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Overview 

Fenbendazole is a broad-spectrum antiparasitic medication that is used to treat gastrointestinal parasites such as roundworms, giardia, hookworms, pinworms, whipworms, strongyles, strongyloides, and paragonimiasis in sheep, horses, cattle, dogs, cats, fish, seals, and rabbits.

Livestock, pets, and poultry can all be treated with fenbendazole as they seem to tolerate this medication quite well.

Fenbendazole is not intended for use by humans but there is a reported case where the drug has been successfully used to manage a parasitic condition known as visceral larva migrans.

Toxicity

Generally, poisoning with fenbendazole is unusual because the medication has a low toxicity and high safety margin and most animal species tolerate the drug very well. It is reported to be quite toxic to aquatic life though so the correct dosage for these organisms is especially important.

There are certain situations where specific animals may be affected though:

  • Administering more than the recommended dosage for certain animals may lead to side effects and unwanted adverse reactions. The safety margin for animals are up to 20mg per kg in cattle, 500mg per kg in sheep, 100mg per kg in horses, 10mg per kg in dogs, and 3mg per kg in cats and pigeons.
  • In pigs, a dose of 125mg per kg per day over five days, or a total of 2000mg per kg over 15 days of administration of the drug may result in reversible leucopenia (decreased white cell count) which resolves 15 days after discontinuation of the medicine.
  • Teratogenic abnormalities were reported in pregnant bitches after they received a treatment of fenbendazole of three times the recommended dosage.
  • Therapeutic doses of the medication in young pigeons can result in damage to their plumage.
The correct use of fenbendazole in animals is unlikely to be detrimental to the environment and will not cause any issues in humans who ingest foods and water exposed to the medicine as well as to coprophagous insects. 

Side effects and adverse drug reactions 

The following are side effects and adverse reactions that may occur in animals who are administered fenbendazole:

  • Occasional vomiting was reported in cats and dogs at therapeutic doses.
  • Allergic reactions may occur due to the sudden death of parasitic worms as a result of the release of large amounts of allergens. 
  • When administered together with bromsalan derivatives, the medication may cause acute intoxication in sheep and cattle.

Precautions and warnings

  • Pregnant queens (cats) and bitches shouldn't be treated with fenbendazole.
  • During the first 3 months of gestation, pregnant sows should also not be treated with the medication.
  • Birds that are molting should not be treated with fenbendazole. 
  • Small dogs should not be treated with fenbendazole preparations developed for larger dogs. Sometimes, in order to save money, pet owners buy larger tablets and divide them up to use in smaller dogs. This can lead to an overdose of the medication and the animals will experience unwanted side effects.
  • Preparations for dogs should not be administered to cats as the medication may contain ingredients that are toxic to the latter.
  • Products developed for livestock should not be used in cats or dogs unless prescribed by a veterinary doctor.

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