Bartonella henselae (Cat-scratch disease and Bacillary angiomatosis)

Bartonella henselae is a zoonotic bacteria which may cause Cat-scratch disease, or CSD, in humans.

Zoonotic bacteria refers to any bacteria which can be transmitted between animals and people.

Now, Bartonella henselae spreads between cats with the help of a cat flea, called Ctenocephalides felis, which feeds on the blood of an infected cat. Then, it drops its feces, which contain Bartonella henselae, on the cat’s body.

Finally, when the cat grooms or scratches, Bartonella henselae gets on its teeth and claws, and the cat may infect a human by scratching or biting - hence the name, “cat scratch disease”, or “cat scratch fever”.

There’s also bacillary angiomatosis, which is a severe form of cat scratch disease, that develops primarily in immunocompromised individuals.

Now, Bartonella henselae is a gram-negative bacillus, in other words, it's a rod-shaped bacteria that stains red or pink with Gram staining.

This is largely due to the fact that Bartonella henselae has a thin peptidoglycan wall that doesn’t retain crystal violet dye during Gram staining.

Sometimes, though, it doesn’t readily gram stain, so the silver nitrate-based Warthin-Starry stain is used for direct visualization.

Bartonella henselae is a facultative intracellular bacteria, which means that it's adapted to live inside cells to avoid the immune system.

Specifically, it hides inside endothelial cells lining up the interior surface of the blood vessels and lymph vessels.

Once inside the endothelial cell, Bartonella henselae can trigger an increased production of interleukin-10, which suppresses the action of immune cells; and interleukin-8, which promotes endothelial cell proliferation.

In humans, the response to infection depends on the immune system.

Immunocompetent individuals develop a simple form of the cat-scratch disease.

The most characteristic symptom is the swelling of a single lymph node or a cluster of lymph nodes, usually in the armpits, which are extremely painful to the touch.

This is known as regional lymphadenopathy, and it’s a consequence of Bartonella henselae triggering a never ending T-cell response, which creates an area of inflammation known as granulomas in the lymphoid tissue.