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Selamectin is a broad-spectrum antiparasitic active ingredient used in veterinary medicine in dogs and cats against external parasites (lice, mites, fleas, etc.) as well as against internal parasites (e.g. roundworms). It is not used against agricultural and household pests. It belongs to the chemical class of the macrocyclic lactones.

Common name: SELAMECTIN

Type: veterinary medicine
Chemical class: macrocyclic lactone

CHEMICAL STRUCTURE

Molecular structure of SELAMECTIN

 

 

 

 

 

 

 

 

 

 


EFFICACY AGAINST PARASITES

Type of action: Systemic & contact broad-spectrum ectoparasiticide and endoparasiticide.
Main veterinary parasites controlled: Roundworms (nematodes), incl. heartworm (Dirofilaria spp) prevention, fleas, lice, mites, and certain tick species

Efficacy against a specific parasite depends on the delivery form and on the dose administered.

Click here for general information on features and characteristics of PARASITICIDES.


DOSING

Click here to view the article in this site with the most common dosing recommendations for selamectin used in domestic animals.


SAFETY

Oral LD50, rat, acute*:  >1600 mg/kg
Dermal LD50, rat, acute*: not found
* These values refer to the active ingredient. Toxicity has to be determined for each formulation as well. Formulations are usually significantly less toxic than the active ingredients.

MRL (maximum residue limit): Not applicable: not approved for livestock
Withholding periods for meat, milk, eggs: Not applicable: not approved for livestock

WARNING: Dogs of some breeds do not tolerate macrocyclic lactones or other medicines (e.g. emodepside) that can cross the blood-brain barrier. They can suffer more or less serious adverse effects if treated at dose rates slightly higher than the recommended ones. Consequently dosing must be as accurate as possible. This is the case for Collies and related breeds, which have a mutation in the MDR-1 gene that affects the blood-brain barrier and makes it more permeable to such compounds than in dogs without this mutation. Besides Collies, other dog breeds have shown similar problems, although the MDR-1 mutation has not been confirmed in all of them. The breeds more affected by this mutation are (% frequency): Collie (70%), Long-haired Whippet (65%), Australian Shepherd (50%, also mini),  McNab (30%), Silken Windhound (30%), English Shepherd (15%), Shetland Sheepdog (15%), English Shepherd (15%), German Shepherd (10%), Herding Breed Cross (10%). Other less affected breeds are: Old English Sheepdog, Border Collie, Berger Blanc Suisse, Bobtail, Wäller. The only way to be sure that a dog is affected or not is to test for it. As more dogs are tested it is likely that the mutation is discovered in other breeds, or that the frequencies change.

Learn more about selamectin safety.

General information on the safety of veterinary antiparasitics is available in specific articles in this site (click to visit):

WARNING: It is obvious that veterinary products are not intended for and should never be used on humans!!!


MARKETING & USAGE

Decade of introduction: 1990
Introduced by: PFIZER
Some original brands: REVOLUTION, STRONGHOLD
Patent: Expired (particular formulations may be still patent-protected)

Use in LIVESTOCK: No
Use in HORSES: NO
Use in
DOGS and CATS: Yes, moderate
Main delivery forms: 

Use in human medicine: No
Use in
public/domestic hygiene: No
Use in
agriculture: No
Generics available: 
Not yet...

COMMERCIAL BRANDS WITH SELAMECTIN


PARASITE RESISTANCE

In pets: Yes, reported to ivermectin in heartworm microfilariae in the USA (so far particularly in the South), with cross-resistance to macrocyclic lactones, most likely including also selamectin.

Learn more about parasite resistance and how it develops.


SPECIFIC FEATURES

Selamectin is a semi-synthetic macrocyclic lactones obtained from fermentation extracts of the soil microorganism Streptomyces avermitilis. Selamectin is the pet endectocide from ZOETIS (formerly PFIZER ANIMAL HEALTH), which has another endectocide specific for livestock, doramectin. In fact, selamectin is a derivative of doramectin.

Efficacy of selamectin

Selamectin is a special case among the macrocyclic lactone. At the therapeutic dose it is the only one that is effective against fleas and provides a certain control of a few pet ticks as well (Rhipicephalus sanguineusDermacentor spp), but not of other tick species (e.g. Amblyomma spp, Ixodes spp, etc.) important for dogs.

Otherwise selamectin is an effective heartworm (Dirofilaria spp) preventative and controls a few other pet roundworms as well (e.g. Toxocara canisAncylostoma spp) It also controls a few mite and lice species. However, selamectin is ineffective against tapeworms, flukes, mosquitoes and any flies.

Pharmacokinetics of selamectin

In dogs, after topical spot-on administration selamectin spreads rapidly through the dogs hair-coat within the first 24 hours. Only a small part of the administered dose is absorbed through the skin into the bloodstream. Highest blood levels are achieved about 70 hours after administration. In cats the amount absorbed into blood is significantly higher, probably due to the thinner cat skin and to oral intake after grooming. As a consequence, bioavailability is much higher in cats (~75%) than in dogs (~5%).

Excretion runs faster in dogs than in cats. In both dogs and cats, most selamectin is excreted through feces and only a small amount through urine, mostly in the form of the parent molecule.

Mechanism of action of selamectin

As all macrocyclic lactones selamectin acts as agonist of the GABA (gamma-aminobutyric acid) neurotransmitter in nerve cells and also binds to glutamate-gated chloride channels in nerve and muscle cells of invertebrates. In both cases it blocks the transmission of neuronal signals of the parasites, which are either paralyzed and expelled out of the body, or they starve. It also affects the reproduction of some parasites by diminishing oviposition or inducing an abnormal oogenesis.

In mammals the GABA receptors occur only in the central nervous system (CNS), i.e. in the brain and the spinal chord. But mammals have a so-called blood-brain barrier that prevents microscopic objects and large molecules to get into the brain. Consequently macrocyclic lactones are much less toxic to mammals than to the parasites that do not have such a barrier, which allows quite high safety margins for use on livestock and pets. A notable exception to this are those dogs that carry the MDR-1 mutation previously mentioned.

Click here to view the list of all technical summaries of antiparasitic active ingredients in this site.