Milbemycin Oxime
Milbemycin oxime is a **macrolide anthelmintic** (macrocyclic lactone) widely used in veterinary medicine. - **Primary use**: Monthly heartworm (*Dirofilaria immitis*) prophylaxis and intestinal parasite control (hookworms, roundworms, whipworms). - **Off-label uses**: Treatment of generalized demodicosis, scabies, cheyletiellosis, and nasal mites. > **Clinical Pearl**: While generally safe, caution is required when using high, daily doses for demodicosis in breeds with the **ABCB1 (MDR1) mutation** (e.g., Collies, Australian Shepherds, Shelties), as they are more susceptible to neurologic toxicity. It is often considered a safer alternative to ivermectin for these breeds, but genetic testing is still recommended before initiating high-dose therapy.
Mechanism: Milbemycin acts primarily on the nervous system of invertebrates. - Binds to **glutamate-gated chloride channels** and **GABA receptors** in the parasite's nervous system. - This binding increases membrane permeability to chloride ions โ **hyperpolarization** of nerve and muscle cells. - Results in flaccid paralysis and death of the parasite. *Note*: Mammals are generally protected because their GABA receptors are confined to the central nervous system, and intact P-glycoprotein (P-gp) pumps at the blood-brain barrier prevent the drug from accumulating in the CNS.
Dosing by species
- Prevention of heartworm; treat adult hookworm and adult roundworms ยท 2 mg/kg PO once monthly ยท PO ยท q30d
- Heartworm prophylaxis, control of adult hookworms, roundworms, and whipworms ยท 0.5 mg/kg PO once a month ยท PO ยท q30d ยท Minimum dosage. Can also be used with nitenpyram for flea control.
- Microfilaricide chemotherapy (in adulticide-pretreated dogs) ยท 0.5 mg/kg PO ยท PO ยท once ยท Repeat in 2 weeks if necessary. Give approximately one month after melarsomine.
- Treatment of generalized demodicosis ยท 1-3 mg/kg/day PO ยท PO ยท q24h ยท Until resolved ยท Begin at 1 mg/kg/day with skin scraping after 30 days. Increase to 2 mg/kg if minimal improvement, then 3 mg/kg if needed. Dogs homozygous for MDR1 mutation can show neuro side effects at 1-2.2 mg/kg/day.
- Treatment of cheyletiellosis ยท 2 mg/kg PO every 7 days for 3 doses ยท PO ยท q7d ยท 3 doses
- Treatment of scabies ยท 2 mg/kg PO every 7 days for 3 doses or 0.75 mg/kg once daily for 30 days ยท PO ยท q7d or q24h ยท 3 doses or 30 days
- Chronic rhinitis caused by Pneumonyssus caninum ยท 1 mg/kg PO once every 10 days for 3 treatments ยท PO ยท q10d ยท 3 treatments
- Nematodes ยท 0.5-1 mg/kg PO; repeat in 2 weeks ยท PO ยท once, repeat in 14 days ยท Until clear ยท If 14 days after second dose fecal is positive, a third dose is given. Appears to be safe in chelonians (unlike ivermectin).
Routes of administration
Contraindications
- Puppies less than 4 weeks of age or less than 2 lbs
- Kittens less than 6 weeks of age or less than 1.5 lbs
Adverse effects
- Transient shock-like syndrome (in microfilaremic animals)
- Ataxia
- Trembling
- Mydriasis
- Hypersalivation
- Lethargy
- Pyrexia
- Seizures
- Coma
Drug interactions
- Benzodiazepines ยท Effects may be potentiated by milbemycin; concurrent use is not advised in humans and warrants caution in animals.
- P-glycoprotein inhibitors (Amiodarone, Ketoconazole, Cyclosporine, Diltiazem, Erythromycin, Clarithromycin, Quinidine, Spironolactone, Tamoxifen, Verapamil) ยท May inhibit the efflux pump at the blood-brain barrier, increasing the risk of milbemycin neurotoxicity, particularly in dogs at risk for the ABCB1-1 (MDR1) mutation.
Monitoring
- Heartworm status (prior to initiation of therapy)
- Neurologic signs (especially in susceptible breeds or at high doses)
- Skin scrapings (if treating demodicosis)
Overdose
Beagles have tolerated a single oral dose of 200 mg/kg (200 times monthly rate). Rough-coated collies have tolerated doses of 10 mg/kg (20 times labeled) without adversity. Toxic doses can cause mydriasis, hypersalivation, lethargy, ataxia, pyrexia, seizures, coma, and death. There is no specific antidotal treatment; supportive therapy is recommended.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturerโs current label.