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Veterinary Forum April 2007 (Vol 24, No 4)

Clinical Report: "Fecal Centrifugation"

by Sophia Yin, DVM, MS (Animal Science)

    Dogs and cats commonly carry intestinal parasites that can pose a major zoonotic threat to humans. According to the Centers for Disease Control and Prevention (CDC), a survey of dogs in animal shelters revealed that almost 36% harbor such helminths as hookworms, roundworms and whipworms. In addition, an estimated 10,000 human cases of Toxocara infection caused by either Toxocara canis or T. cati occur annually in the United States, and each year more than 700 people infected with Toxocara organisms experience permanent or partial loss of vision (source: www.cdc.gov/healthypets/diseases/toxocariasis.htm).

    Veterinarians may routinely screen for zoonotic parasites using fecal flotation tests. However, if a passive fecal flotation is conducted, zoonotic infections are likely being drastically underdiagnosed.

    The need to centrifuge

    Anne Zajac, DVM, PhD, associate professor of parasitology at Virginia-Maryland Regional College of Veterinary Medicine, Virginia Tech, indicated that "the biggest mistake made when conducting fecal flotation is failing to use a centrifuge to spin the samples. Use of a passive bench-top incubation system provides considerably less-sensitive results." According to Zajac, although the zinc sulfate solution used in some of the passive tests was originally described in the 1930s and 1940s for use in a centrifugation technique, no studies have been conducted on its sensitivity in the passive flotation test.

    With the standard flotation technique, feces are mixed using a solution with a specific gravity that is higher than that of common parasite eggs so that the eggs float to the top of the mixture. With the centrifugation technique, the mixture is then centrifuged to increase the efficiency of the flotation. Both Zajac and Michael Dryden, DVM, MS, PhD, professor of veterinary parasitology at Kansas State University College of Veterinary Medicine, have compared the passive and centrifugation techniques and have achieved similar results.

    "We have found that passive flotation techniques can miss up to 32% of positive whipworm samples, greater than 50% of all positive Coccidia and Taenia tapeworm eggs and up to 25% of positive roundworm samples," Dryden explained. The difference is striking to anyone who is making the comparison. "At wet labs held during the NAVC Conference, participants frequently found more eggs and egg types under one low-power-field than they did under the entire coverslip of a passive testing procedure. It's not just a matter of finding more eggs," he emphasized. "Centrifugation also allows us to confirm a greater number of positive samples."

    Not really a time-saver

    Practitioners frequently elect to conduct simple flotation with or without a commercial test kit because of the perceived timesavings. Although the centrifugation procedure includes an extra step compared with the commercial test kit procedures, the overall test time is about the same. Feces should be centrifuged for 5 minutes at 1,200 rpm using a fixed- or swing-head centrifuge. The solution should stand for another 5 to 10 minutes before the coverslip is examined under a microscope — first under 10x and then at higher power if needed. The overall time is 10 to 15 minutes, whereas with passive flotation, the samples must stand for 15 minutes.

    Another problem with commercial assays is that they encourage examination of small fecal samples. One gram is the standard recommendation, but both Zajac and Dryden recommend collecting larger amounts. "Five grams of feces, which is about 1 teaspoon, is ideal," Zajac said. "Because of the small sample size, a negative result for a fecal loop sample is meaningless."

    In addition, the type of solution used makes a difference. Dryden's research has shown that the Sheather's sugar solution is better able to recover Taenia tapeworm eggs because of its higher concentration.

    So what do these findings translate to in private practice? "During the past couple of years," Zajac said, "practitioners have told me they had no idea there were so many parasites out there until they started doing centrifugation flotation."

    Dryden's experience is similar: "Practitioners who have switched from passive to centrifugation [testing procedures] have reported an increase of 50% to 100% in positive fecal testing results."

    For more information:

    Dryden MW, Payne PA, Ridley RK, Smith VE: Gastrointestinal parasites: the practice guide to accurate diagnosis and treatment. Compend Contin Educ Pract Vet 2006;28:7(A)suppl3-3.

    Dryden MW, Payne PA, Ridley RK, Smith VE: Comparison of common fecal flotation techniques for the recovery of parasite eggs and oocysts. Vet Ther 2005;6(1):15-28.

    Zajac AM, Johnson J, King SE: Evaluation of the importance of centifugation as a component of zinc sulfate fecal flotation examinations. JAAHA 2002;38(3):221-224.

    CDC: www.cdc.gov/healthypets/diseases/toxocariasis.htm.

    NEXT: Dental Dilemma: "Full-Mouth Extraction in a Dog with Multiple Dental Anomalies"

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