Evaluation of the Efficacy of Doramectin in the Control of Strongyle (Strongylidae,Cyathostominae) Infestation in Horses

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Israel Journal of Veterinary Medicine  Vol. 69 (2)  June 2014 83 Dectomax in Horses
INTRODUCTION
Diseases caused by parasites are still among the most com-
mon illnesses in equine populations despite great advances
in anti-parasite drug development and use (1-4). Parasitic
infestations cause digestive disorders, which result in ema-
ciation, weakness, colic and even death. Nematodes of the
families Strongylidae and Cyathostominae are the most com-
mon parasites of horses. Because of their wide distribution
they are found in practically every team of horses, regardless
of bioclimatic conditions, farm management methods or age.
Even the best managed horses may be at risk of coming into
contact with invasive forms of parasites (5). Parasitic diseases
usually have a subclinical course, hence they often go unno-
ticed by both breeders and veterinarians.
One of the side efects of the widespread (prolonged and
intensive) use of chemotherapeutic drugs in horses has been
the emergence of drug resistance by parasites. Other factors
that may lead to the development of parasite drug resistance
in horses includes drug under-dosing, alternating admin-
istration of drugs belonging to the same class of chemical
compounds, parasites coming into frequent contact with the
active substance, and the fact that animals infect one anoth-
er with drug-resistant strains (6). Te European Medicines
Agency indicates an increase of helminth resistance to three
major classes of anthelmintics, namely macrocyclic lactones,
imidazoles and benzimidazoles. Te development of new an-
thelmintic drugs in the near future with a novel mode of
action is unlikely to occur, making eforts to prevent drug
resistance a priority (7). Modern anthelmintics should be ef-
fective, non-toxic, easy to administer, have a broad spectrum
of activity, and cause no adverse efects.
Doramectin is an avermectin derived from the class of
macrocyclic lactones. Te substances of this group have no
efect on protozoa, trematodes and tapeworms, but at low
doses they are efective against nematodes and arthropods.
Studies with cattle have shown that doramectin has a very
Evaluation of the Efcacy of Doramectin in the Control of Strongyle
(Strongylidae, Cyathostominae) Infestation in Horses
Prokulewicz, A., Pilarczyk, B.* and Tomza-Marciniak, A.
Department of Animal Reproduction Biotechnology and Environmental Hygiene, Faculty of Biotechnology and Animal
Husbandry, West Pomeranian University of Technology, Szczecin Doktora Judyma St. 6, 71-466 Szczecin, Poland.
*Corresponding author: Dr. hab. Bogumiła Pilarczyk, Department of Animal Reproduction Biotechnology and Environmental Hygiene, West Pomeranian
University of Technology, Szczecin. Tel: +48 91 449 67 91. Email: bogumila.pilarczyk@zut.edu.pl.
ABSTRACT
Te aim of the study was to evaluate the efcacy of doramectin (Dectomax) in the control of strongyle
(Strongylidae, Cyathostominae) infestations in horses. Te research involved 24 horses of various breeds (stallions
and mares), aged 1.5 to19 years of age. Te prevalence and intensity of infestation were determined based on
coproscopic examination using Willis-Schlaf and Mc-Master methods. Drug resistance was determined by
fecal egg count reduction test (FECRT). Horses were injected subcutaneously with doramectin at a dose of
0.2 mg/kg bodyweight. Doramectin had an efcacy of 99.6% on day 14 and strongyles reappeared 2 months
after doramectin administration. Subcutaneously injected doramectin proved highly efective in the control
of strongyle (Strongylidae, Cyathostominae) infestations in horses.
Keywords: Horses,equine, Strongylidae, Cyathostominae, Dectomax, Doramectin, drug resistance.
Israel Journal of Veterinary Medicine  Vol. 69 (2)  June 2014 Pilarczyk, B. 84
wide margin of safety. No side efects have been reported af-
ter using 25 times the recommended dose (8).
Research on the use of doramectin against nematode in-
fections in horses has been carried out by Schuman (8), Perez
et al. (9), Clarke et al. (10), and Davies and Schwalbach (11).
In each research project, authors evaluated the reduction in
eggs at 14 days after administration of doramectin. Te ef-
fcacy of the drug in those studies ranged from 96-100%.
No studies about efcacy of doramectin in horses have been
carried out in Poland until now. Moreover doramectin is
not registered as an active substance for horse treatment in
Poland.
Te aim of this study was to evaluate the efcacy
of doramectin in the control of strongyle (Strongylidae,
Cyathostominae) infestation in horses.
MATERIAL AND METHODS
Coproscopic examination was performed in 24 horses (Table
1) from an agri-tourism farm located in West Pomerania,
NW Poland. A total of 264 fecal samples were analyzed. Te
horses were kept in loose boxes on shallow bedding. Stable
manure was removed every day. When weather conditions
permitted, the animals were kept on sand paddocks during
the day. Te horses were maintained in accordance with the
requirements of animal welfare (12). Tey were subjected to
anthelmintic treatment on the farm twice a year (in spring
and autumn). In the autumn, the horses were dewormed with
an anthelmintic including ivermectin as the active substance.
Te horses were dewormed in the spring (April) with
doramectin. Tis drug is a broad-spectrum antiparasitic reg-
istered in Poland for veterinary use in cattle, sheep and pigs.
Doramectin was administered to each horse individually, as a
one-time injection at a dose of 0,2 mg/kg bodyweight. After
the horses were dewormed, the stable and its equipment were
washed and disinfected. During the tests the health of the
horses was monitored daily by a veterinarian who took care
of all the animals at the farm.
During the study no deviations were noticed in heart
and respiration rate and rectal temperature. Also no colic
occurred due to obstruction of gastrointestinal tract or diar-
rhea was observed and mucous membranes remained nor-
mal. Tere were no symptoms of apathy or anxiety.
Fecal samples were collected from each horse prior to
doramectin administration and on days 7, 14 and 21 after
anthelmintic treatment. In addition, horse feces were also
examined for intestinal parasites 1, 2 and 3 months after
doramectin administration. Drug resistance was deter-
mined by Fecal Egg Count Reduction Test (FECRT). Tis
method, recommended by the World Association for the
Advancement of Veterinary Parasitology (WAAVP), com-
pares the mean egg count per gram of feces before admin-
istration of a drug and at certain periods of time after treat-
ment (13). Tis test is used to estimate the efcacy of a drug
activity. For the horses studied, efcacy of doramectin was
determined after 14 days.
Te prevalence and intensity of infection were determined
based on coproscopic examination using Willis-Schlaf (quali-
tative method) and Mc-Master (quantitative method) meth-
ods, previously described by Ziomko et al. (14). Only positive
Table1: Breed, age and sex of the study group of horses
HORSE’S
NAME
A
L
A
D
A
R
E
L
I
T
A
F
E
L
A
G
R
U
N
W
A
L
D
H
A
R
F
A
H
A
T
L
A
H
A
W
A
N
A
H
O
N
D
A
H
O
R
N
E
T
A
K
A
G
E
R
A
K
A
I
R
K
A
R
M
O
N
A
K
O
M
E
T
A
K
O
P
E
R
E
K
K
O
R
O
N
A
L
I
L
A
O
B
E
L
L
E
O
K
T
A
W
I
A
P
A
R
Y
S
P
A
S
J
A
P
I
R
A
T
R
E
N
A
R
E
T
O
R
A
S
E
K
A
T
O
R
AGE 7 6 6 19 5 5 5 5 7 9 9 6 7 9 1.5 6 7 4 3 12 5 6 7 9
SEX G F F G F F F F F F M F F G F F M F G F G F F G
BREED
H
B
H
B
H
A
F
L
I
N
G
E
R
S
H
B
H
B
H
B
H
B
H
B
H
B
M
A
L
O
P
.
H
B
H
B
P
O
N
Y
H
B
H
A
F
L
I
N
G
E
R
F
R
I
E
S
.
H
B
H
B
H
B
H
B
H
B
H
B
H
B
BODY
WEIGHT [kg]
6
0
0
3
0
0
3
0
0
6
0
0
5
0
0
5
0
0
5
0
0
5
0
0
5
0
0
5
0
0
5
0
0
5
0
0
5
0
0
3
0
0
3
0
0
3
0
0
6
0
0
5
0
0
5
0
0
5
0
0
5
0
0
5
0
0
5
0
0
6
0
0
F-female, M-male, G- gelding, HB - half-bred, S- Silesian, MAŁOP.- Malopolski, FRIES.- Friesian
Research Articles
Israel Journal of Veterinary Medicine  Vol. 69 (2)  June 2014 85 Dectomax in Horses
fecal samples were tested using the quantitative method. Te
results obtained were used to calculate the mean prevalence
of infection and eggs per gram of feces, and to determine the
percentage of small and large strongyles in the parasitic gas-
trointestinal fauna of the horses. Eggs were identifed based
on morphological characteristics – shape, envelope structure,
number and size of blastomeres and biometry using a key ac-
cording to Foreyt (15) and Tienpoint et al. (16).
Te experiment complied with the current laws of Poland.
Te material collected during routine veterinary procedures
(subcutaneous injections) performed by veterinarians. In
such cases, the agreement of the Local Ethical Committee
on Animal Experiments is not required.
RESULTS
In this study the presence of nematodes of the families
Strongylidae and Cyathostominae was found. Prior to de-
worming, mean prevalence of infestation with gastrointesti-
nal parasites in the horses was 100% and mean intensity of
infestation was 844 eggs per g of feces (Table 2).
Our results showed large diferences between the preva-
lence of infestation with large strongyles (Strongylinae) and
small strongyles (Cyathostominae). Strongylinae were found
in 12% of the examined animals and Cyathostominae in 92%.
After the treatment (14 day) with the anthelmintic that
included doramectin (Dectomax® Injectable Solution, Pfzer,
New York, USA) as an active ingredient, the number of ani-
mals infested with Strongylinae declined to zero, while in the
case of Cyathostominae to four horses. Seven and fourteen days
after the horses were treated with doramectin, single nema-
tode eggs were found in 4 horses from the investigated group.
During this period, the doramectin had an efcacy of 99.6%
(for examined Strongylidae) (Fig.1). However, its efcacy de-
creased to 82.4% at 28 days after administration. Two months
after the treatment, nematode mean egg count raised consider-
ably to 588 eggs per gram of feces (range: 350-1400 eggs per
gram). During the same period, the prevalence of infection also
increased to 75%. Tree months after the treatment egg counts
were similar to those before the treatment (750 eggs per gram).
DISCUSSION
Te present study showed that prior to deworming all ex-
amined animals were infested with gastrointestinal parasites,
which is comparable with the results of other authors (1, 17-
Table 2: Te efcacy of doramectin in diferent periods of research.
Before dewormed After dewormed (day)
7 14 21 28 58 90
EPG 844 3 3 4 150 588 750
Range 500-2500 0-50 0-50 0-50 0-300 350-1400 350-1650
Prevalence (%) 100 16.67 16.67 16.67 42 75 91.67
Figure 1: Evaluation of Dectomax (Pfzer) efcacy in the studied horses.
Research Articles
Israel Journal of Veterinary Medicine  Vol. 69 (2)  June 2014 Pilarczyk, B. 86
19). Te presence of nematodes of the families Strongylidae
and Cyathostominae were found. Te prevalence of infec-
tion with small strongyles (Cyathostominae) was much high-
er than infection with large strongyles (Strongylinae) (92%
vs.12%). Gawor and Kita (20) and Gawor et al. (21) consider
this phenomenon to beassociated with the high efcacy of
the commonly used anthelmintics against migrating larvae of
nematodes of subfamily Strongylinae. As reported by Lyons
et al. (22) and Tarigo-Martinie et al. (23), it is impossible
to completely remove all small strongyles (Cyathostominae)
from the population of infested horses. Tis is caused by hy-
pobiosis, i.e. the presence of arrested-stage larvae (L4) in the
gut mucosa.
After the treatment with doramectin, the number of ani-
mals infested with parasites (Strongylinae) declined to zero.
However, deworming did not remove Cyahtostominae para-
sites from all the hosts, but considerably limited the preva-
lence of infestation in these animals. Tis could be associ-
ated with the restoration of the parasite population in the
animal’s body by arrested larvae that survived anthelmintic
treatment (24). Te highest efcacy (99.6%) of doramectin
against the examined Strongylidae was observed at 7 and 14
days after treatment. Similar fndings were reported by (25)
for donkeys in Sudan, where doramectin had an efcacy of
99.24% at day 14. Daviesand Schwalbach (11) showed 100%
efcacy in horses on day 14 after intramuscular injection of
doramectin. Similar results were also obtained by Schumman
(8) in horses after intramuscular treatment. Slightly lower ef-
fcacy was noted by Clarke et al. (10)after intramuscular ad-
ministration, reaching a level of 96% in one year old horses.
A comparison with the literature Monahan and Klei (26)
indicates that doramectin has a similar efcacy to ivermec-
tin and moxidectin. According to those authors, the efcacy
of ivermectin and moxidectin against luminal cyathostomins
was 99%, although as emphasized by Monahan and Klei (26)
ivermectin did not act against hipobiotic larvae.
Te World Association for the Advancement of
Veterinary Parasitology reports that drug resistance is con-
sidered to exist when FECRT is below 90%. According to
Watson (27), this does not mean that there are no drug-re-
sistant parasites in the population.
Tere was a reinfestation of parasites in the horse group
studied. We observed that the efcacy of doramectin de-
creased to 82.4% on day 28 and 47.8% on 58 day. Tis in-
dicates a shorter time of protection against these parasites
compared to other avermectins. Taylor and Kelly (28), Jacobs
et al. (29) and Monahan and Klei (26) reported that the egg
reappearance period for moxidectin was 15-24 weeks, and
only 8-14 weeks for ivermectin. Because doramectin is not
registered for horses in Poland, the manufacturer provided no
information concerning the period of protection against in-
festation. Pérez et al. (9) reported that subcutaneously inject-
ed doramectin is more efective than oral administration and
remains in the body for a longer time periods (3-4 weeks),
thus providing longer protection against parasites. Te au-
thors found that subcutaneously administered doramectin is
excreted more slowly in feces (for a period of 60 days) and
may be more efective in the control of migrating larvae and
those remaining in the tissues of animals being treated.
Subcutaneously injected doramectin proved highly efec-
tive in the control of strongyle (Strongylidae, Cyathostominae)
infestations in horses and no adverse efects were observed.
Te disadvantage of this substance is the short lived protec-
tion of doramectin against the stongyles.
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