Relapsing fever – a forgotten disease revealed

Borrelial relapsing fever was once a major worldwide epidemic disease that made a significant impact on Livingstone during his epic travels through Africa and throughout Europe. Indeed, the term ‘relapsing fever’ was first used to describe clinical cases of this disease in Edinburgh. During the last century, we have witnessed the demise of the louse‐borne infection, largely through improving standards of living resulting in a reduction in body lice, the vector for Borrelia recurrentis [louse‐borne relapsing fever (LBRF)]. The tick‐borne zoonotic form of the disease persists in endemic foci around the world [tick‐borne relapsing fever (TBRF)]. Indeed, TBRF is reportedly the most common bacterial infection from Senegal and listed within the top ten causes of mortality in children under five in Tanzania. In Ethiopia, LBRF is again within the top ten causes of hospital admission, associated with significant morbidity and mortality. Despite these figures, many now regard relapsing fever as an unusual tropical disease. Certainly, recent cases have been imported following travel from endemic zones. More surprisingly, cases have been reported following family reunions in Colorado, USA. A further case was reported from the Mt Wilson observatory in Los Angeles, USA. In many regions, the infection is zoonotic with natural reservoirs in several vertebrate species. In West Africa, infection is again primarily zoonotic. Whether those species found predominantly in East Africa are zoonoses or are infections of humans alone is still debated, however, the life cycle may be determined by the feeding preferences of their arthropod vectors.     

A host species‐informative internal control for molecular assessment of African swine fever virus infection rates in the African sylvatic cycle Ornithodoros vector

African swine fever virus (ASFV) infection in adult Ornithodoros porcinus (Murry 1877, sensu Walton 1979 ) ticks collected from warthog burrows in southern and East Africa was assessed using a duplex genomic amplification approach that is informative with respect to the invertebrate host species and infecting sylvatic cycle virus. DNA extracted from individual ticks was used as template for the simultaneous amplification of a C‐terminal 478‐bp ASFV p72 gene region and a ～313‐bp fragment of the tick mitochondrial 16S rRNA gene, under optimized reaction conditions. Within‐warthog burrow infection rates ranged from 0% to 43% using this approach, and phylogenetic analysis of 16S gene sequences revealed the presence of three geographically discrete O. porcinus lineages, but no support for subspecies recognition. False negatives are precluded by the inclusion of host species‐informative primers that ensure the DNA integrity of cytoplasmically located genome extracts. In addition, infection rate estimates are further improved as false positives arising from carry‐over contamination when performing a two‐step nested polymerase chain reaction are negated by the one‐step approach. Phylogenetic comparison of full‐length virus gene sequences with the partial C‐terminal p72 gene target confirmed the epidemiological utility of the latter in a sylvatic setting. The method is therefore of particular value in studies assessing the prevalence and diversity of ASFV in relation to the African sylvatic tick vector and holds potential for investigating the role of alternative tick species in virus maintenance and transmission.     

Co-infection of Ornithodoros coriaceus with the relapsing fever spirochete, Borrelia coriaceae, and the agent of epizootic bovine abortion

The soft tick, Ornithodoros coriaceus (Koch) (Acari: Argasidae), is a common mammalian parasite of livestock in many arid regions of the western U.S.A. The tick is a known vector of the undescribed bacterial pathogen that causes epizootic bovine abortion (EBA), which results in late-term abortions in beef cattle and subsequent economic loss, which can be considerable, to producers. A second reported bacterial pathogen, Borrelia coriaceae, a member of the relapsing fever complex, has also been identified in this tick and was at one time hypothesized to be the aetiological agent of EBA. In order to test whether bacterial infections in ticks overlapped geographically and to determine the prevalence of co-infection in O. coriaceus populations, we used molecular methods to detect bacterial DNA from ticks collected from a wide variety of habitats in California, Nevada and Oregon. Of the 15 sites at which ticks tested positive for the agent of EBA (aoEBA), eight also contained ticks positive for Borrelia spp. by polymerase chain reaction assay. Additionally, two ticks were co-infected; both of these were collected from the same location. Univariate risk analysis indicated the presence of juniper-dominated habitat at the collection site and geographic location to be significantly associated with infection of the tick vector by either pathogen.     

The persistence of African swine fever virus in field-infected Ornithodoros erraticus during the ASF endemic period in Portugal

African swine fever (ASF) is an important disease of pigs and outbreaks of ASF have occurred in Europe on multiple occasions. To explore the period for which the European soft tick species Ornithodoros erraticus (Acari: Argasidae) is able to act as a reservoir of African swine fever virus (ASFV) after infected hosts are removed, we collected specimens from farms in the provinces of Alentejo and Algarve in Portugal during the endemic period and tested them subsequently using cell culture and experimental infection. We show that ticks from previously infected farms may contain infectious virus for at least five years and three months after the removal of infectious hosts. Furthermore, in two cases infectious virus was successfully isolated from ticks on restocked farms that had not yet suffered a re-emergence of disease. Experimental transmission to pigs was demonstrated in batches tested up to 380 days after an outbreak. These results clarify the epidemiological role of O. erraticus ticks in the persistence of ASFV in the field, provide additional evidence to support its role in the re-emergence of a sporadic outbreak of ASF in Portugal in 1999 and suggest that the current quarantine legislation and restocking advice when these ticks are present on the pig farm premises is appropriate.     

Effect of O. porcinus Tick Salivary Gland Extract on the African Swine Fever Virus Infection in Domestic Pig

African swine fever is a haemorrhagic disease in pig production that can have disastrous financial consequences for farming. No vaccines are currently available and animal slaughtering or area zoning to restrict risk-related movements are the only effective measures to prevent the spread of the disease. Ornithodoros soft ticks are known to transmit the African swine fever virus (ASFV) to pigs in farms, following the natural epidemiologic cycle of the virus. Tick saliva has been shown to modulate the host physiological and immunological responses during feeding on skin, thus affecting viral infection. To better understand the interaction between soft tick, ASFV and pig at the bite location and the possible influence of tick saliva on pig infection by ASFV, salivary gland extract (SGE) of Ornithodoros porcinus, co-inoculated or not with ASFV, was used for intradermal auricular inoculation. Our results showed that, after the virus triggered the disease, pigs inoculated with virus and SGE presented greater hyperthermia than pigs inoculated with virus alone. The density of Langerhans cells was modulated at the tick bite or inoculation site, either through recruitment by ASFV or inhibition by SGE. Additionally, SGE and virus induced macrophage recruitment each. This effect was enhanced when they were co-inoculated. Finally, the co-inoculation of SGE and virus delayed the early local spread of virus to the first lymph node on the inoculation side. This study has shown that the effect of SGE was powerful enough to be quantified in pig both on the systemic and local immune response. We believe this model should be developed with infected tick and could improve knowledge of both tick vector competence and tick saliva immunomodulation.     

Ixodid ticks (Acari: Ixodidae) infesting humans in Tokat Province of Turkey: species diversity and seasonal activity

Ixodid ticks (Acari: Ixodidae) are the major vectors of pathogens threatening animal and human health. Tokat Province, Turkey, is a suitable habitat for extended tick activity with its moderate climate and vegetation. In the present study, we surveyed humans visiting health centers to determine the species diversity, geographical distribution, and seasonal activity of ixodid ticks infesting them. Out of 5,999 adult ticks collected from humans from April to September, 2008, 800 ticks were identified to species, while the remaining were identified to genus according to their distinct morphological characteristics. Hyalomma marginatum, Hyalomma detritum, Rhipicephalus bursa, Rhipicephalus (Boophilus) annulatus, Haemaphysalis punctata, Dermacentor marginatus, and Ixodes laguri were the most prevalent tick species among 24 ixodid tick species infesting humans in the region. One of these tick species, Hyalomma isacii was identified as a new record for the ixodid tick fauna of Turkey. Hyalomma species were the most abundant in summer, while Dermacentor and Ixodes species displayed the lowest frequency. Hyalomma aegyptium infestation was very common on humans in the province. Results indicated that a variety of ixodid tick species infest humans depending on the season in the target area. It is possible that a variety of ixodid tick species may contribute to the spread of tick‐borne diseases such as Crimean‐Congo haemorrhagic fever virus (CCHFV), which is endemic in the region.     

Impact of white‐tailed deer on the spread of Borrelia burgdorferi

There is a public perception that the white‐tailed deer Odocoileus virginianus (Artiodactyla: Cervidae) is the main reservoir supporting the maintenance and spread of the causative agent of Lyme disease, Borrelia burgdorferi. This study examines the pathogen prevalence rate of Borrelia in adult Ixodes scapularis (Ixodida: Ixodidae), the black‐legged tick, collected from white‐tailed deer and compares it with pathogen prevalence rates in adult ticks gathered by dragging vegetation in two contiguous counties west of the Hudson Valley in upstate New York. In both Broome and Chenango Counties, attached and unattached ticks harvested from white‐tailed deer had significantly lower prevalences of B. burgdorferi than those collected from vegetation. No attached ticks on deer (n = 148) in either county, and only 2.4 and 7.3% of unattached ticks (n = 389) in Broome and Chenango Counties, respectively, were harbouring the pathogen. This contrasts with the finding that 40.8% of ticks in Broome County and 46.8% of ticks in Chenango County collected from vegetation harboured the pathogen. These data suggest that a mechanism in white‐tailed deer may aid in clearing the pathogen from attached deer ticks, although white‐tailed deer do contribute to the spatial distribution of deer tick populations and also serve as deadend host breeding sites for ticks.     

Pathobiology of Borrelia theileri in the tropical cattle tick, Boophilus microplus

The development of Borrelia theileri infections in the tick Boophilus microplus was studied during all stages of the tick developmental cycle. Light microscopical examination of hemolymph and ovary smears from ovipositing females allowed identification and separation of infected and uninfected ticks. A Borreli-free tick colony was established. Small numbers of spirochetes were present in larvae, with numbers increasing through the nymphal and adult tick stages. Borreliae occurred in hemolymph, hypodermis, midgut, Malpighian tubules, ovary, Gené's organ, and the central ganglion of engorging and ovipositing females and their eggs. The ovary, central ganglion, and hemolymph seemed to be preferred sites for the spirochete, with extensive multiplication occurring in hemocytes. No measurable effect of spirochete multiplication upon feeding and reproductive performance of ticks could be detected. Infections in cattle caused fever of short duration which coincided with the presence of spirochetes in blood smears. Morphology and size of blood and tick forms were consistent with those of B. theileri reported by other authors. B. theileri is important because infections of invertebrate and vertebrate hosts may interfere with the interpretation of data in various experimental designs, and because it is probably endemic in populations of one or more tick species and their hosts throughout the world.     

Feeding preferences of the red‐billed oxpecker,Buphagus erythrorhynchus: a parasitic mutualist?

The oxpecker–ungulate association of sub‐Saharan Africa is an example of a complicated interspecific association subject to variation in outcome. Oxpeckers (Buphagus spp.) are unusual birds because they not only glean ticks from an array of African ungulates, but they are one of the few avian species known to wound‐feed from their living hosts. The conditions under which oxpeckers wound‐feed and the mechanisms generating variation in this association are unclear. We took a unique approach to studying the relationship by conducting a series of feeding preference experiments on twelve captive red‐billed oxpeckers (B. erythrorhynchus). We assessed how oxpecker feeding behaviour is influenced by changes in tick abundance and tick type. In cafeteria‐style experiments, oxpeckers fed equally on ticks and liquid bovine blood. In experiments using donkeys as the host animal, oxpeckers spent more time wound‐feeding when a less‐preferred tick type was available and when tick abundance was low compared to when a preferred tick type was available and when tick abundance was high. However, oxpeckers still wound‐fed even when offered a large number of the ticks they prefer. Additional experiments incorporating tick species of different stages and multiple ungulate species are necessary to fully reveal the dynamics of this association.     

Borrelia burgdorferi sensu lato and co‐infections with Anaplasma phagocytophilum and Rickettsia spp. in Ixodes ricinus in Hamburg,Germany

To obtain initial data on Borrelia burgdorferi sensu lato (Spirochaetales: Spirochaetaceae) in Ixodes ricinus (Ixodida: Ixodidae) ticks in Hamburg, Germany, 1400 questing ticks were collected by flagging at 10 different public recreation areas in 2011 and analysed using probe‐based quantitative real‐time polymerase chain reaction. The overall rate of infection with B. burgdorferi s.l. was 34.1%; 30.0% of adults were infected (36.7% of females and 26.0% of males), as were 34.5% of nymphs. Significant differences in tick infection rates were observed between the spring and summer/autumn months, as well as among sampling locations. Borrelia genospecies identification by reverse line blotting was successful in 43.6% of positive tick samples. The most frequent genospecies was Borrelia garinii/Borrelia bavariensis, followed by Borrelia afzelii, Borrelia valaisiana, B. burgdorferi sensu stricto, Borrelia spielmanii, Borrelia bissettii and Borrelia lusitaniae. Based on previously published data, co‐infection of Borrelia and Rickettsiales spp. was determined in 25.8% of ticks. Overall, 22.9% of ticks were co‐infected with Rickettsia spp. (Rickettsiales: Rickettsiaceae), 1.7% with Anaplasma phagocytophilum (Rickettsiales: Anaplasmataceae), and 1.2% with both pathogens. Study results show a high prevalence of Borrelia‐positive ticks in recreation areas in the northern German city of Hamburg and the potential health risk to humans in these areas should not be underestimated.     

An update on the ecological distribution of Ixodid ticks infesting cattle in Rwanda: countrywide cross-sectional survey in the wet and the dry season

As part of the epidemiological studies aimed at developing an East Coast fever (ECF) immunisation control strategy, which combines an infection and treatment method with strategic tick control, a countrywide tick survey was carried out in both the dry and the wet season to determine the abundance and the dynamics of the tick populations infesting cattle in Rwanda. Six Ixodid tick species where identified from a total of 12,814 tick specimens collected. Rhipicephalus appendiculatus, the main vector of ECF was the most abundant (91.8%) followed by Boophilus decoloratus (6.1%) and Ambyomma variegatum (1.2%). Few ticks from the three other less economically important Ixodid species (Rhipicephalus compositus, R. evertsi evertsi and Ixodes cavipalpus) were recovered. Both adult and immature stages of the most dominant tick species were found to be widespread with a year round presence. The numbers of ticks were high in low land and medium zones and declined markedly in the higher regions of Rwanda. The geographical distribution of various tick species throughout the country and their epidemiological implications are discussed.     

New World relapsing fever Borrelia found in Ornithodoros porcinus ticks in central Tanzania

Ticks were collected from 8 houses in Mvumi Mission village, near Dodoma, Tanzania. All ticks were examined for Borrelia infestation by flagellin gene-based nested polymerase chain reaction. All houses were highly infested with ticks, and all ticks collected were of the Ornithodoros porcinus species. Fifty-one out of 120 ticks were infected with spirochetes, and a flagellin gene sequence comparison showed that most of the spirochetes belonged to Borrelia duttonii, which is the causative agent of tick-borne relapsing fever in East Africa. The rest of the spirochetes were quite different from B. duttonii and instead resembled the New World tick-borne relapsing fever borreliae. Phylogenetic analysis using 16S ribosomal RNA gene sequences also supported the interpretation that the spirochete was a Borrelia species distinct from previously described members of the genus.     

Effects of infection of the tick Ornithodoros moubata with African swine fever virus

The effects of infection with African swine fever virus (ASFV) on adult and nymphal Ornithodoros moubata Murray (Ixodoidea, Argasidae) ticks were examined. Three groups of ticks were used, an uninfected control group, one group infected with the VIC T90/1 isolate of ASFV and another group infected with the LIV 13/33 isolate of ASFV. Infection with ASFV did not affect the oviposition rates of infected ticks when compared with uninfected ticks. There was no difference between infected and uninfected ticks in progeny hatching rates and first nymphal stage feeding rates. Feeding rates of infected adult ticks were also unaffected. However, a significant increase in mortality rates was observed amongst the adult ticks that fed on an infective bloodmeal compared to ticks fed on an unifected bloodmeal.     

Detection of spotted fever group Rickettsia spp. from bird ticks in the U.K.

Migratory birds are known to play a role in the long‐distance transportation of microorganisms. To investigate whether this is true for rickettsial agents, we undertook a study to characterize tick infestation in populations of the migratory passerine bird Riparia riparia (Passeriformes: Hirundinidae), the sand martin. A total of 194 birds were sampled and ticks removed from infested birds. The ticks were identified as female Ixodes lividus (Acari: Ixodidae) using standard morphological and molecular techniques. Tick DNA was assayed to detect Rickettsia spp. using polymerase chain reaction and DNA was sequenced for species identification. A single Rickettsia spp. was detected in 100% of the ticks and was designated Rickettsia sp. IXLI1. Partial sequences of 17‐kDa and ompA genes showed greatest similarity to Rickettsia sp. TCM1, an aetiological agent of Japanese spotted fever‐like illness, previously described in Thailand. Phylogenetic analysis showed that Rickettsia sp. IXLI1 fitted neatly into a group containing strains Rickettsia japonica, Rickettsia sp. strain Davousti and Rickettsia heilongjiangensis. In conclusion, this research shows that U.K. migratory passerine birds host ticks infected with Rickettsia species and contribute to the geographic distribution of spotted fever rickettsial agents.     

Hyalomma ticks on northward migrating birds in southern Spain: Implications for the risk of entry of Crimean‐Congo haemorrhagic fever virus to Great Britain

Crimean‐Congo haemorrhagic fever virus (CCHFV) is a zoonotic virus transmitted by Hyalomma ticks, the immature stages of which may be carried by migratory birds. In this study, a total of 12 Hyalomma ticks were recovered from five of 228 migratory birds trapped in Spring, 2012 in southern Spain along the East Atlantic flyway. All collected ticks tested negative for CCHFV. While most birds had zero Hyalomma ticks, two individuals had four and five ticks each and the statistical distribution of Hyalomma tick counts per bird is over‐dispersed compared to the Poisson distribution, demonstrating the need for intensive sampling studies to avoid underestimating the total number of ticks. Rates of tick exchange on migratory birds during their northwards migration will affect the probability that a Hyalomma tick entering Great Britain is positive for CCHFV. Drawing on published data, evidence is presented that the latitude of a European country affects the probability of entry of Hyalomma ticks on wild birds. Further data on Hyalomma infestation rates and tick exchange rates are required along the East Atlantic flyway to further our understanding of the origin of Hyalomma ticks (i.e., Africa or southern Europe) and hence the probability of entry of CCHFV into GB.     

Microbial communities and interactions in the lone star tick,Amblyomma americanum

To quantify microbial composition and interactions, we identified prokaryotic communities in the lone star tick (Amblyomma americanum) based on 16S rRNA gene sequences and direct probing. The lone star tick is the vector of emerging diseases and host to additional symbionts of unknown activity, and is representative of other blood‐sucking arthropods. We evaluated the potential for vertical (transovarial) transmission by molecular analysis of microbial symbionts from egg and larval clutches. Direct probing of adults (N = 8 populations from the southeastern and midwestern USA, 900 ticks total) revealed three vertically transmitted symbionts: a Coxiella symbiont occurred at 100% frequency, Rickettsia species occurred in 45–61% of all ticks in every population and an Arsenophonus symbiont occurred in 0–90% of ticks per population. Arsenophonus and Rickettsia exhibited significant heterogeneity in frequency among populations. The human pathogens Ehrlichia chafeensis and Borrelia lonestari were rare in most populations. Additional microbes were detected sporadically. Most ticks (78%) were co‐infected by two or three microbes but statistical analysis indicated no significant deviation from random co‐occurrence. Our findings indicate that microbial communities within lone star ticks are diverse, and suggest that direct probing for a wider range of prokaryotes and application of quantitative polymerase chain reaction (PCR) may provide further insights into microbial interactions within disease vectors. Our results also emphasize the close phylogenetic relationship between tick symbionts and human pathogens, and consistent differences in their prevalence.     

Genomic evidence for host-associated differentiation in an animal parasite,Dermacentor variabilis,the American dog tick

The process responsible for the formation of genetically distinct populations associated with different host species is known as host-associated differentiation (HAD). Many insect parasites of plants have been shown to exhibit HAD but there have been fewer studies of HAD in parasites of vertebrate animals. Previous to this study, HAD has been documented in at least three species of ticks. The American dog tick, Dermacentor variabilis (Say) (Acari: Ixodidae) was chosen as the focal species for this study due to its importance as the vector of tularemia and Rocky Mountain spotted fever. Previous population genetic studies of this tick found the existence of various haplotypes but the tick’s host origins were unknown. In this study, ticks were collected from 15 vertebrate host species to test for HAD using single nuclear polymorphisms (SNPs). In total, 136 individual D. variabilis ticks were sequenced using ddRADseq. Genomic evidence was found to point to D. variabilis exhibiting HAD on eight different hosts. A STRUCTURE analysis showed that the highest posterior probability was obtained with a population size of eight and these populations correlated with host species. Pairwise F_ST values were as high as 0.622 and indicated a range of genetic distinction between host groups. In addition, ticks collected from the vegetation appeared as one homogenous distinct genotype suggesting the existence of nidicolous (nest dwelling) and non-nidicolous genotypes. The identification of host race formation occurring in this animal parasite has implications for the understanding of D. variabilis pathogen transmission and targeted control efforts because genetically distinct populations can differ in traits relevant to these applications.     

Density of Ixodes scapularis ticks on Monhegan Island after complete deer removal: A question of avian importation?

Questing adult blacklegged tick (Ixodes scapularis Say) abundance declined markedly three years after the 1999 removal of white‐tailed deer (Odocoileus virginianus Zimmermann) from Monhegan Island, ME. Since 2000, subadult ticks have not been found on Norway rats (Rattus norvegicus Berkenhout); questing nymphs have not been found since 2002. This suggested I. scapularis was reintroduced annually via bird importation of subadult ticks, but unable to complete its two‐year life cycle on the island due to lack of deer. To investigate this, we used uncertainty analysis to estimate 1) questing adult ticks/ha that would result from avian importation of nymphs, and 2) questing adult ticks/ha on Monhegan Island, using bird capture and tick burden data from Appledore Island, ME, flagged tick data from Monhegan Island, and ten uncertain parameters. During the deer‐fed period (1990–2001), estimated tick density on Monhegan Island was 18 times greater than that of imported ticks. During the post‐deer‐fed period (2002–2008), Monhegan Island tick density was equivalent to imported tick density. This supported the premise that all I. scapularis ticks on Monhegan Island have been bird‐derived since 2002.     

Tick prevalence and species diversity on Aldabran giant tortoises (Dipsochelys dussumieri) in relation to host range and host size in a restored ecosystem, Kenya

Tick species density and diversity on Aldabran tortoises was investigated in relation to the habitat range and size of each tortoise. Identification of tick infestation patterns forms an important aspect of effective tick control. Ten Aldabran tortoises were de‐ticked and monitored over the course of 2 months. Tick species found were Amblyomma sparsum, Amblyomma nuttalli, Amblyomma hebraeum and Boophilus decoloratus, with the most prevalent species found being A. sparsum. Tick loads varied considerably from 20 to 214 ticks per tortoise, with most ticks collected from the head/neck region. Tortoises ranging outside Haller Park had higher tick loads (70–214) compared with tortoises ranging within Haller Park (20–99). Tick load was not correlated with tortoise size. Results indicate that tick loads are related to the habitat range of the tortoises and may indirectly also be related to food preference and host food availability. Implications of the findings and appropriate tick control measures are discussed.     

Borrelia crocidurae in Ornithodoros ticks from northwestern Morocco: a range extension in relation to climatic change?

Tick‐borne relapsing fever (TBRF) is caused by Borrelia spirochetes transmitted to humans by Argasid soft ticks of the genus Ornithodoros. We investigated the presence of Ornithodoros ticks in rodent burrows in nine sites of the Gharb region of northwestern Morocco where we recently documented a high incidence of TBRF in humans. We assessed the Borrelia infection rate by nested PCR and sequencing. All sites investigated were colonized by ticks of the Ornithodoros marocanus complex and a high proportion of burrows (38.4%) were found to be infested. Borrelia infections were observed in 6.8% of the ticks tested. Two Borrelia species were identified by sequencing: B. hispanica and B. crocidurae. The discovery in northwestern Morocco of Ornithodoros ticks infected by B. crocidurae represents a 350 km range extension of this Sahelo‐Saharan spirochete in North Africa. The spread of B. crocidurae may be related to the increasing aridity of northwestern Morocco in relation to climate change.