Pivotal Studies
Five toxicity or tolerance trials, three breeding animal safety trials and
three trials on the acceptability of treating dogs with patent heartworm infections
(heartworm positive dogs), have been conducted. The trials are further identified
in table l.
Because high levels of ivermectin are required for appropriate safety testing
compared to the low dose and dilute nature of the oral tablet market formulation,
the market formulation could not be used to administer the necessary elevated
levels in these trials. Impractical and irrelevant numbers of tablets would
have been required, therefore, other formulations were utilized (as described
below). When lower doses were administered the heartworm-positive-dog studies,
the market formulation was used. The ivermectin used in all of these studies
is the same as the ivermectin in the market formulation.
Toxicity and Tolerance
TT#78-038-0 - Tolerance - Fourteen Week Toxicity Study in Dogs
Twenty male and 20 female beagles from 39 to 43 weeks of age were divided
into five treatment groups of eight dogs each, four of each sex, and were
allocated to treatment. Treatments were administered daily for 94 to 95 consecutive
days by intubation. Three groups received ivermectin at 500, 1,000 or 2,000
mcg/kg/day and two groups served as controls receiving either sesame vehicle
or deionized water, also daily.
The dogs were evaluated based on weights, ophthalmologic examinations, electrocardiograms,
hematologic and serum biochemical studies, urinalyses and appropriate gross
and histopathologic examinations.
No potentially drug-related abnormalities were detected in any examination
or sample, and no adverse reactions were observed in any dog receiving ivermectin
at 500 mcg/kg/day during the 14 week study.
Mydriasis was detected in dogs receiving 1000 mcg/kg/day after 15 days.
A retardation in weight gain was also observed in this treatment group. No
other treatment-related effects were detected during examinations or in samples
from the group.
No abnormalities or adverse reactions were detected in dogs after the first
dose of 2,000 mcg/kg. Mydriasis was first clinical sign observed and was eventually
seen in all dogs in this group, and more severe signs were seen in some dogs
including salivation, tremors, ataxia, anorexia, dehydration and weight loss.
No other treatment-related changes were found.
TT#79-2869 - Acute Toxicity in Dogs
Two male and two female beagle dogs, 10 to 14 months of age, were allocated
to each of four treatment groups: ivermectin at 2,500 5,000 or 10,000 mcg/kg
body weight or vehicle. Sesame oil was the drug solvent or vehicle. The treatments
were administered by gastric intubation. The dogs were observed for 14 days.
Emesis, salivation, mydriasis, absence of pupillary response and tremors
were seen in some dogs following treatment at the two higher dose levels.
One of the dogs given the highest dose became ataxic and sedated. The dog
recovered from the sedation within 48 hours of treatment and from the remaining
signs 72 hours after treatment. Mydriasis and absence of pupillary response
were the only clinically significant signs seen at the lowest ivermectin dose
level (2,500 mcg/kg). None of the signs persisted past the sixth day after
treatment.
TT#81-2500 - Acute Toxicity in Dogs
Each of 2 male and 2 female beagle dogs, 6 to 9 months of age, were allocated
to receive ivermectin at 5,000, 10,000, 20,000, 40,000 or 80,000 mcg/kg body
weight or sesame oil vehicle. The treatments were administered by gastric
intubation. The dogs were observed for 14 days.
Emesis following treatment was seen in at least one dog in all treatment
groups (including vehicle) except 40,000 mcg/kg. Mydriasis was seen in most
dogs at all ivermectin dose levels. Ataxia and tremors were seen at doses
of 10,000 mcg/kg and greater. Salivation was noted in some dogs at the two
highest ivermectin dose levels. One dog given 40,000 mcg/kg and three dogs
given 80,000 mcg/kg were unable to stand beginning four to six hours after
treatment. One of these dogs (at the highest dose level) was up the next morning;
however, the other three remained down, became comatose and died. Three of
four dogs given 40,000 mcg/kg and two of four dogs given 80,000 mcg/kg survived.
All of the surviving ivermectin-treated dogs appeared normal at the end of
the study.
TT#81-025-0 - Acute Toxicity in Young Dogs
Five groups of three male and three female pups (11 to 12 weeks old) were
allocated to receive subcutaneous injections of ivermectin in a micelle solution
at 4,700, 9,400, 18,800, 37,500 or 75,000 mcg/kg body weight while a sixth
group of two female and three male pups received only vehicle.
The 15 day trial included observations for clinical signs and an ophthalmologic
examination of survivors eight days after treatment. All dogs were necropsied.
Mydriasis, negative pupillary response to light and ataxia were detected
the day after treatment in dogs receiving 4,700 mcg/kg. All dogs in this group
appeared normal by the seventh day after treatment.
Emesis was seen in one dog given 9,400 mcg/kg and in one dog given 75,000
mcg/kg. Mydriasis, ataxia, salivation, negative pupillary light response,
decreased activity, tremors and death were experienced in groups receiving
ivermectin at 9,400 mcg/kg and greater doses.
No treatment-related abnormalities were detected in the ophthalmic examination
of surviving dogs eight days after treatment.
Other than agonal changes no treatment-related histologic changes were detected.
ASR 11017 - Acute Toxicity in Collies (Breed)
Sixteen purebred collies, eight of each sex and half of the animals affected
with collie eye anomaly (CEA) and half free of the anomaly, were utilized
to determine if ivermectin was toxic in collies at levels not generally toxic
to other breeds and if collie eye anomaly (CEA) was related to sensitivity.
The dogs were allocated to four treatment groups so that each sex with each
eye status was represented in each treatment group. The four treatment groups
were: untreated control, ivermectin at 50 mcg/kg, 200 mcg/kg or 600 mcg/kg.
Ivemectin was administered orally in a fractionated coconut oil solution.
Frequent clinical neurologic examinations were made for 7 days. Necropsies
and histopathologic evaluations and tissue ivermectin assays were made on
three dogs, two of which where showing severe signs of toxicity and one control.
In a second phase, the three remaining dogs, which had originally served as
controls, were given ivermectin at 50 mcg/kg and examined for 4 days.
Signs of toxicity were detected in a female free of CEA after receiving
ivermectin at 600 mcg/kg and in a CEA-affected male after receiving 200 mcg/kg.
The evolving signs included ataxia with gradually increasing hypermetria,
depression, tremors, paresis, recumbency, mydriasis, drooling, paralysis and
coma with predominantly diaphragmatic respiration. The affected dog in the
highest dose group was euthanized in extremis approximately 28 hours
after treatment, and the other affected dog died approximately 5l hours after
treatment.
The histologic findings, including pulmonary congestion, pulmonary edema
and multifocal hemorrhages in central nervous tissue were all likely related
to agonal death or the euthanasia technique and were not felt to be effects
of ivermectin treatment.
Ivermectin tissue assays showed high concentrations of ivermectin in the
central nervous system.
Two other dogs, one male in the 600 mcg/kg group and one female in the 200
mcg/kg group, both not affected with CEA, showed slight transitory signs.
The signs seen in the affected collies were similar to those seen in the
dogs in the toxicity and tolerance studies at doses of 2500 mcg/kg and greater.
Sensitivity to ivermectin did not appear to be related to CEA. None of the
collies given ivermectin at 50 mcg/kg showed signs of toxicity.
Breeding Animal Safety
Teratology in Laboratory Animals
Ivermectin has been shown to be teratogenic in rats, rabbits, and mice at
or near maternotoxic dose levels. At these high doses, evidence of a teratogenic
effect is limited to cleft palate that occurs at a low frequency in all three
species and clubbing of the forepaws which occurs only in rabbit fetuses.
Mice are the most sensitive species to the effects of Ivermectin with maternotoxlclty
at a dose of 200 mcg/kg/day and teratogenicity at 400 mcg/kg/day. In rabbits
6000 mcg/kg/day was maternotoxic and teratogenlc, and teratogenicity was also
evident at a dose of 3000 mcg/kg/day. The threshold for both maternotoxicity
and teratogenicity in rats was 10,000 mcg/kg/day.
TT#80-704-0 - Teratology in Dogs
A solution of ivermectin in sesame oil was administered by stomach tube
at a dose level of 500 mcg/kg to one group of 14 pregnant beagles on Days
5, 15, 25 and 35 of gestation, and a second group of 15 pregnant beagles received
the same dosage of ivermectin on Days 10, 20, 30 and 40 of gestation. An additional
group of 12 pregnant beagles served as controls and received the vehicle on
Days 5, 10, 15, 20, 25, 30, 35, and 40 of gestation. All dogs were hysterectomized
on Day 48 of gestation and the uterine contents removed and examined. All
fetuses were weighed and examined for external, visceral, and skeletal alterations.
The pregnant females were observed at ]east once daily for clinical signs
of toxicity and were weighed frequently during the trial.
There were no clinical signs of toxicity among pregnant female dogs receiving
ivermectin. Average maternal body weight gains were not affected by ivermectin
treatment.
Ivermectin had no effect on the reproductive status of pregnant bitches
as monitored by the number of implants, resporptions, and live and dead fetuses
per pregnant female.
There was no evidence of a teratogenic effect at external, visceral, or
skeletal examination of fetuses from bitches treated with ivermectin. At initial
examination, there was an apparent increase in sternebral variations in fetuses
from ivermectin-treated bitches. The evaluation of these variations is highly
subjective and a reexamination of the fetuses in this study by additional
pathologists, not advised of the treatment groups, found no meaningful differences
in the incidences of litters with these variations between treated and untreated
control groups. The sternebral ossification process and the size, shape and
orientation of sternebra are highly variable. The type of variations seen
in this study have been reported to occur at high and variable frequencies
in fetuses from untreated bitches in the colony used in this study and in
other colonies. These variations are not detectable after birth.
ASR 11014 - Safety in Breeding and Pregnant Bitches
Thirty non-pregnant beagle bitches of proven reproductive performance were
randomly allocated to two treatment groups with the first ten animals in each
group to come into estrus used to complete the trial. The two treatments were
vehicle or ivermectin at 600 mcg/kg in fractionated coconut oil; both were
administered orally. The bitches were treated monthly (at least twice) until
coming into estrus and being bred. Treatments were then administered on approximately
days 10, 25 and 45 of gestation (from first acceptance of the stud) and continued
on a monthly regimen until the pups were weaned at six weeks of age. Therefore,
each bitch that became pregnant received at least five or as many as ll treatments
depending on when she came into estrus.
All of the bitches were given physical examinations including evaluations
of hematology and serum chemistry and urinalysis prior to the first treatment
and at the end of the study. The animals were frequently observed throughout
the trial.
Pups were closely examined at whelping and at weaning. All dead pups including
any stillborns were necropsied.
No meaningful differences were detected between groups for incidence or
frequency of any clinical observations.
All bitches in the ivermectin group were successfully bred, while nine of
the 10 vehicle-treated bitches became pregnant.
An average of 6 live pups was born to each pregnant ivermectin-treated bitch,
with 93.3% of the pups whelped surviving to weaning. In the vehicle-treated
group, each pregnant bitch whelped an average of 5.44 pups with 85.7% of the
pups whelped surviving to weaning.
No anomalies were found in the bitches in either physical examination. Laboratory
studies detected some values outside of reference 'normal' parameters in both
treatment groups; however, no patterns were evident and the variations were
not attributed to treatment. No clinically meaningful differences were observed
between groups for bitch or puppy weights.
Congenital abnormalities were detected in four puppies from bitches in the
vehicle group and two puppies from the ivermectin group.
No bitches died during the trial. Seven puppies from the vehicle group and
four puppies from the ivermectin group died between birth and weaning.
None of the observations was related to ivermectin treatment, as they occurred
at similar or greater incidence in the control group.
From this trial, it is obvious that ivermectin, even at high and repeated
doses, has no effects on the breeding or reproductive performance of canine
females.
ASR 11015 - Safety in the Breeding Male (Stud) Dog
Twelve healthy male beagle dogs of proven breeding performance were divided
into six replicates of two dogs each in order of pretreatment average sperm
count ranking. Within each replicate, the dogs were randomly allocated to
receive either tap water (controls) or ivermectin at 600 mcg/kg in a fractionated
coconut oil solution. Treatments were administered at 30-day intervals from
the start of the trial to necropsy so that each stud received 8 treatments.
Semen samples were collected from each stud every third day beginning 28
days prior to the first treatment and continuing for 83 days after that treatment.
Standardized techniques were used in the collection and in the evaluation
of the semen samples. Beginning approximately two weeks after the last sample
was collected, each stud was bred to two bitches. The studs were necropsied
after the last of the bitches whelped.
The studs were given a physical examination prior to the first treatment
and at the end of the trial. Hematology, blood and serum chemistry and urinalyses
were performed prior to the first treatment and at the end of the study. The
studs were closely observed throughout the trial. The studs were weighed prior
to each treatment and at the end of the trial. The necropsy evaluation included
the gross examination of the organ systems as well as the histopathological
examination of the testes and epididymides.
The results of each stud being bred to two healthy untreated bitches were
evaluated for breeding behavior, fertility (conception rate), litter size,
puppy body weights and puppy abnormalities. Any puppies that died or were
born dead were closely examined or necropsied.
There were no meaningful differences between treatment groups in incidence
and frequency of significant clinical observations or in stud weight changes.
No anomalies were detected by the physical examinations of the studs before
or after treatment. While certain clinical chemistry and urinalysis values
for individual dogs in both the control and ivermectin treatment groups may
be considered to be outside expected normal values, these variations were
scattered randomly, occurring without apparent patterns. The variations were
not indicative of any clinical syndrome, were seen in both groups before treatment
as well as at the end of the trial and are not attributable to treatment.
No pattern of differences were found between ivermectin-treated dogs and
controls for total sperm counts, percent progressively motile sperm or speed
of progression of sperm. No deviations from normal were experienced for semen
color or pH, or for sperm morphology or abnormalities (primary, secondary
or tertiary).
One bitch bred to an ivermectin-treated stud did not become pregnant. The
first bitch bred to that stud did become pregnant. All other breedings in
both treatment groups were successful. The fertility index (number conceived
per number mated x 100) for the ivermectin group was 91.7 and 100 for the
controls. The mean fertility rate at the colony is 90.2%. Considering the
prolonged period of manual semen collection, no unexpected or breeding behavior
was experienced in the studs.
No abortions were observed in bitches bred to studs of either group.
No meaningful differences were observed between groups for litter size or
for average puppy weight in a litter. The only observed important abnormalities
were in puppies from the control group breeding and included one undershot
jaw, and one cleft palate.
At necropsy, no differences were observed histologically in the testes and
epididymides between the ivermectin-treated and water-placebo-treated-control
groups. The testes and epididymides of both groups were essentially normal.
Spermatogenesis occurred in an organized manner in both groups. No treatment
related pathologic lesions were found in any of the dogs.
This trial demonstrates that repeated administration of ivermectin at doses
many times greater than the dose for heartworm prevention has no clinically
significant effect on dogs and specifically demonstrates that ivermectin is
safe for use in breeding male (stud) dogs.
ASR 10843, 10844 and 10972 - Acceptability in Dogs With
Patent Heartworm Infections (Microfilariae-positive Doqs)
The effects of ivermectin treatment on dogs with patent heartworm infections
as indicated by the presence of circulating microfilariae was examined in
three similar studies (see table l for identification of trials).
In these trials, a total of 80 dogs with naturally acquired DirofiIaria
immitis infections confirmed using a modified Knott technique, were utilized.
The animaIs were mature dogs of both sexes and several different breeds and
crosses.
In each trial, the dogs were randomly allocated to the four treatment groups;
untreated control, ivermectin at 2 or 10 mcg/kg in the swallow tablet formulation
or at 400 mcg/kg in a fractionated coconut oil solution. Each of the ivermectin
treatments was administered orally three times at monthly intervals (28-30
days). The dogs were closely observed throughout the trial. Blood samples
were taken prior to the first treatment and several times during the trial
and evaluated using a modified Knott technique or direct count method. Patent
infections were assumed to continue as no adulticidal agent was administered.
Observations and sampling continued until the end of the trials, approximately
two weeks after the third treatment. The dogs were given a physical examination
prior to the initial treatment and at the end of the trials.
Adverse reactions, possibly related to the destruction of microfilariae
and release of antigen following the oral administration of ivermectin to
heartworm-positive dogs included varying levels of vomiting, soft, poorly-formed
feces and diarrhea (some with blood flecks or a bloody tinge). In ASR 10843,
most of these reactions were seen after the third treatment. More reactions
were observed in the dogs given ivermectin at 400 mcg/kg than in those given
2 or l0 mcg/kg.
The types of reactions seen in these trials are commonly observed in dogs,
were transitory and did not appear to endanger the dogs' health. None of the
dogs experienced the severe DEC-type shock reaction. These results agree with
the finding of Boreham (Boreham, P.F.L. and Atwell, R.B.: Absence of shock-like
reactions to ivermectin in dogs infected with Dirofilaria immitis .
Journal of Helminthology: 57, 279-281,1983).
Conclusions
With a dose of 6 mcg/kg being effective against the tissue 30 larval stage
of Dirofilaria immitis , HEARTGARD (ivermectin oral tablet) has a
wide margin of safety in normal dogs. When administered daily to beagles for
14 weeks, ivermectin at 500 mcg/kg had no adverse effects and 1,000 mcg/kg
had only mild effects. No adverse effects were detected after a single dose
of 2,000 mcg/kg. In acute toxicity studies, deaths did not occur at oral doses
below 40,000 mcg/kg. When toxic levels of ivermectin are administered to dogs,
the signs seen may include mydriasis, loss of pupillary response, loss of
menace reflex, depression, ataxia, tremors, recumbency, salivation, coma and
death.
Even in those individual dogs, particularly of the collie breed, that have
an unusual and extreme sensitivity to the toxic effects of ivermectin, an
adequate safety margin exists with HEARTGARD-30.
For dogs with patent heartworm infections, treatment with HEARTGARD-30 poses
no threat to their health.
Neither the reproductive performance nor the offspring of breeding females
or males were adversely affected by repeated, relatively high (500-600 mcg/kg)
doses of ivermectin.
(Eds. note: The following table consists of 5 columns.)
