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Doramectin is a broad-spectrum antiparasitic active ingredient used in veterinary medicine in livestock against external parasites (lice, mites, 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: DORAMECTIN

Type: veterinary medicine, depending on usage
Chemical class: macrocyclic lactone

CHEMICAL STRUCTURE

Molecular structure of DORAMECTIN

 

 

 

 

 


EFFICACY AGAINST PARASITES

Type of action: Systemic & contact broad-spectrum ectoparasiticide and endoparasiticide.
Main veterinary parasites controlled: Gastrointestinal and pulmonary roundworms, lice, mites, horn flies; myiasis by screwworms, bot flies, warble flies, etc.

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 doramectin used in domestic animals.


SAFETY

Oral LD50, rat, acute*:  50-2000 mg/kg, depending on the vehicle
Dermal LD50, rat, acute*: >2000 mg/kg
* 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) set for animak tissues (e.g. beef, mutton pork or chicken)*:

  • CODEX: Yes
  • EU: Yes
  • USA: Yes
  • AUS: Yes

* This information is an indicator of the acceptance of an active ingredient by the most influential regulatory bodies for use on livestock. MRL's for animal tissues may be set also for agricultural pesticides that are not approved for use on animals but are used on commodities fed to animals. A MRL may be also set in the form of an IMPORT TOLERANCE for active ingredients not approved in a particular country but approved for imported animal commodities.

Withholding periods for meat, milk, eggs, etc. depend on delivery form, dose and national regulations. Check the product label in your country.

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 doramectin safety.

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

WARNING

Never use products for livestock on dogs and cats unless they are explicitly approved for both livestock and pets. Pets may not tolerate livestock formulations.

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: DECTOMAX
Patent: Expired (particular formulations may be still patent-protected)

Use in LIVESTOCK: Moderate
Use in HORSES:
Very scarce
Use in
DOGS and CATS: NO
Main delivery forms

Use in human medicine: No
Use in
public/domestic hygiene: No
Use in
agriculture: No
Generics available: 
Yes, a few


PARASITE RESISTANCE

In livestock: Yes, cross resistance with ivermectin, very frequent in gastrointestinal roundworms of sheep, goats and cattle. There are a few reports on cattle ticks (Boophilus microplus) resistance to ivermectin likely to show cross-resistance to doramectin.

Learn more about parasite resistance and how it develops.


SPECIFIC FEATURES

Doramectin is a semi-synthetic macrocyclic lactone obtained from fermentation products of the soil microorganism Streptomyces avermitilis. It has quite similar features to those of ivermectin. The spectrum of efficacy is almost identical.

Doramectin is ZOETIS' (formerly PFIZER ANIMAL HEALTH) macrocyclic lactone for livestock. It is not used in dogs and cats because PFIZER developed its own endectocide for dogs and cats called selamectin.

Efficacy of doramectin

Doramectin is effective against the major parasitic roundworms: gastrointestinal (e.g. Haemonchus spp, Cooperia spp, Ostertagia spp, Trichostrongylus spp) and pulmonary (e.g. Dictyocaulus spp). It is also effecive against most mites and lice species, and against numerous myiases (e.g. those caused by screwworm flies, bot flies and warble flies) usually regardless of the delivery form (pour-on, injectable, etc).

However, at the usual therapeutic dose, doramectin products are ineffective against tapeworms and flukes, mosquitoesfleas, horse and deer flies, stable flies and several other flying insects. Cattle horn flies are only controlled by pour-on formulations. Cattle ticks (Boophilus spp) are only sufficiently controlled by pour-ons. Other ticks (e.g. Amblyomma spp, Hyalomma spp Dermacentor spp, Ixodes spp, Rhipicephalus spp) are not controlled at all by doramectin products.

Some studies indicate the doramectin (as a pour-on) is slightly more efficacious against cattle ticks (Boophilus microplus) than ivermectin. Injectable doramectin at the usual dose (200 mcg/kg bw) seems to be also more efficient against sheep scab mites (Psoroptes ovis) than ivermectin.

Doramectin has a slightly longer residual effect than ivermectin, but this strongly depends on the formulation, the delivery form and the administered dose.

Nevertheless, the difference between doramectin and other macrocyclic lactone for livestock is rather small. This probably explains why there are thousands of generic ivermectin products, and only a few doramectin generics. Generic ivermectin was available earlier than generic doramectin, and for most uses on livestock ivermectin is good enough.

Pharmacokinetics of doramectin

Halftime for absorption into blood after injection in a sesame oil vehicle is shorter for goats (~0.7 days) than for cattle (~2.4 days) or sheep (~2.7 days).

Doramectin is well distributed throughout the whole body including target organs such as the gastric and gut mucosae. The highest concentrations are found in body fat that acts as a depot from where it is progressively released to blood. Bioavailability of doramectin after injection is almost 100%, but only about 18% after pour-on administration, which requires a higher dose. Excretion goes through bile and feces, 50 to 70% in the form of the unchanged parent molecule. The highest concentration in blood occurs 24 to 48 hours after administration.

Mechanism of action

As all macrocyclic lactone doramectin 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 some dog breeds that carry the MDR-1 mutation.

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