Atovaquone — Veterinary Vademecum Drug Reference

Dosage & Administration Guidelines

Species	Route	Dose	Frequency	Duration & Clinical Notes
Dog	PO	13.3-26.6 mg/kg (for babesiosis, often combined with azithromycin 10 mg/kg q24h)	q8h or q12h	Duration: 10-14 days for babesiosis; 4-6 weeks for toxoplasmosis Notes: Administer with a fatty meal to enhance absorption. For babesiosis, combination therapy with azithromycin is recommended to reduce resistance.
Cat	PO	15-20 mg/kg	q12h	Duration: 4-6 weeks for toxoplasmosis; 7-14 days for coccidiosis Notes: Administer with food. For toxoplasmosis, may be used alone or with clindamycin. Monitor for gastrointestinal upset.
Horse	PO	5-10 mg/kg (often combined with pyrimethamine 0.5-1 mg/kg and sulfadiazine 20-30 mg/kg)	q24h	Duration: 90-120 days for EPM Notes: Administer with a fatty meal or oil. Combination therapy is standard for EPM. Monitor for anemia and neurologic status.
Rabbit	PO	20-30 mg/kg	q24h	Duration: 4-6 weeks for encephalitozoonosis Notes: Administer with a high-fat treat. Limited pharmacokinetic data; use with caution.
Bird (e.g., penguins)	PO	25-50 mg/kg	q12h	Duration: 10-14 days for malaria Notes: Administer with food. Dosing based on anecdotal reports; adjust based on species and response.

Clinical Indications & Species Uses

General Indications

Treatment of protozoal infections caused by susceptible organisms, including Babesia, Toxoplasma, Neospora, Sarcocystis, and Cystoisospora.

Dog (Canine)

Treatment of babesiosis (Babesia canis, Babesia gibsoni)
Treatment of toxoplasmosis (often in combination with other agents)
Treatment of neosporosis (Neospora caninum) (off-label)
Treatment of coccidiosis (Isospora spp.) (off-label)

Cat (Feline)

Treatment of toxoplasmosis (Toxoplasma gondii)
Treatment of coccidiosis (Cystoisospora spp.) (off-label)
Treatment of cytauxzoonosis (Cytauxzoon felis) (off-label, often in combination)

Horse (Equine)

Treatment of equine protozoal myeloencephalitis (EPM) caused by Sarcocystis neurona (often in combination with pyrimethamine/sulfadiazine)

Rabbit & Small Mammals

Treatment of encephalitozoonosis (Encephalitozoon cuniculi) (off-label)
Treatment of hepatic coccidiosis (off-label)

Avian & Poultry

Treatment of coccidiosis in some avian species (off-label)
Treatment of malaria (Plasmodium spp.) in penguins and other birds (off-label)

Exotic & Other Species

Treatment of babesiosis in ferrets (off-label)
Treatment of toxoplasmosis in various exotic species (off-label)

Pharmacology & Mechanism of Action

Drug Class: Antiprotozoal | Pharmacological Group: Hydroxynaphthoquinone

Mechanism of Action: Atovaquone selectively inhibits the mitochondrial electron transport chain in protozoa by binding to the cytochrome bc1 complex (complex III). This disrupts the electron transport and collapses the mitochondrial membrane potential, leading to inhibition of pyrimidine synthesis and energy production, ultimately causing parasite death. The drug is selectively toxic to protozoa because of differences in the binding affinity of the cytochrome bc1 complex compared to mammalian cells.

Pharmacodynamics: Atovaquone is a potent antiprotozoal agent with activity against Plasmodium spp., Toxoplasma gondii, Pneumocystis jirovecii, Babesia spp., and other apicomplexan parasites. It acts primarily on the replicating stages of the parasite. In combination with other agents (e.g., azithromycin or proguanil), it shows synergistic effects. The drug is cidal against susceptible organisms, but resistance can develop if used as monotherapy.

⚡ Pharmacokinetics Summary

Absorption: Atovaquone is poorly absorbed orally, with bioavailability significantly enhanced when administered with a fatty meal. In dogs, oral bioavailability is approximately 20-30% when fasted, but increases to 50-70% when given with food. In cats, absorption is also variable and improved with food. The drug is highly lipophilic and requires bile salts for optimal absorption.

Distribution: Atovaquone is highly protein-bound (>99%) and extensively distributed into tissues, particularly the liver and lungs. It crosses the blood-brain barrier to a limited extent but achieves therapeutic concentrations in tissues where parasites reside. It is distributed into milk and crosses the placenta.

Metabolism: Atovaquone undergoes minimal hepatic metabolism. It is primarily excreted unchanged in the feces via biliary excretion. Enterohepatic recirculation may occur, prolonging its presence in the body.

Excretion: The primary route of elimination is fecal excretion of unchanged drug (over 90%). Renal excretion is minimal (less than 1%). The elimination half-life is long, ranging from 2-3 days in dogs and cats, and up to 4-5 days in some species.

Half-Life: Dogs: 2-3 days; Cats: 2-3 days; Humans: 2-3 days (not commonly used in other veterinary species)

Bioavailability: Oral: 20-30% (fasted) to 50-70% (fed) in dogs; variable in cats, improved with food.

Protein Binding: >99%

Available Formulations & Strengths

Oral Suspension 150 mg/5 mL (30 mg/mL) (PO)

Oral Tablet 250 mg, 500 mg (PO)

Contraindications & Clinical Warnings

Contraindications:

Known hypersensitivity to atovaquone or any component of the formulation
Severe hepatic impairment (use with caution)
Concurrent use with drugs that are highly protein-bound (potential for displacement interactions)

Warnings & Clinical Precautions:

Use with caution in animals with hepatic or renal disease.
Administer with a fatty meal to improve absorption; otherwise, therapeutic levels may not be achieved.
Do not use in pregnant or lactating animals unless benefits outweigh risks; safety not established.
May cause gastrointestinal upset; administer with food to reduce nausea.
Use in combination therapy for babesiosis to prevent resistance.
Monitor for signs of anemia or bone marrow suppression during prolonged therapy.
Not approved for use in food animals; extra-label use requires careful withdrawal time consideration.

Adverse Effects & Reactions

Common:

Gastrointestinal upset (vomiting, diarrhea, anorexia)
Salivation (especially in cats)
Lethargy

Serious / Severe:

Bone marrow suppression (rare)
Hepatotoxicity (rare)
Severe skin reactions (rare)

Rare:

Allergic reactions (urticaria, angioedema)
Neurologic signs (ataxia, seizures) at high doses

Key Drug Interactions

Interacting Drug / Class	Clinical Effect & Mechanism	Severity
Azithromycin	Synergistic antiprotozoal effect; commonly used in combination for babesiosis and toxoplasmosis.	Moderate
Proguanil	Synergistic antimalarial effect; not commonly used in veterinary medicine.	Moderate
Rifampin	May decrease atovaquone plasma concentrations by inducing metabolism.	High
Tetracyclines	May reduce atovaquone absorption; separate administration by 2 hours.	Moderate
Metoclopramide	May reduce atovaquone absorption; avoid concurrent use.	Moderate

Overdose & Toxicity Management

Signs of Toxicity:

Gastrointestinal signs (vomiting, diarrhea)
Lethargy
Ataxia
Respiratory distress (rare)

Emergency Treatment Protocol: There is no specific antidote. Treatment is symptomatic and supportive. Induce emesis if recent ingestion and animal is conscious. Administer activated charcoal to reduce absorption. Provide fluid therapy and supportive care. Monitor for hepatic and renal function. In severe cases, hospitalization may be required.

Food Animal Withdrawal Times

Atovaquone is not approved for use in food animals. Extra-label use in food animals is prohibited in the United States under AMDUCA due to lack of established withdrawal times. Do not use in animals intended for food.

Storage, Handling & Regulatory Information

Storage Temperature: Store at room temperature (15-30°C or 59-86°F).

Handling & Special Conditions: Protect from moisture. Keep container tightly closed. Do not freeze oral suspension.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Not FDA-approved for veterinary use; approved for human use (e.g., Malarone for malaria).

Extra-Label (Off-Label) Use: In the United States, atovaquone is not FDA-approved for veterinary use. Extra-label use in animals is permitted under AMDUCA for non-food animals, but prohibited in food animals. In the EU, similar regulations apply under Article 112 of Regulation (EU) 2019/6.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Atovaquone is a valuable antiprotozoal agent in veterinary medicine, particularly for the treatment of babesiosis and toxoplasmosis. Its efficacy is highly dependent on adequate absorption, which is significantly enhanced when administered with a fatty meal. Combination therapy with azithromycin is recommended for babesiosis to improve cure rates and reduce the risk of resistance. In horses, atovaquone is used as part of a combination protocol for EPM, but its use has declined due to the availability of other effective treatments. Adverse effects are generally mild and gastrointestinal in nature, but serious reactions can occur. Due to its long half-life, dosing intervals are typically 12-24 hours. Atovaquone should be used with caution in animals with hepatic impairment, and monitoring of liver enzymes and blood cell counts is advised during prolonged therapy. It is not approved for food animals, and extra-label use in such species is prohibited.

References & Literature

📚 Plumb's Veterinary Drug Handbook
📚 Papich Veterinary Pharmacology and Therapeutics
📚 Compendium of Veterinary Products (CVP)