Other than the control test where a quantitative count is achieved, fecal egg count (FEC) results are a subjective measure, a snapshot in time, of the parasite level in the animal.
Jerry Woodruff, DVM, Boehringer Ingelheim Vetmedica, Inc., says there are several factors that, if not taken into consideration when interpreting FEC and fecal egg count reduction test (FECRT), may lead to an incorrect conclusion. Those factors are:
1. Seasonality of inhibited Ostertagi—since this most economically significant parasite of cattle is known to arrest development of early larval forms in the animal by hibernating in the abomasal lining during hot summer months in southern climates and cold winter months in more northern climates, inhibited Ostertagi would not be detected by standard analysis of fecal samples.
2. Small or limited sample size—FECs vary greatly within a group of animals, so if an inadequate number of samples are analyzed, a true picture of the parasite level is not obtained. It is recommended to gather 20 samples or 10% of the group in herds larger than 200 head.
3. Sample source—Fecal samples are best obtained by rectal grab rather than picking them up off the ground. Errors in animal identification are minimized with rectal grab technique, which is especially important in FECRT where first and second sample need to be from the same animal. Samples can be picked off the ground only if fresh, no dirt or debris is gathered, and if the animal is positively identified.
4. Vastly different egg producing ability of GI parasites (fecundity)—some parasites are prolific egg producers (Cooperia, Haemonchus) while others (Nematodirus) are not as prolific.
5. Diet and consistency of feces—watery, diarrheal feces may dilute the parasite eggs being shed, such that an underestimation of parasite level is made.