Explain the mechanism of action of Selamectin against ectoparasites.

Selamectin exerts its antiparasitic effects primarily through its action on the nervous system of ectoparasites. The mechanism involves the enhancement of the influx of chloride ions across neuronal membranes, primarily by binding to certain glutamate-gated chloride channels and gamma-aminobutyric acid (GABA) receptors. This leads to an increase in the permeability of these neuronal membranes to chloride ions, resulting in hyperpolarization of the neuron.

When the neurons become hyperpolarized, the normal transmission of nerve impulses is disrupted, leading to paralysis of the parasite, which ultimately results in its death. This mechanism is particularly effective against insects and arachnids, making selamectin a valuable agent in treating infestations by fleas, ticks, and other ectoparasites in pets.

The other choices do not accurately describe the pharmacological action of selamectin. While immune system modulation or nutrient absorption interference are mechanisms seen with other drugs or conditions, they do not pertain specifically to selamectin’s action. Additionally, excessive shedding of the parasite is not a primary mechanism through which selamectin operates.