Читать книгу Infectious Disease Management in Animal Shelters - Группа авторов - Страница 107

A fecal examination is a primary tool for the diagnosis of endoparasitism in companion animals and typically consists of both direct fecal smears and flotation concentration techniques. (Note that fecal antigen detection for select pathogens is also commercially available as discussed in the section on ELISA testing found under primary diagnostic testing.) The direct fecal smear—microscopic evaluation of a small particle of feces mixed with saline—is ideal for the evaluation of delicate nematode larvae (e.g. Aelurostrongylus spp., Ancylostoma spp., Filaroides spp., Strongyloides spp.) and protozoan trophozoites (e.g. Giardia spp., Tritrichomonas spp.) (Bowman 2014) (Table 4.3). Direct smears also allow for the evaluation of organism motility and identification of organisms that are easily distorted by flotation solutions. Due to the small sample size evaluated, direct fecal smears have low sensitivity. That is, while a positive fecal smear can confirm endoparasitism, a negative fecal smear does not rule out infection. Fecal smears can also be stained for more detailed examination and parasite identification, though motility will be lost with such preparation (Zajac and Conboy 2012).

In most cases, direct fecal smears should be performed in tandem with fecal flotation techniques. Such techniques allow for the separation of fecal and food material, fats and dissolved pigments, and parasite eggs and cysts based on the density of the suspension liquid. Flotation techniques that involve centrifugation of the sample will result in higher sensitivity than those that rely on passive gravitational suspension of eggs. This is most important when low numbers of parasites are expected (e.g. Giardia spp., Trichuris spp.). If a passive flotation method is pursued, sodium nitrate (e.g. Fecasol^®, Vetoquinol, Fort Worth, TX) or saturated salt solutions should be used. Though they are ideal for centrifugation techniques, the high viscosity of sugar solutions and the lower specific gravity of zinc sulfate solutions can impede the passive flotation process (Zajac and Conboy 2012). In the absence of centrifugation or proper flotation solutions, the likelihood of inaccurate test results warrants consideration of additional diagnostic testing or empirical treatment of patients exhibiting clinical signs consistent with parasitic infection.

Once the sample is prepared and the surface layer transferred to a coverslip, the sample should be scanned under the 10× objective lens of a microscope. To maximize the contrast between parasites and background debris, the microscope condenser should be lowered and the light intensity and diaphragm should be reduced. Samples prepared in sodium nitrate solution should be evaluated immediately to avoid distortion of any parasites and crystallization of the preparation (Bowman 2014; Zajac and Conboy 2012).