CONDITION
Bartonellosis
Bartonellosis is an infection caused by bacteria of the Bartonella genus, which can affect dogs and cats in different ways. These bacteria are typically transmitted through bites, scratches, or contact with fleas, and they can establish persistent infections that may affect the blood, immune system, heart valves, or other organs. In many cases, infected animals show no signs at all, though some develop fever, lethargy, inflammation in various parts of the body, or less common patterns involving the heart, eyes, or nervous system. Owners often arrive at this page after a positive test result in an animal that appears well, or after episodes of unexplained fever, lameness, or swelling that have been difficult to link to a clear cause. Because the signs can be subtle, intermittent, or absent, and because these bacteria are found in healthy animals as well as unwell ones, the question of what the test means for a particular pet is often central. This page explores what patterns of illness can be associated with Bartonella infection, how the bacteria interact with the body, how infection is detected and interpreted, and what approaches exist when treatment is considered. The aim is to help you understand what may be happening and what conversations may follow.
Why this matters now
Bartonella infection can occur at any age, though younger animals may be more likely to encounter the bacteria through flea exposure or contact with other animals during socialisation. Cats that spend time outdoors, particularly those that hunt or interact with feral populations, tend to have higher exposure rates. In dogs, the pattern is less clear, though working dogs, those in multi-animal households, or animals with a history of tick or flea burden may encounter the bacteria more frequently. There is no strong breed predisposition, though immune function and concurrent illness can influence whether infection leads to signs.
The course of Bartonella infection is highly variable. Many animals carry the bacteria for months or years without ever showing signs, and the infection may resolve spontaneously in some cases. In others, signs may appear weeks to months after exposure, often as intermittent episodes of fever, lameness, or malaise that come and go without obvious pattern. Progression to more persistent or organ-specific illness—such as inflammation of the heart valves, eyes, or nervous system—occurs in a minority of cases and may develop gradually over months, though it can also emerge without preceding signals.
Signals & patterns
Early signals
Intermittent fever without clear cause
The animal may have episodes of raised body temperature that come and go over days or weeks, often without other obvious illness. These fevers may be noticed as warmth to the touch, quieter behaviour, or reduced appetite during the episodes, and they may resolve on their own between episodes.
Shifting lameness or stiffness
Some animals develop lameness that moves from one leg to another, or stiffness that seems to come and go. This can reflect inflammation in joints or bones, and it may be more noticeable after rest or in cooler weather, though the pattern is often unpredictable.
Lethargy or reduced engagement
A general drop in energy or enthusiasm for usual activities may be subtle and gradual. The animal may sleep more, show less interest in play or interaction, or seem slower to respond to familiar routines, though appetite and other functions may remain normal.
Swollen lymph nodes
Lymph nodes under the jaw, in front of the shoulder blades, or behind the knees may become enlarged and firm. This can be felt during handling or grooming and may occur with or without other signs of illness.
Later signals
Heart murmur or exercise intolerance
In some animals, bacteria settle on the heart valves and cause inflammation that interferes with normal blood flow. This may be detected as a new murmur during examination, or the owner may notice reduced stamina, breathlessness, or reluctance to exert themselves as they once did.
Eye redness or vision changes
Inflammation inside the eye or at its surface can cause redness, squinting, discharge, or changes in how the animal navigates familiar spaces. These changes may develop gradually or appear suddenly, and they may affect one or both eyes.
Neurological changes
Less commonly, infection can affect the nervous system, leading to changes in coordination, balance, behaviour, or awareness. These signs may include tremors, head tilt, circling, or altered responsiveness, and they tend to develop over days to weeks.
Click to read about the biological mechanisms
How this is usually investigated
Investigation of possible Bartonella infection typically begins with a detailed history of the animal's environment, flea exposure, contact with other animals, and the pattern of any signs observed. Physical examination may reveal fever, enlarged lymph nodes, or signs pointing to particular organ systems, though many infected animals appear entirely normal. Laboratory testing is then used to look for evidence of infection or its effects, with the choice of tests shaped by what has been observed and what questions remain.
Serological testing (antibody detection)
PCR testing (DNA detection)
Blood culture
Haematology and biochemistry
Imaging and tissue biopsy
Options & trade-offs
Management of Bartonella infection is usually shaped by the individual animal's signs, test results, and the degree of certainty about the role of the bacteria in what is being observed. Approaches range from observation without intervention in animals that are well, to courses of antimicrobials in those with persistent or organ-specific signs, often combined with measures to prevent re-exposure. Different animals and households find different combinations practical, and the response to treatment can itself help clarify whether Bartonella was contributing to the picture.
Antimicrobial therapy
Treatment with antibiotics—commonly doxycycline, azithromycin, or fluoroquinolones—aims to reduce bacterial numbers and resolve signs in animals where infection is thought to be contributing to illness. Courses typically last several weeks, though longer durations are sometimes used for persistent or relapsing cases. The choice of antibiotic and duration may be guided by the species identified, the severity of signs, and the response observed during treatment.
Trade-offs: Bartonella bacteria can be difficult to eliminate completely, and relapse after treatment is recognised. Prolonged courses carry the risk of gastrointestinal upset, secondary infections, and, in some drugs, effects on developing cartilage in young animals. Treatment of asymptomatic carriers is debated, as many resolve infection without intervention and the risk of progression to illness is uncertain.
Supportive care and monitoring
In animals with mild, intermittent signs, or in those that test positive but remain well, an approach centred on observation and symptomatic support may be chosen. This can include rest during febrile episodes, anti-inflammatory medication if discomfort is present, and regular reassessment to detect any change in pattern. Flea control is maintained to reduce the chance of re-exposure or transmission.
Trade-offs: This approach avoids the costs and potential side effects of antimicrobials, but it requires tolerance of uncertainty and the possibility that signs may recur or progress. It is less suitable for animals with evidence of organ involvement or those whose signs interfere significantly with quality of life.
Flea and ectoparasite control
Rigorous, year-round control of fleas is used to reduce the risk of new infection, re-infection, or transmission to other animals in the household. This involves topical or oral products applied to all susceptible animals in the environment, as well as treatment of bedding and living areas where flea life stages may persist.
Trade-offs: Effective flea control reduces exposure but does not treat established infection or reverse damage already done. Compliance can be challenging in multi-animal households or outdoor environments, and some products may not be suitable for very young, pregnant, or unwell animals.
Management of concurrent conditions
When Bartonella infection occurs alongside other illness—such as immune-mediated disease, other infections, or chronic stress—attention to these factors can improve overall resilience and may reduce the likelihood of bacterial flares. This might involve adjusting immunosuppressive medication, addressing nutritional deficits, or treating co-infections that could be complicating the picture.
Trade-offs: Identifying and managing concurrent issues can be time-consuming and may require multiple tests and trials of treatment. The relative contribution of Bartonella versus other factors is often unclear, and improvement may occur even if the bacteria persist.
Common misconceptions
"A positive Bartonella test means the animal is unwell and needs treatment."
Positive serology is common in healthy animals, particularly cats that go outdoors, and indicates exposure rather than active disease. Many animals with positive tests remain well for life, and treatment is usually considered only when there are signs that fit the infection or when organ involvement is identified. The decision to treat rests on the whole picture, not the test result alone.
"If the bacteria are not eliminated completely, the animal will inevitably become ill."
Many animals carry Bartonella for months or years without developing signs, and some clear the infection spontaneously. Persistent low-level infection does not always lead to disease, and the factors that determine which animals progress to illness are not fully understood. Monitoring over time often reveals a stable, asymptomatic state rather than deterioration.
"Bartonella infection in pets poses a significant risk to people in the household."
While Bartonella species can infect humans, transmission from pets is uncommon and typically requires direct inoculation through scratches, bites, or flea contact. Most human cases are typical cat scratch disease, which occurs in otherwise healthy people—often children—and is usually mild and self-limiting. Routine hygiene, flea control, and care when handling animals reduce risk considerably, and more severe or disseminated forms of Bartonella infection are more likely in people with weakened immune systems.
Related conditions
Ehrlichiosis
Ehrlichiosis is another tick-borne infection that, like bartonellosis, involves bacteria living inside blood cells and can cause fever, lethargy, and effects on blood cell counts or clotting. The two conditions may share overlapping signs and can both produce positive blood test results in animals that appear well, making careful interpretation of laboratory findings important in areas where both infections are present.
Anaplasmosis
Anaplasmosis is transmitted by ticks and involves bacteria invading white blood cells, leading to patterns of fever, lameness, and intermittent illness that can resemble those seen in bartonellosis. Both infections may be carried without signs, and both can appear as part of the differential diagnosis when vector-borne disease is being considered.
Immune-Mediated Polyarthritis
Immune-mediated polyarthritis describes inflammation in multiple joints driven by immune activity, and Bartonella infection has been identified as a possible trigger in some cases of polyarthritis in dogs. When joint inflammation occurs alongside evidence of Bartonella, clinicians may consider whether the infection is contributing to the immune response, though the relationship is not always straightforward.
Uveitis
Uveitis refers to inflammation inside the eye, and Bartonella infection can occasionally be associated with this pattern, particularly in cats. When uveitis occurs without an obvious external cause, infectious agents including Bartonella may be considered as part of the investigation, though many cases of uveitis have no infection identified.
Endocarditis
Endocarditis is an infection of the heart valves, and Bartonella species have been identified as one of the bacteria that can, in rare cases, establish infection in this location. When heart valve infection is suspected, particularly in animals with fever or signs of heart compromise, Bartonella may be included among the organisms tested for, though many cases of endocarditis involve other bacteria.
If Bartonella infection has been identified or suspected, the broader context of immune and inflammatory health may be worth exploring, particularly if signs have been intermittent or difficult to characterise. Understanding how infections interact with the immune system, and what other conditions can present with similar patterns, can help frame conversations about what to monitor and when re-assessment might be useful. The pattern of signs over time often clarifies whether the bacteria are playing a central role or whether other factors are more influential.