CONDITION

Endocarditis

Endocarditis is an infection of the inner lining of the heart, most often affecting the heart valves. Bacteria that enter the bloodstream—sometimes from the mouth, skin, or urinary tract—can settle on valve tissue and begin to multiply, forming fragile clumps that interfere with the valve's ability to close properly. Over time, this can disrupt normal blood flow and allow pieces of infected material to break away and travel elsewhere in the body. The course varies widely. In some animals, signs build over weeks: a dog or cat may seem quieter than usual, lose interest in food, or run a fever. In others, particularly when the aortic valve is involved, the illness can progress over days, with sudden heart failure, bacteria spreading through the bloodstream, or infected clots lodging in a limb, the kidneys or the brain. In one case series of 71 dogs, lameness was present in about half, clots lodging elsewhere in the body were recorded in 44 per cent, and more than half of the dogs did not survive. A new murmur is not always heard at first, and many of these signs cannot be told apart from other illnesses without an examination, blood tests and heart imaging. This page explores the patterns that may prompt investigation, the mechanisms by which infection takes hold in the heart, the ways the condition is identified, and the approaches used to manage it. It also covers Bartonella, a cause of endocarditis in dogs that can infect people.

Why this matters now

Endocarditis can appear at any age, but tends to be recognised more often in middle-aged to older dogs, particularly medium and large breeds. Male dogs appear over-represented in many reported case series, though the reasons for this pattern remain unclear. Conditions that allow bacteria to enter the bloodstream—dental disease, skin infections, prostate infections, or procedures that break the skin barrier, including surgery and intravenous catheters—may create opportunities for bacteria to reach the heart valves. Dogs with subaortic stenosis, a narrowing below the aortic valve present from birth, are at increased risk, as are animals whose immune defences are suppressed by illness or by immunosuppressive treatment such as corticosteroids. Degenerative mitral valve disease, the common valve disease of older small-breed dogs, is not regarded as a recognised predisposing factor. In cats, endocarditis is diagnosed far less frequently, and the triggering events are often harder to identify.

The pace of endocarditis varies widely, and it is not always slow. Some animals have an illness that smoulders for weeks, with fever, weight loss and low energy. Others become seriously ill over a few days, with bacteria circulating in the bloodstream, sudden heart failure as a damaged valve begins to leak, or fragments of infected material breaking away and blocking blood supply to a limb, the kidneys or the brain. Infection of the aortic valve tends to carry a poor outlook, while infection confined to the mitral valve more often progresses slowly, over months. In a case series of 71 dogs, more than half did not survive, and clots, low platelet counts and kidney complications were linked with a poorer outcome. Which course an individual animal is following cannot be judged at home, because the valve involved and the extent of the damage are only visible on echocardiography.

Signals & patterns

Early signals

Fever and lethargy

The dog or cat may feel warm, seem flat and withdrawn, or be off food. Lethargy was the most common sign in one case series. Fever in endocarditis can come and go, and a day or two of apparent recovery does not mean the infection has settled, because bacteria can persist on the valve between episodes. Fever cannot be confirmed by touch alone, and dogs with Bartonella endocarditis are more often without a fever, so its absence is not reassurance.

Reduced appetite and weight loss

Food intake may drop gradually, with the animal turning away from meals or eating only small amounts. Over several weeks, this can lead to noticeable weight loss, though the change may be subtle day to day.

Quieter demeanour

The animal may spend more time resting, be less enthusiastic about walks or play, or seem generally less engaged. This shift in energy can be gradual enough that it is attributed to age or weather rather than illness.

Lameness, stiffness or swollen joints

Lameness was present in about half of the dogs in one large case series, and it may shift from one leg to another. It can come from infected fragments lodging in joints or muscles, or from immune-driven inflammation in several joints at once. A sudden, painful lameness in a limb that feels cold or weak can reflect a clot blocking its blood supply. Fever alongside lameness in a dog that is not its usual self is the combination that raises the possibility of endocarditis, and its cause cannot be separated from simpler joint problems without an examination and tests.

Later signals

New or changing heart murmur

As the valve tissue becomes increasingly damaged and fails to close properly, turbulent blood flow may produce a sound audible through a stethoscope. This may be detected during a routine examination prompted by other signs.

Fast or laboured breathing

When a damaged valve leaks severely, fluid can build up in the lungs, in some animals over hours to days, and this is more often seen when the aortic valve is affected. Breathing becomes faster or more laboured at rest, a dog may stand or sit with its elbows held out rather than lie down, and a cat may breathe with its mouth open. A resting or sleeping breathing rate that stays above about 30 breaths a minute is outside the range seen in most dogs and cats with stable heart disease. Gums that look blue, grey or very pale reflect poor oxygen delivery. Fluid on the lungs can progress to respiratory failure and death.

Weakness, collapse or neurological signs

Collapse can arise from an abnormal heart rhythm, a failing circulation, overwhelming infection, or a clot lodging in the brain. Neurological problems such as unsteadiness, confusion or seizures were recorded in about a quarter of dogs in one case series. A single collapse, even one the animal appears to recover from within minutes, can be the first sign of a rhythm disturbance that may cause sudden death, and the cause of a collapse cannot be identified without an examination and heart tests.

Click to read about the biological mechanisms

How this is usually investigated

The investigation of suspected endocarditis typically begins with physical examination findings—a new heart murmur, fever, or signs of illness that do not fit a simpler pattern—and then moves towards tests that can confirm infection, locate it within the heart, and assess the extent of damage. Because the signs are often non-specific, the picture usually sharpens through a combination of imaging, blood tests, and sometimes sampling of the infected tissue itself. No single test confirms or excludes the diagnosis on its own; instead, the pattern across multiple findings builds the case.

Physical examination

Purpose: Listening to the heart may reveal a murmur—often loud, and sometimes changing in character over time—suggesting that a valve is no longer closing properly. Fever, pale gums, or pain on palpation of the limbs or abdomen may also be noted.
Considerations: Not all animals with endocarditis have a murmur at the time of examination, particularly early in the course. Conversely, many murmurs arise from causes unrelated to infection, so this finding prompts further investigation rather than confirming the diagnosis.

Echocardiography

Purpose: Ultrasound imaging of the beating heart can visualise the valves directly, revealing irregular masses attached to the leaflets, thickened or eroded valve tissue, and abnormal movement of blood during the cardiac cycle. It can also assess the size of the heart chambers and the degree of regurgitation through the damaged valve.
Considerations: Image quality depends on the animal's size, body condition, and cooperation, and small or early vegetations may not be visible. A normal echocardiogram does not rule out endocarditis entirely, particularly in the early stages or when technical factors limit the view.

Blood culture

Purpose: Samples of blood are incubated in the laboratory to detect bacteria circulating in the bloodstream, and to identify which species is present. A positive culture can guide the choice of antimicrobial treatment.
Considerations: Bacteria may be released into the blood intermittently, so a negative culture does not exclude infection. Multiple samples taken at different times may improve the chance of detection, and results typically take several days to return. Some organisms, including Bartonella species, rarely grow on routine culture; in one case series Bartonella was found in about one in five dogs whose blood cultures were negative, using other tests such as PCR.

Haematology and biochemistry

Purpose: Blood tests often show markers of inflammation—such as elevated white cell counts or changes in protein levels—and may reveal consequences of embolic events or reduced organ perfusion, including changes in kidney or liver function. Anaemia is common in chronic cases.
Considerations: These findings are non-specific and can occur in many other conditions. They help to assess the overall impact of the illness and to monitor for complications, but they do not confirm that the heart is the source of infection.

Radiography

Purpose: Chest X-rays may show enlargement of the heart silhouette or changes in the lung fields suggestive of fluid accumulation or altered blood flow. They can also help to identify complications such as pulmonary oedema or thromboembolic disease affecting the lungs.
Considerations: Radiographic changes are often subtle or absent in the early stages, and many of the findings overlap with other forms of heart disease. The images provide context but cannot visualise the valves themselves or confirm infection.

Options & trade-offs

Management of endocarditis is usually built from a combination of antimicrobial treatment, support for the failing heart, and monitoring for complications. The approach is individualised, shaped by the severity of valve damage, the presence of embolic events, the species of bacteria identified, and what is practical in a given household. Different animals and different owners find different combinations workable, and the course of treatment is often long and unpredictable.

Long-term antimicrobial therapy

Antibiotics are given for weeks to months, aiming to suppress or eliminate the bacteria embedded within the valve vegetation. The choice of drug is guided by culture and sensitivity results when available, or by the most likely bacterial species when culture is negative. Treatment is often started intravenously in hospital and then continued by mouth at home.

Trade-offs: The bacteria are partially protected by fibrin and clot, so even well-chosen antibiotics may not clear the infection completely. Long courses increase the risk of side effects, require consistent administration, and do not repair the valve tissue that has already been damaged.

Cardiac medications

Drugs that reduce the workload on the heart—such as ACE inhibitors—or that help to remove excess fluid—such as diuretics—are often introduced when the damaged valve leads to heart enlargement or signs of congestive failure. The aim is to ease the strain on the heart muscle and improve circulation.

Trade-offs: These medications manage the consequences of valve damage rather than the infection itself. Dosing often requires adjustment over time, and some animals develop side effects such as changes in kidney function or electrolyte imbalances that require monitoring.

Monitoring and adjustment

Regular reassessment—through repeat echocardiography, blood tests, and observation of clinical signs—allows treatment to be adjusted as the disease progresses or stabilises. This may involve changing antibiotics if the response is poor, altering cardiac drug doses, or identifying new embolic events early.

Trade-offs: Frequent monitoring adds to the cost and time commitment, and the course of the disease can be unpredictable despite close attention. Some animals improve for a period and then deteriorate, and it is not always clear whether changes reflect the infection, the heart failure, or both.

Nutritional and environmental support

Maintaining calorie intake and body condition can help animals tolerate long-term illness and treatment. Reducing physical exertion and stress may ease the demand on a struggling heart, and some households adapt routines to accommodate these needs.

Trade-offs: Not all animals accept changes to diet or routine easily, and quality of life is shaped by more than nutrition and rest alone. These measures support the animal but do not alter the underlying infection or valve damage.

Palliative care

When the infection is extensive, the valve damage severe, or the response to treatment poor, the focus may shift towards keeping the animal comfortable for the time that remains. This can involve pain relief, modest doses of heart medications to ease breathlessness, and close attention to signs of distress.

Trade-offs: This approach accepts that the condition may not be controlled, and prioritises the animal's immediate experience over prolonging life. It suits animals and owners for whom the burden of intensive treatment outweighs the uncertain prospect of longer-term stability.

Common misconceptions

Misconception:

"Endocarditis is caused by poor hygiene or something the owner did wrong."

Reality:

Endocarditis arises when bacteria that are already present in the body—often in the mouth, on the skin, or in the urinary tract—enter the bloodstream and settle on heart valve tissue. The conditions that allow this to happen are varied and often beyond an owner's control; dental disease, minor injuries, and underlying infections are all potential sources, and no single factor is responsible in every case.

Misconception:

"If the heart murmur improves, the infection has cleared."

Reality:

Changes in the character or loudness of a murmur can reflect shifts in blood flow or the position of vegetations, but a quieter murmur does not necessarily mean the infection has resolved. Bacteria may persist within the valve tissue even when clinical signs fluctuate, and ongoing monitoring—including repeat imaging and blood tests—is needed to assess the response to treatment.

Misconception:

"Once antibiotics are started, recovery is straightforward."

Reality:

Endocarditis is one of the more difficult infections to treat, because the bacteria are shielded within layers of fibrin and clot that limit drug penetration. Even with prolonged antimicrobial therapy, the infection may not be fully cleared, the valve tissue may continue to deteriorate, and embolic events can occur during or after treatment. The course is often unpredictable, and many animals require adjustments to their treatment over time.

Related conditions

Mitral Valve Disease in Dogs

Degenerative mitral valve disease is the usual cause of a heart murmur in older small-breed dogs and is not regarded as a recognised predisposing factor for endocarditis. The two can sound similar through a stethoscope, and echocardiography is used to distinguish a thickened, degenerate valve from an infected one, which matters because the treatment and outlook differ.

Congestive Heart Failure in Dogs

Endocarditis can lead to congestive heart failure when valve damage becomes severe enough to disrupt the heart's ability to move blood efficiently, causing fluid to accumulate in the lungs or abdomen. Both conditions may present with similar signs—lethargy, reduced appetite, breathing changes—making it important to piece together the broader clinical picture.

Pyelonephritis

Pyelonephritis and endocarditis can share a common mechanism: bacteria entering the bloodstream from a focus of infection elsewhere in the body, such as the urinary tract, and then settling in another organ. In some cases, investigation for one condition may uncover evidence of the other, particularly when blood cultures identify the same organism.

Glomerulonephritis

Endocarditis can sometimes trigger glomerulonephritis through immune-mediated mechanisms, as circulating immune complexes formed in response to the infection may deposit in the kidney's filtering units and cause inflammation. This overlap means that signs of kidney involvement—such as protein in the urine—may appear alongside the heart-related findings.

Discospondylitis

Both endocarditis and discospondylitis involve bacterial infection reaching a site via the bloodstream, and in some cases the same bacteraemia may seed both the heart valves and the intervertebral discs. Lameness or spinal pain in an animal with fever and malaise may prompt investigation that reveals infection in more than one location.

Aortic Stenosis

Dogs with subaortic stenosis, a narrowing below the aortic valve present from birth, are at increased risk of infective endocarditis. In one long-term study of untreated dogs with the condition, around 6 per cent developed endocarditis, usually later in life.

Bartonellosis

Bartonella species are a cause of endocarditis in dogs whose routine blood cultures are negative, and in one case series they were linked with aortic valve involvement, heart failure and shorter survival. Bartonella can also infect people, mainly through fleas and contact with flea-infested animals such as cats.

The questions that tend to shape the outlook in endocarditis are which valve is affected, whether heart failure or clots have already occurred, and which organism is responsible. These are answered through echocardiography, blood cultures and, where cultures are negative, tests for organisms such as Bartonella. Bartonella can infect people, mainly through fleas and contact with flea-infested animals, which makes flea control across the household's pets relevant to people living with them, particularly anyone with a weakened immune system. The related pages on subaortic stenosis, congestive heart failure and Bartonellosis give context on the conditions most closely tied to this one.

Last reviewed: 13 September 2026 · Dr Alastair Greenway MRCVS