nd safranin-stained smears of Cryptosporidium and Cyclospora oocysts were e qual in staining quality, and results were comparable in the immunofluoresc ence assay and enzyme immunoassay commercial kits. Stool fixed in STF and s tained with trichrome showed less-than-acceptable staining quality compared with stool fixed in PVA, STF provides an excellent substitute for formalin as a fixative in routine examination of stool samples for parasites. Howev er, modifications to the trichrome staining procedures will be necessary to improve the staining quality for protozoal cysts fixed in STF to a level c omparable to that with PVA.