Formalin-fixed stool improves the performance of the Kato-Katz method

Abstract

Background and aim: Parasitic infections are a public health problem worldwide, including in Thailand. An epidemiological survey for helminthiasis based on stool examination uses the Kato-Katz method as recommended by the World Health Organization. Limitations of this method include the need for fresh stool, time requirement, and lack of quality control. The aim of this study was to enhance the efficiency of the Kato-Katz technique using formalin and glycerol solutions and to implement specimen preparation in fieldwork.

Materials and methods: For the Kato-Katz method, stool samples were divided into formalin-fixed and unfixed groups at various time points and processes. Fresh echinostome eggs were added to each stool group. Incubation with glycerol increased the clearing process. Each group was observed and photographed using a light microscope. Parasite eggs were imaged and compared using the standard Kato-Katz method.

Results: Visualization of echinostome eggs from formalin-fixed stool slides was significantly better than that from unfixed stool slides (p < 0.01). Stool samples fixed for 7 days retained normal echinostome eggs morphology. Incubation with glycerol for 1 h resulted in increased Kato-Katz performance by digesting the stool content and enhancing egg observation. Moreover, the results of the Kato-Katz method using fixed and fixed stool plus glycerol for natural helminth infection showed good quality of Opisthorchis viverrini and Taenia egg visualization and normal morphology with a clear background of slides.

Conclusion: Formalin-fixed stool could be more suitable than fresh stool for the Kato-Katz method.

Keywords: Kato–Katz method; formalin-fixed stool; glycerol; parasitology examination; specimen preparation.

Figure-1

The diagram of all assigned protocols.

Figure-2

The scoring of Kato–Katz slide, (a) +1: The fecal particles were larger than +2, (b) +2: The fecal particles were smaller than +1, (c) +3: The fecal particles were smaller than +2.

Figure-3

(a-p) The appearance of echinostome egg in various pH 3, 5, 7, and 9 using simple smear and Kato–Katz technique, respectively.

Figure-4

(a-l) The appearance of echinostome egg from Kato–Katz slide using unfixed stool at various time points; Egfg (E).

Figure-5

The appearance of echinostome egg from Kato–Katz slide using (a-c) unfixed stool and (d-f) formalin-preserved stool at 1, 3, and 6 h; Egg (E).

Figure-6

The appearance of echinostome egg and stool particle of Kato–Katz slide using (a-g) formalin-preserved stool and (h–n) formalin-preserved plus glycerol; Egg (E).

Figure-7

The appearance of Taenia egg and stool particle of Kato–Katz slide at 1^st h using (a) unfixed stool, (b) formalin-preserved stool, and (c) formalin-preserved plus glycerol; Egg (E).

Figure-8

The appearance of Opisthorchis viverrini egg and stool particle of Kato–Katz slide at various time point (1^st, 9^th, 24^th, 36^th, and 48^th h) using unfixed stool (a-e) formalin-preserved stool (f-j) and formalin-preserved plus glycerol (k-o); Egg (E).