A series of well controlled efficacy
studies was submitted with the original NADA 120-648 and also with supplements
to the original NADA 120-648. For this supplemental application, additional
efficacy studies were conducted and are summarized below.
Well controlled trials were conducted
at three different geographical locations in the United States in naturally
infected horses to determine the efficacy of fenbendazole against encysted mucosal
cyathostome larvae.
On each site, twenty (20) horses
were assigned, by restricted randomization, to treatment and control groups,
each comprising 10 horses. Fenbendazole 10% paste, was administered orally to
the horses in the treatment groups at a dose rate of 10 mg/kg body weight for
5 consecutive days.
Following a minimum six week post-treatment
housing period, the horses were euthanized and their cyathostome worm burdens
assessed. Encysted hypobiotic early 3rd stage larvae (EL3) were identified
and enumerated following pepsin HCl digestion of the large intestinal mucosa.
The larger encysted late L3 and L4 were enumerated following
transmural illumination (TMI) and digestion. Efficacy was calculated as the
percentage (%) reduction in geometric mean (GM) worm burdens of the fenbendazole
treated groups relative to the controls as follows:
....................................................Efficacy
(%) = GMc - GMt x 100
...................................................................................
GMc
GMc = ...... mean number of strongyles
in control horses.
GMt = ...... mean number of strongyles
in treated horses
Strongyle larval and adult worm
counts at post-mortem were analyzed for differences using the parametric two-sample
t-test and the 5% significance level. (When the t-test was inappropriate due
to zero variances, where indicated it was possible to use the non-parametric
Fisher exact test). Data were found to be suitably transformed for parametric
analyses using the log (X+1) transformation. The results from the three sites
were pooled and analysis carried out for an overall treatment effect using analysis
of variance and the randomized block design. In the analyses, studies and study
by treatment effects were treated as random effects and treatments as a fixed
effect, and analyses were carried out on log (X+1) transformed data using a
mixed effects model.
Five consecutive treatments with
fenbendazole provided >92% efficacy for the different mucosal cyathostome
larvae stages listed in the INDICATIONS section. Data for the three studies
support the use of fenbendazole, applied orally, at 10 mg/kg daily for 5 days
of the treatment of all stages of encysted cyathostome larvae in the mucosa
of the large intestine of horses.
Dose Confirmation of Efficacy Against
Cyathostome Larvae Encysted In The Large Intestinal Mucosa
A. Study Type
Controlled necropsy studies to demonstrate efficacy against encysted mucosal
cyathostome larvae in naturally infected horses. There were three studies.
B. Investigators:
Site 1- Study Number 120-648-01-02-95-SP
J.A. DiPietro, D.V.M., M.S.
Associate Dean for Research
University of Illinois
College of Veterinary Medicine
2001 South Lincoln Avenue
Urbana, Illinois 61801
Site 2 - Study Number 120-648-02-02-95-SP
Thomas R. Klei, Ph.D.
Department of Veterinary Science
Louisiana Agricultural Experiment Station
LSU-Agricultural Center
Louisiana State University
Baton Rouge, LA 70803-8416
Site 3 - Study Number 120-648-03-02-95-SP
Craig R. Reinemeyer, D.V.M., Ph.D.
Department of Comparative Medicine
College of Veterinary Medicine
University of Tennessee
P.O. Box 1071
Knoxville, TN 37996-1071
C. General Design:
1. Purpose:
To confirm the efficacy of orally administered fenbendazole at10 mg/kg daily
for 5 days against naturally acquired cyathostome infections in particular,
mucosal 3rd and 4th stage larvae.
2. Animals:
The studies included a total of 30 horses/ponies treated with fenbendazole and
a total of 30 control horses/ponies. The mixed breed horses and ponies were
aged between 7 months and three years and weighed between 65 and 398 kg at the
study start.
3. Housing:
The horses/ponies were
housed in their treatment groups (2-5/pen) for the duration of the study to
preclude further infection.
4. Infections:
These were acquired while
at pasture. This was confirmed prior to the start of the studies by fecal examinations
and coproculture of the test horses and by post-mortem larval worm counts of
three (3) "tracer" horses/ponies at each site.
5. Dosage form:
A paste containing 100
mg fenbendazole/g paste.
6. Route:
By dosing syringe, administering
the paste orally on the dorsum of the tongue.
7. Dose:
10 mg fenbendazole/kg
body weight for 5 consecutive days.
8. Study Duration:
The horses/ponies were
necropsied for nematode recovery starting on day 43 of the study.
9. Relevant Parameters Measured:
Early 3rd stage, late 3rd
stage and 4th stage cyathostome larvae encysted in the mucosa of the large intestine
using transmural illumination (TMI) and digestion techniques. For TMI, uniform
sections of mucosa were collected from the caecum, ventral and dorsal colons
and stretched over a light source. Encysted cyathostome larvae (late L3/L4)
were enumerated using a microscope at 10-15 X magnification. For digestion,
uniform sections of mucosa were collected from the three organs.The mucosa and
submucosa were separated from the serosa and transferred to suitable containers
for subsequent digestion with a pepsin HCl mixture at 37° C for at least
2 hours. Aliquots of the digested mixture were stained then examined by stereomicroscopy
(X20) for early 3rd stage, late L3 and L4 cyathostome larva:adults/larvae in
the gut lumen
D. Pooled Data Results:
Efficacy against all stages of encysted mucosal cyathostome larvae was >90%.
SITE 1
|
Mean (GM) Count
|
|
|
Stage
|
Control
|
Fenbendazole
|
% Efficacy
|
|
Mucosal EL3
(hypobiotic)
|
1510.4
|
63.8
|
96
(p = 0.0628)
|
|
Mucosal LL3
L4 (by digestion)
|
2482.9
|
62.4
|
98
(p = 0.0044)
|
|
Mucosal LL3/L4 by (TMI)
|
137.8
|
2.4
|
98
(p = 0.0001)
|
|
Lumen adults / larvae
|
5423.3
|
184.4
|
97
(p = 0.0001)
|
SITE 2*
|
Mean (GM) Count
|
|
|
Stage
|
Control
|
Fenbendazole
|
% Efficacy
|
|
Mucosal EL3 (hypobiotic)
|
23861.8
|
2150.9
|
91
(p <0.0001)
|
|
Mucosal LL3/L4
(by digestion)
|
1409.1
|
89.9
|
94
(p = 0.0485)
|
|
Mucosal LL3/L4
(by TMI)
|
2752.9
|
698.7
|
75
(p = 0.0066)
|
|
Lumen adults / Larvae
|
26598.1
|
8346.8
|
69
(p = 0.0325)
|
*Benzimidazole (BMZ) resistant parasites
present
SITE 3:
|
Mean (GM) Count
|
|
|
|
Stage
|
Control
|
Fenbendazole
|
% Efficacy
|
|
Mucosal EL3 (hypobiotic)
|
4944.9
|
6.2
|
99.9
(p < 0.0001)
|
|
Mucosal LL3/L4
(by digestion)
|
1727.9
|
47.3
|
97.3
(p = 0.0083)
|
|
Mucosal LL3/L4
(by TMI)
|
7415.5
|
705.1
|
90.5
(p <0.0001)
|
|
Lumen adults / Larvae
|
82,689.1
|
6203.7
|
92.5
(p <0.0001)
|
Pooled Data (3 Sites)
|
Mean (GM) Count
|
|
|
|
Stage
|
Control
|
Fenbendazole
|
% Efficacy
|
|
Mucosal EL3 (hypobiotic)
|
5628.2
|
99.1
|
98
(p = 0.0433)
|
|
Mucosal LL3/L4
(by digestion)
|
1821.7
|
64
|
97
(p <0.0001)
|
|
Mucosal LL3/L4
(by TMI)
|
1414.5
|
117.7
|
92
(p = 0.0339)
|
|
Lumen adults / Larvae
|
22,849.2
|
2124.5
|
91
(p = 0.0336)
|
E. Adverse Reactions:
There were no adverse reactions to treatment.
F. Conclusions:
Fenbendazole administered orally at 10 mg/kg for 5 consecutive days to horses/ponies
with naturally acquired cyathostome infection is safe and effective.
