A Ubiquitous Gram-Negative Bacteria with “Apparent” Ocular Manifestations in “Some” Dogs and “Some” Cats

Steve Dugan, DVM, MS, Diplomate of the ACVO

Bartonella is a gram-negative, aerobic bacilli that infects primarily the erythrocytes and vascular endothelial cells of its hosts.  The genus Bartonella contains many species that can induce chronic infections in humans and other animals.  Cats are believed to be the main reservoir host for Bartonella henselae and Bartonella clarridgeiae.  Dogs are believed to be the main reservoir host for Bartonella vinsonii subspecies berkhoffii.  Fleas and ticks are considered to be the primary arthropod vectors for Bartonella.  Exposure to fleas and flea feces is believed to be the most important factor for transmission of Bartonella between cats.  Dependent upon the geographic location assessed, Bartonella species infection of cats and dogs can be common.  However, fleas and ticks are uncommon in Colorado and as such the prevalence of Bartonella infection is low in Colorado versus many other regions of the US.

Because Bartonella are able to induce persistent infections in the host while evoking minimal immunologic recognition, it’s difficult to establish a cause-and-effect relationship between this highly adapted bacteria and a diverse spectrum of disease manifestations.  It is unclear as to why some dogs and some cats develop Bartonella-associated illness and others do not, nonetheless, a myriad of disease manifestations of Bartonella infection have been described in humans, cats and dogs.  Bartonella henselae is the cause of cat scratch disease in humans.  Despite most cats with serologic evidence of exposure to a Bartonella species being clinically normal, Bartonella infection of cats has been associated with fever, lethargy, lymphadenopathy, uveitis, stomatitis and seizures.  Most dogs with clinical Bartonella infection have multiple clinical signs attributed to more than one organ system.  Disease manifestations commonly associated with clinical bartonellosis in dogs include arrythmias, endocarditis, fever, polyarthritis/lameness, lymphadenitis, lethargy, weight loss, anorexia, uveitis, and Meningoencephalitis.

The challenges of diagnosis are especially problematic as Bartonella species are slow growing and extremely difficult to culture, and PCR amplification (microbial DNA amplified from blood) and detection of antibodies have diagnostic limitations.  A number of tests exist to assess cats for Bartonella infection but no individual test can “prove” a cat has clinical bartonellosis.   As per Dr. MR Lappin, “it’s not that Bartonella does not make some cats sick, I believe it does, it’s just hard to prove which ones as none of the currently available Bartonella tests correlate to the presence of illness.”   When assessing clinically ill cats for Bartonella species infection, Dr. Lappin suggests the combination of serologic testing and PCR assay are more accurate than either alone.

Treatment recommendations for dogs and cats have been extrapolated from medical regimens recommended for Bartonella infections in humans.  However, the antibiotic of choice as well as the duration of treatment have not been clearly established for Bartonella species infection in humans, dogs or cats.  Although Azithromycin was previously considered effective in cats, its use in the treatment of Bartonella is now discouraged because of the rapidity with which the organism develops resistance.  As a result, the American Association of Feline Practitioners 2006 Panel report currently  recommends doxycycline at 10mg/kg per os q 24h as the first drug of choice.  If the patient has improved after the first week, continue doxycycline treatment two weeks beyond the resolution of clinical disease or for a minimum of four weeks.  If, on the other hand, a poor response to doxycycline is noted after one week of therapy, then the clinician should consider changing to a fluoroquinolone such as orbifloxacin.  All in all, these treatment regimens may have positive effects in some, but not all, cases.  It has also been suggested that six months following antibiotic therapy, serology be repeated in order to determine if there has been a decrease in the antibody titer.  It is necessary to wait six months from the end of treatment in order to allow the antibody levels to drop via catabolism after removal of the Bartonella infection which serves as the antigenic stimulus.  A two- to four-fold decrease in the antibody titer between the pre- and post-therapy samples indicates successful Bartonella treatment.


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