Laboratory Studies
Stains
Microscopic examination of Giemsa-stained blood smears is used to detect B bacilliformis in patients who may have Oroya fever.
Organisms are rod-shaped and slightly curved, similar to Campylobacter or Helicobacter species.
B bacilliformis organisms often appear to be adherent to erythrocytes, but they may actually be inside erythrocytes.
Other Bartonella species are visible only with silver stains (eg, Warthin-Starry, Steiner, Dieterle), if they stain at all. Bacillary forms also exist. These stains are not specific.
Immunostaining can also aid in the diagnosis of early lesions or atypical manifestations of cat scratch disease. Capnetti at al [40] found that immunohistochemical analysis was positive in 25% of cases, PCR in 38% of cases, and Steiner silver stain in 46% of cases of 22 patients with lymphadenopathy and histopathological findings compatible with cat scratch disease.
Late in the course of B henselae infection, organisms may not be found in areas of necrotizing granulomas.
Cultures
Culture for Bartonella bacteria is not recommended for routine cases of patients with cat scratch disease lymphadenopathy.
Cultures may be useful in patients who have other manifestations of either B henselae or B quintana infection, including fever of unknown origin, neuroretinitis, encephalitis, culture-negative endocarditis, and peliosis or bacillary angiomatosis. Fresh media is required to increase the chance of isolation.
A study by Maggi et al. in 2005 suggested a novel chemically modified liquid medium that supports the growth of seven Bartonella species. [41]
A 2016 study showed that a serum-free media using glucose, sucrose, and hemin as supplements was equivalent to the commercially available media but was easier to prepare. [42]
The lysis-centrifugation system (Isolator) is recommended for blood cultures.
Minced tissues may be cultured on chocolate agar plates in a humid atmosphere with carbon dioxide to facilitate growth.
Antibiotic susceptibility is not routinely tested in patients with bartonellosis because susceptibility studies may fail to predict response to therapy.
Serologic testing
Serologic testing is the most cost-effective method to confirm the diagnosis in most patients with bartonellosis; however, serologic testing can provide negative results or can be otherwise nondiagnostic in immunocompromised patients. In transplant patients, serology was positive on all 23 patients who were tested. [24]
For cat scratch disease and CNS or eye infections, serologic testing provides helpful data. Cross-reactions occur between antibodies for B henselae and B quintana and with both Chlamydia species and Coxiella burnetii. [43]
Some studies have shown that Western immunoblotting appears to have very high sensitivity and specificity for diagnosing Bartonella endocarditis. [15]
An IgM titer of 1:16 or greater is usually considered evidence for early recent Bartonella infection. IgM levels may stay elevated for about 3 months in 50% of patients. A positive IgM finding supports the diagnosis, but sensitivity was low (43%) in one study. [44]
A study in 2017 showed that IgM Western Blot had a sensitivity of 53% (49/92) but a very high specififity (none of the healthy 130 subjects had a positive test). The conventional indirect fluoresence antibody assay had a 28.3 % sensitivity. [45]
An IgG titer of greater than 1:256 is considered evidence of current or past Bartonella infection. Titers of 1:64 to 1:128 are considered equivocal. A titer of 1:800 or more for IgG antibodies to either B henselae or B quintana has a positive predictive value of 0.810 for the detection of chronic bartonellosis in the general population and a value of 0.955 for the detection of bartonellosis among patients with endocarditis. [46]
Patients with delayed decreases in antibody titers may be at risk for endocarditis relapse. When clinical suspicion is high, titers should be repeated in 10-14 days for comparison.
Evidence shows that some patients never mount a detectable antibody response. The serology results of some patients remain positive long after exposure and recovery from their illness.
Polymerase chain reaction
PCR is a molecular technique that involves amplification of Bartonella species genes.
PCR-based detection of various target genes of Bartonella species in tissue specimens is widely accepted in diagnosing cat scratch disease and bacillary angiomatosis, but the results rely heavily on the quality of the laboratory in which the assays are conducted.
In cases of suspected Bartonella endocarditis, PCR on cardiac valve tissue is very sensitive and is not affected by antibiotics treatment prior to surgery.
Because of the cross-reactivity between Bartonella species and other bacteria, PCR analysis of tissue and body fluid is the most specific test, especially in identifying distinct genotypes among Bartonella species.
PCR has played an important role in the diagnosis of Bartonella endocarditis. This test amplifies a 296–base-pair fragment of the 16S ribosomal RNA. Investigators reported PCR results that were positive for Bartonella on cardiac valve tissue in more than 95% of patients with Bartonella endocarditis in one study, despite the fact that more than 60% of these patients underwent valve analysis following the receipt of antibiotics.
A 2017 study found that the sensitivity of Bartonella PCR on valve tissue was 92% (48/52) compared to 33% (20/60) in blood and 36% (25/70) in serum. [47] . Another study showed that PCR testing of blood, plasma, or serum samples taken from confirmed case of Bartonella endocarditis was shown to have a sensitivity of about 58% and specificity of 100%. [48]
Imaging Studies
CT scanning of the chest and abdomen may reveal mediastinal, retroperitoneal, or mesenteric lymph node enlargement.
CT scanning or ultrasonography of the abdomen can demonstrate multiple hypodense lesions in the liver and/or spleen in patients with cat scratch disease
In patients with bacillary angiomatosis/peliosis, CT scanning or MRI of the involved organ shows the enhancing lesions (eg, brain, liver). Radiography of the bone may show osteolytic lesions and bone destruction.
According to some reports, patients with cat scratch disease may require echocardiography to assess for valvular lesions. Patients with valvulopathy and cat scratch disease are at risk for Bartonella endocarditis.
Other Tests
The cat scratch disease skin test is helpful. However, this test may transmit other infectious agents, as it is derived from human pus.
Silver stains and PCR can be performed on pus or tissue collected via fine-needle aspiration or biopsy.
Histologic Findings
Cat scratch disease: Lymph node biopsy demonstrates granulomas with stellate necrosis, which are characteristic of cat scratch disease. Silver stains, such as Dieterle or Warthin-Starry, may show bacilli.
Bacillary angiomatosis and peliosis hepatitis: Both of these disorders show blood vessel proliferation. Bacillary angiomatosis refers to vascular tumors that readily bleed when cut. The microscopic appearance of neovascular proliferation is diagnostic. Silver stains may demonstrate clusters of Bartonella bacilli. Peliosis refers to blood-filled cystic lesions that may be microscopic or up to a few millimeters in diameter. These lesions are characteristically found in the liver but are occasionally found in the spleen. Bacilli may be visible on electron microscopy.
