Epidemiology

Since the first recognition of feline B. henselae infection in 1992,³³² it has been established that natural infections of cats with Bartonella spp. are common. Blood culture and molecular methods have detected natural infections with B. henselae, B. clarridgeiae, B. koehlerae, B. bovis, and B. quintana in cats.^† Serologic and blood culture data indicate that exposure to Bartonella spp., predominantly B. henselae, is most prevalent among cats in temperate regions of the world.²⁹⁵ Prevalence in domestic cats is lowest in Northern Europe and the Rocky Mountain regions of the United States and Canada, and greatest in warmer, more humid regions (Fig. 52-2). There are two main genotypes of B. henselae. The Houston 1 genotype is more prevalent in the Far East, and the genotype Marseille is predominant in western Europe, Australia, and the western United States. Both genotypes are equally prevalent in the eastern United States.⁹⁵ A third genotype, designated Berlin, was identified in a cat from Germany.⁹ Seroepidemiologic studies in cats generally show a higher prevalence of seroreactivity with age, warmer temperature, and higher humidity, and in feral cats and those infested with fleas.^‡ B. henselae bacteremia affects approximately 5% to 40% of cats in the United States, depending on geographic location, and is most common in temperate areas.^{53,86,177,273} Seroprevalence of over 90% is reported in some cat colonies.³⁰⁵ Prevalence rates of B. clarridgeiae infections were reported to be approximately 10% of cats with Bartonella bacteremia evaluated in the United States, 16% to 31% of cats with Bartonella bacteremia in France, and 31% of cats in the Philippines.^91,186,186 B. koehlerae was isolated from 2 cats (from the same household in California), and B. bovis was isolated from 2 cats in Utah and 2 in Illinois.^133,330 Domestic cats are considered the major reservoir and vector for human infections with B. henselae and for B. clarridgeiae. Cattle are the reservoir for B. bovis, and humans are the predominant reservoir for B. quintana. The reservoir for B. koehlerae is unknown although cats are suspected.^58a In France, molecular methods detected B. quintana, B. koehlerae, B. henselae, and B. clarridgeiae in cat fleas, along with rickettsial pathogens.³⁴⁰ This finding suggests that fleas may be involved in the transmission of many of these Bartonella species for humans and animals; however, the fleas may have just fed on an infected host. Additional evidence that arthropods may serve as vectors of some Bartonella species comes from finding these organisms in questing ticks in California, and in Ixodes ricinus ticks in Italy, among other locations.^{76,78,122,298,351} Ticks have been proposed as the vectors for transmission of some Bartonella infections in humans, dogs, and other mammalian hosts.^{23,315,362,391} In addition, there is evidence of transstadial infection of I. ricinus ticks with B. henselae, and B. henselae was detected in the saliva of infected ticks.¹¹¹ Biting flies have also been associated with Bartonella transmission.¹⁰⁴ Interestingly, Bartonella spp. have been isolated from rodents in Alaska, where there are no tick vectors.²⁸¹

Wild felids are also exposed to Bartonella spp.; 18% of panthers (Puma concolor coryi) in Florida and 28% of mountain lions (Puma concolor) in Texas had serum antibodies to B. henselae, and the prevalence of serum antibodies to B. henselae in free-ranging and captive wild felids in California was 30% to 53%.344,404 Prevalence of serum antibodies to B. henselae in mountain lions and bobcats (Lynx rufus) in Mexico, Central America, and South America ranged from 0% to 33%.⁹⁸ Bartonella infections have also been documented in free-ranging wild and captive African lions (Panthera leo) and cheetahs (Acinonyx jubatus), using serology, PCR testing, and blood culture.^95,295 B. henselae infection was identified by PCR in several species of neotropical felids in Brazil.¹⁶⁸

B. henselae contains multiple genetically diverse strains. There are two recognized 16S ribosomal RNA (rRNA) types of B. henselae (Houston 1 [type 1] and Marseille [type II]) and at least two subgroups are within each type.276,406 Cats can be co-infected with B. henselae 16S rRNA types I and II and co-infected with B. henselae and B. clarridgeiae.¹⁷⁸ In cats, co-infection also exists with hemotropic Mycoplasma spp. (see Chapter 31).²⁵⁰ Regional differences in prevalence of infection of cats exist with different genetic types of B. henselae.* Some evidence is suggestive of genomic variation in B. henselae during the course of infection in cats.^22,207 Such variation may enhance the ability of B. henselae to persist in an infected cat for prolonged periods. Whether the 16S rRNA type is the most accurate means of describing genetic differences among B. henselae isolates has not yet been determined. Multiple other methods have been investigated, and the full extent of genetic diversity within B. henselae is still being defined.^† Genetic variation makes vaccine development difficult (see Pathogenesis) but is useful in epidemiologic studies and may also be useful in furthering the understanding of the pathogenicity of various Bartonella isolates.

Pathogenesis

B. henselae is naturally transmitted among cats by cat fleas (Ctenocephalides felis). B. henselae was transmitted among cats by transferring fleas fed on infected cats to specific-pathogen free cats, and by intradermal inoculation of flea excrement.93,150 Cats exposed to B. henselae–infected fleas that were confined to capsules that permitted fleas to feed, but prevented contamination of cats’ skin and haircoat with flea excrement, did not become infected with B. henselae.¹⁵⁰ This finding suggests that transmission does not occur via flea saliva. As noted previously, ticks may also have a role in transmission.

Cats were experimentally infected with B. henselae through intravenous or intramuscular inoculation with infected cat blood,²³³ and by intravenous, subcutaneous, intradermal, or oral routes of inoculation with plate-grown bacteria.^{1,166,167,308} B. henselae transmission has not occurred when infected cats cohabit with uninfected cats in a flea-free environment,^1,167 indicating that transmission among cats does not occur through cat bites, scratches, grooming, or sharing of food dishes and litter boxes. Transmission did not occur when cats were inoculated intramuscularly with urine of bacteremic cats.²³⁰ Transmission also did not occur between bacteremic female cats and males during mating, or to the kittens of infected females either during gestation or in the neonatal period,^1,174 again in flea-free environments.

Bacteremia with B. henselae and B. clarridgeiae is commonly chronic and waxes and wanes; periods occur when bacteremia cannot be detected either by culture or by PCR testing. Although experimentally infected cats were not always followed for extended periods, some maintained B. henselae and/or B. clarridgeiae bacteremia for as long as 454 days.²³³ Relapsing bacteremia occurred at irregular intervals of between 1 and 4.5 months in some of these cats.^175,233 Naturally infected cats maintained recurrent bacteremia for periods as long as 3 years; however, reinfection via fleas of cats living in private homes is a likely cause of prolonged recurrent bacteremia.^10,237 Measured increases in interleukin-4 and serum antibody titers after the peak of bacteremia occurred concurrently with a decrease in the bacteremia to low or undetectable levels in one study.²⁰⁸ However, the effectiveness of antibodies in clearing bacteremia is not certain. In another study, kittens that did not produce measurable anti-Bartonella IgM or IgG antibodies had the same course of bacteremia as did kittens that produced high titers of anti-Bartonella antibodies.¹⁷⁵ A study suggests that cell-mediated immunity is important in reducing the level of bacteremia in experimentally infected cats.²⁰⁹ There was no difference in antibody response or duration of bacteremia in splenectomized cats compared to nonsplenoctomized cats, though the average level of bacteremia in splenectomized cats was tenfold greater in the splenectomized cats.³⁷¹ This suggests that phagocytosis is important in limiting bacteremia in infected cats. In a study of shelter cats, concurrent infection with B. henselae was most frequent in cats coinfected with feline leukemia virus than with feline immunodeficiency virus or feline panleukopenia virus.^69a One conclusion is that latent FeLV infection may predispose cats to B. henselae infection or its persistence.

Immune protection against Bartonella infections has been evaluated in several studies. Complete protection against reinfection is highly specific, even with strains of a given species of Bartonella, and likely in reactivity to the outer membranes of the organism.⁸¹ Cats previously infected with B. henselae 16S rRNA type II were protected from reinfection with B. henselae 16S rRNA type II but were susceptible to infection with B. henselae 16S rRNA type I.⁴⁰¹ Cats infected with B. henselae type I or II were susceptible to challenge infection with B. clarridgeiae, and cats infected with B. koehlerae or B. clarridgeiae were susceptible to challenge infection with B. henselae type I or type II.⁴⁰³ In contrast, cats infected first with B. henselae type I were partially or completely protected against subsequent challenge infection with B. henselae type II.⁴⁰³ It is evident from these studies that the level of bacteremia and degree of susceptibility to reinfection after challenge inoculation vary with strains, as well as with species, of Bartonella.⁴⁰¹

The scope of localization of Bartonella in cats has not been completely determined. Bartonella are intracellular bacteria, and B. henselae have been detected within erythrocytes of naturally infected cats341,355,355 and in vitro (Fig. 52-3).²⁸⁷ Bartonella are also endotheliotropic and may also be located intracellularly in vascular endothelial cells of infected cats, as has been suggested for rodents (Fig. 52-4).¹¹⁷ B. henselae are also found extracellularly in blood and tissues of infected cats.¹⁷⁶

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