Objective—To determine the prevalence and clinical
implications of anthelmintic resistance in cyathostomes
Animals—80 horses on 10 farms in a 5-county region
of northeast Georgia.
Procedure—On each farm, horses were stratified in
descending order according to pretreatment fecal egg
count (FEC), blocked into groups of 4, and then randomly
assigned to 1 of 4 treatment groups: no treatment
(controls), and treatment with pyrantel
pamoate, fenbendazole, or ivermectin. Fecal samples
were collected 24 hours prior to treatment and 2, 4,
and 6 weeks after treatment for determination of
FEC. Mean percentage of reduction in FEC was then
calculated for each treatment group. For horses from
each farm, the efficacy of each anthelmintic was categorized
on the basis of mean percentage of reduction
in FEC at 2 weeks after treatment (< 80% reduction
= ineffective; 80 to 90% reduction = equivocal;
and > 90% reduction = effective).
Results—Pyrantel pamoate was effective at reducing
FEC in horses from 7 farms, ineffective in horses from
2 farms, and equivocal in horses from 1 farm.
Fenbendazole was ineffective at reducing FEC in horses
from 9 farms and equivocal in horses from 1 farm.
Ivermectin was effective at reducing FEC in horses
from all 10 farms.
Conclusions and Clinical Relevance—Results suggest
that cyathostome resistance to fenbendazole is
highly prevalent, and resistance to pyrantel pamoate
is high enough to warrant concern. Resistance to ivermectin
was not detected. On the basis of these data,
it appears that ivermectin continues to be fully effective
in horses. However, too few farms were used in
this study to determine the prevalence of cyathostome
resistance to ivermectin. Therefore, the efficacy
of ivermectin should continue to be monitored closely.
(J Am Vet Med Assoc 2001;218:1957–1960)