Use Laboratory Diagnosis to Determine Deworming Strategies

Wisconsin’s 2016 growing season was ‘good for grass’, as abundant rainfall and optimal temperatures supported forage production. For certain, internal parasites whose life cycle depends upon grass and grazing ruminants, proliferated in response to all that grass.

Efficacy of the most commonly used dewormers today is 59%, and history shows efficacy decreasing every year. Bottom line: 41% of your cattle’s worm may survive treatment if you routinely use a pour-on or injectable dewormer. Gone are the days of simply deworming cattle with a one-size fits all approach, while blissfully ignorant of the complexity involved. Seek veterinary guidance to develop a deworming protocol tailored to your farm’s management. Base your protocols on proper worm diagnosis, which involves performing fecal egg counts (FEC). The WI Modified Sugar Floatation is a preferred FEC method. Test kits used for detecting internal parasites of small animals are not sensitive enough for use with ruminant herbivores.

Life cycle stages of gastrointestinal worms (helminths or nematodes, like Ostertaia, Cooperia and Haemonchus) are found within cattle, in feces, and on grass. The worms need all three environments in which to complete their lifecycle; therefore, several opportunities exist for interrupting their growth. One such opportunity is afforded by Wisconsin’s winter.

Pre-puberty and adult worm stages reside inside cattle, and may overwinter in cattle, serving as a source of eggs defecated on pastures in the spring. Larval stages of the worm survive frozen pasture conditions and also serve as a source of infestation as the grass begins to grow in the spring.

Deworming products are administered to cattle in an attempt to kill worm stages found inside cattle. Deworming late in the fall after several hard freezes will help reduce the number of internal worm stages carried over winter by cattle, and any treated survivors will be depositing their eggs into frozen conditions. Helminth eggs do not survive freezing. Performing a laboratory fecal egg count reduction test (FECRT) along with species identification of the surviving worms will allow you to develop a treatment strategy for those internal, possibly dewormer resistant, survivors.

Deworming itself does not affect the worm stages out on grass at the time the cattle are treated; however, our management of treated cattle does affect what type of, and where, worm eggs are deposited. These resistant survivors probably originated from resistant pasture larvae, who will overwinter outside. FECRT and species identification of the worms over-wintering in cattle provides needed information for developing spring deworming plans for grazing cattle consuming these resistant larvae.

Allow dewormed stock to remain in the same area for at least a week after treatment. Once moved onto another paddock, larvae that survived treatment are providing resistant eggs to this new area. When stockpiled forages or bedding packs are used, resistant eggs may find themselves in micro-environments conductive to their hatching into resistant larvae.

Next spring allow your oldest stock to first graze areas you suspect resistant larvae may reside. Older animals are more tolerant of worms, and with FECs and species identification 30 days after turnout, you will have an indication of the pasture’s larvae infestation. Perform a 14-day post treatment FECRT to know the efficacy of your dewormer investment. To be considered efficacious, the FECRT should be 95% or better.

Written by: Sandy Stuttgen, DVM, UW Extension Agriculture Educator, Taylor County. This article recently appeared in The Wisconsin Agriculturist

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