Bartonellosis

TL;DR. Bartonellosis is a bacterial infection spread by fleas and ticks that can cause silent infections in cats but serious, potentially life-threatening heart and inflammatory conditions in dogs.

Both dogs and cats can contract Bartonella, but they experience the infection very differently.

What is it?

Bartonellosis is an infectious disease caused by bacteria belonging to the genus Bartonella. These specialized bacteria target two primary areas within a pet's body: the endothelial cells (the cells that line the inside of blood vessels) and the erythrocytes (red blood cells). Because the bacteria live inside these cells, they can evade the immune system and persist in the body for long periods.

How the disease behaves depends heavily on the species infected. Cats are considered "host-adapted" reservoirs for several Bartonella species, meaning they have co-evolved with the bacteria. Dogs, on the other hand, are "non-host-adapted" hosts. A leading veterinary internal medicine reference explains this difference:

"–associated illness is not identified in the host adapted species (e. g. , B. henselae, B. clarridgeiae, and B. koehlerae infections in cats) even though large numbers of the organism are detected in blood. In contrast, when Bartonella spp. infect non–host-adapted species, illness can occur with extremely low levels of bacteremia."

This means your cat can carry a massive number of bacteria in their blood without showing any outward signs of illness, while your dog can become severely ill from only a tiny bacterial presence.

Causes & risk factors

Bartonellosis is primarily transmitted by blood-sucking parasites, most notably fleas and ticks. The species Bartonella henselae, Bartonella clarridgeiae, and Bartonella koehlerae are commonly spread among cats by fleas. Pets living in warm, humid climates where fleas and ticks thrive year-round face the highest risk of exposure.

Transmission does not always occur directly through a bite. The bacteria are shed in flea feces (often called "flea dirt" or frass). When a pet scratches or bites at flea bites, they inoculate the bacteria from the flea dirt into their own skin. This is also how the disease is transmitted to humans, commonly known as "Cat Scratch Disease."

"Transmission to humans commonly occurs after cat bites or scratches; the disease appears to be transmitted most commonly from kittens. B. henselae survives in flea frass for days, so the cat's claws and teeth are li[kely contaminated]"

While cats are the primary reservoir, dogs can also harbor the bacteria and pose a transmission risk.

"henselae have been detected in both dogs and human beings, B. henselae has been detected in dog saliva, and cat-scratch disease has been documented in a human being after exposure to a dog (Chen et al, 2007). Care should be taken to avoid bites or scratches or contaminated needle sticks while handling or treating infected dogs."

There are no documented breed predispositions for this infection; any dog or cat exposed to vector parasites can contract bartonellosis.

Signs to watch for

Because the bacteria affect blood vessels and red blood cells, the symptoms of bartonellosis can be incredibly diverse, ranging from mild, vague signs to severe, life-threatening emergencies.

In Dogs

• Common Signs:
  • Fever
  • Heart murmur (a new or changing abnormal heart sound)
  • Arrhythmias (irregular heartbeats)
• Occasional Signs:
  • Lethargy and weakness
  • Syncope (fainting spells due to heart irregularities)
  • Polyarthritis (swollen, painful, and stiff joints)
  • Epistaxis (nosebleeds)
  • Thrombocytopenia (a low blood platelet count, which can lead to bruising or bleeding)

In Cats

Most infected cats remain subclinical, meaning they show no signs of illness at all. However, when cats do develop clinical disease, they may exhibit:

• Occasional Signs:
  • Fever
  • Lymphadenopathy (swollen lymph nodes)
  • Uveitis (painful inflammation inside the eye)
  • Gingivitis (red, inflamed gums)

Uveitis, or internal eye inflammation, is an occasional symptom of bartonellosis in cats.

How vets diagnose it

Diagnosing bartonellosis is notoriously difficult. Because the bacteria live inside cells and are often present in very low numbers (especially in dogs), standard blood tests may fail to detect them. Your vet will use a combination of specialized diagnostics:

• BAPGM Culture with PCR Assay (The Gold Standard): This advanced test uses a special growth medium (Bartonella Alpha-Proteobacteria Growth Medium) to multiply the bacteria in a blood sample before performing a Polymerase Chain Reaction (PCR) test to detect the bacterial DNA. This dramatically increases the chances of a correct diagnosis.
• PCR Assay: A molecular test used to detect the genetic material of the bacteria directly from blood or tissue samples.
• Blood Culture: Attempting to grow the bacteria from a blood sample, though this can be difficult due to the low number of organisms in dogs.
• Serologic Testing: This blood test measures antibodies against Bartonella. A positive result shows that the pet has been exposed to the bacteria, though it does not guarantee an active, ongoing infection.
• Echocardiography: If your vet hears a heart murmur or detects an arrhythmia, they will perform an ultrasound of the heart (an echocardiogram). This allows them to look for "vegetations" (bacterial clumps) on the heart valves, which is a hallmark sign of infective endocarditis.

An echocardiogram (heart ultrasound) is crucial for identifying heart valve damage caused by Bartonella.

Treatment options

Treating bartonellosis requires targeted, long-term antibiotic therapy. The choice of medication depends on the severity of the clinical signs and whether the pet is showing active disease.

First-Line Therapy

• Doxycycline: This tetracycline antibiotic is the standard first-line treatment. It is highly effective at penetrating cells to reach the bacteria and has natural anti-inflammatory properties that help reduce tissue damage.

Second-Line & Adjunctive Therapies

• Enrofloxacin: A fluoroquinolone antibiotic that may be used as an alternative or in combination with other medications to combat stubborn infections.
• Azithromycin: A macrolide antibiotic that is sometimes utilized, particularly in cats.

According to a leading veterinary internal medicine textbook:

"The antibiotics used for the treatment of bartonellosis in cats generally have a broad spectrum, are effective for other infecting organisms that can cause syndromes resembling bartonellosis, and can also have antiinflammatory properties. Treatment
In experimental studies, administration of doxycycline, tetracycline, erythromycin, amoxicillin-clavulanate, or enrofloxacin can limit bacteremia..."

If a dog or cat has developed endocarditis (heart valve infection), treatment becomes much more complex and aggressive, often requiring supportive medications to manage heart failure and arrhythmias alongside long-term intravenous or oral antibiotics.

Prognosis

The prognosis for bartonellosis is highly variable and depends entirely on how the disease manifests:

• Subclinical Cats: For the vast majority of cats who carry the bacteria without showing symptoms, the prognosis is excellent. They can live normal, healthy lives, though they remain a potential source of infection for fleas and humans.
• Pets with Inflammatory Signs: Dogs and cats with fever, joint pain, or eye inflammation generally have a fair to good prognosis, provided they receive a timely diagnosis and complete a full course of appropriate antibiotics.
• Pets with Endocarditis: If the bacteria have infected the heart valves, the prognosis is guarded to poor. Infective endocarditis is a severe, life-threatening condition. Even with aggressive, long-term therapy, permanent damage to the heart valves can lead to progressive congestive heart failure.

Prevention

Because bartonellosis is transmitted by parasites, prevention is highly effective and straightforward:

• Strict Vector Control: Year-round, vet-approved flea and tick preventative medications are the single most important step you can take to protect your pets. As noted in veterinary literature:

"Flea and tick control is likely to lessen transmission of Bartonella"

• Keep Cats Indoors: Keeping cats indoors minimizes their exposure to fleas and wild animal reservoirs.
• Hygiene and Handling: Wash your hands after handling pets, especially if you have scratches or open wounds. Avoid rough play with kittens to prevent scratches, and never allow pets to lick open wounds.

When to call your vet

You should contact your veterinarian if your pet shows persistent lethargy, unexplained fever, or signs of joint pain.

Seek emergency veterinary care immediately if your pet experiences any of the following red-flag symptoms:

• Fainting spells or sudden collapse (syncope)
• Sudden, extreme weakness or inability to stand
• Difficulty breathing or rapid, labored breathing
• Unexplained bleeding, such as nosebleeds or pinpoint purple spots on the gums or skin

Sources

• Small Animal Internal Medicine, 5th Edition, p. 1349, 1350, 1351, 1426.

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