Ixodes

Abstract: Crimean-Congo hemorrhagic fever (CCHF) came to modern medical attention in 1944–1945, when about 200 Soviet military personnel were infected while assisting peasants in war-devastated Crimea (Ukrainian SSR). Subsequent epidemics occurred in Astrakhan (1953–1968) and Rostov Oblasts (1963–1971) of USSR and in Bulgaria (1953–1973). There have been numerous lesser outbreaks in southern USSR and, in 1976, outbreaks in Pakistan. However, it was only in 1967, when Soviet workers first used the generally accepted newborn white mouse (NWM) inoculation technique for CCHF virus isolation and study, that the etiologic agent could be characterized antigenically, physiochemically, and morphologically. Collaboration in 1968 between the Soviet and American experts M. P. Chumakov and Jordi Casals demonstrated the serologically identical properties of virus strains from human CCHF patients and corpses, lower mammals, and ticks from Asian and European areas of the USSR and from Bulgaria, Congo (Zaire), Nigeria, and Pakistan. These results, confirmed and broadened in subsequent studies, enabled serological and other research tools to be developed for producing identifiable antibodies and antigens required in experimental procedures and seroepidemiological surveys and obtaining scientific evidence to demonstrate vector and reservoir species and virus dynamics in nature. CCHF virus, a member (without generic assignment) of the family Bunyaviridae, is the prototype of the CCHF serogroup, which also includes Hazara virus (from Ixodes redikorzevi parasitizing alpine voles in Pakistan). CCHF virus is enzootic in the Palearctic, Oriental, and Ethiopian Faunal Regions, chiefly in steppe, savanna, semidesert, and foothill biotopes where 1 or 2 Hyalomma species are the predominant ticks parasitizing domestic and wild animals. Presence of the virus has been demonstrated by isolations from humans, other mammals, and/or ticks, or by seroepidemiological survey results, in western and southern India, Pakistan, Afghanistan, Iran, Soviet Middle Asia (Turkmen, Uzbek, Kazakh, Kirgiz, and Tadzhik SSR), Transcaucasia (Armenian and Azerbaijan SSR), European USSR (Ukrainian and Moldavian SSR, Kalmyk and Daghestan ASSR, Astrakhan and Rostov Oblasts, and Krasnodar and Stavropol Regions of RSFSR), Bulgaria, Yugoslavia, Greece, Hungary, France, Senegal, Nigeria, Central African Empire, Zaire, Uganda, Kenya, Ethiopia, Tanzania, and Egypt. The ecologically atypical CCHF foci in Moldavian deciduous forest habitats of Ixodes ritinus and Dermatentor and Rhipicephalus species may represent a spillover phenomenon associated with environmental changes created by humans. CCHF virus is a true tick-associated arbovirus; it survives transstadially (from larva to nymph to adult) and interseasonally in several tick species and is transmitted transovarially to the F1 generation (in some cases to F2) in Hyalomma m. marginatum, H. marginatum rufipes, Dermacentor marginatus , and Rhipicephalus rossicus . Twenty-five tick species and subspecies have been reported to be CCHF virus reservoirs/vectors (the single record from an argasid, the birdparasitizing Argas persicus, remains to be confirmed). One-host ticks, Boophilus annulatus, B. microplus, B. decoloratus (and probably B. geigyi ), appear to maintain intense virus interaction for many weeks or months between several tick species infesting artiodactyls (especially cattle). The 2-host vectors are Hyalomma m. marginatum, H. mmginatum turanicum and H. marginatum rufipes (and probably H. marginatum isaaci ); they feed as immatures on birds, hares, or hedgehogs and, as adults, chiefly on artiodactyls (often also on humans). Other 2-host vectors, H. anatolicum anatolicum, H. detritum , and Rhipicephalus bursa , feed both as immatures and adults on artiodactyls. The H. marginatum complex, and H. a. anatolicum , are especially important in causing epidemics and outbreaks of human CCHF owing to their great numbers during certain periods and to their aggressiveness in seeking human hosts. Others, including 13 species of 3-host ticks [ Haemaphysalis punctata, Amblyomma variegatum, Dermacentor (2 spp.), Hyalomma (5 spp.), and Rhipicephalus (4 spp.)], which generally seek human hosts less aggressively than the cited hyalommas, serve chiefly to maintain enzootic foci of CCHF virus circulation between ticks and wild and domestic mammals. Ground-feeding birds are often hosts of CCHF virus-infected ticks but birds apparently do not become viremic; the epidemiological role of these birds is to support populations of certain vector species and to disseminate these species intracontinentally and/or intercontinentally. CCHF epidemics have developed on a background of favorable climatic factors and environmental changes beneficial for survival of large numbers of hyalommas and of the hosts of both their immature and adult stages. The environmental changes have been wartime neglect of agricultural lands, introduction of susceptible military personnel or new settlers into infected foci, widescale collectivization of agriculture, changing pasture patterns, converting floodplains and marshy deltas to farmland and pastures, flood control, etc. Unusually severe winter-spring weather, resulting in decimation of Hyalomma populations and also of hosts of immature stages, appears to have been largely responsible for virus circulation to revert from epizootic (epidemic) to enzootic intensity. Humans become infected when bitten by infected ticks, or when crushing these ticks in their bare hands or shearing tick-infested sheep. Household and nosocomial cases resulting from contamination by bloody discharges from CCHF patients have been especially numerous and severe, often with great mortality, in villages and hospitals where the disease was unrecognized. Other cases have occurred from laboratory accidents and from handling infected animal carcasses. Mild, moderate, and severe disease courses are described. A certain number of human infections may be clinically inapparent. Mortality rates in CCHF patients have ranged from 15 to 40% or more. Despite the absence of specific drugs for treating CCHF, the Leshchinskaya regimen of hospital care has resulted in appreciable reduction in mortality rates. There are no scientific data to indicate that the virus is less virulent in Africa than in Eurasia. An apparently effective vaccine has been developed to prevent infections in persons in high-risk situations. Various measures applied to prevent human illness have had mixed success. The sensitivity of serological tests for CCHF antibodies needs to be improved to obtain more reliable survey results and to determine whether there are significant differences between CCHF virus strains. The fluorescent antibody technique (FAT) is useful for determining the presence of antibodies to the virus in vertebrate tissues and in ticks (but the dynamics of the virus in ticks has not been investigated). The indirect FAT is an important candidate for research in relation to epidemiological surveys. Most CCHF strains yield no agglutinating antigens (unlike all other arboviruses causing significant human disease, except Colorado tick fever virus); CCHF virus also replicates poorly or not at all in most cell cultures and no visible cytopathogenic effect has been described. Addendum . In 1978, while this manuscript was in press, CCHF isolates were reported from Hyalomma dromedarii in Turkmenia and Rhipicephalus appendiculatus in Uganda. Thus, 27 tick taxa have been associated with CCHF virus.

Abstract: Lymedisease spirochetes, Borrelia burgdor- feri sensu lato, aremaintained inzoonotic cycles involving ticks andsmall mammals. Inunfed ticks, thespirochetes produce oneoutersurface protein, OspA,butnotOspC.During infection inmammals, immunological datasuggest thatthe spirochetes havechanged their surface, nowexpressing OspC butlittle ornoOspA.Wefindbyinvitro growth experiments that this change isregulated inpartbytemperature; OspCis produced byspirochetes at32-370C butnotat24°C. Further- more,spirochetes inthemidgutofticks thathavefully engorged onmicenowhaveOspContheir surface. Thustwo environmental cues, anincrease intemperature andtick feeding, trigger amajoralteration ofthespirochetal outer membrane. Thisrapidsynthesis ofOspCbyspirochetes during tick feeding mayplay anessential roleinthecapacity ofthesebacteria tosuccessfully infect mammalianhosts, including humans, whentransmitted byticks. Manyinfectious agents pathogenic inhumansaremaintained innatural zoonotic cycles involving wildvertebrates and obligate blood-feeding arthropods (1,2).Although muchis knownabout theclinical description anddiagnosis ofthese humandiseases, thephysiological andmorphological adapta- tions ofthese agents, especially bacteria, while intheir arthro- podvectors arenotwell understood. Several non-vector-borne bacterial pathogens, including species ofSalmonella, Shigella, andBordetella, display animpressive repertoire ofadaptive molecular responses toenvironmental signals onentry into mammalian hosts (3-5). Specific changes bybacterial patho- gensinarthropod vectors during feeding onbloodhavenot beendescribed. Identifying suchevents wouldbroaden our knowledge ofhowthese agents areperpetuated andtransmit- tedinnatureandassist inthedevelopment ofeffective vaccines anddiagnostic tests. Borrelia burgdorferi isoneofatleast three closely related species ofspirochetes thatcauseaspectrum ofclinical syn- dromesinhumans, collectively called Lymedisease orLyme borreliosis (6-8). Thesespirochetes aremaintained inzoo- notic cycles involving adiversity ofwildmammalsandticks primarily inthegenus Ixodes (9). Thesemicrobes' adaptation totickandmammalian environments likely involves very different surface components soastoensure their transmis- sionandsurvival intwoverydifferent hosts. Several lipopro- teins havebeendescribed onthesurface ofB.burgdorferi (10-16), someofwhicharevariably expressed during serial passage inculture (17-19). Theapparent flexibility inthe spirochete's synthesis ofsomeouter surface proteins (Osps) in vitro mayhaverelevance toitsalternation ofhosts. Previous studies demonstrate that B.burgdorferi produces onesurface protein, OspA,andlikely OspB,inthemidgut ofIxodes ticks thathavenotyetengorged onblood(10). However, itis unclear howlongspirochetes continue toproduce this protein after entering mammalian hosts because fewanimals, includ- inghumans, makeantibodies tothis protein during infection (20-22). Yet,another surface protein, OspC,stimulates an early antibody response inhumans(23,24),although its expression inticks isunknown. Inthis report wedemonstrate that theLymedisease spirochete alters its outer surface during tick feeding andthat this switch isalso controlled, inpart, by anincrease intemperature.

Abstract: In little more than 30 years, Lyme disease, which is caused by the spirochaete Borrelia burgdorferi, has risen from relative obscurity to become a global public health problem and a prototype of an emerging infection. During this period, there has been an extraordinary accumulation of knowledge on the phylogenetic diversity, molecular biology, genetics and host interactions of B. burgdorferi. In this Review, we integrate this large body of information into a cohesive picture of the molecular and cellular events that transpire as Lyme disease spirochaetes transit between their arthropod and vertebrate hosts during the enzootic cycle.

Abstract: Utilitarian arguments concerning the value of biodiversity often include the benefits of animals, plants, and microbes as sources of medicines and as laboratory models of disease. The concept that species di- versity per se may influence risk of exposure to disease has not been well developed, however. We present a conceptual model of how high species richness and evenness in communities of terrestrial vertebrates may re- duce risk of exposure to Lyme disease, a spirochetal ( Borrelia burgdorferi ) disease transmitted by ixodid tick vectors. Many ticks never become infected because some hosts are highly inefficient at transmitting spirochete infections to feeding ticks. In North America, the most competent reservoir host for the Lyme disease agent is the white-footed mouse ( Peromyscus leucopus ), a species that is widespread and locally abundant. We suggest that increases in species diversity within host communities may dilute the power of white-footed mice to in- fect ticks by causing more ticks to feed on inefficient disease reservoirs. High species diversity therefore is ex- pected to result in lower prevalence of infection in ticks and consequently in lower risk of human exposure to Lyme disease. Analyses of states and multistate regions along the east coast of the United States demonstrated significant negative correlations between species richness of terrestrial small mammals (orders Rodentia, In- sectivora, and Lagomorpha), a key group of hosts for ticks, and per capita numbers of reported Lyme disease cases, which supports our " dilution effect " hypothesis. We contrasted these findings to what might be expected when vectors acquire disease agents efficiently from many hosts, in which case infection prevalence of ticks may increase with increasing diversity hosts. A positive correlation between per capita Lyme disease cases and species richness of ground-dwelling birds supported this hypothesis, which we call the " rescue effect ." The reservoir competence of hosts within vertebrate communities and the degree of specialization by ticks on par- ticular hosts will strongly influence the relationship between species diversity and the risk of exposure to the many vector-borne diseases that plague humans.