Pharmacotherapeutic group: endectocide (milbemycin family)
ATCvet code: QP54AB02
Pharmacodynamic properties
Moxidectin is a parasiticide active against a wide range of important internal and external parasites. It is a second generation macrocyclic lactone of the milbemycin family. Its principal mode of action is interference with the GABA (gamma amino butyric acid) receptors involved with neuromuscular transmission.
Moxidectin stimulates the release of GABA and increases its binding to the postsynaptic receptors. The net effect is to open the chloride channels on the postsynaptic junction to allow the inflow of chloride ions and induce an irreversible resting state. This results in flaccid paralysis and eventual death of parasites exposed to the drug.
Pharmacokinetic particulars
Following pour-on application, the drug is distributed throughout the body tissues (except muscle) but due to its lipophilicity the concentrations in fat are 5-15 times those in other tissues.
Moxidectin undergoes partial biotransformation by hydroxylation in the body and the only significant route of excretion is the faeces, where the parent compound accounts for approximately 50%.
Environmental properties
Moxidectin fulfils the criteria for a (very) persistent, bioaccumulative and toxic (PBT) substance. In particular, in acute and chronic toxicity studies with algae, crustaceans and fish, moxidectin showed toxicity to these organisms, yielding the following endpoints:
Organism | EC50 | NOEC |
Algae | S. capricornutum | >86.9 μg/l | 86.9 μg/l |
Crustaceans (Water fleas) | Daphnia magna (acute) | 0.0302 μg/l | 0.011 μg/l |
Daphnia magna (reproduction) | 0.0031 μg/l | 0.010 μg/l |
Fish | O. mykiss | 0.160 μg/l | Not determined |
L. macrochirus | 0.620 μg/l | 0.52 μg/l |
P. promelas (early life stages) | Not applicable | 0.0032 μg/l |
| Cyprinus carpio | 0.11 μg/l | Not determined |
EC50: the concentration which results in 50% of the test species individuals being adversely affected, i.e. both mortality and sub-lethal effects.
NOEC: the concentration in the study at which no effects are observed.
This implies that when allowing moxidectin to enter water bodies, this may have a severe and lasting impact on aquatic life. To mitigate this risk, all precautions for use and disposal must be adhered to.