Susan E. Shaw, BVSc (Hons), BSc, DACVIM, DECVIM, FACVSc, MRCVS
Although the names are similar, these feline infections are associated with different epidemiology, pathogenesis and disease patterns.
Haemobartonella felis organisms have long been recognised as cat microparasites and are Gram-negative, haemotropic, bacteria lacking a cell wall. They attach to the surface of host erythrocytes and are at present, unculturable. DNA sequence analysis has shown that these bacteria are most closely related to the genus Mycoplasma and they have recently been reclassified accordingly. Two species have now been identified, Mycoplasma haemofelis and M. haemominutum; the former is a larger parasite and is associated with clinical disease in cats while the latter smaller organism although common, appears to be non-pathogenic.
Transmission and prevalence
There is minimal information available on the epidemiology of feline haemoplasma infection. The mode of transmission has not been determined although arthropod transmission is incriminated. In addition, although infection is recognised world-wide in cats, the prevalence of infection/exposure is unknown due to the unavailability of serological testing.
Previous reports that infection results in mild or inapparent disease may relate to M. haemominutum. Severe haemolytic anaemia is associated with M. haemofelis infection and clinical signs include pallor, lethargy, anorexia weight loss and depression. Intermittent fever may be present in the acute stage of the disease while icterus is occasionally seen later in its progression. Splenomegaly and lymphadenopathy may also occur. Haematological signs compatible with a marked regenerative anaemia are commonly present. Both Coombs positivity and cold agglutinating antibody have been reported in association with infection and although pathogenesis of the anaemia is thought to have an immune-mediated component, this has not been characterised. Chronic persistent infection is reported and may be associated with minimal clinical signs.
Diagnosis by microscopic identification of organisms in blood smears is now complicated by the recognition of the apparently apathogenic species M. haemominutum. Although close morphologic examination may be able to distinguish the large from the smaller species of haemoplasma, the cyclic nature of the parasitaemia makes this method of detection insensitive. The organism cannot be cultured at present and so development of a reliable serological test has been hampered. Molecular diagnosis using polymerase chain reactivity (PCR) analysis is definitive and recommended.
Doxycycline (5-10 mg/kg) or enrofloxacin (5-10mg/kg) administered orally for 3-4 weeks is recommended for cats with clinical anaemia and should be combined with blood transfusions and prednisolone in severely affected Coombs positive cases. Although clinical response is often achieved, parasitological cure may be less certain and successfully treated cats may become asymptomatic carriers.
Bartonellosis is caused by fastidious, Gram-negative, intraerythrocytic, arthropod-transmitted bacteria of the genus, Bartonella. Several species are pathogenic in cats (Bartonella henselae, B. koehlerae, B. clarridgeiae). Asymptomatic infection with B. henselae or B. clarridgeiae is common in cats, which are therefore considered to be a major reservoir for human infection. In humans, B. henselae and B. clarridgeiae have been shown to be the agents of the common, but usually self-limiting cat scratch disease (CSD). However, less frequently, B. henselae has been associated with more profound syndromes such as the vasculo-proliferative disorders, bacillary angiomatosis and peliosis hepatis, as well as endocarditis, prolonged bacteraemia and various ocular disorders including Parinaud oculoglandular syndrome, neuroretinitis and chorioretinitis. Genotypic and phenotypic (serological) variations have been demonstrated among strains of B. henselae in domestic and wild cats and those from different geographical locations. At present, the most significant division within the species delineates strains into one of two subtypes on the basis of their 16S-rDNA gene sequence.
Cat fleas are considered the main vector of B. henselae in cats and recent work has shown transmission by skin inoculation of infected flea faeces. However, the role of the cat flea in the transmission of B. henselae from cats to humans has not been proven.
Asymptomatic infection with Bartonella henselae (and B. clarridgeiae) is common in cats; 40-70% with seropositivity and 9-90% with bacteraemia. Variability in reported figures may be a consequence of small survey sizes, differences in cat population characteristics (cattery, stray, feral and captive wild cats) and seasonal variation in prevalence as well as true differences in geographic prevalence. The prevalence of infection appears to be higher in young to middle-aged cats but there is no breed or gender predisposition. Although geographic environments with warm temperatures and high humidity are reportedly associated with the highest exposure; the prevalence in cool temperate climates is also relatively high. In recent UK surveys for B. henselae, 11% of cats surveyed were culture positive and 41% of cats were seropositive. The effect of climatic factors on the ecology of Bartonella infection may be blurred as in colder countries, animals are kept in heated domestic or confined environments, facilitating the maintenance of the flea life cycle.
Pathogenicity and clinical signs
Disease association with naturally occurring feline Bartonella infection is difficult to determine. Although clinical disease (fever, lethargy, transient anaemia, lymphadenomegaly, neurological dysfunction or reproductive failure) has been reported following experimental infections with B. henselae and B. clarridgeiae, naturally occurring disease associated with infection is more difficult to define because of its high prevalence in apparently asymptomatic cats. In naturally infected cats, there is a statistical correlation with stomatitis and urinary tract disease. Uveitis associated with intraocular Bartonella DNA and ocular IgG production has also been reported in cats. Although Bartonella infection has been associated with other clinical syndromes such as endocarditis based on positive blood culture, the association is difficult to evaluate unless the presence of lesional organisms is confirmed.
Treatment of bartonellosis and elimination of bacteraemia is problematic. Doxycycline, amoxicillin, amoxicillin/clavulanate used at higher than recommended dose rates have been successful in suppressing bacteraemia in experimental infections. Rifampicin and enrofloxacin are also reportedly effective. However, total elimination of infection may be impossible despite the use of combination therapy such as rifampicin and doxycycline, and prolonged duration (4-6 weeks) of therapy. In addition, the risk of re-exposure is high.
CONTROL AND PREVENTION OF BOTH INFECTIONS
No vaccination is currently available for either group of organisms. The occurrence of multiple strains with lack of cross -reactivity in the case of Bartonella, suggests that any vaccine would require incorporation of multiple epitopes. Vaccine candidates will also need to induce cellular as well as antibody-mediated immunity.
The prevalence of Bartonella bacteraemia in cats and the risk of Bartonella-associated disease in pet owners, is decreased by a vigorous integrated flea control programme. When uninfected cats are housed with B. henselae bacteraemic SPF cats in an ectoparasite-free environment, there is no evidence of Bartonella transmission between cats. Although there is no definitive proof of arthropod transmission of haemoplasma species, aggressive flea control is often recommended for control.