Clinicopathologic differences of the many Bartonella spp are not well-documented,² and factors such as virulence among species and strains, mode of transmission, host immune response, concurrent disease, and immunosuppression may all contribute to clinical signs and degree of disease manifestation.³

Following are the author’s top 5 Bartonella spp and their significance in humans.

B henselae can be found worldwide,⁴ and although cats are its primary reservoir,^5,6 other animals (eg, dogs, raccoons, mongooses) may also be sources of infection.² B henselae is transmitted among cats through flea feces (Ctenocephalides felis) and scratches from nails contaminated with flea feces.^2,4,7,8 B henselae transmission to humans from dog bites^2,9 and cat bites and scratches^5,6 appears to be a low-risk source for human infection unless the animal is concurrently infested with fleas or the nails or saliva are contaminated with flea feces.² Ticks, spiders, and mites may also play a role in transmission of B henselae to animals and humans.^2,10,11 Bacteremia has been identified in various animals²; in cats (stray or pet), infection prevalence varies considerably by geographic area.⁶ Infection prevalence in cats is highest in warm, moist areas with high flea burdens.^4,5,12 A North American study found seroprevalence rates to be higher in warmer regions as compared with cooler regions.¹² Overall, 27.9% of cats tested in that study were seropositive. Bacteremia in cats can be intermittent,¹⁰ occurs primarily in cats younger than 2 years, and can last a year or longer.^6,7 The seroprevalence rate of B henselae in dogs in the southeastern United States has been recorded to be 10% in healthy dogs and 27% in sick dogs.¹³ Broader studies have found 2% to 3.8% of dogs to be seropositive for B henselae.^14,15

B henselae infection in humans and animals can be subclinical; this is particularly common in cats due to their coevolution with B henselae.^2,7,16 Although association between infection and clinical signs in cats is equivocal,¹⁷ signs can include endocarditis,^2,6 myocarditis,^2,7 fever,¹⁸ lethargy,^3,18 gingivostomatitis,¹⁹ uveitis, conjunctivitis, and others.³ In dogs, B henselae can cause fever, endocarditis, granulomatous hepatitis, peliosis hepatitis, lymphadenomegaly, panniculitis, and vasoproliferative lesions, as well as others.^2,3,5,20

B henselae is the predominant cause of cat scratch disease (CSD) in humans.^10,21 There are an estimated 500 hospitalizations and 12 000 outpatient cases of CSD each year in the United States.⁸ In humans, at the site of inoculation, a cutaneous papule or pustule develops 3 to 10 days after contact with an infected animal and may last 1 to 3 weeks.⁴ Accompanying signs may include regional lymphadenopathy, fever,¹⁸ fatigue, malaise, headache, sore throat, and/or rash.⁴ More severe signs may include neuralgia, endocarditis, osteomyelitis, Parinaud’s oculoglandular syndrome, pneumonia, hepatitis, splenitis, and/or encephalopathy.⁴ B henselae and B quintana are causative agents of bacillary angiomatosis, a neovascular disorder that occurs primarily in immunocompromised individuals and involves the skin, lymph nodes, and a variety of organs.^4,22,23 Although CSD is often self-limiting, B henselae can cause chronic or intermittently clinical illness accompanied by persistent bacteremia.²⁴

B quintana, and to a lesser extent B henselae, was the causative agent of trench fever during World War I and has recently been the cause of urban trench fever, which occurs predominately among homeless and marginalized populations.^25-28 B quintana can be found worldwide and is transmitted among humans via feces of the human body louse (Pediculus humanus).^2,26 Humans are considered incidental hosts for all Bartonella spp, with the exception of B quintana, for which humans are reservoir hosts.^4,17 New findings, however, support the role of C felis in transmission and suggest that cats, dogs, and, possibly, the cynomolgus monkey—all accidental hosts—may also serve as sources of infection.^{2,3,10,17,29,30} B quintana transmission to humans has rarely been associated with cat bites and scratches, and transmission from dogs to humans has never been documented.¹⁰

B quintana does not appear to cause illness in cats, despite evidence of infection.^3,21,31 B quintana infection in dogs is typically subclinical but has been associated with endocarditis.^2,32 Prevalence of these pathogens in cats and dogs is not well-known. A study in Germany demonstrated a B quintana seroprevalence rate of 18% in stray cats.³ In a study in Israel, 2.6% of cats were found to be seropositive only to B henselae, 20.2% only to B quintana, and 39.5% to both B henselae and B quintana. In another study in North Carolina, 8.7% of cats were found to be seropositive only to B henselae, 7% only to B quintana, and 40.4% to both B henselae and B quintana.³³

In humans, B quintana infection typically results in a self-limiting febrile illness, with fever lasting up to 5 days, fatigue, conjunctival congestion, headache, myalgia, bone pain (particularly shin pain), rash, nystagmus, endocarditis, pericardial effusion, bacillary angiomatosis, and/or bacillary peliosis, particularly in individuals infected with human immunodeficiency viruses.^4,23,26,34 B quintana and, rarely, B henselae can persist long-term if not treated properly in humans and are known causes of culture-negative endocarditis in humans.⁴

Humans are the only known reservoir of B bacilliformis, which is transmitted among humans via the bite of sand flies (Lutzomyia spp) located in the Andes mountains, particularly in Peru, Colombia, and Ecuador.^{2,5,17,25,35,36} B bacilliformis can cause Carrión’s disease, a biphasic illness in humans that consists of acute and chronic phases. The acute phase (ie, Oroya fever) is characterized by a wide array of signs (eg, fever, chills, headache, intense myalgia, mental status changes, seizures, profound hemolytic anemia) and has a high fatality rate when untreated. The chronic phase (ie, verruga peruana [ie, Peruvian warts]) appears weeks to months after resolution of the acute phase and is characterized by angioproliferative skin lesions prone to ulceration and bleeding.^4,35 Children and pregnant women may be particularly affected; complications can include fetal death, miscarriage, and/or premature birth.³⁵

An animal reservoir of B Bacilliformis has not been discovered but may still exist³⁵; there have been no documented cases of infection in dogs or cats.^33,36,37

Cats are the primary reservoir of B koehlerae transmission to humans,^3,5,6,24 although B koehlerae and B quintana are less frequently isolated in cats as compared with B henselae.³ Evidence of B koehlerae infection has also been found in dogs and the fleas of wild gerbils (Meriones lybicus).^2,3,38 C felis (and possibly other flea species) is critical in the transmission of B koehlerae between animals and humans^2,17; however, additional arthropod vectors and other documented means of transmission of Bartonella spp are emerging.¹

Infection in cats can lead to subclinical infection and persistent bacteremia⁴ or, rarely, epithelioid hemangioendothelioma and systemic reactive angioendotheliomatosis.² B koehlerae infection in dogs has been associated with endocarditis and splenic disease.^2,20,39,40 A serosurvey of dogs in North America identified a seropositivity rate of 2.4%.¹⁵

B koehlerae is an uncommon cause of illness in humans⁴ but has been associated with regional pain syndrome type I, hallucinations, sensory neuropathy, peripheral visual deficits, endocarditis, and other clinical conditions.^2-5

Coyotes, dogs,⁴¹ and foxes are the main reservoirs of B vinsonii subsp berkhoffii,^2,3 but the subspecies has also been found in cats, deer, horses, humans, a steer, and a red wolf.^2,42,43 A serosurvey of dogs in the United States found a seropositivity rate of 1.5%¹⁴ and a higher rate of 3.6% in clinically ill dogs.⁴⁴ Signs in dogs may be subclinical or include anemia, arrhythmias, endocarditis, epistaxis, fever, hemangiosarcoma, myocarditis, splenomegaly, uveitis, and other signs; infection with this subspecies may also cause death.^3,17,45,46

B vinsonii subsp berkhoffii is rare in cats and humans. In cats, signs can include endocardial fibrosis complex, endomyocarditis, and osteomyelitis.^3,42 Reported signs in humans include endocarditis and neurologic symptoms.^2-4,23,47,48 Vectors for transmission include C felis and, possibly, ticks and Pulex spp fleas.² It is probable that B koehlerae and B vinsonii subsp berkhoffii are underrecognized, as testing for Bartonella spp other than B bacilliformis, B quintana, and B henselae is rarely performed.³

Although new species of Bartonella are discovered frequently, the most widespread and well-described in humans and animals is B henselae. Many Bartonella spp can produce similar clinical signs in a host, but these infections can be subclinical and chronic. Vectors play a significant role in the epidemiology of all Bartonella spp; transmission by other routes, particularly bites and scratches, makes Bartonella spp an occupational risk to those working closely with animals, especially those with a flea infestation. The occupational risks for Bartonella spp infection in veterinary staff can be reduced through a better understanding of Bartonella spp and by adhering to good prevention measures when working with animals (see Occupational Hazard).