Doxycycline for dogs and cats (part 1)

Doxycycline (Ronaxan, Tabernil Doxycycline, Vibravenos) is a broad-spectrum antibiotic that is very effective at eliminating gram-positive and gram-negative microorganisms. It is an oxytetracycline derivative and both belong to the tetracycline group.

Doxycycline for dogs

Doxycycline has a similar mechanism of action to other tetracyclines, since it inhibits mRNA and tRNA binding by blocking the 30S subunit. This means the bacteria cannot synthesise proteins. The 30S subunit binding site differs from the one employed by aminoglycosides, so there is no competition between the two.

Doxycycline selectively attacks prokaryotic cells without damaging the dog’s eukaryotic cells.

Differences compared to other tetracyclines

Doxycycline is the most fat-soluble drug within the tetracycline group. The active form of the molecule enters the infectious agents directly through their lipid membrane.

Its significant capacity to penetrate inside microorganisms reduces the frequency of resistance because it is effective where other tetracyclines fail.

Pharmaceutical forms of doxycycline

Doxycycline is available in two forms:

• Oral format: 20, 100 and 300 mg tablets. There is also a powder formulation in a 260 mg sachet.
• Injectable format: 20 mg/mL, long acting.

The usual doses of doxycycline for dogs and cats are 10 mg/kg given orally every 24 hours with food, or 5 mg/kg given orally every 12 hours for 3 weeks for chlamydiosis in cats.

Pharmacokinetics of doxycycline for dogs

Doxycycline absorption is not affected by the presence of food in the digestive tract. It also penetrates most tissues very efficiently because of its excellent liposolubility.

It has a high protein binding capacity and a plasma half-life of 18–24 hours, so it is considered a long-acting drug and therefore offers the advantage of only requiring one daily dose.

Doxycycline is mainly eliminated through the intestines, so it can still be indicated for animals with kidney failure.

Doxycycline for dogs and cats: indications

Doxycycline is used as a specific treatment for the following conditions:

• Bartonellosis caused by Haemobartonella felis and canis.
• Pneumonia and bronchopneumonia caused by Staphylococcus spp., Streptococcus spp., Haemophilus spp., Bordetella bronchiseptica andMycoplasma spp.
• Pharyngitis, tonsillitis, otitis, tracheitis, bronchitis and sinusitis caused by different strains of Streptococcus, Staphylococcus, Mycoplasma and Chlamydia.
• Genitourinary tract infections caused by Klebsiella, Staphylococcus spp., Streptococcus spp., Escherichia coli and Mycoplasma.
• Leptospirosis
• Borreliosis caused by Borrelia burgdorferi.
• Ehrlichiosis, tetanus and brucellosis
• Gastrointestinal infections caused by Escherichia coli, Salmonella spp. and Campylobacter jejuni.
• Skin infections, abscesses and cellulitis caused by Staphylococcus aureus and Streptococcus spp.
• Otitis caused by different strains of Staphylococcus, Streptococcus, Mycoplasma and Chlamydia.
• Postoperative prophylaxis or wounds infected with Staphylococcus spp., Streptococcus spp., Corynebacterium spp., Escherichia coli or Pasteurella multocida.
• Joint infections, arthritis and abscesses caused by Staphylococcus spp., Streptococcus spp. and Corynebacterium spp.
• Pododermatitis caused by Fusobacterium spp. or Staphylococcus spp.
• Periostitis and gingivitis caused by Fusobacterium, Streptococcus, Staphylococcus and Bacteroides.

Doxycycline for dogs and cats: side effects

Tetracyclines can have a negative impact on bone growth and development in animals. They may also stain the teeth if given to females in the last 2–3 weeks of pregnancy or puppies during the first few weeks of life. Doxycycline appears to cause fewer side effects than other tetracyclines because it does not bind to calcium as strongly.

Similarly, doxycycline should not be given to lactating females, as significant amounts pass into maternal milk and then onto puppies. Otherwise the puppies can develop problems such as enamel hypoplasia, photosensitivity reactions and longitudinal bone growth inhibition.

Unlike in puppies, photosensitivity reactions are very rare in adult dogs.

Doxycycline: modes of administration

As we have already mentioned, food does not affect the bioavailability of doxycycline. Tablets can be administered whole, crushed or even dissolved in a little liquid. If the tablets are crushed and dissolved in liquid, they must be ingested immediately.

If a single daily dose is prescribed, 10 mg/kg should be given; however, the dose should be 3–5 mg/kg if the drug is administered every 12 hours.

Treatment duration

In the case of bartonellosis, ehrlichiosis, leptospirosis and brucellosis, treatment is administered for at least 30 days at the vet’s discretion. Dogs should be given 10 mg/kg/day and cats should receive 5–10 mg/kg/day.

Indications are for 5–10 mg/kg body weight/day for 10–15 days to treat respiratory, genitourinary and intestinal infections.

Contraindications

• Doxycycline for dogs should not be given during pregnancy to avoid undesirable effects on the foetuses.
• Nor should it be given to animals undergoing treatment with phenytoin or barbiturates.
• Do not administer doxycycline in combination with antacids, since this will impair its absorption.
• Adjust the doxycycline dose in cases of liver failure because its delayed metabolism may increase the drug’s half-life and cause toxicity.
• Doxycycline doses do not need to be adjusted in the case of kidney failure.