Transovarial transmission of African swine fever virus in the argasid tick Ornithodoros moubata

The aim of this study was to determine filial infection prevalence of experimentally infected colony Ornithodoros moubata Walton (Ixodoidea: Argasidae) ticks for African swine fever virus (ASFV). Three groups of ticks were used: an uninfected control group, one group orally infected with the VIC T90/1 isolate and another group orally infected with the LIV 13/33 isolate of ASFV. The results show that filial infection prevalences were not constant but were highly variable between egg batches from different ticks and between successive egg batches from the same tick. Filial infection prevalences ranged from 1.8% to 31.8% for ticks infected with the VICT90/1 isolate and from 1.2% to 35.5% for ticks infected with the LIV 13/33 isolate. A similar pattern was noted after the third feed. Immunohistochemisty showed that virus replicates in the developing larval cells and not in the yolk sac cells or within the outer layers of the eggs. The results show that ASFV can replicate to a high titre (10(5.1)log10HAD50) within the larval cells of the developing egg.     

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.     

Enhancement of tick-borne encephalitis virus transmission by tick salivary gland extracts

Abstract. To investigate the role of ticks in TBE virus transmission, salivary gland extract (SGE) was derived from partially fed female Ixodes ricinus, Dermacentor reticulatus and Rhipicephalus appendiculatus ticks. Guinea-pigs were infested with uninfected R.appendiculatus nymphs and inoculated with a mixture of TBE virus and SGE or with virus alone. The number of ticks which on average acquired virus from feeding on animals inoculated with TBE virus and SGE from partially fed ticks was 4-fold greater than the number that became infected by feeding on animals inoculated with virus alone or virus plus SGE from unfed I.ricinus. Viraemia was detected in 67% of guinea-pigs inoculated with virus plus SGE compared to 30% of guinea-pigs inoculated with virus alone. Virus titres in the blood were similar for both groups of animals [range 2.0-2.8 log₁₀ plaque-forming units (PFU)/ml of blood]; however, the number of ticks that became infected was significantly higher on animals inoculated with virus plus SGE from partially fed ticks. No significant difference was observed with respect to the tick species used to derive SGE. The results indicate that TBE virus transmission is enhanced by factor(s) associated with the salivary glands of feeding ticks, and that these factor(s) may facilitate efficient transmission of TBE virus between infected and uninfected ticks even when they feed on hosts that have no detectable viraemia.     

Salivary fluid secretion in the ixodid tick Rhipicephalus appendiculatus is inhibited by Thogoto virus infection

Adult Rhipicephalus appendiculatus ticks, infectedwith Thogoto (THO) virus or control, were fed on guinea pigs and removed atintervals throughout the feeding cycle. Salivary fluid secretion was measuredbyan in vitro technique. The salivary glandsof infected, partially-fed ticks secreted fluid in vitro at about 75% the rateof controls, but the difference between infected and controls among engorgedticks was not statistically significant. Basal and DA-stimulated levels ofcyclic AMP (cAMP) were determined in isolated glands and were significantlyaffected by THO virus infection. The differences in secretory rate amongcontroland infected ticks could not be explained in terms of altered cAMP levels.Haemolymph volume was measured by a tracer-dilution technique using³H-inulin. The mean haemolymph volume for both THO-infected andcontrol groups was between 23–24% body weight throughout the feedingcycle, indicating that infection by this arbovirus did not influence salivaryfluid secretion via altered haemolymph volume. The mechanism by which THO virusaffects secretory activity of its tick vector remains unknown.     

Replication of Colorado tick fever virus within human hematopoietic progenitor cells.

Significant neutropenia, as well as thrombocytopenia and a mild anemia, occurs in patients infected with Colorado tick fever virus. In this study, human bone marrow CD34+ cells and KG-1a cells, a human hematopoietic progenitor cell line, were infected in vitro with Colorado tick fever virus. The time course and morphological appearance of viral replication in human progenitor cells were similar to those seen in erythroblasts and in HEL cells and suggest one possible mechanism for the clinical hematologic findings.     

Role of small mammals in the persistence of Louping-ill virus: field survey and tick co-feeding studies

Louping-ill (LI) is a tick-borne viral disease of red grouse, Lagopus lagopus scoticus Lath. (Tetraonidae: Galliformes), and sheep, Ovis aries L. (Bovidae: Artiodactyla), that causes economic loss to upland farms and sporting estates. Unvaccinated sheep, grouse and mountain hares, Lepus timidus L. (Leporidae: Lagomorpha), are known to transmit LI virus, whereas red deer, Cenrus elaphus L. (Cervidae: Artiodactyla), and rabbits, Oryctolagus cuniculus L. (Leporidae: Lagomorpha), do not. However, the role of small mammals is unknown. Here, we determine the role of small mammals, in particular field voles, Microtus agrestis L. (Muridae: Rodentia), in the persistence of LI virus on upland farms and sporting estates in Scotland, using field sampling and non-viraemic transmission trials. Small mammals were not abundant on the upland sites studied, few ticks were found per animal and none of the caught animals tested seropositive to LI virus. Laboratory trials provided no evidence that small mammals (field voles, bank voles, Clethrionomys glareolus L. (Muridae: Rodentia), and wood mice, Apodemus sylvaticus L. (Muridae: Rodentia), can transmit LI virus between cofeeding ticks and, in the field, LI virus was prevalent only in areas with known LI virus competent hosts (grouse, mountain hares or unvaccinated sheep) and absent elsewhere. In contrast to the case of tick-borne encephalitis (TBE) virus in Europe, it is concluded that small mammals seem to be relatively unimportant in LI virus persistence.     

Ixodes ricinus tick saliva modulates tick-borne encephalitis virus infection of dendritic cells

Tick-borne encephalitis virus is an important human pathogen, naturally delivered into host skin via a tick bite. To examine the effects of the virus on dendritic cell biology, we cultured dendritic cells with two tick-borne encephalitis virus strains of different virulence in the presence of Ixodes ricinus tick saliva. Tick saliva treatment increased proportion of virus-infected cells, led to a decrease in virus-induced TNF-α and IL-6 production and to reduced virus-induced apoptosis. Our data indicate that tick saliva modulate virus-mediated alterations in dendritic cells, thus probably being involved in the early infection process in the host.     

Characterization of the stop codon readthrough signal of Colorado tick fever virus segment 9 RNA

Termination codon readthrough is utilized as a mechanism of expression of a growing number of viral and cellular proteins, but in many cases the mRNA signals that promote readthrough are poorly characterized. Here, we investigated the readthrough signal of Colorado tick fever virus (CTFV) segment 9 RNA (Seg-9). CTFV is the type-species of the genus Coltivirus within the family Reoviridae and is a tick-borne, double-stranded, segmented RNA virus. Seg-9 encodes a 36-kDa protein VP9, and by readthrough of a UGA stop codon, a 65-kDa product, VP9'. Using a reporter system, we defined the minimal sequence requirements for readthrough and confirmed activity in both mammalian and insect cell-free translation systems, and in transfected mammalian cells. Mutational analysis revealed that readthrough was UGA specific, and that the local sequence context around the UGA influenced readthrough efficiency. Readthrough was also dependent upon a stable RNA stem-loop structure beginning eight bases downstream from the UGA codon. Mutational analysis of this stem-loop revealed a requirement for the stem region but not for substructures identified within the loop. Unexpectedly, we were unable to detect a ribosomal pause during translation of the CTFV signal, suggesting that the mechanism of readthrough, at least at this site, is unlikely to be dependent upon RNA secondary-structure induced ribosomal pausing at the recoded stop codon.     

An experimental assessment of the tampan tick Ornithodoros moubata as vector of hepatitis B virus

Wild-caught and colonized tampan ticks, Ornithodoros moubata (Murray), were fed on hepatitis B virus (HBV)-positive blood-means in a series of four experiments. Hepatitis B surface antigen (HBsAg) persisted in nymphal and adult ticks for up to 779 days, while the epsmark antigen (HBeAg) persisted in mature nymphs up to 13 days, in adult males up to 11 days and in adult females up to 16 days. HBsAg was transmitted trans-stadially through two moults during the life cycle but transovarial transmission did not occur. The surface antigen was transmitted by two out of fifteen single ticks into 0.4 ml aliquots of HBV-negative blood, although six groups of ticks failed to transmit into 5.5 ml aliquots of blood: this antigen was not transmitted to hamsters. HBsAg was detected in samples of the ticks' coxal and rectal fluid secretions always at the infecting feed and usually at the second feed. HBeAg was only detected in one of two samples of coxal fluid collected at the infecting feed. The results as a whole indicate that no biological multiplication of virus occurs in O.moubata but that mechanical transmission from ticks to man could occur by: (i) contamination of a person when crushing infected ticks; (ii) infection by bite; (iii) contamination with coxal fluid, especially by scratching bites. This is thought to take place among the Kavango tribe in their village huts in north-eastern Namibia where infestations of infected O.moubata occur.     

Distribution of E and NS1 proteins of TBE virus in mammalian and tick cells

Four monoclonal antibodies recognizing TBEV proteins were prepared. Three of them (2/H6, 8/A10, 5/F9) specifically recognize the structural E protein and the fourth binds to the nonstructural NS1 protein. These antibodies were used for an immunofluorescence study of TBEV protein distribution in infected mammalian host and tick vector tissue culture cells. Any differences in the distribution of the E and NS1 proteins were revealed. In porcine PS cell line the proteins were localized in the cytoplasm with a distinct increase of the signal in the perinuclear area 16 h post-infection (p.i.). The area gradually expanded and at 24 h p.i. covered the major part of the cytoplasm. The localization of the proteins in tick RA-257 cells revealed a uniform spread of the proteins in whole cytoplasm of the infected cells. The signal was very weak at 16 h p.i. and gradually increased during progressing time p.i., although, the distribution of the proteins did not exhibit any changes. Difference in viral protein distribution in the two cell types points to the possibility that the maturation process of TBEV exhibits different features in mammalian and tick cells.     

Colorado tick fever virus in cell culture. II. Physical and chemical properties

Trent, Dennis W. (University of Oklahoma School of Medicine, Oklahoma City), and L. Vernon Scott. Colorado tick fever virus in cell culture. II. Physical and chemical properties. J. Bacteriol. 91:1282-1288. 1966.-Heat-inactivation kinetics for Colorado tick fever (CTF) virus grown in L cells indicated that more than one rate constant was involved for inactivation at each exposure temperature. An Arrhenius plot of the data indicated the inactivation rate constants to be dependent on the absolute temperature. The energy of activation, for thermal inactivation of the virus, was 17,289 calories per mole, with the Q(10) being 2.6. The optimal pH range for maintenance of CTF viral infectivity was determined to be 7.5 to 7.8. The infectivity of CTF virus was stable to freezing and thawing in diluents which contained: 50% calf serum, 20% glucose, 20% glycerol, 10% bovine serum albumin, 20 mm glutamine, and 2% gelatin. CTF virus replication was insensitive to inhibition by 5-fluoro-2'-deoxyuridine and 5-bromo-2'-deoxyuridine, whereas herpes simplex virus was markedly inhibited, as reported by others. Actinomycin D inhibited CTF virus replication when cells were pretreated for 24 and 12 hr prior to infection, but not when the inhibitor was added at the time of infection. The nucleic acid of CTF virus appears to be of the ribose type.     

The major tick salivary gland proteins and toxins from the soft tick,Ornithodoros savignyi,are part of the tick Lipocalin family: implications for the origins of tick toxicoses

The origins of tick toxicoses remain a subject of controversy because no molecular data are yet available to study the evolution of tick-derived toxins. In this study we describe the molecular structure of toxins from the soft tick, Ornithodoros savignyi. The tick salivary gland proteins (TSGPs) are four highly abundant proteins proposed to play a role in salivary gland granule biogenesis of the soft tick O. savignyi, of which the toxins TSGP2 and TSGP4 are a part. They were assigned to the lipocalin family based on sequence similarity to known tick lipocalins. Several other tick lipocalins were also identified using Smith-Waterman database searches, bringing the tick lipocalin family up to 20. Phylogenetic analysis showed that most tick lipocalins group within genus-specific clades, suggesting that gene duplication and divergence of tick lipocalin function occurred after tick speciation, most probably during the evolution of a hematophagous lifestyle. TSGP2 and TSGP3 show high sequence identity and group terminal to moubatin, an inhibitor of collagen-induced platelet aggregation from the tick, O. moubata. However, no platelet aggregation inhibitory activity is associated with the TSGPs using ADP or collagen as agonists, suggesting that TSGP2 and TSGP3 duplicated after divergence of O. savignyi and O. moubata. This timing is supported by the absence of TSGP2-4 in the salivary gland extracts of O. moubata. The absence of TSGP2 and TSGP4 in salivary gland extracts from O. moubata correlates with the nontoxicity of this tick species. The implications of this study are that the various forms of tick toxicoses do not have a common origin, but must have evolved independently in those tick species that cause pathogenesis.     

Immunocytochemical mapping of FMRFamide-like peptides in the argasid tick Ornithodoros parkeri and the ixodid tick Dermacentor variabilis

FMRFamide-like immunoreactivity was studied in the argasid tick Ornithodoros parkeri and the ixodid tick Dermacentor variabilis using immunocytochemistry based on the peroxidase-antigeroxidase method. FMRFamide-like immunoreactive cells are widely distributed in various regions of the tick synganglion including protocerebral, cheliceral, stomodeal, palpal, pedal I–IV, and opisthosomal regions in both species. However, there is one layer of immunoreactive cells located on the dorsal surface of the postoesophageal part of the synganglion that is found only in D. variabilis. Besides the immunoreactivity within the cell body and its axons, the neuropile and the neural lamella (the extracellular sheath of the synganglion) are rich in immunoreactive materials. Some coxal muscles are innervated by the FMRFamide-like immunoreactive processes of the nerve from the pedal ganglion.     

Persistence of tick-borne virus in the presence of multiple host species: tick reservoirs and parasite mediated competition.

Tick-borne viruses in tropical and temperate parts of the world have a significant impact on human, livestock and wildlife hosts both directly, through mortality/morbidity, and economically. Since the ticks have multiple life stages and can utilize a large range of host species our understanding of the dynamics of these infections is often not clear. In this paper we consider the impact of a population which is a tick host but non-viraemic on one which is both a tick host and viraemic. We present two simple deterministic models and use joint threshold density curves to illustrate the basic reproductive ratios of both the ticks and the virus. We find that the non-viraemic hosts can have considerable impact on the viraemic host. Either they amplify the tick population and cause the virus to persist, or they dilute the infection and cause it to die out. A general model framework is presented here but a special case of this model describes the red grouse-hare-Louping-ill system.