Acyclovir

Antiviral for equine herpesvirus (EHV-1, EHV-4) including herpesviral myeloencephalopathy; feline herpesvirus-1 ocular disease. Note: poor oral bioavailability in horses; valacyclovir generally preferred. Synthetic purine nucleoside analogue antiviral. Mechanism: phosphorylated by viral thymidine kinase to active triphosphate form, which selectively inhibits viral DNA polymerase and terminates viral DNA chain elongation. Active against herpesviruses (EHV-1, EHV-4, FHV-1, HSV). Poor oral bioavailability in horses (~3%); IV or topical preferred. Not toxic to uninfected cells. Key information Nephrotoxic — ensure adequate hydration during IV use. Primarily used in birds (parrots) and exotic species; limited equine use for EHV. Not absorbed orally in horses. Store at room temperature.

Oral: Administer orally. May be given with food to improve compliance and reduce GI upset. Tablets may be crushed. Horses: oral bioavailability is very poor — IV administration preferred for systemic therapy; dilute in 0.9% NaCl and infuse over 1 hr. Topical ophthalmic: apply 1 cm ribbon to conjunctival sac q4–6h. Store at room temperature; protect from light. If a dose is missed Administer as soon as you remember. If it is almost time for the next scheduled dose, skip the missed dose and resume the normal schedule. Never double up doses to make up for a missed one. Complete the prescribed course.

Nephrotoxicity (crystalluria/tubular obstruction) with IV use — ensure adequate hydration. GI: nausea, vomiting, diarrhea. CNS: lethargy, ataxia, tremors at high doses. Horses: poor oral bioavailability limits efficacy but also reduces systemic side effects with oral dosing.

For animals Poor oral bioavailability in horses (~3%) — IV route preferred for systemic therapy. Ensure adequate hydration during IV use (nephrotoxicity risk from crystalluria). Monitor renal function. Dose-reduce in renal impairment. For people handling this medication Wash hands thoroughly with soap and water after handling. Avoid contact with eyes; if contact occurs, flush with water for 15 minutes and seek medical attention. Keep out of reach of children. If accidental ingestion occurs, contact Poison Control (1-800-222-1222). Antiviral — minimize skin contact. Wash hands thoroughly with soap and water after handling. Keep out of reach of children. Pregnant women and women of childbearing age should avoid contact.

Contact your veterinarian. Acyclovir overdose causes kidney damage (crystalluria). Aggressive hydration required. Contact your veterinarian, a veterinary emergency clinic, or the ASPCA Animal Poison Control Center (888-426-4435) immediately. Do not induce vomiting unless specifically directed by a veterinary professional.