Doxycycline
Doxycycline is a broad-spectrum, semi-synthetic **tetracycline antibiotic** derived from oxytetracycline. It is widely used in veterinary medicine due to its favorable pharmacokinetic profile compared to older tetracyclines. **Key Clinical Advantages:** * **High Lipophilicity:** Penetrates tissues exceptionally well, including the central nervous system (CSF), prostate, and eye. * **Renal Safety:** Unlike other tetracyclines, doxycycline is primarily excreted in the feces as an inactive chelate. It does **not** accumulate in patients with renal dysfunction, making it the tetracycline of choice for azotemic patients. * **Heartworm Adjunct:** Doxycycline is a critical component of modern canine heartworm (*Dirofilaria immitis*) treatment protocols. It eliminates ***Wolbachia***, an endosymbiotic bacterium essential for heartworm survival and reproduction, thereby weakening the adult worms and reducing post-adulticide pulmonary pathology. It is highly effective against atypical organisms including *Borrelia* (Lyme disease), *Leptospira*, *Rickettsiae*, *Chlamydia*, *Mycoplasma*, *Bartonella*, and *Bordetella*.
Mechanism: Doxycycline is a **bacteriostatic** antibiotic. * **Primary Mechanism:** It reversibly binds to the **30S ribosomal subunit** of susceptible organisms โ prevents the binding of aminoacyl transfer-RNA to the mRNA-ribosome complex โ **inhibits bacterial protein synthesis**. * **Secondary Mechanisms:** It may also reversibly bind to 50S ribosomes and alter cytoplasmic membrane permeability. * **Anti-inflammatory/Immunomodulatory Effects:** Beyond its antimicrobial properties, doxycycline inhibits matrix metalloproteinases (MMPs) and scavenges reactive oxygen species, which contributes to its efficacy in treating chronic inflammatory conditions (e.g., idiopathic rhinitis, arthritis).
Dosing by species
- Hemotropic mycoplasmosis ยท 5-10 mg/kg PO once daily for 14 days ยท PO ยท q24h ยท 14 days ยท Round dose to nearest whole tablet or capsule.
- Bartonellosis ยท 50 mg (total dose) PO q12h for 14-28 days ยท PO ยท q12h ยท 14-28 days
- Systemic infections, bacteremia ยท 5-11 mg/kg PO or IV q12h as long as necessary ยท PO/IV ยท q12h ยท As long as necessary
- Ehrlichiosis or Anaplasmosis ยท 5-10 mg/kg PO q12h for 21 days ยท PO ยท q12h ยท 21 days
- Clinical hemoplasmosis or bartonellosis ยท 10 mg/kg PO q12-24h ยท PO ยท q12-24h
- Toxoplasma gondii ยท 5-10 mg/kg PO q12h for 4 weeks ยท PO ยท q12h ยท 4 weeks
- Susceptible mycobacterial, L-Forms, or mycoplasma infections ยท 5-10 mg/kg PO q12h ยท PO ยท q12h
- Nocardia (N. nova) infections ยท 5 mg/kg or higher PO twice daily ยท PO ยท q12h ยท Combination therapy with amoxicillin and clarithromycin.
- Feline chlamydial infections (C. felis) ยท 10 mg/kg PO once daily for a minimum of 3-4 weeks ยท PO ยท q24h ยท 3-4 weeks
- Susceptible bacterial infections ยท 10 mg/kg ยท PO ยท q24h ยท As directed ยท Administer with food or a water bolus to prevent oesophageal ulceration.
- Feline chlamydiosis ยท 10 mg/kg ยท PO ยท q24h ยท 3-4 weeks ยท Administer with food or a water bolus.
Routes of administration
Contraindications
- Hypersensitivity to tetracyclines
- Intravenous administration in horses (associated with fatal cardiac arrhythmias)
- First half of pregnancy (can retard fetal skeletal development and discolor deciduous teeth, though less likely than other tetracyclines)
- Pregnant animals
- Animals with evidence of oesophagitis or dysphagia
Adverse effects
- Vomiting
- Diarrhea
- Anorexia
- Elevated liver enzymes (ALT, ALP)
- Esophageal strictures (especially in cats if dry-pilled)
- Superinfections (non-susceptible bacteria or fungi)
- Photosensitivity (reported in humans)
- Cardiac arrhythmias, collapse, and death (if given IV to horses)
- Nausea
- Diarrhoea
- Oesophagitis
- Oesophageal ulceration/stricture (especially in cats)
- Tooth discoloration (if given during tooth development, though risk is lower than other tetracyclines)
Drug interactions
- Oral Antacids (Aluminum, Calcium, Magnesium, Zinc, Bismuth) ยท Chelates with divalent/trivalent cations, significantly decreasing doxycycline absorption. Give at least 1-2 hours apart.
- Bismuth Subsalicylate, Kaolin, Pectin ยท May reduce gastrointestinal absorption of doxycycline.
- Oral Iron ยท Decreases tetracycline absorption. Give iron 3 hours before or 2 hours after doxycycline.
- Penicillins / Cephalosporins / Aminoglycosides ยท Bacteriostatic drugs (doxycycline) may theoretically interfere with the bactericidal activity of these antibiotics.
- Phenobarbital ยท May decrease doxycycline half-life and reduce serum levels. ยท moderate
- Warfarin ยท Tetracyclines may depress plasma prothrombin activity; anticoagulant dosage may need adjustment.
- Antacids ยท Reduced absorption of doxycycline ยท moderate
- Calcium salts ยท Reduced absorption of doxycycline ยท moderate
- Magnesium salts ยท Reduced absorption of doxycycline ยท moderate
- Iron salts ยท Reduced absorption of doxycycline ยท moderate
- Phenytoin ยท May increase doxycycline metabolism, decreasing plasma levels ยท moderate
- Antacids (Calcium, Magnesium, Iron salts) ยท Reduced absorption of doxycycline (though less marked than with water-soluble tetracyclines) ยท moderate
Monitoring
- Clinical efficacy
- Adverse effects (GI upset)
- Liver enzymes (ALT, ALP) with prolonged use
- Clinical response to therapy
- Signs of dysphagia, regurgitation, or oesophagitis (especially in cats)
- Gastrointestinal upset
Overdose
Doxycycline is generally quite safe in mild overdose situations (except for IV administration in horses). * **Oral Overdose:** Most likely associated with gastrointestinal disturbances (vomiting, anorexia, diarrhea). * **Treatment:** Oral administration of divalent or trivalent cation antacids (e.g., calcium, magnesium, aluminum) can bind the drug in the gut and reduce GI distress. Monitor and replace fluids/electrolytes if severe emesis or diarrhea occurs. * **IV Overdose:** Rapid intravenous injection has induced transient collapse and cardiac arrhythmias in several species, presumably due to chelation with intravascular calcium ions.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturerโs current label.