Ticks are not Insects: Consequences of Contrasting Vector Biology for Transmission Potential

Tick-borne viruses

At least 38 viral species are transmitted by ticks. Virus–tick–vertebrate host relationships are highly specific and less than 10% of all tick species (Argasidae and Ixodidae) are known to play a role as vectors of arboviruses. However, a few tick species transmit several (e.g.Ixodes ricinus,Amblyomma variegatum) or many (I. uriae) tick-borne viruses. Tick-borne viruses are found in six different virus families (Asfarviridae, Reoviridae, Rhabdoviridae, Orthomyxoviridae, Bunyaviridae, Flaviviridae) and at least 9 genera. Some as yet unassigned tick-borne viruses may belong to a seventh family, theArenaviridae. With only one exception (African swine fever virus, family Asfarviridae) all tick-borne viruses (as well as all other arboviruses) are RNA viruses. Tick-borne viruses are found in all the RNA virus families in which insect-borne members are found, with the exception of the family Togaviridae. Some tick-borne viruses pose a significant threat to the health of humans (Tick-borne encephalitis virus,Crimean-Congo haemorrhagic fever virus) or livestock (African swine fever virus,Nairobi sheep disease virus). Key challenges are to determine the molecular adaptations that allow tick-borne viruses to infect and replicate in both tick and vertebrate cells, and to identify the principal ecological determinants of tick-borne virus survival.

Ecological mechanisms that promote arbovirus survival: a mathematical model of Ross River virus transmission

Many assessments of host and vector competence for arboviruses focus on level and length of infectivity and ignore ecological mechanisms that contribute to virus survival. In this paper, mathematical models are used to compare local survival mechanisms for a range of scenarios, using Ross River virus as a case study. Ross River virus is an Australian arbovirus with many mosquito vectors and reservoir hosts. The mechanisms for maintaining long-term transmission of the virus vary between salt and freshwater mosquito vectors, and according to the availability of susceptible hosts. The models demonstrate that overwintering of virus in adult freshwater mosquitoes requires a large host population, while overwintering of virus in infected eggs of saltwater mosquitoes is an effective survival strategy when filial infection rates are high. The virus survives longer when both salt and freshwater mosquito species are included in the model than when only one mosquito species is present. When the marsupial host is replaced by a host with higher birth rate and shorter infectious period, the virus survived longer under all models. This suggests that birth rate can be a key factor when assessing the competence of reservoir hosts to maintain virus transmission.

Evidence that climate change has caused 'emergence' of tick-borne diseases in Europe?

Even though tick-borne disease systems are highly susceptible to climatic influences, climate change to date is not necessarily the cause of the marked increased incidence of a variety of tick-borne diseases in many parts of Europe over the past two decades. To test for causality, rather than coincidence, we need to examine whether the right sorts of climate change have occurred at the right time and in the right places to account for the observed heterogeneous temporal and spatial patterns of tick-borne disease 'emergence'. Tick-borne encephalitis (TBE) incidence, for example, showed a 3-fold step increase from 1983 to 1986 in Sweden, doubled in 1993 in the Czech Republic, increased even more dramatically in the same year in Lithuania and Poland, but declined markedly in 1997 in Hungary, Croatia and Slovenia. Within each country, TBE incidence has changed to different degrees in different regions. Because other tick-borne diseases, notably Lyme borreliosis, has commonly 'emerged' in parallel with TBE, we should first examine climate variables predicted to have a general effect on tick abundance, which has indeed increased in the past decade. These include temperature and moisture stress, which have seasonally differential impacts. Monthly mean records for 1960-2000 from the UK Climate Research Unit's interpolated global climate surface reveal that mean spring, spring-autumn and winter temperatures have all increased gradually over the past 40 years, but apparently most sharply in the late 1980s, when moisture stress also increased. These climate data do not reveal any obvious differences between sites where TBE did or did not 'emerge', and in Sweden increases in TBE pre-dated the onset of warmer springs and winters. If recorded climate changes cannot yet satisfactorily explain the temporal and spatial patterns of tick-borne disease change in Europe, the impact of biotic factors, such as increases in deer abundance and changing habitat structure, and of socio-political changes following the end of communist rule, demand more detailed quantitative analyses.

Effects of Anaplasma phagocytophilum infection on the molting success of Ixodes scapularis (Acari: Ixodidae) larvae

We assessed the effects of sympatric (occupying the same or overlapping geographic areas) and allopatric (occurring in separate geographic areas) isolates of Anaplasma phagocytophilum on the survival of Ixodes scapularis Say larvae that were derived from ticks collected in Bridgeport, CT. Seven isolates of A. phagocytophilum, originating from different geographic regions of the United States, were tested: four isolates from the northeast (Bridgeport, Dawson, Gaillard, and NY-8), two from the Midwest (Webster and Sp-Is), and one from California (MRK). BALB/c mice were infected with each of the seven isolates via exposure to infected I. scapularis nymphs, whereas uninfected nymphs fed upon control mice. Both infected and control mice were infested with uninfected larvae at 1, 2, 3, 4, 6, and 9 wk after nymphal infestation. The molting success in cohorts of infected and uninfected ticks was calculated as the percentage of larvae successfully molting into nymphal stage, and the prevalence of infection in molted nymphs was determined by polymerase chain reaction. In ticks that became infected with the Bridgeport or Sp-Is isolates, the molting success decreased with an increase in the prevalence of infection. Ticks that fed upon mice infected with six allopatric isolates (Dawson, Gaillard, NY-8, Sp-Is, Webster, and MRK) showed significantly lower levels of survival than those fed upon control mice, regardless of the prevalence of infection, whereas in ticks fed upon mice infected with a sympatric isolate (Bridgeport), the overall molting success was similar to the control. Thus, some but not all of the A. phagocytophilum isolates have adverse effects on ticks. Ticks exposed to harmful isolates may experience higher levels of bacterial metabolism, and/or reduced quality of their blood meal, thereby reducing their survival. Noted differences between isolates may be due to the origin of a particular isolate and/or the degree of coadaptation between the pathogen and its vector on the population level.

Empirical evidence for key hosts in persistence of a tick-borne disease

An important epidemiological consequence of aggregated host-parasite associations occurs when parasites are vectors of pathogens. Those hosts that attract many vectors will tend to be the focus of transmission. But to what extent, and can we identify characteristics of these key hosts? We investigated these questions with respect to the host-tick relationship of the yellow-necked mouse, Apodemus flavicollis, a critical host in the maintenance of the zoonotic disease, tick-borne encephalitis. Transmission of the virus occurs when ticks feed in a 'co-feeding' aggregation. Thus, the number and frequency of co-feeding groups provides an estimate of the potential rate of virus transmission. We recorded the spatio-temporal variations in co-feeding on a population of rodents in conjunction with recording individual host characteristics. Using Lorenz curves, we revealed conformation of tick-borne encephalitis transmission potential to the 20/80 Rule, where the 20% of hosts most infested with ticks were accountable for 80% of transmission potential. Hosts in the transmission cohort were identified as the sexually mature males of high body mass. Therefore control efforts targeted at this group would substantially reduce transmission potential compared to non-targeted control of the population, which resulted in a linear reduction in transmission potential. Focusing on the 'wrong' functional group would have little impact upon transmission potential until a considerable proportion of the population had been subject to control. However, individuals can change their functional status over time making it difficult to predict the contribution of these individuals to future transmission.

Association of Borrelia garinii and B. valaisiana with songbirds in Slovakia

In Europe, 6 of the 11 genospecies of Borrelia burgdorferi sensu lato are prevalent in questing Ixodes ricinus ticks. In most parts of Central Europe, B. afzelii, B. garinii, and B. valaisiana are the most frequent species, whereas B. burgdorferi sensu stricto, B. bissettii, and B. lusitaniae are rare. Previously, it has been shown that B. afzelii is associated with European rodents. Therefore, the aim of this study was to identify reservoir hosts of B. garinii and B. valaisiana in Slovakia. Songbirds were captured in a woodland near Bratislava and investigated for engorged ticks. Questing I. ricinus ticks were collected in the same region. Both tick pools were analyzed for spirochete infections by PCR, followed by DNA-DNA hybridization and, for a subsample, by nucleotide sequencing. Three of the 17 captured songbird species were infested with spirochete-infected ticks. Spirochetes in ticks that had fed on birds were genotyped as B. garinii and B. valaisiana, whereas questing ticks were infected with B. afzelii, B. garinii, and B. valaisiana. Furthermore, identical ospA alleles of B. garinii were found in ticks that had fed on the birds and in questing ticks. The data show that songbirds are reservoir hosts of B. garinii and B. valaisiana but not of B. afzelii. This and previous studies confirm that B. burgdorferi sensu lato is host associated and that this bacterial species complex contains different ecotypes.

Habitat-specific diversity of Borrelia burgdorferi sensu lato in Europe, exemplified by data from Latvia

The distribution of Borrelia burgdorferi sensu lato genospecies in questing Ixodes ricinus ticks from ecologically distinct habitats in Latvia was analyzed. A significant variation in the frequency of the genospecies across sites was observed, pointing to the importance of the host community in the ecology of Lyme borreliosis.

An empirical quantitative framework for the seasonal population dynamics of the tick Ixodes ricinus

The wide geographic and climatic range of the tick Ixodes ricinus, and the consequent marked variation in its seasonal population dynamics, have a direct impact on the transmission dynamics of the many pathogens vectored by this tick species. We use long-term observations on the seasonal abundance and fat contents (a marker of physiological ageing) of ticks, and contemporaneous microclimate at three field sites in the UK, to establish a simple quantitative framework for the phenology (i.e. seasonal cycle of development) of I. ricinus as a foundation for a generic population model. An hour-degree tick inter-stadial development model, driven by soil temperature and including diapause, predicts the recruitment (i.e. emergence from the previous stage) of a single cohort of each stage of ticks each year in the autumn. The timing of predicted emergence coincides exactly with the new appearance of high-fat nymphs and adults in the autumn. Thereafter, fat contents declined steadily until unfed ticks with very low energy reserves disappeared from the questing population within about 1 year from their recruitment. Very few newly emerged ticks were counted on the vegetation in the autumn, but they appeared in increasing numbers through the following spring. Larger ticks became active and subsequently left the questing population before smaller ones. Questing tick population dynamics are determined by seasonal patterns of tick behaviour, host-contact rates and mortality rates, superimposed on a basal phenology that is much less complex than has hitherto been portrayed.

Host association of Borrelia burgdorferi sensu lato--the key role of host complement

Borrelia burgdorferi sensu lato (s.l.), the tick-borne agent of Lyme borreliosis, is a bacterial species complex comprising 11 genospecies. Here, we discuss whether the delineation of genospecies is ecologically relevant. We provide evidence that B. burgdorferi s.l. is structured ecologically into distinct clusters that are host specific. An immunological model for niche adaptation is proposed that suggests the operation of complement-mediated selection in the midgut of the feeding tick. We conclude that vertebrate hosts rather than tick species are the key to Lyme borreliosis spirochaete diversity.

The shifting landscape of tick-borne zoonoses: tick-borne encephalitis and Lyme borreliosis in Europe

The two major vector-borne diseases of northern temperate regions, tick-borne encephalitis (TBE) and Lyme borreliosis (LB), show very different epidemiological patterns, but both have increased significantly in incidence since the 1980s. Insight into the temporal dynamics of TBE, gained from statistical analysis of spatial patterns integrated with biological explanation, suggests that the recent increases in TBE cases in Central Europe and the Baltic States may have arisen largely from changes in human behaviour that have brought more people into contact with infected ticks. Under forecast climate change scenarios, it is predicted that enzootic cycles of TBE virus may not survive along the southern edge of their present range, e.g. in Slovenia, Croatia and Hungary, where case numbers are indeed decreasing. New foci, however, are predicted and have been observed in Scandinavia. At the same time, human impact on the landscape, increasing both the habitat and wildlife hosts of ticks, has allowed tick populations to multiply significantly. This probably accounts for a genuine emergence of LB, with its high potential transmission rate, in both the USA and Europe, although the rate of emergence has been exaggerated by improved surveillance and diagnosis.

Ticks and tick-borne disease systems in space and from space

Analyses within geographical information systems (GISs) indicate that small- and large-scale ranges of hard tick species (Ixodidae) are determined more by climate and vegetation than by host-related factors. Spatial distributions of ticks may therefore be analysed by statistical methods that seek correlations between known tick presence/absence and ground- or remotely-sensed (RS) environmental factors. In this way, local habitats of Amblyomma variegatum in the Caribbean and Ixodes ricinus in Europe have been mapped using Landsat RS imagery, while regional and continental distributions of African and temperate tick species have been predicted using multi-temporal information from the National Oceanic and Atmospheric Administration-Advanced Very High Resolution Radiometer (NOAA-AVHRR) imagery. These studies illustrate ways of maximizing statistical accuracy, whose interpretation is then discussed in a biological framework. Methods such as discriminant analysis are biologically transparent and interpretable, while others, such as logistic regression and tree-based classifications, are less so. Furthermore, the most consistently significant variable for predicting tick distributions, the RS Normalized Difference Vegetation Index (NDVI), has a sound biological basis in that it is related to moisture availability to free-living ticks and correlated with tick mortality rates. The development of biological process-based models for predicting the spatial dynamics of ticks is a top priority, especially as the risk of tick-borne infections is commonly related not simply to the vector's density, but to its seasonal population dynamics. Nevertheless, using statistical pattern-matching, the combination of RS temperature indices and NDVI successfully predicts certain temporal features essential for the transmission of tick-borne encephalitis virus, which translate into a spatial pattern of disease foci on a continental scale.

Biodiversity series: The function of biodiversity in the ecology of vector-borne zoonotic diseases

This is a critical evaluation of the influence of species diversity within communities of vertebrates on the risk of human exposure to vector-borne zoonoses. Vertebrates serve as natural reservoirs of many disease agents (viral, bacterial, protozoal) that are transmitted to humans by blood-feeding arthropod vectors. We describe the natural history of the Lyme disease zoonosis to illustrate interactions among pathogens, vectors, vertebrate hosts, and risk to humans. We then describe how the presence of a diverse assemblage of vertebrates can dilute the impact of the principal reservoir (the white-footed mouse, Peromyscus leucopus) of Lyme disease spirochetes (Borrelia burgdorferi), thereby reducing the disease risk to humans. Exploring the logic of what we call the dilution effect reveals four conditions that are necessary for it to apply generally to vector-borne zoonoses: (1) the feeding habits of the vector are generalized; (2) the pathogen is acquired by the vector from hosts (as opposed to exclusively transovarial transmission); (3) reservoir competence (the ability of a particular host species to infect a vector) varies among host species; and (4) the most competent reservoir host tends to be a community dominant, as defined by the proportion of the tick population fed by that species. When these conditions are met, vertebrate communities with high species diversity will contain a greater proportion of incompetent reservoir hosts that deflect vector meals away from the most competent reservoirs, thereby reducing infection prevalence and disease risk. Incorporating the likelihood that the abundance of competent reservoirs is reduced in more diverse communities, owing to the presence of predators and competitors, reinforces the impact of the dilution effect on the density of infected vectors. A review of the literature reveals the generality, though not the universality, of these conditions, which suggests that the effects of diversity on disease risk may be widespread. Issues in need of further exploration include (i) the relative importance of diversity per se versus fluctuating numbers of particular species; (ii) the relevance of species richness versus evenness to the dilution effect; (iii) whether the dilution effect operates at both local and regional scales; and (iv) the shape of empirically determined curves relating diversity to measures of disease risk. Further studies linking community ecology with epidemiology are warranted.

Impact of microclimate on immature tick-rodent host interactions (Acari: Ixodidae): implications for parasite transmission

Rodents play a significant role in enzootic cycles of tick-borne pathogens, notably, in the northern hemisphere, tick-borne encephalitis virus and Lyme borreliosis spirochaetes. The relative numbers of nymphal and larval ticks feeding on rodents are crucial variables in determining the probability of rodent infection and the degree of amplification of infection prevalence in the tick population. Manipulation of the microclimate within quasinatural experimental arenas revealed that under increasingly dry conditions the numbers of unfed nymphal Ixodes ricinus L. questing in upper layers of the herbage decreased, whereas the rate of fat use and the numbers of nymphs feeding on small rodents, both increased. This is consistent with nymphs descending to the moist lower vegetation layers for water replenishment, where they would come into contact with small hosts. Very few larvae quested or fed on rodents under the dry conditions, but many more did so once the humidity increased, suggesting that larvae escape desiccation by becoming quiescent. The ratio of larvae to nymphs feeding on rodents thus increases with increasing humidity, contributing to the seasonal and geographical variation in disease transmission dynamics.

Epidemiological uses of a population model for the tick Rhipicephalus appendiculatus

The spatial and temporal risk of tick-borne disease depends fundamentally on the distribution, abundance and seasonal dynamics of the vector ticks. The latter factor exerts a major quantitative influence on the transmission dynamics of tick-borne parasites. The population model for Rhipicephalus appendiculatus applies throughout the range of this tick in eastern Africa, and predicts all three fundamental risk factors on the basis of the local temperature and rainfall conditions. Satellite imagery can provide more detailed, real-time measures of environmental conditions over extensive areas than climatic data. There is preliminary evidence to suggest that the population model could be driven by satellite-derived surrogates of its climatic predictors, thus providing wide-scale predictive risk maps of theileriosis.

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.