High levels of hybridization between molecular forms of Anopheles gambiae from Guinea Bissau

In the malaria vector Anopheles gambiae Giles sensu stricto, two molecular forms denoted M and S are considered units of incipient speciation within this species. Very low hybrid frequencies and significant genetic differentiation have been found in sympatric M- and S-form populations. We studied the molecular form composition and the degree of genetic differentiation at 15 microsatellites in two samples of An. gambiae collected in two consecutive years from Bissau, Guinea Bissau. High frequencies of M/S hybrids (19-24%) were found in this area. Coincidently, very low levels of genetic differentiation were detected between forms when analysis involved microsatellites mapped at chromosome-3 (mean Fst, 0.000-0.002). The single exception was the X-linked AGXH678, for which high differentiation was measured (Fst, 0.158-0.301). This locus maps near the centromere of chromosome X, a low recombination region in which selection is likely to promote divergence between M and S forms. These results strongly suggest that the degree of isolation between M and S forms, considered the units of incipient speciation within An. gambiae, is not homogenous throughout the species distribution range.

New and earlier records of ticks (Acari: Ixodidae, Argasidae) from Guinea-Bissau

Ticks were collected from pastures and domestic and wild vertebrates during the rainy seasons of 1994 and 1996 in Guinea-Bissau. We collected the following species: from pastures Rhipicephalus lunulatus Neumann, R. muhsamae Morel & Vassiliades, R. senegalensis Koch, and R. sulcatus Neumann; from reptiles Amblyomma nuttalli Dönitz and Aponomma flavomaculatum (Lucas); and from mammals Amblyomma paulopunctatum Neumann, Amblyomma splendidum Giebel, Amblyomma variegatum (F.), Hyalomma truncatum Koch, R. muhsamae, R. sanguineus (Latreille), R. senegalensis, and R. sulcatus. New host records are as follows: Amblyomma nuttalli on Senegal flap shelled turtle (Cyclanorbis senegalensis Dumeril & Bibron), Aponomma flavomaculatum on West African dwarf crocodile (Osteolaemus tetraspis Cope), and R. sulcatus on bushbuck (Tragelaphus scriptus Pallas). We reviewed the literature for earlier records of ticks from Guinea-Bissau. The regional distributions of the 21 tick species hitherto recorded in Guinea-Bissau are listed.

Comparison of plant products and pyrethroid-treated bed nets for protection against mosquitoes (Diptera: Culicidae) in Guinea Bissau, West Africa

In Guinea Bissau, West Africa, the shrub Hyptis suaveolens Poit. (Lamiaceae) and smoke of the bark of the tree Daniellia oliveri Rolfe (Caesalpiniaceae) traditionally are used to reduce numbers of mosquitoes indoors at night. We recorded the numbers of mosquitoes in the bedrooms of 30 households in a rural village, Mandinka-Rá in central Guinea Bissau. Each household was provided with bed nets and allocated randomly to 1 of 6 treatments as follows: (1) lambda-cyhalothrin-treated bed nets (10 mg/m2), (2) permethrin-treated bed nets (500 mg/m2), (3) burning of H. suaveolens, (4) burning of the bark of D. oliveri, (5) fresh H. suaveolens, and (6) control group using untreated bed nets and no plant products. In treatment groups 1-4, the mean number of mosquitoes was significantly less than the mean of the control. These results indicated that the use of burning H. suaveolens or D. oliveri indoors at night repelled endophagic mosquitoes and may contribute significantly to reducing the prevalence of diseases caused by pathogens transmitted by house-frequenting, nocturnally blood-feeding arthropods.

Rickettsia helvetica in Ixodes ricinus ticks in Sweden

In the present study further characterization of the amplified sequence of the citrate synthase gene of the spotted fever group Rickettsia isolated from Ixodes ricinus ticks in Sweden showed that it has 100% homology with the deposited sequence of the citrate synthase gene of Rickettsia helvetica. The restriction fragment length polymorphism (RFLP) pattern of an amplified 382-bp product of the citrate synthase sequence, defined by primers RpCS877 and RpCS1258, yielded fragments for our isolate that could be visualized as a double band that migrated at approximately 44 bp, another double band at 85 bp, and a single band at nearly 120 bp after digestion with the restriction enzyme AluI. When calculating a theoretical PCR-RFLP pattern of the sequence of the citrate synthase gene of R. helvetica from the known positions where the AluI enzyme cuts, we arrived at the same pattern that was obtained for our isolate, a pattern distinctly different from the previously published PCR-RFLP pattern for R. helvetica. Investigation of 125 living I. ricinus ticks showed a higher prevalence of rickettsial DNA in these ticks than we had found in an earlier study. Rickettsial DNA was detected by amplification of the 16S rRNA gene, for which a seminested primer system consisting of two oligonucleotide primer pairs was used. Of the 125 ticks, some were pooled, giving a total of 82 tick samples, of which 20 were found to be positive for the rickettsial DNA gene investigated. When considering the fact that some of the positive samples were pooled, the minimum possible prevalence in these ticks was 20 of 125 (16%) and the maximum possible prevalence was 46 of 125 (36.8%). These prevalence estimates conform to those of other studies of spotted fever group rickettsiae in hard ticks in Europe.

Plant products used as mosquito repellents in Guinea Bissau, West Africa

By standardized interviews of people in 23 rural villages, in the Oio region of Guinea Bissau, we collected data on which plant species and plant derived products or methods people use to reduce mosquito biting activity. The following plants were used to reduce numbers of mosquitoes indoors at night: fresh or smouldering Hyptis suaveolens Poit. (Lamiaceae), smoke of the bark of Daniellia oliveri Rolfe (Caesalpiniaceae), smoke of the infructescence of Elaeis guineensis Jacq. (Arecaceae), smoke of the seed capsules of Parkia biglobosa (Jacq.) Benth. (Mimosaceae), smoke of the leaves of Azadirachta indica A.Juss. (Meliaceae) and Eucalyptus sp. (Myrtaceae), fresh Ocimum canum Sims (Lamiaceae), and fresh Senna occidentalis (L.) Link (Caesalpiniaceae). In two field experiments we estimated the 'repellent activity' of certain of these plants and compared their efficacies with those of two commercially available mosquito repellents, i.e. 'positive' controls. In the first experiment we tested: smouldering H. suaveolens (85.4% repellency); fresh H. suaveolens (73.2%); burning of the bark of D. oliveri (74.7%); and smoke of the leaves of Eucalyptus (72.2%). In the second experiment we tested: smouldering H. suaveolens (83.6% repellency); fresh H. suaveolens (66.5%); burning of the bark of D. oliveri (77.9%); smoke of the leaves of A. indica (76.0%); smoke of the infructescence of E. guineensis (69.0%); fresh O. canum (63.6%); and fresh S. occidentalis; (29.4%). All the products tested, except S. occidentalis were significantly more effective than the negative control.

Increasing geographical distribution and density of Ixodes ricinus (Acari: Ixodidae) in central and northern Sweden

The geographical distribution and density of Ixodes ricinus (L.) in the 2 northern regions, Svealand and Norrland, in Sweden were studied by using a questionnaire in Swedish magazines for house owners and dog owners, and in provincial newspapers. Analysis of the approximately 1,200 answers revealed that ticks are present in all parts of Svealand (except northern Värmland and northern and western Dalarna), the southeastern part of Norrland (i.e., Gästrikland and Hälsingland), and along the Baltic Sea coast of central and northern Norrland. The proportion of answers reporting ticks and the estimated tick density (i.e., the number of ticks infesting dogs and cats) decreased from south to north. The answers to the questionnaire and data from field sampling of ticks suggest that tick density decreased distinctly along a narrow boundary zone, coinciding with the biological limit of Norrland (Limes Norrlandicus), crossing Sweden through southern Värmland, southeastern Dalarna, and central Gästrikland. The results of the questionnaire suggest that I. ricinus ticks are more widespread today than in the early 1980s, especially in Värmland, western and central Dalarna, Hälsingland, and the coastal areas of Medelpad, Angermanland, and Västerbotten, and that the proportion of the human population at risk for tick-borne pathogens in Svealand and Norrland is increasing.

The palpal ratio method compared with PCR to distinguish between Anopheles gambiae s.s. and A. melas from Guinea Bissau, West Africa

We compared the palpal ratio method with the polymerase chain reaction (PCR) to distinguish between Anopheles gambiae s.s. and A. melas. At the end of the rainy season of 1995, female mosquitoes were collected indoors in the Antula area of Bissau, Guinea Bissau. A subsample of 354 mosquitoes were identified first with PCR and then with the palpal ratio method (study A). Subsequently, another 195 mosquitoes were identified first with the palpal ratio method and then with PCR (study B). The highest percentage (100%, n = 16) of correctly identified A. melas was obtained at the palpal ratio cut-off point of 0.83. However, at this point 4.0% (14/347) and 11.3% (21/186) of the A. gambiae were erroneously identified as A. melas in study A and B, respectively. This suggests that the palpal ratio method is not sufficiently reliable to distinguish between A. gambiae and A. melas from the Bissau area.

Ixodes ricinus strains in Europe

An analysis of cuticular hydrocarbons (CH) of unfed adult Ixodes ricinus ticks collected throughout Europe showed that there are 10 distinct I. ricinus groups. Studies on the seasonal and annual consistency of CH composition and possible effects of host and environmental factors suggested that CHs may be used as a genuine genetic marker for I. ricinus. Preliminary studies compared the vector competence of ticks from three of the most separated I. ricinus groups and the results suggested that there may be significant differences in tick susceptibility to Borrelia afzelii.

European reservoir hosts of Borrelia burgdorferi sensu lato

Existing knowledge on reservoir hosts of Borrelia burgdorferi sensu lato was collated and reviewed and several species, particularly birds, were identified as reservoir competent. At the present time, 9 small mammals, 7 medium-sized mammals and 16 bird species, including passerines, sea birds and pheasants, appear to be capable of transmitting spirochaetes to ticks and thus of participating in the natural circulation of B. burgdorferi s.l. in Europe. The house mouse, Mus musculus is strongly suspected of reservoir competence and many other small rodent species, particularly in eastern Europe and Russia, have been implicated. Ungulates are not thought to play a major role as reservoir hosts, though co-feeding transmission may permit some tick infection. The criteria for establishment of reservoir status are outlined and a method for identification of host blood meals of previous instars of unfed ticks, developed in a participant laboratory, is briefly described.

Lyme borreliosis habitat assessment

Tick ecologists throughout Europe provided descriptions of Lyme borreliosis habitats according to a standardised format and data for 105 habitats in 16 countries were received. The data showed that high risk situations, as defined by the presence of large numbers of B. burgdorferi sensu lato-infected nymphal I. ricinus, occur in heterogeneous deciduous woodland, usually with a recreational function and with a diverse fauna, usually including deer. Large numbers of ticks occurred in some other habitats, but infection prevalence was usually low. The situation for adult I. ricinus was similar but less clearly defined. Tick infection rates were found to be lower in western Europe than in the east, and the infection rate in I. persulcatus, the most easterly vector species, was markedly higher than in I. ricinus. In the vast majority of habitats the infection rate in adult I. ricinus was greater than in nymphs. Larvae were rarely found to be infected.

Lyme borreliosis awareness

A Lyme borreliosis information leaflet has been produced to promote awareness amongst the general public. It was designed to provide a framework for similar material throughout Europe and complements a questionnaire produced to measure awareness of Lyme borreliosis. This questionnaire can be used to determine the impact of educational campaigns using material such as the leaflet. Feasibility studies showed that the questionnaire successfully highlighted predictable differences between sample groups and also that the leaflet performed well in increasing knowledge in low-awareness groups.

Infestation of mammals by Ixodes ricinus ticks (Acari: Ixodidae) in south-central Sweden

Infestation by Ixodes ricinus ticks on rodents, hares and cervids was examined at Bogesund, 10 km north of Stockholm, in south-central Sweden during 1991-1994 and on varying hares (Lepus timidus) at Stora Karlsö and Gotska Sandön in the Baltic Sea during 1992-1993. At Bogesund, there were great differences between two consecutive years in the number of I. ricinus larvae infesting bank voles (Clethrionomys glareolus). The seasonal pattern of infestation by I. ricinus larvae and nymphs on bank voles was unimodal in 1991, with peaks in June-July and bimodal in 1992, with peaks in June and August. Male bank voles, compared to females and older voles, compared to young voles, harboured greater numbers of I. ricinus ticks. Apodemus mice, compared to bank voles, harboured greater numbers of I. ricinus ticks. Ixodes ricinus larvae engorged on Apodemus mice were heavier than larvae engorged on bank voles and resulted in larger nymphs. However, there was no difference in the proportions of viable nymphs resulting from larvae engorged on mice or voles. The ranges in the numbers of I. ricinus ticks infesting individual hosts were 1-451 for rodents, 16-2374 for hares and 428-2072 for roe deer (Capreolus capreolus). These ranges of tick numbers are estimated to represent potential blood losses from individual hosts of approximately 0.2-65% for rodents, 0.2-13% for hares and 0.3-9.0% for roe deer. Within the populations of all host species examined, the distributions of all stages of I. ricinus were clumped, with most host individuals harbouring few ticks and only a few individuals harbouring many ticks. The data suggest that, even though a small proportion of tick hosts may be severely affected, the direct effects of feeding by I. ricinus are unlikely to play an important role on mammal population dynamics.

Transmission of mixed malaria species and strains by mosquitoes, as detected by PCR, in a study area in Guinea-Bissau

Parasites present in blood samples of asymptomatic carriers and in the midgut of mosquitoes collected within a few days from the same households, have been analysed by PCR. A high prevalence (32%) of infected mosquitoes was observed and, in half of these, two parasite species were found simultaneously. The distribution of parasite species in the mosquito correlated with that found in the infected persons. Genotype patterns of Plasmodium falciparum populations were however found to be different in the two sets of samples. These results and the potential of PCR are discussed with reference to investigations of the dynamics of malaria transmission.

Characterization of a spotted fever group Rickettsia from Ixodes ricinus ticks in Sweden

A spotted fever group rickettsia isolated from the common tick, Ixodes ricinus, was genetically characterized by PCR and genomic sequencing. This study was performed with nymphal and adult ticks collected in southern and central Sweden. I. ricinus is the only North European tick species of medical importance which is regularly collected from humans. No species of the genus Rickettsia has previously been found in Scandinavian ticks, nor has any case of domestic rickettsial infection in humans or animals been reported. According to the nucleotide sequencing, the present Rickettsia sp. belongs to the spotted fever group of rickettsiae. Ticks are the most common arthropod reservoirs and vectors of the rickettsiae of this group. Among 748 ticks investigated, 13 (1.7%) were positive for a Rickettsia sp. Borrelia burgdorferi was detected in 52 (7%) of the ticks, a prevalence similar to or somewhat lower than that previously been recorded in other Swedish studies. There was no evidence of ehrlichial or chlamydial DNA in these ticks. The Rickettsia sp. was further characterized by 16S ribosomal DNA (rDNA) sequencing and restriction fragment length polymorphism (RFLP). The 16S rDNA sequencing resulted in a sequence identical to that described for Rickettsia helvetica, but the pattern obtained with RFLP of the citrate synthetase gene diverged from previously known patterns. The rickettsial agent of one tick which was positive by PCR was confirmed by transmission electron microscopy. The morphology of this rickettsia was similar to that of the spotted fever and typhus group rickettsiae. This represents the first documented isolate of a Rickettsia sp. from Swedish ticks.

Relationship between Ixodes ricinus density and prevalence of infection with Borrelia-like spirochetes and density of infected ticks

During 1994-1995 we examined the density of questing Ixodes ricinus (L.) nymphs and their prevalence of infection with Borrelia-like spirochetes at 43 localities in south-central Sweden with median nymphal densities ranging from 0.1 to > 50/100 m2. The general pattern was that the infection prevalence in nymphs increased with nymphal density at < 10 nymphs per 100 m2, peaked (20-30%) at 10-20 nymphs per 100 m2, and decreased at higher nymphal densities. A low infection prevalence (7-10%) in nymphs at 4 of the 5 localities with the highest nymphal densities (> 35 nymphs per 100 m2) at the small island of Bedarön was most likely related to the presence of large numbers of fallow deer, Dama dama (L.), which are reservoir-incompetent for Borrelia burgdorferi s.l. Johnson, Schmid, Hyde, Steigerwalt & Brenner. However, because the increase in nymphal density at Bedarön was greater than the decrease in infection prevalence, 5 of the 6 highest densities of infected nymphs occurred in the 5 localities at Bedarön. We suggest that reservoir-incompetent cervids, being more important hosts for adult 1. ricinus than for the larvae, indirectly increase the density of infected nymphal ticks by feeding large numbers of adult ticks, thereby increasing the number of larvae feeding on reservoir-competent shrews, rodents, and hares.

Seasonal variations in density of questing Ixodes ricinus (Acari: Ixodidae) nymphs and prevalence of infection with B. burgdorferi s.l. in south central Sweden

Seasonal variations in the density of questing Ixodes ricinus (L.) nymphs and their prevalence of infection with Borrelia burgdorferi s.l. Johnson, Schmid, Hyde, Steigerwalt & Brenner were studied during 1991-1993 at a locality in southcentral Sweden. The seasonal pattern of the density of questing I. ricinus nymphs was variable; there was a bimodal pattern with peaks of similar magnitude in 1991, a bimodal pattern with a small spring peak and a greater autumn peak in 1992, and a unimodal pattern with a spring peak and lower values during summer and autumn in 1993. The seasonal pattern of prevalence of Borrelia infection in questing nymphs was similar during all 3 yr, with a peak infection prevalence in late spring or early summer, followed by lower infection prevalences in late summer and autumn. There was a significant correlation between the monthly density of Borrelia-infected nymphs and the monthly density of nymphs. This suggests that, within a certain range of nymphal densities, it may be possible to assess the density of Borrelia-infected I. ricinus nymphs without examining nymphs for B. burgdorferi s.l.

Lyme borreliosis spirochetes in Ixodes ricinus (Acari:Ixodidae) and the varying hare on isolated islands in the Baltic, Sea

We investigated isolated ecosystems to determine if Lyme borreliosis is maintained in the absence of reservoir-competent small mammals. Human cases of Lyme disease have been reported on the isolated islands of Gotska Sandön and Stora Karlsö in the Baltic Sea. The varying hare, Lepus timidus, is the only terrestrial vertebrate species capable of serving as a host for all stages of Ixodes ricinus (L.) on these islands. In August, mean larval infestation on 5 hares from each island was 548 with a maximum of 2,276 larvae on 1 hare. Smaller numbers of nymphal and female ticks were also engorging on the hares. During August-September, B. burgdorferi s.l. was detected in 11-24% of nymphal I. ricinus fed as larvae on hares and in 8-19% of host-seeking nymphal I. ricinus collected from the vegetation. We conclude that L. timidus serves as a maintenance host for B. burgdorferi s.l. and its vector, I. ricinus, on both islands.

Transhemispheric exchange of Lyme disease spirochetes by seabirds

Lyme disease is a zoonosis transmitted by ticks and caused by the spirochete Borrelia burgdorferi sensu lato. Epidemiological and ecological investigations to date have focused on the terrestrial forms of Lyme disease. Here we show a significant role for seabirds in a global transmission cycle by demonstrating the presence of Lyme disease Borrelia spirochetes in Ixodes uriae ticks from several seabird colonies in both the Southern and Northern Hemispheres. Borrelia DNA was isolated from I. uriae ticks and from cultured spirochetes. Sequence analysis of a conserved region of the flagellin (fla) gene revealed that the DNA obtained was from B. garinii regardless of the geographical origin of the sample. Identical fla gene fragments in ticks obtained from different hemispheres indicate a transhemispheric exchange of Lyme disease spirochetes. A marine ecological niche and a marine epidemiological route for Lyme disease borreliae are proposed.

Is the small mammal (Clethrionomys glareolus) or the tick vector (Ixodes ricinus) the primary overwintering reservoir for the Lyme borreliosis spirochete in Sweden?

We determined the capacity of bank voles (Clethrionomys glareolus) to infect feeding Ixodes ricinus ticks with Borrelia burgdorferi sensu lato (infectivity), during June to October 1991 and June to September 1992 in south-central Sweden. In both years, the infectivity of older voles to ticks was higher in August to September (48% to 59%) than in June to July (20% to 32%). We propose that the infectivity of bank vole populations in Sweden decreases during winter and spring due to death of highly infective voles and recruitment of uninfective young ones, and that the tick vector, rather than the mammalian host, is the primary overwintering reservoir of B. burgdorferi.

Prevalence of Borrelia burgdorferi sensu lato-infected ticks on migrating birds

The prevalence of Lyme disease Borrelia-infected ticks on migrating birds was studied in Scandinavia. A total of 22,998 birds were caught at eight different bird observatories and examined for ticks. Five different species of ticks were found infesting the birds. The dominant species, Ixodesricinus, constituted 98.3% of the ticks collected. The presence of spirochetes was determined by an immunofluorescence assay of tick larvae and DNA amplification by PCR on all ticks. To determine which Borrelia burgdorferi sensu lato species were present, a species classification was performed by DNA amplification with species-specific 16S rDNA primers and by DNA sequencing (rDNA is DNA coding for rRNA). Flagellin gene sequences of all species of B. burgdorferi sensu lato previously recorded in Europe were observed. Borrelia garinii was the most prevalent Lyme disease Borrelia species in ticks collected from birds arriving from the South or Southeast in the spring, whereas the distribution was more heterogeneous in ticks from birds migrating from the Southwest. These data support the notion that birds are partly responsible for the heterogeneous distribution of Lyme disease Borrelia spirochetes in Europe.

Prevalence of Borrelia burgdorferi sensu lato infection in Ixodes ricinus in Sweden

Between 1988-1991, a total of 3,141 Ixodes ricinus ticks, 2,740 adults and 401 nymphs, was collected from different localities in 23 of the 25 provinces of Sweden. The ticks were identified, dissected and examined for the presence of Borrelia spirochetes. Indirect immunofluorescence was performed, using an antiserum obtained from rabbits, immunized with sonicated, whole Borrelia burgdorferi spirochetes isolated from Swedish Ixodes ricinus ticks. Borrelia-positive I. ricinus were found in all 23 provinces. The prevalence of infection in adults ranged from 3% in Jämtland to 23% in Södermanland. In nymphs, the infection prevalence ranged from 0% in 9 provinces to 15% in Södermanland. A significantly greater proportion of the adult ticks were found to be positive for Borrelia in the southern and central parts of Sweden as compared to the northern part (Norrland). No significant difference in prevalence could be demonstrated between the western and eastern parts of Sweden. On average, 10% of the nymphs and 15% of the adult I. ricinus were positive for Borrelia. Of 41 non-I. ricinus ticks, none was positive for Borrelia. This study shows that Borrelia burgdorferi sensu lato is present throughout the distributional area of I. ricinus in Sweden. This should lead to increased awareness of signs and symptoms compatible with Lyme borreliosis in persons living in or visiting areas where I. ricinus is present.

Control of endophagic Anopheles mosquitoes and human malaria in Guinea Bissau, West Africa by permethrin-treated bed nets

We compared the anti-mosquito and antimalarial potentialities of placebo-treated versus permethrin-impregnated bed nets in north-western Guinea Bissau. Baseline, pre-intervention entomological and parasitological data were collected during the rainy season of 1990 and bed nets were distributed shortly before the rainy season of 1991. Pairs of 3 ethnically different villages were investigated. The villages in each pair were at least 2 km apart but belonged to the same ethnic group in an ecologically similar area. After one year permethrin-treated bed nets were provided to all people in one village of each pair and placebo-treated bed nets to the other villages. About 98% of mosquitoes caught in bedrooms belonged to Anopheles gambiae and A. melas, which we consider to be the main malaria vectors in the study villages. Mean Plasmodium falciparum sporozoite rate in A. gambiae (9.6%) and A. melas (12.4%) was highest during October-November. The Plasmodium index in children 2-9 years old in the 6 villages, at the end of the rainy season 1990, ranged between 44% and 79%. Of these, 98% were identified as P. falciparum, 1% as P. malariae and 1% as mixed infections of these species. Significant reductions of Anopheles indoor resting densities and malaria parasite rates in humans were recorded in villages which had received permethrin-treated nets, but not in the control villages. The mean number of P. falciparum-infective mosquito bites received indoors in untreated villages during the rainy season was estimated to be about 4 per child and 20 per adult. This inoculation rate was reduced by at least 78% by the use of permethrin-impregnated bed nets. The malaria parasite rates and proportions of people experiencing 'disease with fever' decreased significantly in villages provided with permethrin-treated nets but not in the control villages. Impregnated nets may be an important tool to reduce disease and death due to malaria in Guinea Bissau.

Transmission of Borrelia burgdorferi s.l. from mammal reservoirs to the primary vector of Lyme borreliosis, Ixodes ricinus (Acari: Ixodidae), in Sweden

Factors regulating prevalence of Borrelia burgdorferi s.l. Johnson, Schmid, Hyde, Steigerwalt & Brenner in Ixodes ricinus (L.) were examined during 1991-1992 at Bogesund, near Stockholm in south-central Sweden. Nine species of small and medium-sized mammals (Sorex araneus L., S. minutus L., Neomys fodiens Pennant, Clethrionomys glareolus [Schreber], Microtus agrestis [L.], Apodemus sylvaticus [L.], A. flavicollis [Melchior], Lepus europaeus Pallas, L. timidus L.) were found to infect feeding tick larvae with B. burgdorferi s.l., whereas two species of large mammals (Capreolus capreolus L., Alces alces L.) failed to infect feeding tick larvae with this spirochete. The most important mammalian reservoirs at the study locality were S. araneus and rodents, accounting for 91% of all I. ricinus larvae infected. In view of the great number of potentially effective reservoirs for B. burgdorferi s.l. in Sweden, control of Lyme disease by reduction of abundance of reservoir hosts will be difficult to achieve. We also found that infectivity of a rodent species is related to the number of infesting, potentially infective nymphal I. ricinus. Insectivores and rodents were the most important hosts of larval I. ricinus, whereas most nymphal ticks fed on hares and cervids. Adult I. ricinus were frequently found on all species of hares and cervids examined but never on insectivores and rodents. No single species seemed to be of paramount importance as a source of blood for female ticks. Therefore, control of Lyme disease by reduction of abundance of mammal hosts available for female tick engorgement will probably require massive reductions of numbers of both C. capreolus and L. timidus.

Geographical distribution, host associations, and vector roles of ticks (Acari: Ixodidae, Argasidae) in Sweden

This review covers the geographic distribution and host relationships of the tick species in Sweden. Ixodes uriae White, I. caledonicus Nuttall, I. unicavatus Neumann, I. arboricola Schulze & Schlottke, and I. lividus Koch are ornithophagous species. I. trianguliceps Birula, I. canisuga Johnston, I. hexagonus Leach, and Argas vespertilionis (Latreille) are mammalophagous. I. ricinus (L.) and Haemaphysalis punctata Canestrini & Fanzago feed on both birds and mammals. All these tick species may be considered to be permanently present in Sweden. I. persulcatus Schulze, Hyalomma marginatum Koch, and the brown dog tick, Rhipicephalus sanguineus (Latreille), may be regarded as not indigenous to Sweden although they may be regularly introduced by spring-migrating birds or imported dogs, respectively. The first European record of the American dog tick, Dermacentor variabilis (Say), is reported. There are several records of Hyalomma aegyptium (L.) from imported tortoises in Sweden. Excluding other ticks imported on exotic pets and zoo animals, another 13 tick species are listed that may occur, at least occasionally, in Sweden. Because of its wide geographic distribution, great abundance, and wide host range, I. ricinus is medically the most important arthropod in northern Europe. I. ricinus is common in southern and south-central Sweden and along the coast of northern Sweden and has been recorded from 29 mammal species, 56 bird species, and two species of lizards in Sweden alone. The potential introduction to Sweden of exotic pathogens with infected ticks (e.g., I. persulcatus and H. marginatum on birds or Dermacentor spp. and R. sanguineus on mammals) is evident.

Seasonal variation in the capacity of the bank vole to infect larval ticks (Acari: Ixodidae) with the Lyme disease spirochete, Borrelia burgdorferi

Seasonal variation in the capacity of bank voles, Clethrionomys glareolus (Schreber), to infect larval Ixodes ricinus (L.) ticks with Borrelia burgdorferi Johnson, Schmid, Hyde, Steigerwalt & Brenner was examined from May through October 1991 at Bogesund, near Stockholm in south-central Sweden. Although larval infestations of bank voles were greatest in June and July, nearly 70% of all larval ticks infected with spirochetes by bank voles at this site became infected during August and September. Seasonality of infectivity was related to the degree of earlier nymphal infestation on voles as well as to the age composition of the vole population. These factors may influence the infectivity of other rodent reservoirs of B. burgdorferi, both in Europe and North America. Moreover, in determining the reservoir potential of tick hosts, a host population's spirochetal infectivity should be determined for the entire period of larval infestation rather than just during the period of peak larval infestation.

A Lyme borreliosis cycle in seabirds and Ixodes uriae ticks

The Lyme disease spirochaete, Borrelia burgdorferi s.l., is the only Borrelia known to infect both mammals and birds. The main vertebrate reservoirs of B. burgdorferi are thought to be various small and intermediate size mammals, but the importance of birds as a reservoir has not been thoroughly explored. In the Northern and Southern Hemispheres the seabird tick, Ixodes uriae, is prevalent and closely associated with many species of colony-nesting marine birds. Here we report the presence of spirochaetes, demonstrated by immunofluorescent assay, by polymerase chain reaction and in culture, in I. uriae infesting razorbills on an island in the Baltic Sea. This island is free from mammals. The protein profile of the spirochaetes and the sequences of their flagellin and ospA genes are identical to those of the Lyme disease spirochaete, Borrelia burgdorferi s.l., previously isolated from I. ricinus on a nearby island. In biopsies from the foot web of razorbills, B. burgdorferi-specific DNA was detected after amplification by polymerase chain reaction. Our results suggest that birds play an important part in the maintenance of B. burgdorferi and that mammals may not be a prerequisite for its life cycle.

Seasonal prevalence of Borrelia burgdorferi in Ixodes ricinus in different vegetation types in Sweden

The aim of this investigation was to estimate the seasonal risk of contracting human Lyme disease in different vegetation types in southern Sweden. Host-seeking Ixodes ricinus ticks (Acari: Ixodidae) were collected with standardized methods during May-September 1988 and March-October 1989 at 10 different sampling sites. Tick abundance was greatest during May-June and August-September. Prevalence of Borrelia burgdorferi infection of the ticks was assessed by phase-contrast microscopy complemented by immunology. Spirochetal infection of tick larvae was not detected. The prevalence of infection among nymphal ticks differed significantly between years and between sampling sites. Infection prevalence was greater in adult females than in nymphs, but was similar in female and male ticks. Among all vegetation types studied, the greatest Lyme disease risk was deemed to be from I. ricinus nymphs during May and September in mixed forest vegetation.

Maintenance by hares of European Borrelia burgdorferi in ecosystems without rodents

During May-October 1991 shrews (Sorex spp.), small rodents (Apodemus spp., Clethrionomys glareolus [Schreber]), and hares (Lepus spp.) were sampled near Stockholm, Sweden. Nymphal Ixodes ricinus (L.) derived from blood-engorged larvae collected from these mammals were investigated by phase-contrast microscopy and immunofluorescence assay (IFA) using the monoclonal antibody H6831 directed against the OspB protein of Borrelia burgdorferi Johnson, Schmid, Hyde, Steigerwalt & Brenner. Three rodent species, previously known as vertebrate reservoirs for B. burgdorferi in Switzerland, are competent reservoirs of Lyme borreliosis also in Sweden. The first data are presented showing Lyme disease reservoir competency of European insectivores (Sorex araneus L., S. minutus L.) and lagomorphs (Lepus europaeus Pallas, L. timidus L.). Lagomorphs are the only European vertebrates known to be both competent reservoirs for B. burgdorferi and a source of blood for all stages of I. ricinus. We propose that Lyme borreliosis can be maintained for prolonged periods on islands, without rodents and insectivores, if lagomorphs are present; and, in such ecosystems, fluctuations of lagomorph population levels may influence the numbers of B. burgdorferi-infected ticks and hence the risk of human Lyme disease infection.

Molecular characterization of Borrelia burgdorferi isolated from Ixodes ricinus in northern Sweden

Ixodes ricinus ticks, harbouring Borrelia burgdorferi, were found in an area in northern Sweden, not thought to be endemic for Lyme borreliosis. This investigation took place at Norrbyskär, an island situated in the Bothnian Gulf, 63 degrees 33'N/19 degrees 52'E. One of 42 nymphal and 8/43 adult I. ricinus ticks collected carried spirochetes as seen by phase contrast microscopy. Pure bacterial cultures were obtained from 2 of the ticks. Western blot analysis using species-specific monoclonal antibodies showed that the isolated spirochetes were B. burgdorferi. The identity of the isolated spirochetes was confirmed by DNA amplification using B. burgdorferi OspA and flagellin gene specific oligonucleotides as well as partial DNA sequencing of the respective OspA and flagellin genes. The 2 isolated spirochaete populations were different as shown by their protein profiles in sodium dodecyl sulphate polyacrylamide gels. Moreover, the demonstration of Lyme borreliosis in a patient from the island of Norrbyskär indicates the need for clinical consideration of this disease in northern Sweden.

The epidemiology of lyme borreliosis

The tick-transmitted bacterial infection known as Lyme disease, or Lyme borreliosis, has recently emerged as the leading arthropod-borne disease in Europe and North America. Several thousand new cases of human Lyme disease are reported each year from Europe and the USA. The causative agent, Borrelia burgdorferi, has been isolated from several species of mammals, birds, ticks and insects and, in this article, Thomas Jaenson discusses the geographical differences in the epidemiology of the infection.

Ecologic studies of mosquitoes and birds as hosts of Ockelbo virus in Sweden and isolation of Inkoo and Batai viruses from mosquitoes

Field studies were conducted in central Sweden from 1983 through 1985 to obtain information on the etiologic agent of Ockelbo disease, described in Sweden in the 1960s and probably identical to Pogosta disease in Finland and to Karelian fever in the western USSR. Mosquitoes (63,644) collected during this 3 year period yielded 21 virus strains. Ockelbo virus isolations were from Culiseta morsitans (5 strains), Culex pipiens and/or Cx. torrentium (6 strains), and Aedes cinereus (3 strains). Inkoo (6 strains) and Batai (1 strain) viruses were recovered from Ae. communis. Blood samples collected March-May from migrating birds on the southeast and est coast of Sweden and in July and August from resident birds in east-central Sweden were tested for neutralizing antibody to Ockelbo virus. Antibody was not detected in 328 birds sampled during spring migrations. Two of 58 (3.4%) birds bled in July and 8 of 78 birds (10%) bled in August had antibody to Ockelbo virus. Ockelbo virus circulates in a mosquito-bird-mosquito cycle, with Cs. morsitans and Cx. pipiens and/or Cx. torrentium as enzootic vectors. Antibody was detected in passerine birds. Other classes of birds or other vertebrates were not sampled. Aedes cinereus may serve primarily to transmit virus to people. The role of other mosquito species as vectors for people is unknown.

First isolations of Borrelia burgdorferi from rodents collected in northern Europe

Spirochetes were found in 13% of Ixodes ricinus collected from an island, near Stockholm where human borreliosis is endemic. Borrelia burgdorferi was cultivated from the kidney and/or spleen of wild rodents (Clethrionomys glareolus and Apodemus flavicollis) from the same island. Spirochetes were identified as Borrelia burgdorferi by indirect immunofluorescence assays using species and genus specific monoclonal antibodies. In these tests the spirochetes could not be differentiated from strains previously cultured from Swedish patients with Ixodes-borne borreliosis. The results show that small rodents in Europe may harbour borreliae and indicate that C. glareolus and A. flavicollis may be important reservoirs for the spirochetes causing Ixodes-borne borreliosis in humans and domestic animals in Europe.

Virus-like rods associated with salivary gland hyperplasia in tsetse, Glossina pallidipes

Long, virus-like rods were discovered in hyperplastic salivary glands of Glossina pallidipes Austen from Kibwezi Forest (2 degrees 27' S, 37 degrees 55' E), Kenya. The glands were enlarged up to four times the normal diameter. This increase in size was due to a cellular proliferation of the glandular epithelial cells and hypertrophy of their nuclei and cytoplasm. Nuclear and cytoplasmic inclusions were present in the enlarged cells but were not found in cells of normal-sized glands. Electron microscopy revealed many virus-liked rods in the abnormal glands. Males with such glands were often completely sterile. Abnormal growth of the ovarioles was a significant feature of young females with hyperplastic glands. Both sexes of wild and laboratory-bred flies were found with enlarged glands. One way of transmission of the trait seems to be from mother to progeny.