Avoiding Common Pitfalls in Microscopic Fecal Examinations
A “fecal” may seem like one of the more humble tasks performed in a veterinary hospital. That does not diminish the importance of this examination, which can provide valuable information on the health status of veterinary patients. Parasite diagnosis and monitoring are vital to pet health and the health of pet owners, given the zoonotic potential of many parasites.
Skill in the conduct and interpretation of fecal examinations is important if internal parasites are to be accurately diagnosed and effectively treated. The performance of reliable and accurate fecal examinations requires a knowledge of the procedures, a thorough familiarity with the important parasites of dogs and cats, and an understanding of how to use this information in a reliable parasite control strategy.
It’s easy to become complacent about parasite management—especially management of internal parasites. We now have highly effective products that prevent gastrointestinal parasites. Nevertheless, research indicates common canine and feline gastrointestinal parasites remain prevalent, due to everything from poor owner compliance to the limitations of “seasonal” prevention. Whatever the reason, the only way to monitor pets for the presence of gastrointestinal parasites is to conduct fecal examinations regularly and properly.
CAPC Guidelines Address Timing, Technique
I have been a member of the Companion Animal Parasite Council (CAPC) since it was formed in 2002. Fellow members include parasitologists from other veterinary colleges, as well as specialists in veterinary internal medicine, public health, veterinary practice and veterinary law. We share a common concern that current practices have not adequately addressed the prevention, treatment and monitoring of internal and external parasites.
How well a fecal examination is conducted can profoundly affect the health of veterinary patients and their families. It also can directly impact the success of a practice itself, particularly if prevention and diagnosis of parasites have been overlooked.
Surprisingly, a number of factors involved in fecal examinations can directly affect the accuracy of results. Consider some of the following examples:
Procedure type. One of the most important factors in proper fecal examination technique is the type of procedure employed. Fecal examination procedures likely to be accepted and implemented in most veterinary practices include flotation (centrifugal or simple), sedimentation and direct examination. While simple flotation is the most common examination procedure used in veterinary hospitals, it is not as sensitive and accurate as centrifugal flotation, the method recommended in the CAPC guidelines. The reason is that centrifugal flotation more effectively separates parasites from fecal debris and decreases the time required to do this.
In a recent study conducted at Kansas State University by CAPC member Dr. Michael Dryden, veterinary students were given positive roundworm and hookworm samples and asked to perform fecal examinations using one of three techniques: (1) direct smear, (2) simple flotation or (3) centrifugal flotation. In every instance, only centrifugal flotation techniques achieved an acceptable level of accuracy.1 In some cases, less than a third of the positive samples were identified using the other techniques.
Fecal sample size and consistency. Size matters, and too small a sample can compromise results. While fecal loops or rectal thermometers often are used to obtain samples, the average sample size obtained with these methods is only about one-tenth of a gram. In fact, the ideal sample size for testing is one gram of formed feces (a cube measuring approximately one-half inch on a side). Examination of feces containing a higher fluid content (soft, unformed or diarrheic feces) requires a larger sample, since liquid dilutes parasite eggs.
In a number of cases, sample size and consistency cannot be controlled. What’s important to remember is that the likelihood of false negatives can rise substantially when the sample size is inadequate.
Flotation solutions. The density of the different flotation solutions can affect parasite egg and larvae recovery. The desired specific gravity of a flotation solution is 1.18 to 1.20 g/mL; the range of densities of common canine and feline internal parasites is 1.06 to 1.20. It’s important to remember that whipworm ova are among the densest parasite eggs. If you do not prepare and maintain fecal flotation solutions properly, you run the risk of underdiagnosing infections.
Improving the Accuracy of Fecal Microscopic Examinations
What seems like an easy and commonplace exam in a veterinary hospital is actually a very complex diagnostic procedure that depends on a variety of factors for accuracy.