An overview of calf diarrhea - infectious etiology, diagnosis, and intervention

Cho, Yong-il; Yoon, Kyoung-Jin

Fecal flotation and direct microscopy are commonly used to diagnose parasite eggs or oocysts. The principal of fecal flotation is simply based on the density difference between a flotation solution (≥1.24) and oocysts (1.05∼1.24) [5] . A centrifugation step is commonly included in the testing procedure to increase detection sensitivity since centrifugation concentrates the target for easy viewing under a microscope. Direct microscopy can also be performed for fecal smears without centrifugation. Oocysts in clinical specimens may be difficult to visualize without special staining. C. parvum oocysts are reported to be positive for acid-fast staining [93] . Modified acid-fast stains are applied to fecal smears to detect these organisms. Unlike the Ziehl-Neelsen modified acid-fast stain, the modified Kinyoun acid-fast stain contains a more concentrated fuchsin dye and lipid solvent, and does not require heating the reagents used for staining [79, 131] . In brief, one to two drops of feces is smeared on a clean glass slide and air-dried. The sample is fixed with absolute methanol, and subsequently stained with carbol fuchsin and 1% sulfuric acid. The specimen is then counterstained with methylene blue or brilliant green and examined under a light microscope with oil immersion. The red or purple stained C. parvum oocysts 4 to 6 μm in diameter should appear against a blue or green background. This modified acid-fast staining method is widely used to detect C. parvum in feces. The sensitivity of this technique is low because the procedure requires approximately 500,000 oocysts per 1 g of feces to confirm the presence of C. parvum oocysts [3] .