FENBENDAZOLE
FENBENDAZOLE
Fenbendazole, methyl[5-(phenylthio)-1H-benzimidazol-2-yl] carbamate, has a
broad spectrum, is very safe, and is widely used against mammalian nematodes.
ACTIVITY Fenbendazole is active against all important nematodes, including
inhibited larvae. It is useful for treating hypobiotic larvae of Toxocara in
bitches prior to whelping. It also has some tapeworm activity.
PHARMACOKINETICS
Fenbendazole
is only marginally absorbed after oral administration. After oral dosing in
calves and horses, peak blood levels of 0.11 micrograms/mL and 0.07
micrograms/mL, respectively, were measured. Absorbed fenbendazole is
metabolized (and vice-versa). In sheep, cattle, and pigs, 44 – 50% of a dose of
fenbendazole is excreted unchanged in the feces, and <1% in the urine.
METABOLISM Fenbendazole is slowly absorbed from the gut, and peak blood levels are
attained 28-30 h post-treatment. Amongst the many metabolites are the corresponding
sulphone and sulphoxide. The latter is identical to oxfendazole and may be
responsible for its activity. Up to 35% of the dose is eliminated in the urine,
most is excreted in faeces. The withdrawal period for fenbendazole is 14 days
for meat and 72 h for milk.
TOXICITY Fenbendazole is extremely safe with a safety factor in excess of 100 times
the recommended dose. Cattle with heavy lungworm infections may suffer an allergic response to treatment as a result
of killing worms in situ. Fenbendazole
is not embryotoxic.
INDICATIONS. Gastro-intestinal roundworms in horses, ruminants, pigs,
dogs, cats, and pigeons; Type II ostertagiosis; transplacental roundworm
transmission in dogs; lungworms in ruminants, pigs, dogs, and cats; Trichostrongylus tenuis in grouse; tapeworms in
ruminants; Taenia in dogs and cats; Giardia in dogs.
Fenbendazole
is indicated for the removal of the following parasites in dogs: ascarids (Toxocara canis, T. leonina), Hookworms (Ancylostoma caninum, Uncinaria steno cephala), whipworms (Trichuris vulpis), and tapeworms (Taenia pisiformis). It is not effective against Dipylidium caninum. Fenbendazole has also been used clinically to treat Capillaria aerophilia, Filaroides hirthi, and Paragonimus
kellicotti infections in dogs.
Fenbendazole
is indicated for the removal of the following parasites in cattle: Adult forms
of: Haemonchus contortus, Ostertagia ostertagi,
Trichostrongylus axei, Bunostomum phlebotomum, Nematodirus helvetianus,
Cooperia spp., Trichostrongylus colubriformis,
Oesophagostomum radia tum, and Dictyocaulus
vivaparus. It is also effective against most immature stages of
the above listed parasites. Although not approved, it has good activity against
Moniezia spp., and arrested
4th stage forms of Ostertagia ostertagi.
Fenbendazole is indicated for the removal of the following parasites in
horses: large strongyles (S. edentatus, S. equinus, S. vulgaris), small strongyles (Cyathostomum spp., Cylicocylus spp., Cylicostephanus spp., Triodontophorus spp.), and pinworms (Oxyuris equi).
Fenbendazole
is labeled for the removal of the following parasites in swine: large round
worms (Ascaris suum), lungworms (Metastrongylus pair), nodular worms (Oesphagostomum denta tum, O. quadrispinolatum), small stomach worms (Hyostrongylus rubidus), whipworms (Trichuris suis), and kidney worms (Stephanurus dentatus; both mature and immature).
Although
not approved, fenbendazole has been used in cats, sheep, goats, pet birds, and
llamas.
DOSAGE. By mouth.
CATTLE: 7.5 mg/kg as a
single dose or in divided
doses over 5 or 10 days (may not be effective against Trichuris, Strongyloides)
For removal/control of Haemonchus contortus, Ostertagia ostertagi, Trichostrongylus axei,
Bunostomum phlebotomum, Nematodirus helvetianus, Cooperia spp., Trichostrongylus colubri formis, Oesophagostomum
radiatum, and Dictyocaulus
vivaparus:
a) 5
mg/kg, PO
b) 7.5
mg/kg, PO
c) 4 mg/kg PO; under conditions of
continuous exposure to parasites, animals may need to be retreated after 4 – 6
weeks,
For Moniezia spp., and arrested 4th stage forms of Ostertagia
ostertagi:
a) 10
mg/kg, PO
For
giardiasis in calves:
a) 15 mg/kg PO for 3 successive days
and then moved to a pen that was thoroughly cleaned and disinfected with 10%
ammonia.
SHEEP: 5 mg/kg as a single
dose
For
susceptible parasites:
a) 5 mg/kg
in feed for 3 days
PIGS: roundworms, 5 mg/kg as a single dose Trichuris, Metastrongylus apri, 5 mg/kg in
divided doses over 7 days
For
susceptible parasites:
a) 5 mg/kg
PO; 3 mg/kg in feed for 3 days; 10 mg/kg for ascarids
b) For
whipworms in potbellied pigs: 9 mg/kg PO for days
HORSES 5-10 mg/kg.
Roundworms, 7.5 mg/kg as a single dose;
Larval Trichonema (cyathostomes), 30 mg/kg as a single dose or 7.5 mg/kg daily for 5 days; Migrating strongyles, 60
mg/kg as a single dose or 7.5 mg/kg
daily for 5 days;
Strongyloides westeri in foals, 50 mg/kg as a single dose
For
susceptible parasites:
a) For
control of large and small strongyles, and pinworms in adult horses: 5 mg/kg
PO;
For foals and weanlings (less than 18
months of age) where ascarids are a common problem: 10 mg/kg PO;
For control of encysted early 3rd
stage, late 3rd stage and 4th stage cyathostome larvae and 4th stage
Strongylus vulgaris larvae) 10
mg/kg PO for 5 consecutive days.
For
treatment of migrating large strongyles:
a) 50
mg/kg PO for 3 consecutive days, or 10 mg/kg for 5 consecutive days
For
mucosal stage of small strongyles:
a) 7.5 – 10 mg/kg PO once daily for 5
days; a single dose of 30 mg/kg is effective against older encysted stages
DOGS 100 mg/kg or 20 mg/kg for 5 days.
For roundworms, tapeworms, Giardia, treatment, (adults) 50 mg/kg daily for 3
days
prophylaxis, (< 6 months of age) 50
mg/kg daily for 3 days; (adults) 100 mg/kg as a single dose
Transplacental transmission, 25 mg/kg
daily from day 40 of pregnancy until 2 days post partum
Lungworms, 50
mg/kg daily for 7 days
1) For susceptible ascarids, hookworms,
whipworms, and tapeworms (Taenia spp. only):
a) 50
mg/kg, PO for 3 consecutive days
b) 55
mg/kg, PO for 3 days (5 days for Taenia)
b. To prevent transplacental and
transmammary transmission of somatic T. canis and A. caninum:
2) 50 mg/kg PO once daily from the 40th day
of gestation to the 14th day of lactation.
3) For Capillaria
plica:
a) 50
mg/kg once daily for 3 days; repeat a single 50 mg/kg dose 3 weeks later
b) 50
mg/kg, PO daily for 3 – 10 days
4) For Capillaria
aerophilia:
a) 25 – 50
mg/kg q12h for 10 – 14 days
b) 50
mg/kg PO once daily for 10 – 14 days
5) For Filaroides
hirthi:
a) 50 mg/kg, PO once daily for 14 days.
Symptoms may worsen during therapy, presumably due to a reaction when the worm dies.
b) 50
mg/kg PO once daily for 10 – 14 days
6) For Paragonimus
kellicotti:
a) 25 – 50
mg/kg PO twice daily for 10 – 14 days
b) 50
mg/kg PO once daily for 10 – 14 days
c) 50 mg/kg, PO once daily for 3
consecutive days; repeat in 2 – 3 weeks and again in 2 months
7) For Crenosoma
vulpis:
a) 50
mg/kg PO once daily for 3 days
8) For Giardia:
a) 50
mg/kg PO once daily for 3 days
b) 25
mg/kg PO q12h for 3 – 7 days
9) For Eucoleus
boehmi:
a) 50
mg/kg PO once daily for 10 – 14 days; improvement may only be temporary
CATS: roundworms,
tapeworms, treatment, (adults) 50 mg/kg daily for 3 days
prophylaxis, (< 6 months of age) 50
mg/kg daily for 3 days; (adults) 100 mg/kg as a single dose
Pregnant
queens, 100 mg/kg as a single dose
Lungworms, 50
mg/kg daily for 3 days
CATS, DOMESTIC:
1. For
susceptible ascarids, hookworms, strongyloides, and tapeworms (Taenia spp. only):
a) 50
mg/kg, PO for 5 days
2. For
lungworms (Aelurostrongylus abstrusus):
a) 25 – 50
mg/kg q12h for 10 – 14 days
b) 50
mg/kg, PO for 10 days
c) 20
mg/kg PO once daily for 5 days; repeat in 5 days
3. For
lungworms (Capillaria aerophilia):
a) 50
mg/kg, PO for 10 days
b) 50
mg/kg PO once daily for 10 – 14 days
4. For Capillaria feliscati:
a) 25
mg/kg, twice daily PO for 3 – 10 days
b) 25
mg/kg, PO q12h for 10 days
5. For Paragonimus kellicotti:
a) 25 – 50
mg/kg PO twice daily for 10 – 14 days
b) 50
mg/kg PO once daily for 10 – 14 days
6. For Eurytrema procyonis (pancreatic fluke):
a) 30
mg/kg, PO daily for 6 days
7. For
Giardia:
a) In
young kittens: 50 mg/kg PO (using the suspension) once a day for 3 – 5 days
b) 50
mg/kg PO q24h for 3 – 5 days
CATS, LARGE (EXOTIC):
For
labeled parasites:
a) 10
mg/kg PO once daily for 3 consecutive days.
GROUSE: Trichostrongylus, by addition to feed, 7–10 mg/kg body-weight given in divided
doses over 14 days; 1 kg/tonne feed
PIGEONS: 20 mg/kg
BEARS (URSIDAE):
For
labeled parasites:
a) 10
mg/kg PO once daily for 3 consecutive days.
SMALL MAMMALS/RODENTS:
a) For
pinworms in mice, rats, hamsters, gerbils and rabbits: 50 mg/kg PO once
b) For
Giardia in Chinchillas: 25 mg/kg PO once a day for 3 days
c) Mice,
Rats, Gerbils, Hamsters, Guinea pigs, Chinchillas:
20 – 50
mg/kg PO once daily for 5 days (Higher dose is for Giardia)
LLAMAS:
For
susceptible parasites:
a) 10 – 15
mg/kg PO (as paste or suspension)
b) 5 – 10 mg/kg PO for 1 – 3 days.
Fenbendazole and ivermectin are the most effective and safest anthelmintics for
use in llamas.
BIRDS:
a) For Ascarids: 10 – 50 mg/kg PO once;
repeat in 10 days. Do not use during molt (may cause stunted feathers) or while
nesting.
For flukes
or microfilaria: 10 – 50 mg/kg PO once daily for 3 days
For Capillaria: 10 – 50 mg/kg PO once daily for 5 days. Is not effective against
gizzard worms in finches.
b) For nematodes, some trematodes: 10 –
50 mg/kg PO once daily for 3 – 5 days; 20 – 100 mg/kg oral single dose range;
125 mg/L of drinking water for 5 days (50 mg/L for 5 days in finches); or 100
mg/kg of feed for 5 days. Not recommended to be used in breeding sea son during
molting.
c) Ratites: 15 mg/kg PO once daily for
3 days. Has efficacy against ostrich tapeworm. (Houttuynia struthionus)
REPTILES:
For
susceptible infections:
a) For
most species: 50 – 100 mg/kg PO once; repeat in 2 – 3 weeks; very effective
against Strongyloides.
CONTRA-INDICATIONS. Administration within 14 days of treatment for liver fluke;
administration of ruminal boluses to non-ruminating cattle or cattle less than
100 kg bodyweight and 3 months of age, concurrent administration of other
ruminal boluses; treatment of grouse after March; Fenbendazole is not approved for use in horses intended for food purposes.
ADVERSE EFFECTS
At
usual doses, fenbendazole generally does not cause any adverse effects. hypersensitivity
reactions secondary to antigen release by dying parasites may occur, particularly
at high dosages. Vomiting may infrequently occur in dogs or cats receiving
fenbendazole. Pancytopenia has been reported in one dog. Single doses (even at
exaggerated doses) are not effective in dogs and cats; must treat for 3 days.
REPRODUCTIVE/NURSING SAFETY
Fenbendazole
is considered safe to use in pregnant bitches and is generally considered safe
to use in pregnancy for all species. In a system evaluating the safety of drugs
in canine and feline pregnancy, this drug is categorized as in class: A (Probably safe. Although specific studies may not have proved the safety
of all drugs in dogs and cats, there are no reports of adverse effects in
laboratory animals or women.)
OVERDOSAGE/TOXICITY
Fenbendazole
is apparently well tolerated at doses up to 100X recommended. The LD50 in
laboratory animals exceeds 10 grams/kg when administered PO. It is unlikely an
acute overdosage would lead to clinical signs.
WARNINGS. If cattle are vaccinated against lungworm, the ruminal
bolus should not be administered until 14 days after the second dose of
vaccine; treatment of pigeons when rearing
young and
during the main moult not recommended
DRUG INTERACTIONS
BROMSALAN
FLUKICIDES (dibromsalan, tribromsalan; not available in
the USA): Oxfendazole or fenbendazole should not be given concurrently with the
bromsalan flukicides; abortions in cattle and death in sheep have been reported
after using these compounds together.
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