Amblyomma variegatum (Acari: Ixodidae): mechanism and control of arbovirus secretion in tick saliva

Saliva is considered to be the conduit by which pathogens are transmitted from blood-sucking arthropod vectors to their vertebrate hosts, but supporting evidence for this is fragmentary. To determine if Thogoto (THO) virus, a tick-borne member of the influenza virus family, is transmitted via tick saliva, and whether virus replication is a prerequisite for such transmission, two experimental conditions were compared: (1) "biological transmission" and (2) "mechanical transmission." In (1), THO virus was allowed to infect and replicate in a natural vector, Amblyomma variegatum: virus was detected in saliva collected from 3/22 (14%) ticks. In (2), virus was inoculated directly into the hemocoel with the drug used to induce salivation and saliva was collected immediately to preclude the possibility of virus replication: virus was detected in saliva collected from 31/170 (18%) ticks. The results demonstrate that THO virus is secreted in tick saliva and that virus can pass from the hemolymph to the salivary glands independently of viral replication within the tick. The comparatively low numbers of ticks that yielded virus-positive saliva samples together with the results from assays of serial saliva samples suggested that virus secretion may not be a continuous process during salivation. Ticks in which THO virus had established an infection showed an impaired secretory response compared with uninfected ticks and ticks used for mechanical transmission.

[Argas and arbovirus update]

At the opportunity of the recent discovery of a new Phlebovirus, Tunis virus, the authors discuss the importance of the genus Argas as for its potential role in the transmission of arboviruses: twelve species of Argas are known to carry about twenty viruses. The wide distribution of Argas reflexes hermanni prompt us to explain its position in the Argas reflexes "complex" group: this species seems to be more liable to infection by viruses. Some ecological and entomological data are added to facilitate its typing. The Tunisian specimens recollected are remarkable by the lack of seta n degree 5 on basis capituli.

Arbovirus surveillance in Italy

A high number of different arboviruses have been demonstrated to be present in Italy, due to the coexistence of climates that are characteristic of continental areas in the North of the country and of subtropical areas in the South. Viruses circulating in Central Europe, such as the Tick-Borne Encephalitis (TBE) virus, are present in Central and Northern Regions, whereas viruses circulating in Mediterranean areas, such as the Sandfly fever viruses are present in Central and Southern Italy. Viruses have been isolated from mosquitoes, ticks, sandflies and vertebrates (birds, rodents, humans). For some of them an association with human disease has been demonstrated.

[The interrelationships of ixodid ticks (Ixodidae) with the causative agents of transmissible vertebrate infections]

The ixodid ticks are the vectors of several hundreds of agents of transmission diseases of vertebrates and human. This agents are represented by viruses, rickettsiae, bacteria, protozoans and filariae. For the majority of these agents the ticks are not only vectors, but also are intermediate or final hosts. In the system agent-tick the antagonistic relationships dominate, and therefore the agent is actually a parasite of its vector. A wide spectrum of parasitic relationships form the high pathogenicity and lethality of an agent for its vector to the fast and complete elimination of a microorganism in a tick could be observed within the system in question. Somewhat balanced type of relationships occurs most often, thus pathogen agent causes a minimal damage to the tick-vectors and could stay in their organism and hold a capability of the transmission to the vertebrate hosts and within the population of ticks.

Serological study of pigs for antibody against African swine fever virus in two areas of southern Malawi

A serological survey was conducted in July 1991 on domestic pigs in two areas of southern Malawi which were severely affected by the African swine fever (ASF) epizootic in 1989-1991. Sixty-six of the 216 owners questioned reported having witnessed ASF in their pigs. Forty-seven owners had pigs with antibodies against ASF virus, and the overall prevalence of pigs with anti-ASF virus antibodies was found to be 12.4%, in 445 pigs sampled in 35 villages. Spread of ASF was thought to occur principally through the slaughter and sale of infected animals, and due to the free-ranging of pigs. Permanent penning of pigs significantly reduced the attack rate (chi 2 = 7.59, P < 0.01, 1df) in pig pens in Thyolo, an area where permanent penning of pigs was widely practised. Feeding of kitchen scraps did not appear to have been an important means of virus spread. Ornithodoros ticks were found in only 1 of the 35 villages. Although virus was not isolated from 203 pooled sera from pigs in the Mulanje district or from collected ticks, the seroconversion of a small proportion of pigs born after the last reported date of ASF occurrence suggests that the virus had continued to circulate to a limited extent in this area.

The complete nucleotide sequence and genome organization of the mite-transmitted brome streak mosaic rymovirus in comparison with those of potyviruses

A virus isolate, designated as 11-Cal, originating from southern France has been identified as an isolate of the mite-transmitted brome streak mosaic rymovirus (BrSMV) by serological and morphological properties. BrSMV is a member of the genus Rymovirus of the family Potyviridae. The complete nucleotide sequence of the RNA genome of BrSMV has been determined. The assembled RNA is 9672 nucleotides in length, excluding a 3'-terminal poly(A)sequence. The RNA contains one open reading frame (ORF) of 9282 nucleotides coding for a polyprotein of 3093 amino acids. A comparison with typical potyvirus showed that BrSMV has a similar genome organization. The predicted cleavage sites of the polyprotein of BrSMV are similar to those of potyviruses. Nevertheless, unusual dipeptides are proposed in two cases. Based on the proposed location of the cleavage sites nine mature proteins are predicted. Specific motifs, described for potyviral polyproteins, are almost all present in the polyprotein of BrSMV, too. However, only an incomplete zinc-finger motif is present in the potential helper component and the motif for aphid transmission in the coat protein is not found. Several alignments of amino acid sequences showed less similarity between BrSMV and potyviruses than between different potyviruses.

Persistence and transmission of tick-borne viruses: Ixodes ricinus and louping-ill virus in red grouse populations

The population dynamics of tick-borne disease agents and in particular the mechanisms which influence their persistence are examined with reference to the flavivirus that causes louping-ill in red grouse and sheep. Pockets of infection cause heavy mortality and the infection probably persists as a consequence of immigration of susceptible hosts. Seroprevalence is positively associated with temporal variations in vectors per host, although variation between areas is associated with the abundance of mountain hares. The presence of alternative tick hosts, particularly large mammals, provides additional hosts for increasing tick abundance. Grouse alone can not support the vectors and the pathogen but both can persist when a non-viraemic mammalian host supports the tick population and a sufficiently high number of nymphs bite grouse. These alternative hosts may also amplify virus through non-viraemic transmission by the process of co-feeding, although the relative significance of this has yet to be determined. Another possible route of infection is through the ingestion of vectors when feeding or preening. Trans-ovarial transmission is a potentially important mechanism for virus persistence but has not been recorded with louping-ill and Ixodes ricinus. The influence of non-viraemic hosts, both in the multiplication of vectors and the amplification of virus through non-viraemic transmission are considered significant for virus persistence.

[A new look at transmission and circulation of tick encephalitis virus in nature]

The author summarizes briefly the results of experiments which provide evidence of non-viraemic transmission of the virus from infected to non-infected ticks during co-feeding on a non-viraemic host. This mode of transmission originally found in the Thogoto virus and its vector, the tick Rhipicephalus appendiculatus, was proved also in the virus of tick-borne encephalitis and its vector Ixodes ricinus. Based on this fact some conclusions are drawn revising the accepted view regarding the life cycle of tick-borne encephalitis virus in nature and in particular the relationship: vector-host-virus.

Two Viruses Isolated from Rodents (Clethrionomys gapperi and Microtus pennsylvanicus) Trapped in St. Lawrence County, New York

Four strains of C. gapperi virus were isolated from 3 Clethrionomys gapperi and 47 strains of Microtus virus from 15 Microtus pennsylvanicus and 1 Mus musculus. One of the Microtus strains was isolated from a pool of 20 mites while the others were from rodent tissues. These agehts were insensitive to ether and sodium desoxycholate, withstood freezing at -70 C for 3 years and lyophilization without loss of titer, and were not killed when heated at 60 C for 1 hour. Their size as determined by filtration was less than 50 mg and greater than 20-35 mmicro. The strains within each group appear to be similar. The illness induced in suckling mice by the C. gapperi agents had a 5-day incubation period followed by prostration and death with a histologic picture of extensive encephalomalacia. The incubation period in mice for the Microtus agents was 9 to 11 days followed by convulsions and death. Histopathology showed meningeal infiltration and necrosis of the molecular layer. No antigenic similarity was detected between the C. gapperi and Microtus viruses by cross complement-fixation test.