Dealing with anthelmintic
resistance in small ruminants
SHEEP & GOAT SPECIALIST
UNIVERSITY OF MARYLAND EXTENSION (UME)
WESTERN MARYLAND RESEARCH & EDUCATION CENTER (WMREC)
SSCHOEN@UMD.EDU - WWW.WORMX.INFO - WWW.SHEEPANDGOAT.COM
Gastro-intestinal parasites (worms) are the
primary health problem affecting small ruminants.
The barber pole worm (Haemonchus
contortus) is the primary parasite affecting
small ruminants in warm, moist climates.
The barber pole worm has developed resistance
to all anthelmintics and all anthelmintic classes.
When an anthelmintic
treatment fails to
reduce fecal egg
counts by 95% or more.
Anthelmintic resistance was (is) inevitable.
No treatment will kill 100% of worms.
Numerous on-farm practices have accelerated the
development of resistant worms in small ruminants.
Frequent deworming, especially without regard to clinical need.
Underdosing drugs (failure to dose based on accurate weights).
Injecting an anthelmintic instead of using a drench.
Pouring an anthelmintic on the back instead of using a drench.
Improper administration of drenches, e.g. depositing drug in mouth
Use of persistent-activity dewormers (e.g. moxidectin).
Treating all animals in flock/herd, leaving no refugia.
Putting treated animals onto a clean pasture: no refugia.
Treating animals when pasture contamination is low.
Giving more than one dewormer at a time.
Introduction of resistant worms to a farm via new animals; failure to
Is not fully appreciated by some producers
Varies by geographic region and farm.
Is result of past deworming practices.
On most farms, resistance is probably still
at a level where there is still time to slow it
down and enable the continued use of
Most small ruminant producers do not
know which dewormers work on their farm.
Dealing with anthelmintic resistance
on the farm.
The first thing you need to do is
determine which dewormer(s) still
work on your farm.
All producers need to devise
production/ management systems
that minimize the need for
There are two methods to determine
anthelmintic (dewormer) resistance.
Fecal egg count
reduction test (FECRT)
Comparison of pre- and post-
treatment fecal egg counts.
Must compare egg counts for
each individual anthelmintic.
development assay (LDA)
In vitro test for anthelmintic
resistance (for all drugs).
Fecal egg count reduction test (FECRT)
Takes 7-14 days (or more) to get results,
longer if done by someone else.
Cost of testing varies
(15 samples x 4 anthelmintics + control group)
75 x free labor = no cost
75 x $5/sample = $350
75 x $10/sample = $750
Requires a lot more animals (ideally,
12-15 per treatment group); many
producers don’t have enough
animals for accurate testing.
% reduction (whole flock/herd)
Results from individual animals can be quite
development assay (LDA)
Takes 3-4 weeks to get results
Only one lab in US that does test:
Dr. Ray Kaplan’s lab at the University
of Georgia College of Veterinary
$450 per sample
Suspected resistance (SR)
There are many things a producer can do to
potentially reduce the number of animals that require
deworming and/or the frequency of deworming.
Pasture and grazing management
Targeted selective treatment (TST)
The foundation of parasite control is good
pasture and grazing management.
Evasive grazing: pasture rest and rotation
Maintaining minimum grazing heights (> 3 in.)
Mowing, haying, cropping
Mixed species grazing w/alpacas, llamas
Low/reduced stocking rates ****
Certain management practices may
reduce the need for deworming.
Winter or fall lambing/kidding
Delay grazing until after dew lifts
Put treated animals in dry lot for 48
hours after deworming
Manage pastures so that plants are in a vegetative
stage for grazing.
Plant annuals, legumes, and warm season plants to
improve nutrition of pasture.
Provide supplemental nutrition when pasture
quantity or quality is low/poor and/or to susceptible
Distiller’s grains (DDSG)
By-pass protein (?)
Increase protein in late gestation ration to counter
periparturient egg rise.
Maintain animals in good body condition (BCS > 2.5)
Genetic selection can reduce the
number of animals that require deworming.
Goats are usually more susceptible
to parasites than sheep.
Some sheep breeds are more
resistant to parasites.
Hair sheep of tropical origin:
St. Croix, Barbado, Katahdin
Native breeds of the Southeast:
Other breeds: Texel (?)
Some goat breeds seem to be
more resistant to parasites than
others: Kiko, Spanish, Myotonic.
80:20 (or 70:30) rule
20-30 percent of flock/herd is responsible
for 70-80% of pasture contamination
Can select any breed for improved
resistance to parasites
Select individuals with lower FECs.
Use EBVs (estimated breeding values) to
choose resistant males for breeding.
Parasite resistance (fecal egg counts) is
a moderately heritable trait (20-40%):
heritability of prolificacy is only 10%.
Resilience is less heritable.
Effect of copper oxide wire particles on the parasite status of
bucks in the 2014 Western Maryland Pasture-Based Meat Goat
Performance Test (0.5 g COWP administered on d-42)
d (-6) d-0 d-14 d-28 d-42 d-56 d-70 d-84
Test - COWP
Study - no COWP
Tannins are plant compounds that bind to proteins
and other molecules.
Effects of tannins vary depending upon type,
concentration, and the animal consuming the tannin.
Tannins can have both negative or positive effects.
Sericea lespedeza (AU Grazer™) is a high-tannin
forage (warm season legume) that has been
scientifically-proven to reduce parasite burdens in
sheep and goats.
Leaf meal (pellets) [simsbrothers.com]
“Natural” dewormers (IMO)
Most likely effect of “natural” dewormers will be to reduce the
number of animals that require deworming by . . .
Disrupting the free-living stage of the parasite (e.g. egg hatching, larval development)
Improving the immune function of the host.
THANK YOU. QUESTIONS? COMMENTS.
American Consortium for Small Ruminant Parasite Control - www.wormx.info