Bacterial Infection (Actinomycosis) in Dogs

Actinomyces is an anaerobic, gram-positive bacteria that lives in the mouth and nasal passages of many animals. It has a rod shape and often forms fungus-like branches or hyphae. Some species can cause an actinomycosis infection. This is more common in cattle, but it is also found in dogs, and, rarely, in humans.  Actinomycosis has a higher incidence in large-breed outdoor dogs, especially hunting dogs. It is an opportunistic infection that takes advantage of a cut or wound which allows bacteria to penetrate the skin and grow unchecked in an anaerobic environment. Infections are commonly subcutaneous (in the deep layers of the skin, below the dermis), and are often associated with contaminating grass particles that find their way into a cut. Particles that are inhaled or ingested can also cause pneumonia and gastrointestinal lesions. In some cases, the bacteria can spread from the original site causing systemic infection including CNS disease or meningitis. The infection progresses slowly, often developing fibrous mass-like abscesses which are sometimes mistaken for cancerous neoplasm. Cervicofacial infections (in the head or neck area) may spread into the mandibular bone causing teeth to fall out and making it difficult for a dog to eat or breathe. Treatment of actinomycosis is difficult because the anaerobic bacteria penetrate so deeply and surgery is often necessary to remove the infected tissue. Oral antibiotics will need to be taken for a number of weeks to eliminate all the bacteria. Very severe cases, especially those that involve the bone, may be difficult to treat.

Actinomyces are an anaerobic species of bacteria that can cause infection under the skin or elsewhere in the body. They are found normally in the oral cavities and nasal passages of dogs, but cuts or tiny grass particles can allow them to penetrate more deeply and proliferate to form an abscess. Veterinarians call this actinomycosis.

Symptoms can be quite varied depending on the infection site. Take your dog to see a veterinarian immediately if you notice any of the following signs.

• Subcutaneous sores and abscesses
• Drainage tracts close to the site of infection
• Sores and abscesses in the mouth, face, or neck
• Serosanguinous drainage (containing blood and plasma)
• Pus drainage that contains sulphur-like grains
• Pain or swelling in the jaw
• Lameness
• Coughing
• Fever
• Weight loss
• Difficult or rapid breathing
• Pyothorax (infection of the chest cavity)
• Pneumonia
• Orbital infection
• Urinary infection (rare)
• Changes in behavior and consciousness
• Seizures

Types

There are many different types of actinomyces bacteria. These are three species commonly identified in canine infections. Actinomyces bovis often causes infection from wound contamination, especially cuts in the mouth. It is more commonly associated with lumpy jaw in cattle, but it can cause the same symptoms in dogs. Actinomyces hordeovulneris is often associated with penetrating grass awns (usually of the species Horedum). It can cause internal abscesses in the lungs, liver and spleen as well as septic arthritis and systemic infection. Actinomyces viscosus causes subcutaneous infection from a wound as well as pyothorax and pneumonia.

Actinomycosis is an opportunistic infection, so it is commonly associated another factor that makes it easier for the bacteria to proliferate.

• Cut or wound (especially in the mouth)
• Penetration or swallowing of a foreign body
• Exposure to long grass
• Chronic periodontal disease
• Poor dental hygiene
• Lung cancer
• Prior surgery
• Concurrent ear or eye infection
• Spending time outside
• Higher incidence among large breed hunting dogs

Your dog’s symptoms will help the veterinarian diagnose actinomycosis. Subcutaneous infection with orange colored granular discharge is a typical sign. Abnormal lung sounds will usually be noticeable with a chest infection. Lameness, jaw pain, exophthalmos, or swelling can also show which areas are affected. Dogs often have fever with systemic infection.

A definitive diagnosis is made by taking a culture of the infected tissue. A biopsy may be necessary for deep or internal infections. Rod-shaped gram-positive bacteria are visible upon examination of a stained culture. X-rays may be needed to diagnose fibrous mass-like lesions. This can help to find the source of internal infection and determine how far the condition has progressed. Radiographs of the head and neck area are common, as well as of the lungs or abdomen. These can also be used to eliminate some other causes and diagnose any concurrent diseases like cancer. Since some actinomycosis abscesses can resemble cancer, a biopsy may be necessary to tell the two apart.

A history of your dog’s activities and recent injuries can help the vet determine the likelihood that actinomycosis is the problem. Outdoor activities, especially in wilderness areas are more conducive to actinomycosis. A recent wound or surgery can also increase the risk.

Actinomycosis is treated with a systemic antibiotic for a minimum of four to six weeks. Beta-lactam penicillin-type antibiotics are frequently prescribed, but other antibiotics have also proved effective and may be recommended by your vet. Oxygen might be needed for dogs with serious pneumonia.

It can be difficult for any antibiotic to penetrate deep infections and abscesses, so in many cases the veterinarian will also recommend surgical removal of infected tissue. With subcutaneous infection this will be relatively minor. The veterinarian will cut away the skin tissue under anesthetic to allow drainage. More invasive surgery could be needed to remove infected masses from the abdomen, liver or spleen. Infections that have progressed into the bone are rarely treated successfully due to the difficulty with penetrating and removing the infected tissue.

Recovery will depend on the severity of your dog’s condition. Localized infections usually respond to antibiotic treatment. Infections of the chest cavity can also often be treated if an antibiotic is prescribed early in the progression of the disease. Severe systemic infections that have been developing for a while can be harder to treat. Your dog’s chances will be evaluated by a veterinarian upon diagnosis.

In order to prevent the disease, avoid grassy wilderness as much as possible, check your dog’s mouth frequently for cuts and scrapes, and clean and sterilize any wounds or sores frequently. Seek treatment immediately if you notice any signs of infection. Actinomycosis develops slowly, so treating the condition right away will give your dog a better chance of recovery.