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 4^th 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.