2017 Station ReportUniversity of Maryland

DETERMINING ANTHELMINTIC RESISTANCE ON SHEEP FARMS IN THE SOUTHEASTERN US

S. SCHOENIAN1, D. O’BRIEN2 AND N. WHITLEY3

1UNIVERSITY OF MARYLAND, 2VIRGINIA STATE UNIVERSITY, 3FORT VALLEY STATE UNIVERSITY

This study was made possible by funding from the Let’s Grow committee of the American Sheep Industry Association (ASI).

Introduction• Gastro-intestinal nematodes are the primary health problem

affecting small ruminants in warm, moist climates and during periods of summer rainfall.

• Worms have developed varying degrees of resistance to alldewormers and dewormer classes.

• Resistance varies by geographic region and farm.

• Anthelmintic resistance has not been evaluated in the Southeast for more than five years.

• It is recommended that producers test for anthelmintic resistance every 2-3 years.

• There are two primary methods for determining anthelmintic resistance: FECRT and DrenchRite® larval development assay.

Materials and Methods

• ASI’s Let’s Grow Program funded a project to determine anthelmintic resistance on 30 commercial sheep farms in the southeastern US.

1. Maryland, via University of Maryland2. Virginia, via Virginia State University3. Georgia, via Fort Valley State University

• The project cost-shared the DrenchRite® test for producers.• $225 paid by Let’s Grow funds

• $225 paid by producer

DrenchRite® larval development assay (LDA)

• In vitro test for determining anthelmintic resistance.

• Determines anthelmintic resistance to all dewormer classes from a single pooled fecal sample.

• Sample 10-15 animals (FAMACHA© 3,4,5)

• Minimum FEC of 500 epg

• Available through Dr. Ray Kaplan’s lab at the University of Georgia (jscb@uga.edu).

DrenchRite® larval development assay

• Eggs are isolated from sample and cultured to L3s

in specialized assay plates in the presence of

varying (doubling) concentrations of the drugs.

• The critical well is the number of the well in which

50% of the eggs were inhibited from developing to L3.

• Critical well values which correlate to fecal egg

count reductions are used to determine resistance.

• Resistance is present if the percentage reduction in fecal egg count is less than 95%.

• Data from ivermectin wells is used to determine moxidectin resistance, since both drugs are

in the same class.

Results (n=26)

500

10850

8800

3900

6400

7950

1650

2650

1000

7100

1500

6500

500

2850

5000

600

4200

2750

6250

1150

3450

900

6300

3600

7850

500

53

100 99

93 9397 96

84

94

85

54

90

38

72

97

52

92 9288

97

91 91 91

59

96

42

0

2000

4000

6000

8000

10000

12000

0

10

20

30

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100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

FEC % HC

ResultsPercent farms with anthelmintic resistance (<95% FECR)

100% 92.3% 42.3%84.6%

0

10

20

30

40

50

60

70

80

90

100

Benzimidazoles Ivermectin Moxidectin Levamisole

MarylandVirginiaGeorgia

Interpretation of test results

Depending upon level of resistance,

actual efficacy may vary from 0% to as high as 95%.

Farm #5

BENZIMIDAZOLESBZ

1. Fenbendazole(SafeGuard®, Panacur®)

2. AlbendazoleValbazen®

IVERMECTINIVM

1. IvermectinIvomec®

2. DoramectinDectomax®

3. EprinomectinEprinex®

MOXIDECTIN MOX DD

Cydectin®

LEVAMISOLELEV

Prohibit®Leva-med®

Conclusions

• All farms had resistance to one or more dewormers.

• Some farms have resistance to all dewormerclasses.

• Resistance varies by geographic region and farm; worse the further south you go.

• Farmers should test for anthelmintic resistance.

• Due to widespread resistance, combination treatments are now recommended.

This study was made possible by funding from the Let’s Grow committee of the American Sheep Industry Association (ASI).