Detection of Cryptosporidium oocysts in fresh calf faeces: characteristics of two simple tests and evaluation of a semi-quantitative approach

Abstract

The objective of the present study was to evaluate the characteristics of two rapid tests, namely, a faecal smear staining method (Heine staining) and a commercially available immunochromatographic (IC) assay, against a direct immunofluorescent antibody test (IFAT) for the diagnosis of Cryptosporidium infections in 917 faecal samples from calves aged <3 weeks. These rapid tests were performed on non-concentrated faeces using a semi-quantitative approach using a 6-point scale (0-5) for Heine staining according to the number of oocysts per microscopic field, and a 4-point scale (0-3) for the IC assay reflecting the intensity of the positive line compared to the control line. Direct IFAT was performed following a diethyl ether concentration and results were expressed as oocysts per g of faeces (opg). Heine staining showed a sensitivity of 76.7% and a specificity of 90.7%. For faecal samples with ≥ 10,000opg, sensitivity increased to 90.0%. The sensitivity of the IC assay was lower (61.8%) but the specificity was 100%. For faeces with ≥ 100,000opg, the sensitivity of the IC assay reached 81%, indicating some limitation for clinical cryptosporidiosis diagnosis. Additional scoring (1-5) of the Heine staining correlated with the corresponding direct IFAT results, particularly for ranges of 1000 to >1,000,000opg. Additional scoring from 1 to 3 according to the thickness of the sample line for the IC test correlated with increasing levels of Cryptosporidium opg measured by IFAT in the range of 10,000 to >1,000,000opg. In conclusion, both Heine staining and the IC test can be reliably used through a simple semi-quantitative scale for grossly quantifying oocyst output in calf faecal samples.

Keywords: Calf; Cryptosporidiosis; Cryptosporidium; Diagnostic methods; Faeces.