Composite Sampling Technique to Estimate Worm Burdens in Cattle Herds and Sheep Flocks
By Dr. Jeba Jesudoss Chelladurai
Gastrointestinal worms infect grazing livestock throughout the world. In the United States, we see significant production loss due to the ‘barber pole worm’ Haemonchus in sheep and the intestinal worm Cooperia in cattle. These parasites pose an economic challenge to producers due to their ability to become resistant to broad-spectrum dewormers (anthelmintics). There has been a recent increase in the number of reports of anthelmintic resistance in pastured animals. Deworming of entire herds/flocks is common but must be discouraged as this practice eliminates only dewormer-susceptible worms, leaving behind dewormer-resistant worms. Parasite control treatments must be strategically and selectively given to the animals that have the highest worm burden in order to make the best use of money and to ensure that dewormers will still be effective in the coming years.
Animals that need treatment can be identified using a quantitative fecal test which gives results expressed as worm eggs per gram of feces. Egg counts can help estimate the worm burden in the animal, monitor pasture contamination, determine the effectiveness of the dewormer used and even help identify the animals with the best worm-resistance genes for breeding.
Egg count tests on fecal samples can be performed in-clinic or at a diagnostic lab such as KSVDL. The procedure starts with a pre-weighed amount of fresh feces (usually about 3 grams per animal), performing a fecal centrifugation technique using a sugar floatation solution (with 1.27 specific gravity), counting the number of eggs and dividing by the starting amount of feces (three grams) to obtain the number of eggs per gram of feces.
Parasites tend to be over-dispersed in the herd/flock. Typically, a few animals have a majority of the parasites and a majority of the animals have a few parasites. While it is good to perform egg counts for each individual animal in the herd, it is often an expense that the producer does not want to spend money on. A solution to this is the composite sampling method. Composite sampling egg counts of the herd/flock has been shown to have high agreement with individual sampling egg counts in a recent study1.
The steps to performing a composite fecal sampling are:
- Wear rectal gloves and collect fresh fecal samples from each animal’s rectum from groups of 15 animals. If samples cannot be collected from the rectum, a group of animals may be quietly cornered to a clean corner of a pen for 15-30 minutes and then allowed to wander away. Feces on the ground must be picked up immediately.
- Mix each individual sample with a tongue-depressor stick or by squeezing the rectal glove for 15 seconds to homogenize.
- Weigh one gram of feces from each individual sample with the help of a small weighing scale and place into a container/zip-lock bag labelled to hold the composite sample. The final composite sample should be 15 grams total (from 15 animals).
- Mix the composite sample with a tongue-depressor stick or by squeezing the zip-lock bag for one minute to homogenize it completely.
- Perform the fecal egg count test in-clinic or mail the sample to a diagnostic lab. Request a quantitative fecal exam in the diagnostic lab form and also indicate that the sample is a composite sample.
- If there is a delay in the mailing or in-clinic testing, store fecal samples at refrigerator temperature (4°C) to prevent worm egg development and hatching.
Results will be reported as worm eggs per gram of feces.
- George, M. M., Paras, K. L., Howell, S. B., & Kaplan, R. M. (2017). Utilization of composite fecal samples for detection of anthelmintic resistance in gastrointestinal nematodes of cattle.Veterinary parasitology,240, 24-29.
Jeba Jesudoss Chelladurai, BVSc, MS, PhD, is board certified by the American College of Veterinary Microbiology (parasitology) and a parasitologist in the KSVDL.