An Overview of Bartonella vinsonii subspecies berkhoffii Infection in Dogs

Jessie Hopkins, DVM; Bruce E. LeRoy, DVM, PhD; Kenneth S. Latimer, DVM, PhD; Holly Moore, DVM

Class of 2006 (Hopkins) and Department of Pathology (LeRoy, Latimer, Moore), College of Veterinary Medicine, University of Georgia, Athens, GA 30602-7388

Introduction

Bartonella spp. (formerly Rochalimaea) are members of the family Bartonellaceae, order Rhizobiales of the alpha subdivision of Proteobacteria. They are small, rod-shaped, gram-negative, hemotropic, aerobic bacteria. Bartonella spp. cause chronic persistent intracellular infections of host erythrocytes, macrophages, and vascular endothelial cells, and stimulate vasoproliferative lesions. Their intracellular location and immunomodulatory effects help them to avoid host immune responses. Several novel Bartonella spp. recently have been implicated in a number of emerging zoonotic and domestic animal diseases. The spectrum of clinical presentations in the mammalian host can be extremely variable, ranging from asymptomatic infection to systemic disease to sudden death. The first case of B. vinsonii subsp. berkhoffii infection was documented in a dog with endocarditis in 1993.¹ Since then, B. vinsonii subsp. berkhoffii has been isolated from the blood of healthy and sick dogs and one human with endocarditis.² Bartonella vinsonii subsp. berkhoffii is believed to be transmitted to dogs via the brown dog tick, Rhipicephalus sanguineus. This speculation is based on the frequent coinfection with other tick-borne pathogens transmitted by R. sanguineus coupled with a 14-fold increased risk of B. vinsonii subsp. berkhoffii infection with tick infestations.³

Epidemiology and Seroprevalence

Risk factors for development of B. vinsonii subsp. berkhoffii infection in dogs include living in a rural area, roaming freely outdoors, and a history of heavy tick infestation. Presumably, ticks transmit B. vinsonii subsp. berkhoffii to dogs, but the precise species is not currently known. Dogs that are seropositive to B. vinsonii subsp. berkhoffii are also often seropositive to Ehrlichia canis and/or Babesia canis, both of which are transmitted by the tick vector, Rhipicephalus sanguineus. Of dogs that were naturally infected with and tested seropositive (reciprocal titer = 64) for B. vinsonii subsp. berkhoffii, 36% were also seropositive for E. canis and 57.1% were seropositive for B. canis. However, dogs experimentally infected with E. canis were not seroreactive to B. vinsonii subsp. Berkhoffii, indicating that there was no cross-reactivity between their antigens.³ These findings suggest that R. sanguineus is a likely vector of B. vinsonii subsp. berkhoffii, but this supposition has not yet been proven. Additionally, 20% of adult Ixodes pacificus ticks living in Bartonella-endemic regions of California have tested positive for Bartonella DNA.²

The seroprevalence of B. vinsonii subsp. berkhoffii in dogs is usually low, but seems to vary with tick exposure. Of the sick dogs that presented to the North Carolina State University (NCSU) Veterinary Teaching Hospital and the NCSU Vector-Borne Disease Diagnostic Laboratory in a one year period, 3.6% (69 of 1920) and 11.4% (90 of 790) were seropositive for B. vinsonii subsp. berkhoffii, respectively. ^{3, 4} A similarly low seroprevalence of disease has also been detected in Israel and Thailand, suggesting that B. vinsonii subsp. berkhoffii may have a world-wide distribution.⁵ Low seroprevalence would suggest that very few dogs should be bacteremic. Thus dogs are probably not the reservoir for B. vinsonii subsp. berkhoffii. However, a 2000 study reported 28% of coyotes in central coastal California were bacteremic and 76% were seropositive for B. vinsonii subsp. berkhoffii, suggesting that coyotes may be an important wildlife reservoir of B. vinsonii subsp. berkhoffii.⁶

Clinical Presentation

The clinical manifestation of B. vinsonii subsp. berkhoffii infection in dogs is extremely variable and can range from asymptomatic infection to sudden death. There is also a large variation in duration of illness with B. vinsonii subsp. berkhoffii infections, ranging from 1 month to 2 years.⁴ The diversity of clinical signs of disease and duration of infection may be due to varying degrees of host immunocompetency, differences in parasitic virulence, or presence of co-infections or other debilitating diseases. The low virulence, persistent bacteremia, and mild tissue invasion seen with Bartonella infection in some hosts may create a chronic infection that persists for months to years before diagnosis and treatment.⁸ Evidence suggests that B. vinsonii subsp. berkhoffii infection creates chronic intravascular infections that can decompensate acutely due to endocarditis, myocarditis, or central nervous system (CNS) disease.⁴ Most dogs exhibit several clinical signs of disease because intravascular infection affects different organ systems. The most common clinical signs are nonspecific and include lethargy, weight loss, anorexia, and/or inappetance. Other signs of disease are less common and include granulomatous lymphadenitis, granulomatous rhinitis, epistaxis, lameness, hind limb paresis, polyarthritis, cutaneous vasculitis, anterior uveitis, hyphema, chorioretinitis, anemia, thrombocytopenia, endocarditis with a predilection for the aortic valve, myocarditis, arrhythmias, cardiogenic pulmonary edema, collapse, seizures, meningoencephalitis, and sudden death.^1,4,7,9,10 Breitschwerdt et al. reported that of the sick dogs that were seropositive for B. vinsonii subsp. berkhoffii, 50% were thrombocytopenic, 33% were anemic, 33% exhibited muscle and/or joint pain, and 16% presented with neurological disorders. This latter finding suggested that B. vinsonii subsp. berkhoffii could establish chronic CNS infections. Fever is infrequent with B. vinsonii subsp. berkhoffii infection in dogs and when present usually suggests endocarditis, lymphadenitis, or bone and joint inflammation.⁴

Clinical Laboratory Findings

In one study of seropositive dogs with clinical disease, 50% were thrombocytopenic, 50% had a neutrophilic leukocytosis (half exhibiting a left shift), 33% were anemic, 33% had a monocytosis, and 29% had an eosinophilia.⁴ Both regenerative and nonregenerative anemias have been reported with up to 21% (5 out of 24) of the dogs in one study having clinical or laboratory evidence of immune-mediated hemolytic anemia (IMHA).⁴ Immune-mediated thrombocytopenia (ITP) also has been documented in dogs with chronic B. vinsonii subsp. berkhoffii infection. Dogs with both IMHA and ITP have been reported and typically have a poor prognosis.⁴ Other laboratory abnormalities include mild to moderate increases in ALP and ALT activities.

Pathophysiology

B. vinsonii subsp. berkhoffii causes chronic infections by establishing intracellular infection in erythrocytes and endothelial cells, thereby escaping the acquired humoral and cell-mediated immune defenses of the host. Bartonella spp. are capable of producing vasoproliferative lesions. It is believed that Bartonella spp. trigger the expansion and migration of the local endothelial cell population by inducing endothelial cells and/or macrophages to secrete endothelial growth factors (such as vascular endothelial growth factor). Cutaneous vasculitis in some dogs may be due to thickened but weak vessel walls due to increased endothelial cell replication or damaged endothelial cells from intracellular infection by the bacterium.⁷ Specific-pathogen-free dogs experimentally infected with B. vinsonii subsp. berkhoffii developed chronic infections that resulted in defects in monocytic phagocytosis, impaired antigen presentation in lymph nodes due to reduced MHC II expression by B lymphocytes, increased numbers of phenotypically naïve CD4+ T lymphocytes, and reduced numbers of CD8+ T lymphocytes with decreased adhesion molecule expression.⁸ Defective phagocytosis can lead to granuloma formation and this may support the finding that Bartonella spp can, in some hosts like the dog, lead to the development of granulomatous inflammation. Increased numbers of naïve CD4+ T lymphocytes suggests that a chronic, but insufficient, cell-mediated immune response develops with B. vinsonii subsp. berkhoffii infection. Therefore, these chronic immunosuppressive effects of B. vinsonii subsp. berkhoffii may predispose dogs to secondary infections and may be another explanation for the variation of clinical signs seen in dogs with this disease.

Diagnosis

Once Bartonella establishes chronic intracellular infection, its replication rate slows, antibody titers decline, and the pathogen is hard to isolate from the blood because of the low bacteremia.⁸ The low bacteremia with B. vinsonii subsp. berkhoffii-infection makes blood culture a relatively insensitive means of isolating the pathogen and diagnosing infection. Polymerase chain reaction (PCR)-restriction fragment length polymorphism (RFLP) analysis and/or sequencing the gltA gene and/or 16S rRNA and/or 16-23S rRNA interspacer transcribed genes are techniques that can be used for disease diagnosis.^6,7 However, serology using immunofluorescent antibodies (IFA) is the most sensitive means of diagnosis. Since the seroprevalence of B. vinsonii subsp. berkhoffii in sick dogs in endemic regions is generally low (<4%), serology can be used as an effective screening test to detect active infection or past exposure.⁵ Experimentally-infected dogs maintained IgG titers =1:64 for the duration of a 132-day study.⁸ Because of the low seroprevalence and the chronicity of the disease in dogs, patients are considered seropositive with a titer =1:64 and should be treated for B. vinsonii subsp. berkhoffii. A high serum titer (>1:512) against Bartonella combined with echocardiographic lesions associated with endocarditis, especially of the aortic valve, are highly suggestive of infectious endocarditis caused by B. vinsonii subsp. berkhoffii.⁹

Transmission electron micrograph of Bartonella vinsonii subspecies berkhoffii (image courtesy of web.umr.edu/~microbio/ BIO221_2002/Bartonella.jpg)

Treatment

Currently, there is no set treatment protocol for B. vinsonii subsp. berkhoffii because the appropriate antibiotic and duration of administration has not yet been established. However, an antibiotic capable of crossing lipid membranes and reaching high intracellular concentrations would be indicated, such as azithromycin, doxycycline, and enrofloxacin. Macrolides, like azithromycin, have shown the most therapeutic promise. Treatment of several weeks duration may be needed to eliminate Bartonella infections.

For dogs with endocarditis or myocarditis where there is serious risk associated with bacterial infection, aminoglycosides may be indicated.⁵ Even with treatment, dogs with endocarditis may not improve due to permanent scarring of the heart valves. There is also a high likelihood of co-infection with other pathogens in dogs that are seropositive to B. vinsonii subsp. berkhoffii. Therefore, it is recommended that a tick titer panel be performed to detect other pathogens such as E. canis, B. canis, and Rickettsia rickettsii. These pathogens also are transmitted by the tick R. sanquineus and have been seen in high correlation with Bartonella infection in dogs. Follow up titers should be done at 4- to 8-week intervals after the initiation of treatment to assess the efficacy of treatment. Treatment should be continued until the titer for the offending pathogen is negative.

Disease Prevention

Dogs should be placed on an effective, monthly acaricide such as Frontline Plus® (Fipronil, methoprene) or Advantix® (Imidocloprid, permethrin) to protect them from fleas and tick infestations. Although fleas are not highly suspected of transmitting B. vinsonii subsp. berkhoffii, they are the known vectors of B. henselae, which can also infect dogs. For owners living in wooded and/or grassy areas with known tick infestations, it is recommended that dogs be inspected for ticks after every walk or at least daily. Tick eradication is almost impossible because the life cycle is spent on different hosts. Unfortunately, a vaccine against B. vinsonii subsp. berkhoffii is not available.

Public Health Considerations

Several Bartonella spp. cause zoonotic diseases that can be transmitted from domestic or wild mammals to humans through various insect vectors. Some of the zoonotic Bartonella spp. can cause severe disease in people, especially in individuals who are immunocompromised. B. vinsonii subsp. berkhoffii has been isolated from the blood of both healthy and sick dogs, as well as from a person with endocarditis whom had been bitten by a coyote. It is not yet known if transmission from a dog to a human can occur. People should take precautions against fleas and tick exposure. When walking in grassy and wooded areas during the warmer months, people should wear their socks rolled over their pant legs, wear lighter colors that help to detect ticks on clothing, use a DEET-containing spray as a protectant, and check for ticks on clothes or attached to the skin after walks.

Conclusion

Based on the previously mentioned clinical and laboratory findings, B. vinsonii subsp. berkhoffii infection should be considered for unexplained epistaxis, endocarditis, myocarditis, granulomatous inflammation, lymphadenitis, cutaneous vasculitis, meningoencephalitis, uveitis, anemia, thrombocytopenia, and polyarthritis.

Dogs are the only known animal host of a Bartonella spp. that produce clinical signs comparable to those seen in humans with bartonellosis.

References

1. Breitschwerdt EB, Kordick DL, Malarkey DE, et al. Endocarditis in a Dog Due to Infection with a Novel Bartonella Subspecies. J Clin Microbiol 1995;33:154-160.

2. Guptill L. Bartonellosis. Vet Clin Small Anim 2003;33:809-825.

3. Pappalardo BL, Correa MT, York CC, et al. Epidemiologic evaluation of the risk factors associated with exposure and seroreactivity to Bartonella vinsonii in dogs. Am J Vet Res 1997;58:467-471.

4. Breitschwerdt EB, Blann KR, Stebbins ME, et al. Clinicopathological Abnormalities and Treatment Response in 24 Dogs Seroreactive to Bartonella vinsonii (berkhoffii) Antigens. J Am Anim Hosp Assoc 2004;40:92-101.

5. Breitschwerdt EB. Bartonellosis. In: Ettinger SJ, Feldman EC, editors. Textbook of Veterinary Internal Medicine: Diseases of the Dog and the Cat, 6^th ed. St. Louis, Missouri: Elsevier Saunders; 2005. pp 636-637.

6. Chang CC, Kasten RW, Chomel BB, et al. Coyotes (Canis latrans) as the Reservoir for a Human Pathogenic Bartonella sp.: Molecular Epidemiology of Bartonella vinsonii subsp. berkhoffii Infection in Coyotes from Central Coastal California. J Clin Microbiol 2000;38:4193-4200.

7. Breitschwerdt EB, Hegarty BC, Maggi R, et al. Bartonella Species as a Potential Cause of Epistaxis in Dogs. J Clin Microbiol 2005;43:2529-2533.

8. Pappalardo BL, Brown TT, Tompkins M, et al. Immunopathology of Bartonella vinsonii (berkhoffii) in experimentally infected dogs. Vet Immunol Immunopathol 2001;83:125-147.

9. MacDonald KA, Chomel BB, Kittleson MD, et al. A Prospective Study of Canine Infective Endocarditis in Northern California (1999-2001): Emergence of Bartonella as a Prevalent Etiologic Agent 2004;18:56-64.

10, Michau TM, Breitschwerdt EB, Gilger BC, et al. Bartonella vinsonii subspecies berkhoffii as a possible cause of anterior uveitis and choroiditis in a dog. Vet Ophthalmol 2003;6:299-304.

Acknowledgements

Painting found at www.aportraitbyjocelyn.com

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