Discospondylitis in dogs: spinal infection and back pain

Overview

Intervertebral discs lie between the vertebrae of the spine. In discospondylitis, inflammation affects both the disc and the bony parts of the adjacent vertebrae. It may affect one disc space or several sites along the spine. This differs from infection confined to a vertebral bone, in which the disc is not involved.

Back pain may be by far the most obvious symptom, without fever or other marked signs of general illness. In a series of 25 dogs followed with MRI, spinal pain was found in 96%. In the same group, 40% had partial limb weakness, or paresis. The median duration of symptoms before hospital admission was 30 days.

Besides the lumbosacral region, infections often occur in the neck and at the junction of the thoracic and lumbar spine.

Causes

A pathogen reaches the disc area

The most common route is for bacteria or fungi to travel in the blood from another infection site to the spine. A microbe can also enter tissue directly through a deep bite or puncture wound. Spinal surgery is another possible route of direct contamination. Infection may also involve a migrating foreign body, such as a grass awn.

A foreign body can enter through the respiratory tract, digestive tract or a penetrating wound. If it carries bacteria or fungi, a focus of infection may develop in the spine. Staphylococci are the bacteria most commonly isolated. Other possible causes include Brucella canis, Escherichia coli and Aspergillus fungi.

The original bacterial infection may be in the urinary tract, prostate, mouth or skin, for example. Heart valve infection can also be a source of infection spreading through the blood.

Inflammation can compress nerve tissue

Inflammation may spread into soft tissues around the vertebrae. A collection of pus extending into the spinal canal can compress the spinal cord. Additional tissue produced by inflammation can also cause compression. Less commonly, neurological signs arise when infection spreads into the spinal cord or a vertebra weakened by inflammation fractures.

Risk factors

Discospondylitis occurs particularly in large dogs. Long-standing infections, such as chronic skin infections, can increase susceptibility. Disease or medication that weakens the immune system may also increase risk. In a monitored case series, the disease was particularly seen in entire males at the junction of the lumbar spine and sacrum.

The disease can also develop after spinal surgery. In a small MRI follow-up group, 20% of cases developed after spinal surgery. These dogs were diagnosed 9–102 days after the operation. This proportion in a selected group does not represent the risk for all dogs undergoing spinal surgery.

Both younger and older dogs can be affected, so age alone does not rule it out.

Typical symptoms

Early signs at home most often involve back pain and changes in willingness to move.

• Your dog moves stiffly and avoids bending its back.

• Your dog is reluctant to jump onto the sofa, into the car or down from a raised surface.

• Turning or touching a particular area of the back causes pain.

• The legs are weak or do not support your dog as usual.

• Your dog has a fever, is tired or is quieter than usual.

• Appetite decreases or weight starts to fall.

Not every affected dog has a fever or loses weight. Symptoms may develop slowly, so the first change can look like simple stiffness. Severe disease can cause paralysis. If brucellosis is the underlying cause, eye symptoms may occur at the same time.

Symptoms may fluctuate and worsen after physical exertion. Some dogs have little pain, or no clearly detectable pain, at the time of examination.

Complications and consequences

Spinal cord compression can disrupt neurological function and make movement difficult. Bone-destroying infection can rarely cause a pathological fracture, which occurs without the usual major injury. The outlook for bacterial discospondylitis is often good when treatment starts early. However, the disease can recur.

The outlook for fungal disease is less certain than for a typical bacterial infection. Some dogs with fungal infections do not respond to medication. Brucella canis usually causes a long-standing and most often lifelong infection. This bacterium can also infect people.

A vertebral fracture or dislocation can cause sudden neurological deterioration in a disease that began slowly. Spread of fungal infection elsewhere in the body may worsen the outlook.

Diagnosis at the vet

Imaging locates the infection

X-rays look for destruction of the vertebral endplates beside the disc and narrowing of the disc space. Longer-standing changes may include bone spur formation. X-ray changes may not become visible until 3–6 weeks after symptoms start. Imaging may therefore need repeating if pain persists and the first X-ray does not explain it.

MRI may detect infection more sensitively than X-rays. CT or other additional imaging is used when needed. Imaging also assesses surrounding soft tissues and any collection of pus in the spinal canal.

Measured from the time of infection, X-ray changes may take 2–6 weeks to develop.

Bone around the affected area of a vertebra may become denser, known as sclerosis.

Vertebral bodies damaged by infection may also gradually shorten.

Samples identify the pathogen

Blood, urine or a sample from the infected disc area can be collected for bacterial culture. The aim is to select treatment that works as effectively as possible against the cause. A disc sample can be taken under imaging guidance or during surgery. Direct sampling is invasive, so it is not performed automatically in every dog.

A negative culture does not always rule out infection. Previous antibiotics may reduce the chance of finding bacteria. A slow-growing bacterium or an inadequate sample may also explain a negative result. Special fungal cultures or other pathogen-specific tests are sometimes needed.

In a small follow-up study, blood culture was positive in 5 of 13 dogs tested. Urine culture was positive in 3 of 14 dogs tested. Culture of a disc sample collected during surgery was positive in 5 of 10 samples.

Routine urinalysis complements culture when looking for a possible infection source and pathogen. Cerebrospinal fluid sampling may be considered as part of further investigations. The sample can be examined for abnormal cells, protein, bacteria and fungi, for example.

Considering brucellosis

A dog with discospondylitis is also tested for Brucella canis infection.

A report from northern Europe documented a canine Brucella canis infection in 2008. The bacterium was isolated from a tissue sample taken from an imported dog's intervertebral disc. Antibody testing is the main screening method for brucellosis. A positive screening result may need confirmation because closely related bacteria can cause cross-reactions.

Treatment

Treatment directed at the cause

Bacterial infections are treated with a suitable antibiotic and fungal infections with an antifungal. The vet tailors treatment to the culture result and your dog's condition. Pain relief is part of treatment. The painkiller is chosen according to pain severity and your dog's other health needs.

An appropriate antimicrobial may ease symptoms within about 5 days. However, rapid improvement is not a reason to stop medication yourself. Stopping too early increases the risk of recurrence.

During early recovery, exercise is usually restricted for 4–6 weeks after diagnosis. If symptoms do not begin to ease within 3–5 days of starting treatment, investigations may be extended to include tissue sampling or fungal testing.

When is surgery considered?

Surgery may be necessary if medication does not produce enough improvement. It can remove tissue compressing the spinal cord and relieve pressure on nerve tissue. Any foreign body can be removed and the infected area cleaned. A culture sample can also be taken directly from the infection site.

If neurological damage is severe, surgery may also require stabilisation of the spine.

Follow-up involves more than the appearance of images

Follow-up X-rays may be taken about 6–8 weeks after treatment starts. X-ray improvement may lag weeks behind clinical recovery. MRI changes interpreted as inflammation may also remain after symptoms have resolved. Follow-up images are therefore interpreted alongside symptoms and other test results.

Of 25 dogs followed with MRI, 64% were symptom-free at follow-up. Nevertheless, depending on the assessor, images were judged to show changes classified as active in 72–80%. The benefit of routine follow-up MRI in symptom-free dogs remained unclear. The small, retrospectively studied group limits how widely the results can be applied.

During healing, a bony bridge may form between adjacent vertebrae, fusing them together.

Prevention

When brucellosis is confirmed, careful hand hygiene helps prevent exposure to secretions. Some national guidance on Brucella canis prevention emphasises testing imported dogs and dogs from abroad used for breeding. Some national guidance recommends euthanasia of a dog carrying Brucella canis to prevent spread of infection. Improvement in symptoms does not always stop bacterial shedding or remove the subsequent infection risk.

A dog with confirmed brucellosis should avoid contact with other dogs and should not attend shows or dog events.

If brucellosis is found in a kennel, disinfecting the environment is also part of control.

Important for dog owners

Record when back pain began and how walking or getting up has changed. Bring a list of previous medicines and details of surgery, wounds, travel and importation. Also mention long-standing skin infections or other infections.

Antibiotics given before culture sampling can affect whether the pathogen is detected.

Alongside pain, monitor appetite, weight and how well the legs support your dog. Ask for a written follow-up plan and details of changes requiring immediate contact. Do not alter medication solely because your dog feels better or because of one imaging result without your vet's assessment.

In some countries, Brucella canis is a notifiable animal infection, and the vet handles the reporting required of them.

Frequently asked questions

Can a dog have discospondylitis without a fever?

Yes: back pain is the most common finding, while fever, lethargy and weight loss are less common. Symptoms can begin gradually, so your dog may initially seem mainly stiff. Later, limb strength may decline or severe disease may cause paralysis.

Can the first X-ray look normal?

Yes: bone changes sometimes become visible on X-rays only 3–6 weeks after symptoms start. If pain persists, imaging may need repeating. MRI can detect infection more sensitively than X-rays, and the choice of investigation depends on your dog's situation.

Does a negative bacterial culture rule it out?

No: cultures can be negative in discospondylitis. Previous antibiotics, a slow-growing microbe or too small a sample can explain this. The vet assesses culture results alongside the history, symptoms and imaging, and considers other samples when needed.

Why does medication continue when my dog feels better?

Symptoms may improve within about 5 days of appropriate treatment. This does not yet show that infection has cleared, and stopping medication too early can lead to recurrence. Improvement on X-rays may also lag weeks behind recovery in wellbeing.

Why is my dog also tested for brucellosis?

Brucella canis is one possible cause of discospondylitis. It can cause a long-standing, often lifelong infection. The bacterium can also infect people. In a reported case from northern Europe, the bacterium was isolated from an intervertebral disc tissue sample.