This study tested for anthelmintic resistance on 30 sheep farms in the southeastern US. Fecal samples were tested using the DrenchRite larval development assay to determine resistance levels to various dewormer classes. Results found that all farms had resistance to at least one dewormer class, with 100% resistance to benzimidazoles. Resistance was worst in Georgia and better but still prevalent in Maryland and Virginia. The conclusions recommend that farmers test for resistance levels and use combination dewormer treatments due to widespread resistance issues.

1. 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).

2. Introduction
• Gastro-intestinal nematodes are the primary health
problem affecting sheep in warm, moist climates
and during periods of summer rainfall.
• Haemonchus contortus (barber pole worm) is the
most pathogenic parasite species, having high
morbidity and mortality.
• H. contortus is usually the most numerous species
in small ruminant parasitic infections.

3. Worms have developed varying degrees of resistance to all
dewormers and dewormer “chemical” classes.
1 2 3
Benzimidazoles
Macrocylic lactones Nicotonic agonists
imidazothiazolesAvermectins Milbimycins
Fenbendazole
SafeGuard®
Ivermectin
Ivomec®*
Moxidectin
Cydectin®*
Quest®
Levamisole
Prohibit®*
Leva-Med®*
Albendazole
Valbazen®*
Doramectin
Dectomax®
Oxfendazole
Synanthic®
Eprinomectin
Eprinex®
*FDA-approved for use in sheep.anthelmintic = dewormer

4. Two methods for determining anthelmintic resistance
FECAL EGG COUNT REDUCTION TEST
(FECRT) “Gold standard”
• Old protocol
Compare fecal egg counts from animals that
have and have not been treated.
• New protocol
Compare pre-and post-treatment fecal egg
counts.
• Requires individual or pooled composite
samples from 15 animals with minimum
FEC of 250 epg.
• Large number of animals required if multiple
anthelmintics are to be tested.
• Can be labor intensive and expensive.
DRENCHRITE® TEST
LARVAL DEVELOPMENT ASSAY (LDA)
• In-vitro laboratory test that determines
resistance to all dewormer “chemical”
classes simultaneously from a single pooled
fecal sample from 10-15 animals with a
minimum FEC of 500 epg.
• Larvae also identified, e.g. % HC
• $450/sample
• Done exclusively by Dr. Ray Kaplan’s lab at
the University of Georgia.

5. Materials and Methods
• In 2016, the American Sheep Industry Association’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 Maryland (1-10)
2. Virginia, via Virginia State University (11-16)
3. Georgia, via Fort Valley State University (17-26)
• The project cost-shared the DrenchRite® test for producers
and facilitated the collection and shipment of samples.
• $225 paid by Let’s Grow funds
• $225 paid by producer

6. 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 are 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%. As the
percentage declines, the drug becomes less effective as the sole treatment. It is not effective
below 50 percent.
• Data from ivermectin wells is used to determine moxidectin resistance, since both drugs are
in the same class.

7. Results (n=26): 82.1 ± 3.8% Haemonchus contortus
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 93
97 96
84
94
85
54
90
38
72
97
52
92 92
88
97
91 91 91
59
96
42
0
2000
4000
6000
8000
10000
12000
0
10
20
30
40
50
60
70
80
90
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

8. Results
Percent 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
Maryland
Virginia
Georgia

9. Interpretation of test results
Depending upon
level of resistance,
actual efficacy
may vary from 0%
to as high as 95%.
Farm #5

10. BENZIMIDAZOLES
BZ
1. Fenbendazole
(SafeGuard®, Panacur®)
2. Albendazole
Valbazen®

11. IVERMECTIN
IVM
1. Ivermectin
Ivomec®
2. Doramectin
Dectomax®
3. Eprinomectin
Eprinex®

12. MOXIDECTIN
MOX DD
Cydectin®

13. LEVAMISOLE
LEV
Prohibit®
Leva-med®

14. Conclusions
• All farms had resistance to one or more dewormers and
dewormer classes.
• Some farms have resistance to all dewormers and
dewormer classes.
• Resistance varies by geographic region and farm; worse
the further south you go.
• Farmers should test for anthelmintic resistance to
determine resistance levels.
• Many farms still have efficacy of one or more
dewormers and need to take steps to maintain their
effectiveness.
• Due to widespread resistance, combination treatments
are now recommended for clinically-parasitized animals.

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