Acyclovir
Acyclovir is a guanosine analogue with activity against susceptible herpesviruses. Poor oral exposure limits its value in horses, and efficacy and systemic dosing in dogs are poorly established.
It is unsuitable for systemic treatment in cats because of toxicity and poor activity against feline herpesvirus. IV use in horses has pharmacokinetic support, but controlled evidence of clinical benefit is limited.
Monitor renal function and hydration during prolonged or IV treatment. Oral exposure in horses is too unreliable to infer efficacy from an oral dose alone.
Group
Antiviral; nucleoside analogue
Mechanism of Action
Viral thymidine kinase initiates phosphorylation; host-cell enzymes then form the active triphosphate. The triphosphate inhibits viral DNA polymerase and terminates viral DNA elongation.
Dose Rates
Horses
| Use / stage | Dose | Route | Schedule | Instructions |
|---|---|---|---|---|
| Herpesvirus infection | 10 mg/kg | IV | Every 12 h | Do not use in: Compromised renal function, Animals intended for food; Infuse over one hour with appropriate dilution and hydration. Clinical efficacy is uncertain; oral acyclovir does not provide reliable systemic exposure. |
Dogs
| Use / stage | Dose | Route | Schedule | Instructions |
|---|---|---|---|---|
| Herpesvirus infection | 3–10 mg/kg | PO | Up to 10 days; Do not use in: Compromised renal function, Animals intended for food; Historical extrapolated regimen: five administrations daily. Systemic efficacy and an optimal canine dose are not established. | |
| Herpesvirus infection | 10–20 mg/kg | IV | Every 8 h | Do not use in: Compromised renal function, Animals intended for food; Historical extrapolated regimen; infuse over one hour. Systemic efficacy is not established. |
Birds
| Use / stage | Dose | Route | Schedule | Instructions |
|---|---|---|---|---|
| Pacheco’s disease | 80 mg/kg | PO | Every 8 h | Up to 14 days; Do not use in: Dehydration, Compromised renal function; Psittacine birds; a 7-14 day course is described. Combine with supportive care and transmission control. |
| Pacheco’s disease | 40 mg/kg | IM | Every 8 h | Up to 3 days; Do not use in: Dehydration, Compromised renal function; Highly irritating historical alternative with tissue-necrosis risk; oral treatment is preferable where feasible. Do not continue parenteral treatment beyond 72 hours. |
Indications
- Selected equine herpesvirus treatment using a slow IV infusion; efficacy is uncertain.
- Historical extrapolated canine herpesvirus regimens, with limited supporting evidence.
- Pacheco’s disease in psittacine birds, with isolation and supportive care; treatment does not eliminate the risk of persistent herpesvirus carriage.
Contraindications
- Cats: poor efficacy and risk of marrow, hepatic and renal toxicity.
- Avoid in animals with compromised renal function under the regimens listed here.
- Do not administer to animals intended for food.
- Dehydration increases renal risk. Assess neurologic disease or previous reactions to related antivirals before systemic treatment.
- Avian IM administration is highly irritating and can cause tissue necrosis; do not continue the described IM regimen beyond 72 hours.
- Pregnancy and lactation data in veterinary species are limited; placental and milk transfer can occur.
Interactions
- Other nephrotoxic drugs increase renal risk.
- Do not mix injectable acyclovir with blood products or bacteriostatic diluents containing preservatives.
- Zidovudine may increase CNS depression.
- Reported infusion incompatibilities include dopamine, dobutamine, fludarabine, foscarnet, meperidine and morphine; avoid unverified admixtures.
Adverse effects
- Renal injury or crystalluria, especially with dehydration or rapid administration.
- Phlebitis at injection sites.
- Sweating or muscle tremors during rapid equine infusion.
- Myelosuppression, hepatotoxicity and nephrotoxicity in cats.
- Gastrointestinal upset and rare neurologic adverse effects during systemic exposure.
Toxicity
Renal injury is the principal systemic concern. Use adequate hydration and slow infusion; monitor urea, creatinine and urine output. Feline toxicity can involve bone marrow, liver and kidneys.
Preparations
| Preparation | Form | Strength | Route |
|---|---|---|---|
| Zovirax 200 mg Tablets Store in a tight, light-resistant container at room temperature. | Tablet | 200 mg / tablet | PO |
| Zovirax 400 mg Tablets Store in a tight, light-resistant container at room temperature. | Tablet | 400 mg / tablet | PO |
| Zovirax 800 mg Tablets Store in a tight, light-resistant container at room temperature. | Tablet | 800 mg / tablet | PO |
| Zovirax Oral Suspension 400 mg/5 mL This strength differs from 200 mg/5 mL acyclovir suspensions; confirm the denominator. Store at room temperature. | Oral Liquid | 400 mg / 5 mL | PO |
| Acyclovir Injection USP 50 mg/mL (Fresenius Kabi) Fresenius Kabi USA, LLC Acyclovir sodium expressed as acyclovir. IV infusion only: dilute before use; not an IM preparation. Available as 500 mg/10 mL and 1000 mg/20 mL single-dose vials. Store at 20-25 C and discard unused opened-vial contents. | Injection | 50 mg / mL | IV |
| Acyclovir Ointment USP 5% Cutaneous ointment, 50 mg/g; not an ophthalmic preparation. Do not substitute it for a sterile ophthalmic product. | Other | 50 mg / g | TOPICAL |
