[Lyme borreliosis: research gaps and research approaches. Results from an interdisciplinary expert meeting at the Robert Koch Institute]. / Lyme-Borreliose: Forschungsbedarf und Forschungsansätze. Ergebnisse eines interdisziplinären Expertentreffens am Robert Koch-Institut.

Lyme borreliosis is currently the most frequent tick-transmitted zoonosis in the northern hemisphere. Germany and other European countries are regarded as highly endemic areas; therefore the burden of disease and consequently the costs for the health systems are considered to be high. This report summarises the results of an interdisciplinary workshop on Lyme borreliosis which aimed to identify research deficits and to prioritise areas which need to be addressed. Research needs have been recognised for different areas: diagnosis, epidemiology, immunology, clinics, ecology and health services research. Examples of research areas which have priority are the standardisation of diagnostic tests, the development of markers to detect an active infection, the improvement of the epidemiological database and the analysis of the burden of disease.

Mating strategies and spermiogenesis in ixodid ticks.

Mate-seeking and sperm-transfer in the ixodid hard ticks, which include important vectors of zoonotic pathogens, generally reflect their peculiarly prolonged pattern of feeding. The metastriate ticks, including Dermacentor, Amblyomma, and Rhipicephalus, invariably attain sexual maturity and mate solely on their hosts. The more primitive prostriate Ixodes ticks, however, may copulate both in the absence of hosts and while the female engorges. These expanded opportunities for insemination complicate the mating systems of the Ixodes ricinus complex of species. In these ticks, autogenous spermatogenesis must precede host contact, whereas anautogenous oogenesis requires that the females store sperm. All hard tick males undergo a courting ritual before they can deposit their spermatophores within the female's genital tract. These diverse and prolonged patterns of sexual interaction provide opportunities for interactions between populations and individuals that may be relevant to the role of ticks as vectors of zoonotic pathogens.

Relative incompetence of european rabbits for Lyme disease spirochaetes.

To determine whether rabbits may serve as reservoir hosts for Lyme disease spirochaetes in Europe, we compared their competence as hosts for Borrelia afzelii, one of the most prevalent European spirochaetal variants, with that of the Mongolian jird. To infect rabbits or jirds, at least 3 nymphal or adult Ixodes ricinus ticks infected with spirochaetes fed to repletion on each animal. Whereas jirds readily acquired tick-borne Lyme disease spirochaetes and subsequently infected vector ticks, rabbits exposed to tick-borne spirochaetes rarely became infectious to ticks. Only the rabbit that was infectious to ticks developed an antibody response. To the extent that I. ricinus ticks feed on European rabbits, these mammals may be zooprophylactic by diverting vector ticks from more suitable reservoir competent hosts.

Competence of American robins as reservoir hosts for Lyme disease spirochetes.

To explore the competence of American robins as a reservoir for Lyme disease spirochetes, we determined the susceptibility of these birds to tickborne spirochetes and their subsequent infectivity for larval vector ticks. Robins acquired infection and became infectious to almost all xenodiagnostic ticks soon after exposure to infected nymphal ticks. Although infectivity waned after 2 months, the robins remained susceptible to reinfection, became infectious again, and permitted repeated feeding by vector ticks. In addition, spirochetes passaged through birds retained infectivity for mammalian hosts. American robins become as infectious for vector ticks as do reservoir mice, but infectivity in robins wanes more rapidly.

Genospecies diversity of Lyme disease spirochetes in rodent reservoirs.

To determine whether particular Borrelia burgdorferi s.l. genospecies associate solely with rodent reservoir hosts, we compared the genospecies prevalence in questing nymphal Ixodes ticks with that in xenodiagnostic ticks that had fed as larvae on rodents captured in the same site. No genospecies was more prevalent in rodent-fed ticks than in questing ticks. The three main spirochete genospecies, therefore, share common rodent hosts.

Characteristics of garden dormice that contribute to their capacity as reservoirs for lyme disease spirochetes.

To describe the contribution of garden dormice to the epizootiology of Lyme disease, we compared their reservoir capacity for these pathogens to that of other sympatric hosts. Garden dormice are trapped most abundantly during early spring and again during midsummer, when their offspring forage. They are closely associated with moist forests. Garden dormice serve as hosts to nymphal ticks far more frequently than do other small mammals. Spirochetal infection is most prevalent in dormice, and many more larval ticks acquire infection in the course of feeding on these than on other rodents in the study site. Mature dormice appear to contribute more infections to the vector population than juveniles do. Replete larval ticks generally detach while their dormouse hosts remain within their nests. The population of garden dormice contributes five- to sevenfold more infections to the vector population than the mouse population does. Their competence, nymphal feeding density, and preference for a tick-permissive habitat combine to favor garden dormice over other putative reservoir hosts of Lyme disease spirochetes.

Effect of prior exposure to noninfected ticks on susceptibility of mice to Lyme disease spirochetes.

To determine whether prior exposure to Nearctic Ixodes vector ticks protects native reservoir mice from tick-borne infection by Lyme disease spirochetes, we compared their infectivities for white-footed mice and laboratory mice that had been repeatedly infested by noninfected deer ticks. Nymphal ticks readily engorged on tick-exposed laboratory mice, but their feeding success on white-footed mice progressively declined. Tick-borne spirochetes readily infected previously tick-infested mice. Thus, prior infestation by Nearctic ticks does not protect sympatric reservoir mice or Palearctic laboratory mice from infection by sympatric tick-borne spirochetes.

Failure of Ixodes ticks to inherit Borrelia afzelii infection.

To define conditions promoting inherited infection by Lyme disease spirochetes in Ixodes ticks, we variously infected ticks with Borrelia afzelii and examined their progenies by dark-field microscopy, immunofluorescence, PCR, and serial passage. No episode of inherited infection was evident, regardless of instar or gender infected or frequency of exposure. We suggest that these spirochetes rarely, if ever, are inherited by vector ticks.

Diversity of European Lyme disease spirochetes at the southern margin of their range.

We determined whether the genospecies diversity of Lyme disease spirochetes in vector ticks questing on a subtropical island is as broad as that in Central Europe. Although spirochetes infected < 1% of the ticks sampled on Madeira Island, these infections included all three genospecies implicated in human disease. Therefore, spirochetal diversity is as great at the southern margin as it is in the center of this pathogen's range.

Competence of urban rats as reservoir hosts for Lyme disease spirochetes.

To determine whether urban rats serve efficiently as reservoir hosts for the agent of Lyme disease, we recorded the frequency of infection in nymphal Ixodes ricinus (L.) ticks that had fed as larvae on experimentally infected Norway rats, Rattus norvegicus (Berkenhout), or on black rats, R. rattus (L.), and evaluated the nidicolous venue of transmission. Subadult vector ticks attached readily to Norway rats as well as black rats and virtually all became infected in the course of feeding. Larval ticks detached when these nocturnally active hosts were at rest. Rats appeared to be competent reservoir hosts of Lyme disease spirochetes in a transmission cycle in urban sites.

Risk of urban Lyme disease enhanced by the presence of rats.

To determine whether Norway rats contribute to the risk of human Lyme disease in a central European city park, densities of endemic rodents were compared as were feeding densities of vector ticks and prevalence of infection by the Lyme disease spirochete. Only Norway rats and yellow-necked mice were abundant, and three times as many mice as rats were present. More larval ticks fed on rats than on mice, and far more nymphs engorged on the rats. All rats but only about half of the mice infected ticks. Each rat was more infectious than each infectious mouse. Infected rats were distributed throughout the city. Spirochetes infected about a quarter of the questing nymphal ticks. The capacity of rats to serve as reservoir hosts for the Lyme disease spirochete, therefore, increases risk of infection among visitors to this and other urban parks.

Etiology of the acrodermatitis chronica atrophicans lesion in Lyme disease.

Spirochete diversity in acrodermatitis chronica atrophicans lesions in a closely defined central European site was compared to that in the local vector population, in human erythema migrans lesions, and in cerebrospinal fluid by amplifying and sequencing a segment of the gene of outer surface protein A directly from sampled tissues. Borrelia garinii, Borrelia afzelii, and Borrelia burgdorferi acutely infect human skin and invade internal tissues. Only B. afzelii, however, is associated with acrodermatitis chronica atrophicans lesions, persisting chronically where the skin has atrophied.

Characteristics of Lyme disease spirochetes in archived European ticks.

To determine whether the characteristics of the Lyme disease spirochete (Borrelia burgdorferi) in Europe may have changed during the past century, DNA was amplified from archived Ixodes ricinus ticks. Tick DNA could be amplified, even when ticks had been stored under museum conditions for nearly a century. Spirochetal DNA was detected by polymerase chain reaction in 6 ticks preserved for as long as a century; the oldest was collected in 1884. Borrelia garinii, which predominates in modern ticks in the region, infected 3 of these older ticks, and the presently infrequent B. burgdorferi sensu stricto infected 2. These data indicate that residents of Europe have been exposed to diverse Lyme disease spirochetes at least since 1884, concurrent with the oldest record of apparent human infection.

The irradiated cercariae vaccine model: looking on the bright side of radiation.

Schistosomes infect between 200 and 300 million people at any one time. A major strategy to reduce the impact of schistosomiasis on human health is the development of a defined antigen vaccine. Protective immunity induced in mice by irradiated cercariae may serve as a model for the development of a vaccine. In such vaccinated mice, worm burdens resulting from challenge infection can be reduced by more than 90% compared to non-vaccinated mice. During the past three decades, the irradiated-carcariae vaccine model has been dissected in the detail in order to determine factors that may be relevant to vaccination, such as the participating immune compartments, the site and kinetics of the immune response, and the antigens recognized. In this review, Dania Richter, Donald A. Harn and Franz-Rainer Matuschka highlight the research on the vaccine model, focusing on the murine model using gamma-irradiated cercariae of Schistosoma mansoni.

Nondifferentiation between Lyme disease spirochetes from vector ticks and human cerebrospinal fluid.

To determine whether Lyme disease neuropathogenesis may result from infection by a particular segment of the locally extant population of spirochetes, genetic markers of spirochetes found in cerebrospinal fluid (CSF) of 12 pediatric patients were compared with those in spirochetes from 40 vector ticks sampled in the vicinity of their homes. The primary structure of the outer surface protein A served as the marker of variation; a fragment of the corresponding gene was amplified by nested polymerase chain reaction and the products sequenced. Tick-derived variants clustered in seven distinct categories, of which four were present in CSF. One of the CSF variants differed from any found in ticks. Coinfection by different spirochete variants was infrequent in ticks and absent in human samples. Spirochetal neuropathology in children in our study site does not correlate with a particular segment of the tickborne pathogens present in nature.

Vaccination against Lyme disease caused by diverse Borrelia burgdorferi.

Diversity and mutations in the genes for outer surface proteins (Osps) A and B of Borrelia burgdorferi sensu lato (B. burgdorferi), the spirochetal agent of Lyme disease, suggests that a monovalent OspA or OspB vaccine may not provide protection against antigenically variable naturally occurring B. burgdorferi. We now show that OspA or OspB immunizations protect mice from tick-borne infection with heterogeneous B. burgdorferi from different geographic regions. This result is in distinct contrast to in vitro killing analyses and in vivo protection studies using syringe injections of B. burgdorferi as the challenge inoculum. Evaluations of vaccine efficacy against Lyme disease and other vector-borne infections should use the natural mode of transmission and not be predicated on classification systems or assays that do not rely upon the vector to transmit infection.