A study to determine if anthelmintic-resistant internal parasites exist in Vermont sheep flocks

Final Report for ONE06-061

Project Type: Partnership
Funds awarded in 2006: $10,000.00
Projected End Date: 12/31/2007
Matching Non-Federal Funds: $1,305.00
Region: Northeast
State: Vermont
Project Leader:
Chet Parsons
University of Vermont Extension
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Project Information


This study was carried out to determine if internal parasites (IP) of sheep were resistant to dewormer medications (anthelmintics) on four Vermont sheep farms. These farms were chosen based on deworming results from the previous year. Twenty sheep on farm A and B and ten sheep on farm C and D were weighed in June of 2006 and a fecal sample was collected from each sheep. One half of the sheep were treated with fenbendazole and one half of the sheep were treated with ivermectin as recommended by the Vermont State Veterinarian. Fecal samples were sent to a parasitology lab to determine fecal egg counts (FEC). Fecal samples were obtained from the same sheep two weeks later and sent to the same pasasitology lab to determine FEC. The results of the FEC were compared to determine if the dewormer was effective in reducing the number of IP eggs in the sheep feces. From these results, farm A, B and D, did not appear to have IP that were resistant to anthelmintics. The results from farm C would suggest that some IP on that farm were resistant to anthelmintics. Future studies need to be done during the month of August to determine if a higher incidence of IP infectivity exist latter in the summer and if there is greater resistance to anthelmintics at that time.

Four classes were held for sheep producers in 2007 on the control of internal parasites in sheep. Information gained from the study as well as current information on controlling IP of sheep was provided to producers with the assistance of two veterinarians from England. These classes also include training in the use of the FAMACHA card that was developed in South Africa to control anthelmintics resistant IP in sheep and goats. This grant has provided the foundation for continued education in parasite control in sheep and goats.


Vermont and other Northeast states are in an ideal position to supply sheep products to the large market that exists in the Eastern United States. Although it is difficult to compete on the commodity market, supplying a specialty market that pays a premium price is financially viable. Marketing organizations such as Vermont Quality Meats, Vermont Fancy Meats, Vermont Shepherd, Green Mountain Spinnery and others, have been successful in supplying these markets with lamb, wool and ewe’s milk cheeses.

As the demand for these products increases, there is a greater pressure on producers to increase the supply. As the sheep numbers become more concentrated, the greater is the risk of sheep, especially lambs, of becoming infected with internal parasites (IP). This is especially true of lambs that are grazed on pasture. Internal parasites lay eggs that are passed out of the animal in the manure. When ideal conditions exist, these eggs hatch into larvae on pastures. As animals graze and ingest infective larvae, their number of internal parasites can increase to the point of being detrimental to the animal. Animals infected with large amounts of IP do not grow well and start to show signs of ill thrift. In some cases, infections can become so great that they will kill the animal. Control of IP is the most important health-related concern facing the sheep industry (Kaplan, 2004).

To prevent IP from causing a problem, there is or is going to be more reliance on anthelmintics (dewormers) to control sheep IP. Unfortunately, after constant use of an anthelmintic, the IP may become resistant to the active drug. Even though there are several drugs available, after extended use, parasites can become resistant to all drugs used (Kimberling, 1988). Literature reviews indicate that some of the major sheep producing countries such as Australia and New Zealand, are experiencing resistance to all of these drugs (Larson, 2005; Scott, 2005; Kaplan, 2004; McKellar, 1997)

Literature Review
Kaplan, R. M., Parasites, Their Resistances and Alternate Methods. 2004. Proceedings: 7th World Sheep and Wool Congress. Quebec City, Quebec, Canada, 2004. p 285-293

Kimberling, C.V., Jensen and Swift’s Diseases of Sheep, 3rd Ed., 1988. p 91-92.

Larsen, J.W.A., N. Andersen, J. Webb Ware, and C. de Fegely. The Productivity of Merino Flocks in South-Eastern Australia in the Presence of Anthelmintic Resistance. Proceedings: 6th International Sheep Veterinary Congress, Hersonissos, Crete, Greece, 2005. p 79-83.

McKellar, Q. A., Control of Parasitism: the Global Perspective. Proceedings: Sustainable Control of Internal Parasites in Ruminants, Animal Industries Workshop, Lincoln Univeristy, New Zealand, 1997. p 1-10.

Project Objectives:

The objectives of this study were to test a sample of sheep on four different Vermont sheep farms to determine if the internal parasites of the sheep were resistant to two different commonly used anthelmintics (dewormers).
The producers from the study and 30 sheep and goat producers will learn how to control internal parasites in sheep and goats and how to control internal parasites from becoming resistant to anthelmintics.


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  • Harvey Levin
  • Dave Martin
  • Kathryn Morse
  • Kerry Rood
  • Erica Zimmerman


Materials and methods:

Five percent of the sheep in four participating flocks were identified and weighed three weeks after they had been grazing on pasture (June, 2006). A rectal fecal sample was taken, by digital manipulation, at that time. One half of the sheep identified were treated with the anthelmintic that the producer had been using and one half was treated with another family of anthelmintics. The recommended dose of anthelmintic was used based on the weight of the animal. Seven days later, a second fecal sample was taken from the sheep identified and previously treated. Fecal samples were placed on ice in the field and shipped to Myers Parasitology Services in Magnolia, Kentucky, for fecal egg counts (FEC) for gastrointestinal nematodes (IP). Results of the FEC were shared with respective producers and recommendations were made on their future management methods. The Vermont State Veterinarian reviewed the results of the laboratory reports and concluded that one of the farms had internal parasites that were resistant to chemical dewormers. Four workshops were held to report findings and educate sheep and goat producers how to manage their animals to minimize the occurrence of internal parasites that are resistant to anthelmintics. Training on the use of FAMACHA cards was incorporated into the workshops.

Research results and discussion:

Testing of sheep on the four test farms did not indicate large numbers of internal parasites, nor did it indicate large numbers of internal parasites that are resistant to anthelmintics. The four farms were selected base on conservations with the owners that would indicate that the anthelmintics they were using were not working. It may make sense in future studies to pre-test selected farms to determine what the worm load is on their respective animals before the actual tests start. Results may differ if all of the farms in the test experienced large numbers of internal parasites. Also, delaying the sampling until August may have shown a larger number of internal parasites. Response to the workshops was higher than anticipated. One could assume that the producers realize that a problem with anthelmintic resistant internal parasites could exist and they are seeking out the information that is need to prevent this from happening. There was also a great interest in the FAMACHA system of determining if animals have a high degree of parasitism based on the animals showing anemia. This interest would indicate that there is a need for future workshops.

Research conclusions:

The greatest impact was the unanticipated turnout for the parasite workshops. Over fifty producers were educated in internal parasite management and trained to use the FAMACHA card.

Participation Summary

Education & Outreach Activities and Participation Summary

Participation Summary:

Education/outreach description:

A five page handout was produced to accompany the workshops entitled, “Understanding Internal Parasites of Sheep and How to Control Them.” This publication will also be used for future classes on internal parasites. Fifty producers attended workshops on how to manage internal parasites.

Project Outcomes

Project outcomes:

Economic analysis could only be inferred from this study. However, if the producers only treated the animals that needed to be treated, this would be an immediate savings in purchased anthelmintics. The long term savings by preventing animals from becoming infected with resistant internal parasites could be considerable.

Farmer Adoption

Fifty farmers adopted the use of the FAMACHA card to use as a tool to control internal parasites.

Assessment of Project Approach and Areas of Further Study:

Areas needing additional study

In conversations with a local veterinarian, there are still producers who are losing sheep to internal parasites. Typically, the parasite that is responsible for deaths in sheep is Haemonchus contortus (sheep barberpole worm). As this is usually considered a parasite of hot, humid regions, e.g. Southeastern United States, a study to determine the degree of infectivity of this species in Northern United States is needed. With information from such a study, workshops or classes could be present to educate more of the sheep and goat producers in the northern states such as Vermont, New Hampshire and Maine.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.