Acyclovir

Overview

Acyclovir is a nucleoside analog antiviral drug used in veterinary medicine primarily for the management of herpesvirus infections. Its most established clinical role is in avian species, particularly for the treatment and prevention of psittacid alphaherpesvirus-1 (PsHV-1), also known as Pacheco’s Disease.

The drug has been investigated in several mammalian species, including horses and dogs; however, clinical efficacy outside avian medicine is variable and often limited by poor oral bioavailability. In horses, acyclovir has shown limited effectiveness against equine herpesviruses, and alternative agents with improved bioavailability are generally preferred. In dogs, its use is largely adjunctive and restricted to specific neonatal or experimental indications.

Mechanism of Action (MOA): Acyclovir is a guanine analogue that is selectively activated within virus-infected cells by viral thymidine kinase. Once converted to its active triphosphate form, it is incorporated into viral DNA, where it inhibits viral DNA polymerase and halts viral replication with minimal effect on uninfected host cells

Despite mild antiviral activity against feline herpesvirus, acyclovir must not be used in cats because of a high risk of myelosuppression and significant renal and hepatic toxicity. Careful patient selection, appropriate dosing, and attention to renal function are essential when this drug is used in any species.

Indications

Acyclovir is indicated for the treatment and prevention of selected herpesvirus infections in veterinary patients. Its clinical usefulness varies significantly by species and is most established in avian medicine.

• Psittacid alphaherpesvirus-1 (PsHV-1, Pacheco’s Disease) in birds: Primary indication for acyclovir use. It may be effective when administered early in the course of infection, ideally before the onset of clinical signs, and is used for both treatment and outbreak control (prophylaxis).
• Other avian herpesvirus infections: Used in various avian species for suspected or confirmed herpesvirus infections. Specific extra-label regimens: Quaker parakeets 80 mg/kg PO q8h × 7 days; Raptors 330 mg/kg PO q12h; food-incorporated administration 240 mg/kg applied to feed. Clinical response varies depending on timing and severity.
• Equine herpesvirus infections (investigational use): Studied for equine herpesvirus type 1 (EHV-1) myeloencephalopathy and equine herpesvirus type 5 (EHV-5); however, clinical efficacy has not been conclusively demonstrated. Valacyclovir is generally preferred due to superior bioavailability.
• Canine herpesvirus (adjunctive use): Used as an adjunctive treatment in neonatal puppies with canine herpesvirus infection, particularly when initiated early.
• Canine parvovirus (experimental prophylaxis): Investigated as a prophylactic agent in experimentally induced parvovirus infections in puppies, where it reduced viral replication and prevented clinical disease.
• Cat FHV-1 ocular infection (TOPICAL ophthalmic ONLY): Apply 0.5% ophthalmic ointment 5 times daily to the affected eye for a maximum of 3 weeks. The systemic contraindication in cats does NOT extend to topical ophthalmic use. Ganciclovir and famciclovir should be considered before aciclovir.

Dosage (Reference)

Dog

In dogs, acyclovir use is limited and considered adjunctive rather than primary therapy. It has been used mainly in neonatal puppies for canine herpesvirus infections or experimentally for parvovirus prophylaxis.

Cat

Acyclovir must NOT be used in cats. Despite mild in vitro activity against feline herpesvirus type 1 (FHV-1), clinical use is contraindicated due to severe toxicity and poor bioavailability.

For feline herpesvirus infections, alternative antiviral agents with better safety profiles (e.g., famciclovir) should be considered instead.

Warnings & Precautions

Acyclovir use in veterinary patients requires careful consideration of species, route of administration, and renal function. Adverse effects are largely related to renal toxicity, local tissue irritation, and improper administration techniques.

• Species-specific toxicity (cats): Acyclovir is contraindicated in cats due to a high risk of myelosuppression, renal tubular necrosis, and hepatic necrosis. No safe or effective dosing regimen has been established in this species.
• Renal toxicity: Acyclovir may cause nephrotoxicity, particularly with rapid IV administration, high doses, dehydration, or concurrent use of nephrotoxic drugs. Dose adjustment and close monitoring are recommended in patients with renal insufficiency.
• IV administration precautions: Injectable acyclovir has an alkaline pH (≈11) and must be diluted to a concentration ≤7 mg/mL and administered as a slow IV infusion (over approximately 1 hour) to reduce the risk of renal injury and vascular irritation.
• Injection site reactions: Parenteral administration may cause thrombophlebitis. In birds, IM administration can result in severe local tissue necrosis and should be avoided when possible.
• Hydration status: Adequate hydration is essential during therapy to reduce the risk of crystalluria and renal tubular precipitation, especially with IV dosing.
• Neurologic effects: Use cautiously in patients with pre-existing neurologic disease, hypoxia, or electrolyte abnormalities, as encephalopathic signs have been rarely reported.
• Hepatic impairment: Use cautiously in animals with hepatic disease, as altered metabolism and increased risk of adverse effects may occur.
• Limited efficacy in some species: Poor oral bioavailability limits effectiveness in horses and some mammalian species; alternative antivirals with better pharmacokinetics are often preferred.
• Pregnancy & lactation: Crosses placenta (no teratogenic effects in rodents); excreted in milk (no known adverse effects in nursing offspring). Use only when maternal benefits outweigh fetal/neonatal risks.

Drug Interactions

The following drug interactions with acyclovir have been reported or are theoretical in humans or animals and may be clinically relevant in veterinary patients. Concurrent use is not always contraindicated, but potential risks should be carefully weighed and additional monitoring performed when appropriate.

• Aminophylline / Theophylline: Concurrent use may increase theophylline serum concentrations; monitor theophylline levels and watch for signs of toxicity.
• Meperidine: May increase concentrations of meperidine and its metabolite normeperidine, increasing the risk of lethargy, agitation, and seizures; avoid concurrent use.
• Mycophenolate: Oral coadministration may increase serum concentrations of both drugs; the risk is greater in patients with impaired renal function. Use cautiously and monitor for toxicity.
• Nephrotoxic medications: Concurrent use of IV acyclovir with nephrotoxic drugs (e.g., aminoglycosides such as amikacin or gentamicin, amphotericin B, cyclosporine) may increase the risk of nephrotoxicity, particularly with dehydration or pre-existing renal impairment. Monitor renal function closely throughout therapy and ensure adequate hydration.
• Probenecid: Inhibits renal tubular secretion of acyclovir; concurrent use increases systemic exposure and may prolong the half-life. Consider dose reduction or extended dosing interval if combined.
• Zidovudine: Concurrent use may cause additional CNS depression, including profound somnolence and lethargy. Monitor mentation if combined.

Side Effects & Overdose

Side Effects

Adverse effects associated with acyclovir are generally dose-related and more likely to occur with parenteral administration, rapid IV infusion, or in patients with underlying renal compromise.

• Gastrointestinal effects: Vomiting, diarrhea, anorexia, and general GI upset may occur with oral or parenteral therapy.
• Renal toxicity: Acute kidney injury may develop, particularly with IV administration, dehydration, or concurrent use of nephrotoxic drugs.
• Injection-related reactions: Thrombophlebitis may occur following IV administration.
• Neurologic effects: Rare encephalopathic signs have been reported. In horses, IV administration over 15 minutes has caused tremors, sweating, and colic.
• Tissue irritation: IM administration in birds may cause severe tissue necrosis and should be avoided when possible.
• Species-specific toxicity: In cats, acyclovir may cause severe myelosuppression, hepatic necrosis, and renal tubular necrosis and should not be used.

Overdose

Acute oral overdose of acyclovir is unlikely to cause severe toxicity in most species, but higher doses increase the risk of renal and neurologic complications.

• Mild to moderate overdose (<150 mg/kg): GI signs such as vomiting, diarrhea, and lethargy are most common.
• High-dose exposure: Acute kidney injury may occur due to drug precipitation within renal tubules. crystalluria and increased renal values occurred specifically at acyclovir 188.7 mg/kg in a dog. Consider GI decontamination at ingestions ≥150 mg/kg.
• Management: Consider GI decontamination for ingestions ≥150 mg/kg, provide aggressive IV fluid therapy, and monitor renal function closely.
• Dialysis: Acyclovir can be removed by hemodialysis but not by peritoneal dialysis.
• Supportive care: Treatment is largely supportive with attention to hydration, renal monitoring, and management of clinical signs.

Key Notes

Practical clinical considerations that support safe and appropriate use of acyclovir in veterinary patients:

• Early intervention is critical: Antiviral efficacy is greatest when acyclovir is administered early in the course of herpesvirus infection, particularly before the onset of clinical signs.
• Species-dependent usefulness: Clinical benefit is best documented in avian species, especially psittacines; effectiveness in mammals is limited by poor oral bioavailability.
• Formulation matters: Oral acyclovir has low and variable bioavailability in many species; alternative prodrugs (eg, valacyclovir) may be preferred when available.
• Administration technique: Slow IV infusion and proper dilution are essential to reduce the risk of adverse vascular and renal effects.
• Supportive care remains essential: Antiviral therapy should be combined with appropriate supportive and husbandry measures, especially in birds with systemic viral disease.
• Not a broad-spectrum antiviral: Activity is primarily limited to herpesviruses; acyclovir should not be expected to be effective against non–DNA viruses.