An inherent problem with FEC EBV is having a sufficient population of pasture parasites to generate a FEC in your flock. The timing of lambing, climate and production scheme can affect the presence of infective worm larvae. These complications limit our ability to identify parasite resistant lambs using FEC, particularly at weaning.

Weaning or post-weaning FEC EBV are based on samples collected from animals that have had an opportunity to be exposed to strongylid parasites (e.g.: Haemonchus, Trichostrongylus and Telodorsagia). Each of these parasites have their own ideal climatic conditions, but generally they desire warm and humid environments. Often these conditions are not present when weaning or post-weaning fecal samples are being collected. This condition occurs with winter or early spring-born lambs and affects producers’ ability to collect weaning fecal samples.

Lack of a weaning fecal sample is not the “end of the world.” Collection of a post-weaning fecal sample will provide data to produce a PFEC EBV and through genetic correlation, a WFEC can be calculated. This is possible if lambs are retained long enough to collect a post-weaning sample. Retaining lambs to just collect a post-weaning fecal sample will impact potential sales and increase cost, thus the only lambs retained are those meant to become replacements. To address this problem, you’d need to align your lambing season to ensure pre-weaning lambs exposure to strongylid parasites. For seedstock producers, late spring-born lambs are too small for most seedstock sales. What to do?

To study parasite infection in sheep, we “artificially” infect lambs with Haemonchus contortus. Most studies use 10,000 infective stage larvae on lambs post-weaning to evaluate parasite resistance. This is a proven means to demonstrate differences in immune responses generated by parasite resistant sheep. Could this methodology be used in seedstock operations where climate, production system or lambing time limit natural pasture exposure to strongylid parasites?

The Haemonchus larvae we culture at WVU were initially isolated from a ram that died unexpectedly (there’s a good story behind this – ask me sometime and I’ll tell you!). For over 10 years we have used these larvae for a variety of experiments, provided infective larvae for ram and buck performance tests, and offered to researchers across the US for their experiments. These larvae are susceptible to levamisole (Prohibit™), but resistant to macrolides (Ivomec ™ or Cydectin ™) and benzimidazoles (Valbazen™).

This year we have marketed larvae to producers in climates that don’t support worm infection in the spring, who were aiming to enter rams into performance tests and wanted to be able to generate WFEC EBVs. Here we provided a producer with sufficient larvae to infect 18 lambs with a dosage rate of 1,000 Haemonchus contortus larvae per 10lbs of body weight. We had the producer wait 5 weeks until a fecal sample was collected. Of the 18 samples we were only able to evaluate 15 but those samples had an average FEC of 2,760 eggs/g and ranged from 200 to 10,200 eggs/g.

The benefit of an artificial infection is that a standard dose given to each animal and the knowledge that each lamb was exposed to larvae. Additionally, using an artificial infection excludes the impact of grazing behavior on FEC, and tests a lamb’s ability to innately resist infection. This approach is a perfect fit for weaning FEC EBV. The intent behind the WFEC EBV was to assess innate immunity to parasite infection. This artificial infection does exactly that and still allows producers to collect post-weaning fecal samples on naturally infected replacement lambs.

There are downfalls to artificial infection, mainly, it’s not the same as natural infection. It’s reasonable to expect that spring-born lambs, in the correct environment, would be constantly exposed to worm parasites prior to weaning. By weaning, those lambs would have had opportunity to develop some resistance to these parasites. Additionally, using lab-grown larvae introduce new parasites to your operation. If you decide to keep them grazing during the artificial infection this may affect anthelmintic resistance of worms on your pasture.

A WFEC EBV can be a valuable component of your selection priorities and if you raise sheep in an environment where it is difficult to generate an infection in lambs prior to weaning, this may be a useful tool to generate a WFEC EBV.

By: Scott Bowdridge, Ph.D

Professor and Livestock Extension Specialist, West Virginia University