Cat Scratch Disease and Bartonella: What Pet Owners Should Know

Bartonella henselae is a small, Gram-negative, intracellular bacterium that causes cat scratch disease (CSD) in humans and a range of subclinical to mild conditions in cats. It is one of the most important zoonotic (animal-to-human) bacterial infections associated with cat ownership, with an estimated 12,000 cases of CSD diagnosed in the United States annually and approximately 500 hospitalisations. In the UK, CSD is less common but still occurs, particularly in households with kittens or flea-infested cats.

Dr. Sarah Chen, DVM: "Bartonella is a fascinating example of how a bacterium can be essentially harmless to its natural host — the cat — while causing significant disease in an accidental host — humans. The single most effective thing you can do to protect your family from cat scratch disease is rigorous flea prevention for your cats. No fleas, no Bartonella transmission."

Cats become infected with Bartonella henselae through flea faeces (flea dirt). The transmission cycle works as follows:

1. The cat flea (Ctenocephalides felis) feeds on an infected cat, ingesting Bartonella bacteria with the blood meal.
2. Bartonella replicates in the flea's gut and is excreted in flea faeces (the dark, gritty material often found in a cat's fur).
3. Flea faeces contaminate the cat's claws and fur as the cat scratches at flea bites and grooms itself. The bacteria can survive in dried flea faeces for days.
4. When the cat scratches or bites a human (or another cat), Bartonella-contaminated flea faeces are inoculated into the wound, establishing infection.

Importantly, cats do not transmit Bartonella directly through their saliva. The bacterium is transmitted via flea faeces on the claws or in the fur. This is why flea prevention is the cornerstone of CSD prevention. For comprehensive flea control strategies, see our complete flea and tick prevention guide.

Approximately 40% of cats carry Bartonella at some point in their lives, with higher prevalence in:

• Kittens under 1 year of age (up to 75% seroprevalence in shelter kittens)
• Feral and stray cats with heavy flea burdens
• Cats in warm, humid climates where fleas thrive year-round
• Multi-cat households with inadequate flea control

One of the most striking features of Bartonella henselae infection in cats is how clinically silent it typically is. The vast majority of infected cats show no symptoms whatsoever, which is why the infection often goes undetected until a human family member develops cat scratch disease.

Bartonella Bacteraemia in Cats

After initial infection — usually acquired as a kitten through flea exposure — Bartonella henselae establishes a prolonged bacteraemia (bacteria circulating in the bloodstream). This bacteraemia can persist for months to years, during which time the cat appears perfectly healthy. The bacteria reside primarily inside red blood cells and endothelial cells (the cells lining blood vessels), where they are protected from the immune system. Over time, most cats mount an immune response that eventually clears the bacteraemia, though reinfection can occur with subsequent flea exposure.

When Cats Do Show Symptoms

While most infected cats remain asymptomatic, a small percentage may develop clinical signs, particularly if they are immunocompromised or carry very high bacterial loads. Reported clinical signs include:

• Fever: Transient, mild fever during the initial bacteraemic phase
• Lymphadenopathy: Mildly enlarged lymph nodes, usually temporary
• Lethargy: Mild, often attributed to other causes
• Uveitis: Inflammation of the eye's uveal tract, which can cause redness, squinting, and changes in pupil size. This is the most commonly recognised clinical manifestation in cats and should prompt Bartonella testing.
• Gingivitis/stomatitis: Some studies have suggested an association between Bartonella and chronic oral inflammation in cats, though this remains debated.
• Endocarditis: Inflammation of the heart valves — rare but serious, and one of the more concerning potential consequences of chronic Bartonella infection in cats.

Diagnosing Bartonella in Cats

Testing cats for Bartonella can be challenging because the bacteraemia is intermittent — the bacteria cycle in and out of the bloodstream, making single blood tests unreliable. Available tests include:

• Blood culture: Specialised Bartonella blood cultures can detect the organism but require prolonged incubation (up to 6 weeks) and are not widely available.
• PCR (polymerase chain reaction): Tests for Bartonella DNA in blood. More sensitive than serology but can still yield false negatives due to intermittent bacteraemia.
• Serology (antibody testing): Detects antibodies against Bartonella. A positive result indicates past or current exposure but does not confirm active infection. Serology is most useful for screening purposes.

Because of these diagnostic limitations, routine testing of healthy cats is generally not recommended. Testing is most appropriate when a cat has clinical signs suggestive of bartonellosis (particularly uveitis) or when a household member has been diagnosed with cat scratch disease.

While this is primarily a pet health resource, understanding cat scratch disease in humans is essential for cat owners — particularly those with young children, elderly family members, or immunocompromised individuals in the household.

Classic Cat Scratch Disease

The typical course of CSD in otherwise healthy individuals follows a predictable pattern:

1. Inoculation site (3–10 days after scratch or bite): A small, red papule (bump) develops at the site of the scratch or bite. It may blister or crust over. Many people do not notice this lesion or dismiss it as a normal wound.
2. Regional lymphadenopathy (1–3 weeks later): This is the hallmark of CSD. The lymph nodes draining the area of the scratch become significantly enlarged, tender, and warm. If scratched on the hand or arm, the axillary (armpit) or epitrochlear (elbow) lymph nodes swell. If scratched on the face or neck, the cervical (neck) or submandibular (jaw) nodes are affected. Swollen nodes can reach 5 cm or more in diameter and may persist for 2–4 months before gradually resolving.
3. Systemic symptoms: Low-grade fever, fatigue, headache, and body aches are common. Most people feel generally unwell but not severely ill.

In 85–90% of cases, CSD is self-limiting — the immune system eventually clears the infection without antibiotic treatment, though the swollen lymph nodes can take months to fully resolve. However, antibiotic treatment (typically azithromycin) can shorten the duration of illness and is recommended for moderate-to-severe cases.

Complications and Atypical CSD

In approximately 10–15% of CSD cases — particularly in children and immunocompromised individuals — complications can develop:

• Parinaud oculoglandular syndrome: If Bartonella-contaminated material enters the eye (e.g., rubbing the eye after handling a cat), conjunctivitis and preauricular lymph node swelling develop. This occurs in about 5% of CSD cases.
• Hepatosplenic CSD: Multiple granulomas (inflammatory nodules) form in the liver and spleen, causing prolonged fever and abdominal pain. This is more common in children.
• Encephalopathy: A rare (1–2% of cases) but serious neurological complication causing seizures, confusion, and altered consciousness. Most patients recover fully, but it can take weeks to months.
• Bacillary angiomatosis and peliosis: In severely immunocompromised individuals (particularly those with HIV/AIDS), Bartonella can cause proliferative vascular lesions in the skin (bacillary angiomatosis) and blood-filled cysts in the liver or spleen (peliosis hepatis). These are serious, potentially life-threatening conditions that require prolonged antibiotic therapy.

The treatment of Bartonella infection differs significantly between cats and humans, reflecting the very different clinical significance of the infection in each species.

Treating Bartonella in Cats

This is one of the more nuanced areas of feline medicine. Because most infected cats are clinically healthy, the question of whether to treat an asymptomatic Bartonella-positive cat is debated among veterinary experts.

When treatment IS recommended:

• Cats showing clinical signs attributable to Bartonella (uveitis, endocarditis, chronic fever)
• Cats in households with immunocompromised individuals (HIV/AIDS, organ transplant recipients, chemotherapy patients) where reducing zoonotic risk is a priority
• Cats that have caused documented CSD in a household member

When treatment is generally NOT recommended:

• Healthy, asymptomatic cats with positive serology alone — a positive antibody test indicates exposure but does not necessarily mean the cat is actively bacteraemic or poses a significant transmission risk
• Cats in households without high-risk individuals, where rigorous flea control is maintained

When treatment is pursued, the most commonly used antibiotics include:

• Azithromycin: The most frequently prescribed option, given orally for 7–21 days. It achieves high intracellular concentrations, which is important for targeting bacteria living inside red blood cells.
• Doxycycline: An alternative, given for 14–28 days. Effective but can cause oesophageal strictures in cats if tablets are not followed by water or food.
• Rifampicin: Sometimes used in combination with azithromycin or doxycycline for difficult cases.

It is important to understand that antibiotic treatment does not always eliminate Bartonella from cats. Studies have shown variable clearance rates, and reinfection can occur if flea exposure continues. This underscores why flea prevention — rather than antibiotic therapy — is the primary control strategy.

Guidance for Humans with Cat Scratch Disease

If a family member develops symptoms consistent with CSD — particularly a cat scratch followed by swollen lymph nodes — they should see their doctor promptly. Treatment typically involves:

• Mild cases: Observation and symptomatic relief (pain management, warm compresses for swollen lymph nodes). The infection is self-limiting in most healthy individuals.
• Moderate-to-severe cases: A 5-day course of azithromycin is the standard antibiotic treatment and has been shown to accelerate lymph node resolution.
• Complicated CSD: Longer courses of antibiotics, sometimes with combination therapy, under specialist guidance.

Always inform your doctor that you have cats in the household if you present with unexplained lymph node swelling. For broader guidance on managing zoonotic risks from pets, visit our common parasites guide for additional information.

Prevention of cat scratch disease centres on two strategies: eliminating fleas (which breaks the Bartonella transmission cycle) and reducing scratch and bite exposure (which prevents inoculation of the bacteria into human skin).

Flea Prevention: The Single Most Important Step

Since Bartonella is transmitted via flea faeces, maintaining rigorous flea prevention for all cats in the household is the most effective way to prevent CSD. Key recommendations include:

• Year-round flea prevention: Use veterinary-recommended flea preventatives on all cats in the household, not just those that go outdoors. Indoor cats can still acquire fleas from other pets, visitors, or through open doors and windows. Effective options include topical spot-on treatments (selamectin, fipronil), oral medications (nitenpyram for rapid kill, spinosad for monthly prevention), and long-acting isoxazoline products where available for cats.
• Treat the environment: Flea eggs, larvae, and pupae live in carpets, bedding, and soft furnishings — not on the cat. Vacuum frequently (especially around pet sleeping areas), wash bedding in hot water weekly, and consider environmental flea sprays containing insect growth regulators (IGRs) for heavy infestations.
• Treat all pets: If you have dogs and cats, all animals must be on flea prevention. Dogs can bring fleas into the home that then infest cats. Our flea and tick prevention guide provides species-specific product recommendations.

Safe Cat Handling Practices

Even with excellent flea control, adopting safe handling practices further reduces CSD risk:

• Avoid rough play: Do not use your hands as toys during play. Use wand toys, feather teasers, and other interactive toys that keep your hands away from claws and teeth.
• Wash scratches and bites immediately: If scratched or bitten, wash the wound thoroughly with soap and running water for at least 20 seconds. Apply an antiseptic. This simple step significantly reduces infection risk.
• Keep cat claws trimmed: Regular nail trimming reduces the depth and severity of accidental scratches. Trim claws every 2–3 weeks.
• Discourage cats from licking open wounds: While Bartonella transmission via saliva is not the primary route, it is theoretically possible if the cat's mouth has been contaminated with flea faeces during grooming.
• Supervise children: Young children are at the highest risk of CSD because they are more likely to engage in rough play that leads to scratches and bites. Teach children to handle cats gently and to report any scratches to an adult for immediate cleaning.

Special Considerations for Immunocompromised Individuals

Households with immunocompromised members (HIV/AIDS, organ transplant recipients, cancer patients on chemotherapy, individuals on immunosuppressive medications) should take additional precautions:

• Adopt adult cats rather than kittens, as kittens are more likely to be bacteraemic and more likely to scratch during play
• Maintain impeccable flea control — consider this non-negotiable
• Avoid adopting stray or feral cats with unknown flea/Bartonella history
• Discuss Bartonella testing of household cats with your veterinarian
• Do not declaw cats — this is an unnecessary surgical procedure that does not eliminate Bartonella risk (bacteria can still be transmitted through bites and residual flea faeces on fur)

After learning about Bartonella and cat scratch disease, some cat owners may feel alarmed. It is important to put CSD risk in proper perspective — the benefits of cat ownership far outweigh the risks, and with basic precautions, CSD is highly preventable.

The Risk in Context

Consider these facts:

• Approximately 74 million cats live in US households, yet only about 12,000 CSD cases are diagnosed annually — a rate of roughly 4.5 cases per 100,000 people.
• The vast majority (85–90%) of CSD cases are self-limiting and resolve without complications.
• CSD almost exclusively occurs in the context of flea-infested cats. Cats on effective flea prevention pose minimal risk.
• Indoor-only cats on year-round flea prevention are extremely unlikely to be Bartonella carriers.
• The health benefits of cat ownership — reduced stress, lower blood pressure, companionship, and mental health support — are well documented and substantial.

Should You Get Rid of Your Cat?

No. The overwhelming consensus among physicians, veterinarians, and public health experts is that even immunocompromised individuals do not need to give up their cats. The Centers for Disease Control and Prevention (CDC) explicitly states that immunocompromised individuals can safely own cats, provided appropriate precautions are taken. Rehoming a beloved pet causes significant psychological distress and is not warranted by the level of risk involved.

A Simple Prevention Checklist

• Keep all cats on year-round flea prevention (this alone prevents the vast majority of CSD cases)
• Wash scratches and bites immediately with soap and water
• Trim cat claws every 2–3 weeks
• Avoid rough play — use toys, not hands
• Supervise interactions between cats and young children
• Keep cats indoors to reduce flea exposure and reinfection risk
• Follow a regular veterinary visit schedule to monitor your cat's overall health

By following these straightforward measures, you can enjoy the companionship of your cats with confidence, knowing that the risk of cat scratch disease is minimised to an extremely low level. If you have questions about your cat's health or want to assess any symptoms, our new kitten checklist and vaccination guide provide comprehensive starting points for keeping your feline companion healthy and your family safe.