Transmission of Bartonella henselae by Ixodes ricinus

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Transmission of Bartonella henselae by Ixodes ricinus

 

 

Abstract

Bartonella spp. are facultative intracellular bacteria associated with

several emerging diseases in humans and animals. B. henselae causes

cat-scratch disease and is increasingly associated with several other

syndromes,

particularly ocular infections and endocarditis. Cats are the main reservoir

for B. henselae and the bacteria are transmitted to cats by cat fleas.

However, new potential vectors are suspected of transmitting B. henselae, in

particular, Ixodes ricinus, the most abundant ixodid tick that bites humans in

western Europe. We used a membrane-feeding technique to infect I. ricinus

with B. henselae and demonstrate transmission of B. henselae within I.

ricinus across developmental stages, migration or multiplication of B. henselae

in salivary glands after a second meal, and transmission of viable and

infective B. henselae from ticks to blood. These results provide evidence that

I. ricinus is a competent vector for B. henselae

 

 

 

 

 

Bartonella spp. are facultative intracellular bacteria associated with

several emerging diseases in humans and animals (1). Domestic animals and

wildlife represent a large reservoir for Bartonella spp., and at least 10

species or subspecies have been reported to cause zoonotic infections. B.

henselae causes cat-scratch disease, possibly the most common zoonosis acquired

from domestic animals in industrialized countries and is becoming

increasingly associated with other syndromes, particularly ocular infections

and

endocarditis (2–6). Although cat fleas are well-established vectors for B.

henselae (7–10), transmission by other arthropods, in particular ticks, has

been suggested (11–13). Ixodes ricinus is the most widespread and abundant

ixodid tick in western Europe and is frequently associated with bites in

humans. It is a vector of emerging zoonotic pathogens including Borrelia

burgdorferi sensu lato (14), Anaplasma phagocytophilum (15), and Babesia spp.

(16).

 

Direct proof of transmission of Bartonella spp. by a tick was reported by

Noguchi in 1926 (17), who described experimental transmission of B.

bacilliformis (cause of Oroya fever) to monkeys by Dermacentor andersoni. In

this

study, ticks were allowed to feed on infected monkeys for 5 days. After

removal, partially engorged ticks were placed on healthy monkeys in which

disease then developed. This study showed that ticks could acquire and transmit

the bacteria but did not demonstrate their vector competence or

transtadial transmission throughout the tick's life cycle. Since this early

study,

the role of ticks in Bartonella spp. transmission has been strongly implied

but never definitively demonstrated. Bartonella spp. DNA was detected in

questing and engorged nymphs and adults Ixodes spp. collected in North

America, Europe, and Asia (13,18–26). If one considers that ixodid ticks feed

only

once per stage, Bartonella spp. DNA in questing ticks suggests transtadial

transmission of these bacteria

 

Other observations support Bartonella spp. transmission by ticks.

Co-occurrence of Bartonella spp. with known tick-borne pathogens such as B.

burgdorferi sensu lato, A. phagocytophilum, or Babesia spp. is not a rare event

in

ticks and hosts (13,19,24,27). A study conducted in a veterinary hospital

in the United States (California) demonstrated that all dogs with

endocarditis and infected with Bartonella spp. were also seropositive for A.

phagocytophilum (28). In humans, several case studies have reported patients

with

concurrent Bartonella seropositivity and detection of Bartonella spp. DNA

in their blood, along with B. burgdorferi infection of the central nervous

system after tick bites (11,29). Moreover, Bartonella spp. DNA has been

detected in human blood cells after a tick bite (30), and 3 patients with B.

henselae bacteremia who had no history of contact with cats but had sustained

tick bites were reported in Texas (12). Finally, tick exposure was

determined to be a risk factor associated with B. vinsonii seropositivity in

dogs

(31).

 

Because Bartonella spp. are emerging human pathogens and Ixodes spp. can

transmit a large spectrum of pathogens to humans, the capability of Ixodes

spp. in transmitting human pathogenic Bartonella spp. should be determined.

We used a membrane-feeding technique to infect I. ricinus with B. henselae,

and investigated transtadial and transovarial transmission of viable and

infective bacteria and putative transmission from tick saliva to blood

during artificial blood meals

 

 

 

 

 

Discussion

This study demonstrated transmission of B. henselae by I. ricinus ticks

across different developmental stages, migration and multiplication of viable

and infective B. henselae in SGs after a second blood meal, and

transmission of B. henselae from ticks to blood. These findings indicate that

I.

ricinus is a competent vector for B. henselae.

 

Vector biologists and epidemiologists have suggested that ticks may play a

role in transmission of Bartonella spp. (11,12,19,23,25,28,29). This

suggestion was based on indirect data for detection of bacterial DNA in ticks

(18,19,24), humans exposed to tick bites (30), or serologic evidence of

co-infection of humans with pathogens known to be transmitted by ticks (11,36).

Difficulties in rearing I. ricinus and lack of a rodent model for B.

henselae infection may explain the absence of data demonstrating the role of

this

tick as a vector of B. henselae. Recent development of an artificial

method suitable for feeding ticks (33) enabled us to study experimental

infection of ticks with blood containing B. henselae, to monitor B. henselae

through various tick stages, and to evaluate putative transmission of bacteria

from the tick to blood.

 

To select a tick population with the lowest Bartonella spp. DNA

prevalence, we estimated the prevalence of Bartonella spp. DNA in questing I.

ricinus

collected in different areas in France. The lowest Bartonella spp. DNA

prevalence was in Loire-Atlantique (19 and unpub. data). We thus used ticks

collected in this area for our study.

 

We detected B. henselae DNA in 100% of carcasses from nymphs and 67% of

carcasses from adults fed on ovine blood containing B. henselae at their

preceding stages. No B. henselae DNA was amplified in corresponding SGs in a

nested PCR, which is more sensitive than amplification of the classic citrate

synthase gene. This result demonstrated that bacteria could be ingested by

I. ricinus larvae and nymphs during feeding on artificial skin and that

bacterial DNA was maintained in the tick after molting. However, no or

undetectable numbers reached the SGs.

Although bacterial DNA was detected in eggs laid by females fed on blood

containing B. henselae, larvae obtained from these eggs were PCR-negative

for B. henselae. This finding suggests external contamination of eggs with

DNA rather than transovarial persistence of bacteria.

 

When molted nymphs and female ticks potentially contaminated with B.

henselae at their previous developmental stage were refed on uninfected blood,

viable B. henselae were detected in SGs after 84 h of engorgement. Two

hypotheses could explain the absence of detectable bacterial DNA in SGs after

an

infected blood meal and molting, when it becomes detectable after a

partial refeeding blood meal. The first hypothesis is that the 84-h refeeding

period may act as a stimulus and enable migration of bacteria from the gut to

SGs of the tick, as previously described for B. burgdorferi sensu lato

(14). The second hypothesis is that this refeeding period may stimulate

multiplication of bacteria already present in SGs, but at undetectable levels.

More investigations are needed to validate one of these hypotheses. Bacteria

located in SGs of nymphs and adults are infective because injection of 1

pair of infected SGs into cats induced high levels of bacteremia. Cats became

bacteremic in the first 2 weeks after injection, as described for cat

infection with B. henselae by fleas, and bacteremia levels were similar to

those

observed in cats infected by flea bites (7,8).

 

Viable B. henselae in blood after 72 h of feeding of ticks with B. henselae

–infected ticks demonstrated its transmission from the tick to the blood

by mouthparts of the ticks. The duration of multiplication or migration

described above would explain such a delay in bacterial transmission.

 

Because our results have demonstrated competence of I. ricinus for

transmission of B. henselae, cat models of B. henselae transmission by ticks

are

needed to confirm that cats can be infected with B. henselae by tick bites.

Further investigations are also needed to evaluate the capacity of I.

ricinus to transmit B. henselae to cats and humans. Such transmission could

occur because cats, although not common hosts for I. ricinus, can be infested

with this tick. In France, attached I. ricinus are commonly found on cats

brought to veterinarians (J. Guillot, pers. comm.). In Great Britain, Ogden

et al. (37) reported cats with woodland and moorland habitats as hosts for

I. ricinus. Podsiadly et al. (38) reported B. henselae in cats and in I.

ricinus removed from those cats in Poland.

 

In conclusion, we demonstrated by using feeding on artificial skin that B.

henselae, the cause of cat-

scratch disease in humans, could be transmitted by ticks through saliva.

Although further investigations are needed to clarify the epidemiology of

such transmission, health authorities must take into account the possibility

of bartonellosis in persons exposed to tick bites, and B. henselae must be

identified as a tick-borne pathogen