If you have treated your dog for fleas, chances are you have used methoprene. Methoprene is a topical parasitic insect treatment; you apply it to the skin at the base of the fur. This chemical compound is an insect development inhibitor, halting the growth stages of flea larvae. Often combined with other chemicals that target and kill adult fleas, methoprene works on the earlier pest stages, increasing the product’s effectiveness.
As an insect growth regulator, methoprene in not considered a pesticide as it does not kill the fleas and other insects. Instead, it interferes with normal life cycle, preventing adults from laying eggs, preventing laid eggs from hatching, and making it impossible for larvae to mature into adult fleas. Since it inhibits transformation from one stage to another, it's effectively useless on adult specimens -- so it often serves in combination with other pesticide products that kill adult fleas.
While methoprene is safe to use as a topical treatment when it's used as directed, high doses can cause complications for dogs. These include vomiting, dilated pupils, problems breathing, mobility issues and changes in behavior. Human contact to concentrated doses can result in skin irritation as well as eye or lung irritation. Because methoprene is not an insecticide, it is considered a safer alternative. When used for flea prevention, methoprene can reduce the need for pesticide use to kill adult fleas.
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