Bartonellosis is a group of infections caused by bacteria from the Bartonella family, spread to humans through cat scratches, insect bites, and other forms of contact with infected animals. These microscopic invaders can cause illnesses ranging from mild, self-limiting conditions to severe, life-threatening diseases that affect the heart, brain, and other vital organs.
Understanding Bartonellosis: A Family of Infections
Bartonellosis refers to any illness caused by bacteria belonging to the Bartonella genus. Scientists have identified at least 22 species of these bacteria, with approximately 15 known to infect humans.[1] These are gram-negative bacteria, meaning they have a specific cell wall structure that affects how they respond to antibiotics and how the immune system recognizes them. What makes these bacteria particularly troublesome is their ability to live inside human cells, where they can hide from the body’s defenses.[2]
The three most common types of human bartonellosis have distinct names and characteristics. Cat scratch disease, caused by Bartonella henselae, is the most frequently encountered form in the United States, affecting more than 20,000 Americans each year.[3] Trench fever, caused by Bartonella quintana, earned its name during World War I when it affected soldiers living in crowded trenches. Carrión’s disease, caused by Bartonella bacilliformis, occurs primarily in mountainous regions of South America and has two distinct phases: Oroya fever and verruga peruana (Peruvian warts).[2]
How Common Is Bartonellosis Around the World?
The frequency of bartonellosis varies significantly depending on where you live and which type of infection we’re discussing. Cat scratch disease has a global presence, with an incidence of approximately 6.4 cases per 100,000 adults worldwide. Children are affected more frequently than adults, with the highest rates occurring in children aged 5 to 9 years, who experience about 9.4 cases per 100,000.[2] Within the United States, the southeastern regions report the highest number of cases, with peaks typically occurring in January and during the late summer and fall months.[2]
Trench fever has a more complicated history. During World War I and World War II, massive epidemics occurred, affecting soldiers living in crowded, unsanitary conditions. Today, this infection primarily affects people experiencing homelessness or those living in poverty-stricken areas with limited access to hygiene and sanitation.[6] Outbreaks have been documented worldwide, though they tend to be more common in warm climates where body lice can thrive.[5]
Carrión’s disease has a much more restricted geographic range. This infection is found only in the Andes Mountains region of South America, primarily in Peru, Colombia, and Ecuador. The disease was first identified in 1870 when thousands of railway workers died during construction of a line from Lima to Oroya in Peru.[2] The infection remains a significant health concern in these mountainous areas where the sand fly vectors live.[5]
What Causes Bartonellosis?
Bartonella bacteria are the root cause of all forms of bartonellosis. These bacteria are particularly cunning because they are intracellular parasites, meaning they can invade and live inside human cells. They show a preference for red blood cells, certain immune cells called macrophages, and endothelial cells that line the inside of blood vessels.[3] By hiding inside cells, these bacteria can sometimes evade detection by the immune system, allowing infections to persist for extended periods.
Each species of Bartonella has evolved to take advantage of specific routes of transmission. Bartonella henselae, the cause of cat scratch disease, spreads when cats carry the bacteria on their claws or in their mouths. Cats themselves typically get infected through flea bites and flea droppings. When an infected cat scratches, bites, or licks an open wound on a person, the bacteria can enter the human body.[1] Approximately 40% of cats carry this bacteria at some point in their lives, though many show no symptoms of illness.[5]
Bartonella quintana follows a different path to human infection. This species is transmitted by the human body louse, a parasite that thrives in crowded living conditions where people lack access to regular bathing and clean clothing. When an infected louse feeds on human blood, it can deposit bacteria-containing feces on the skin. If a person scratches the bite area, they can inadvertently introduce the bacteria into their body through tiny breaks in the skin.[6]
Bartonella bacilliformis relies on sand flies to reach human hosts. These small flying insects pick up the bacteria when they feed on infected individuals and can then transmit it to others through their bites. The sand flies that carry this bacteria are active at night and are found only at certain elevations in the Andes Mountains.[5]
Who Is at Higher Risk for Bartonellosis?
Certain groups of people face elevated risks for contracting bartonellosis or developing severe complications from these infections. For cat scratch disease, children under the age of 15 are the most commonly affected group.[5] This makes sense when you consider that children often play more roughly with pets and may be more likely to get scratched. Kittens younger than one year are more likely to carry and transmit the bacteria than adult cats, so families with young kittens should be particularly aware of this risk.[14]
People who work with animals professionally face occupational hazards related to bartonellosis. Veterinarians and veterinary team members have particularly high exposure rates. Studies have found that up to 50% of veterinary professionals tested positive for one or more Bartonella species.[8] This elevated risk comes from frequent contact with cats, potential scratches and bites, and accidental needle sticks contaminated with infected cat blood.
For trench fever caused by Bartonella quintana, people experiencing homelessness face the greatest risk. The disease spreads through body lice, which thrive in situations where people cannot regularly wash their bodies or clothing. Crowded living conditions without proper hygiene facilities create ideal circumstances for body lice to spread from person to person, bringing the bacteria with them.[6] People living in poverty in warm climates are particularly vulnerable, though outbreaks have occurred in colder regions as well.
Individuals with compromised immune systems face the most severe consequences from Bartonella infections. This includes people living with HIV/AIDS, those receiving chemotherapy for cancer, and patients who have received organ transplants and take immunosuppressive medications. In these vulnerable populations, Bartonella can cause serious complications such as bacillary angiomatosis (a condition where abnormal blood vessels proliferate and form tumor-like masses) and endocarditis (infection of the heart valves).[5]
Recognizing the Symptoms of Bartonellosis
The symptoms of bartonellosis depend heavily on which Bartonella species has caused the infection and the strength of the infected person’s immune system. Despite these variations, some symptoms appear across different types of bartonellosis, while others are specific to particular forms of the disease.
In cat scratch disease, the first sign typically appears at the site where the cat scratched or bit. A small red bump or papule develops, usually within a week after the injury. This bump may fill with fluid and eventually form a pustule before drying up and healing over one to three weeks, often leaving a small scar.[4] The most characteristic symptom follows one to three weeks later: swelling of the lymph nodes near the scratch site. These enlarged lymph nodes, a condition called lymphadenopathy, can be found in the armpit if the scratch was on the arm, in the neck if it was on the face or head, or in the groin if it was on the leg. The swollen nodes are typically painful and tender to touch, and they may feel warm. In some cases, they can fill with pus and may need to be drained.[3]
Many people with cat scratch disease also experience general feelings of illness. Fever is common, particularly in children. Body aches, headaches, fatigue, and a general sense of discomfort can accompany the swollen lymph nodes. Most cases remain relatively mild and resolve on their own within two to four months, though the lymph node swelling can persist for as long as a year.[4]
However, cat scratch disease can sometimes affect other parts of the body. Eye involvement can occur, causing conditions like Parinaud’s oculoglandular syndrome or neuroretinitis (inflammation affecting the retina and optic nerve). Some patients develop complications involving the liver, spleen, or bones. In rare cases, the infection can affect the brain, causing encephalopathy (brain dysfunction), seizures, or confusion.[3]
Trench fever presents quite differently. The disease earned its alternative name “five-day fever” because of its characteristic pattern of recurring fevers. The fever comes and goes in episodes, each lasting four to five days, with the pattern repeating three to five times. These fever episodes are accompanied by sudden chills, severe headaches, and pain in the bones, particularly in the shins, back, and neck.[6] Patients may also develop a rash with small bumps across the abdomen and experience enlargement of the spleen or liver. In people with healthy immune systems, the disease typically resolves on its own, though some individuals experience relapses or develop chronic infections where the bacteria persist in the bloodstream.[6]
Carrión’s disease has two distinct phases with very different symptoms. The acute phase, called Oroya fever, is a sudden and potentially life-threatening illness. It causes high fever and severe anemia (low red blood cell count) because the bacteria invade and destroy red blood cells. Without treatment, up to 90% of people with Oroya fever may die during this acute phase, which typically lasts two to four weeks.[2] Some patients develop neurological symptoms including confusion, agitation, difficulty with coordination, or even coma. The chronic phase, called verruga peruana, comes later and causes distinctive nodular skin lesions that are red to purple in color and can bleed easily.[4]
In people with weakened immune systems, Bartonella infections can cause additional severe complications. Bacillary angiomatosis results in the growth of abnormal blood vessels that form raised, blood-filled bumps on the skin. These lesions can be bright red or darker than the surrounding skin, depending on the patient’s skin tone, and they bleed easily when touched. The condition can also affect internal organs, forming blood-filled cysts in the liver (a condition called bacillary peliosis) or masses in bones and other tissues.[5]
Endocarditis, or infection of the heart valves, represents one of the most serious complications of bartonellosis. This condition can occur with both Bartonella henselae and Bartonella quintana infections. Patients may experience fever, fatigue, shortness of breath, and heart murmurs. The infected heart valves can fail, leading to heart failure, and pieces of the infected tissue can break off and travel to other parts of the body, causing additional problems.[6] Globally, endocarditis caused by Bartonella carries an 11.9% mortality rate.[6]
How to Prevent Bartonellosis
Preventing bartonellosis focuses on breaking the chain of transmission from animals or insects to humans. Since different Bartonella species spread through different routes, prevention strategies must be tailored to the specific risks.
For cat scratch disease, the most effective prevention centers on flea control for cats. Cats become infected with Bartonella henselae through contact with fleas and flea droppings. When flea-infested cats scratch at their bites or at the fleas themselves, contaminated flea feces can get under their claws or in their mouths. Using monthly flea prevention medications on cats significantly reduces the risk that they will become infected and subsequently transmit the bacteria to humans.[8]
Handling cats gently and teaching children proper pet interaction techniques can reduce the likelihood of scratches and bites. When scratches or bites do occur, immediately washing the wound thoroughly with soap and water helps reduce the chance of infection. People should avoid rough play with cats and kittens, and should be especially cautious with young kittens under one year of age, as they are more likely to carry the bacteria.[14]
Veterinary professionals and others who work with animals regularly should take additional precautions. Avoiding bites and scratches whenever possible, wearing protective gloves when handling cats, and taking care to prevent accidental needle sticks can reduce occupational exposure. Any scratches or bites that do occur should be washed promptly and thoroughly.[8]
Preventing trench fever requires attention to personal hygiene and control of body lice. Regular bathing, frequent washing of clothing and bedding in hot water, and access to clean clothes can prevent body lice infestations. In communities where people experience homelessness or live in crowded conditions, public health interventions that provide access to showers, laundry facilities, and clean clothing can help control the spread of both lice and trench fever.[6]
For Carrión’s disease, which occurs only in specific regions of South America, prevention focuses on avoiding sand fly bites. Using insect repellent, wearing long sleeves and pants, and using bed nets when sleeping can reduce exposure to these night-biting insects. People living in or traveling to affected areas in the Andes Mountains should be particularly vigilant about these protective measures.[5]
What Happens in the Body During Bartonellosis
Understanding what happens inside the body during a Bartonella infection helps explain why these diseases cause the symptoms they do. The bacteria have evolved sophisticated strategies to survive and multiply within human hosts, and the body’s response to these invaders drives many of the illness manifestations.
When Bartonella bacteria first enter the body through a scratch, bite, or insect feeding, they quickly seek out specific cell types. Red blood cells represent a primary target for many Bartonella species. The bacteria attach to the surface of these cells and can actually invade them, hiding inside where they are partially protected from immune system detection.[3] This invasion and presence of bacteria in red blood cells can lead to their destruction, which explains the severe anemia seen in Oroya fever caused by Bartonella bacilliformis.
Endothelial cells, which form the inner lining of blood vessels, are another favored home for Bartonella. When the bacteria infect these cells, they can cause them to multiply abnormally. This uncontrolled growth of blood vessel cells leads to the proliferation of new, abnormal blood vessels. In cat scratch disease, this process contributes to the formation of the characteristic swollen lymph nodes. In immunocompromised patients, this same mechanism produces the tumor-like masses of bacillary angiomatosis.[3]
The bacteria also infect immune cells called macrophages and dendritic cells. Under normal circumstances, these cells would engulf and destroy invading bacteria. However, Bartonella has developed ways to survive inside these very cells that should kill them. By doing so, the bacteria can use the immune system’s own cells as vehicles to spread throughout the body.[7]
Bartonella bacteria grow slowly compared to many other infectious agents. This sluggish growth helps them avoid triggering a strong immune response early in infection. The bacteria can also suppress certain immune system pathways, including one called the NF-κB pathway that normally helps cells defend themselves against infection and triggers programmed cell death of infected cells.[9] By blocking these defenses, Bartonella can establish long-lasting infections.
In Carrión’s disease, the massive destruction of red blood cells during Oroya fever not only causes severe anemia but also triggers a state of temporary immunosuppression, where the immune system becomes less effective. This weakened immunity makes patients vulnerable to overwhelming infections by other bacteria, particularly organisms from the Enterobacteriaceae family such as Salmonella.[2] This complication accounts for much of the high mortality rate seen in untreated Oroya fever.
The chronic phase of Carrión’s disease, verruga peruana, demonstrates the long-term effects of Bartonella on blood vessel cells. Months after the acute illness, the ongoing infection triggers abnormal blood vessel growth in the skin, forming the characteristic nodular lesions. These growths contain large numbers of newly formed blood vessels and can persist for months or even years.[4]
When Bartonella infects heart valves, causing endocarditis, the bacteria attach to the valve tissue and stimulate the formation of infected clumps called vegetations. These growths damage the valve structure, preventing it from working properly. Pieces can break off and travel through the bloodstream to other organs, where they can block blood vessels or seed new infections. This explains why endocarditis commonly affects the brain, kidneys, and other organs distant from the heart.[6]
