[Taxonomic structure of Orthomyxoviridae: current views and immediate prospects]

Analysis of taxonomic structure of Orthomyxoviridae was undertaken in view of its anticipated evolution. Four concepts of circulation of influenza A viruses in the biosphere are discussed, viz. anthrponose, zooanthroponose, metastrongilose, and protozoan. All of them may be considered in the framework of the general zooantroponose concept. Influenza B and C viruses can not be regarded as strictly anthroponose. Comparative molecular-genetic analysis of the genus Thogotovirus provides a basis for the designation of Thogoto and Batken-Dhori as independent geni. It is speculated that t he proof of transmission of Isaviruses by copepods Caligus elongates and Lepeophtheirus salmonis (Crustacea: Copepoda) may open up a new line of developments in arborvirology since crustacean vectors of viruses have never been described before.

Determination of natural versus laboratory human infection with Mayaro virus by molecular analysis

A laboratory worker developed clinical signs of infection with Mayaro virus (Togaviridae), an arbovirus of South and Central America, 6 days after preparation of Mayaro viral antigen and 10 days after a trip to a rain forest. There was no evidence of skin lesions during the antigen preparation, and level 3 containment safety measures were followed. Therefore, molecular characterization of the virus was undertaken to identify the source of infection. RT-PCR and DNA sequence comparisons proved the infection was with the laboratory strain. Airborne Mayaro virus contamination is thus a hazard to laboratory personnel.

Epidemic O'Nyong-Nyong fever in southcentral Uganda, 1996-1997: entomologic studies in Bbaale village, Rakai District

Entomologic studies were conducted between January 27 and February 2, 1997, in Bbaale village in southcentral Uganda during an o'nyong-nyong (ONN) virus epidemic, which began in mid 1996 and continued into 1997. The objectives were to confirm the role of anophelines in ONN virus transmission and to examine other mosquito species as epidemic vectors of ONN virus. Of 10,050 mosquitoes collected using light traps and pyrethrum knockdown sprays, Anopheles (Cellia) funestus Giles was presumed to be the principal vector because it was the most abundant mosquito species from which a strain of ONN virus was isolated. This virus was isolated for the first time from a culicine species, Mansonia (Mansonioides) uniformis Theobald. Bwamba virus and Nyando virus were also isolated from An. funestus.

[Arbovirus infections. From virus, mosquitoes, animals and humans]

Arboviruses occur throughout the world in plants and animals: reptiles, birds and mammals including man. These relatively recent RNA-containing viruses have great evolutionary potential and are a major cause of epidemics. Arboviruses exhibit a dual life cycle involving continual transmission to and from the vertebrate host and arthropod vector which ingests or inoculates the agent during blood meals. Agents belong to many different viral families and represent an important source of emerging diseases. Because of the mode of transmission is vectorial, spread can enhanced by man-made changes in the ecosystem. This risk is often underestimated. The population explosion provides a great opportunity for the progression of these arboviruses.

Isolation and experimental oral transmission in pigs of a porcine reproductive and respiratory syndrome virus isolate

A virus inducing a cytopathic effect on porcine alveolar macrophages was isolated from the lungs of a pig with respiratory problems and lesions of proliferative and necrotizing pneumonia. The isolate was found to react with porcine reproductive and respiratory syndrome virus (PRRSV) monoclonal antibodies by indirect immunofluorescence and was designated LHVA-93-3. The virus could also be propagated on the MARC-145 cell line. The LHVA-93-3 macrophage-passaged isolate was inoculated orally or intranasally in four-week-old specific pathogen-free pigs. Histologically, focal to multifocal lesions of proliferative, necrotizing and interstitial pneumonia could be observed in the lungs of pigs inoculated orally or intranasally, 6 and 10 days post-inoculation. Virus could be reisolated from essentially the same tissues including serum following both routes of infection. The distribution of PRRSV antigens in fixed tissues as determined by immunogold silver staining (IGSS) was similar in orally or intranasally inoculated pigs. The results of this experimental transmission study indicate that pigs may become infected by PRRSV following oral as well as intranasal exposure.

Inefficient mechanical transmission of Langat (tick-borne encephalitis virus complex) virus by blood-feeding mites (Acari) to laboratory mice

One day after feeding on a viremic mouse, tropical rat mites, Ornithonyssus bacoti (Hirst), transmitted Langat (tick-borne encephalitis virus complex) virus to a naive suckling mouse in one of four trials. However, no transmissions to naive mice by O. bacoti were recorded either immediately after the viremic blood meal (0/4 trials) or on days 4-18 (0/20 trials). After feeding on a viremic mouse, chicken mites, Dermanyssus gallinae (De Geer), failed to transmit Langat virus to naive suckling mice in any trials (0/24). Although virus failed to replicate in either species of mite, it was detectable in 20% (2/10) of O. bacoti individuals 1 d after a viremic blood meal, but only immediately after the viremic blood meal in 20% (2/10) of D. gallinae mites. Neither mite appears to be an efficient vector of Langat virus.

Transovarial transmission of chikungunya virus by Aedes albopictus mosquitoes ingesting microfilariae of Dirofilaria immitis under laboratory conditions

Female Aedes albopictus mosquitoes of the Miki strain were experimentally fed on defibrinated sheep blood containing 5 x 10(7) PFU of chikungunya virus and 20,000 microfilariae of Dirofilaria immitis per milliliter. Fully engorged mosquitoes transmitted the virus to a small percentage of the F1 progeny, but females of the F1 generation did not transmit the virus to the F2 progeny. The control mosquitoes that ingested the virus without microfilariae did not transmit the virus to their eggs, larvae, or pupae in the F1 or F2 generations. These results showed that A. albopictus of this strain that concurrently ingested the virus and microfilariae transmitted the virus by the transovarial route under experimental conditions.

An outbreak of epidemic polyarthritis (Ross River virus disease) in the Northern Territory during the 1990-1991 wet season

OBJECTIVE

To describe the epidemiology of a large outbreak of epidemic polyarthritis in the Northern Territory during the wet season of 1990-1991.

DESIGN, SETTING AND PARTICIPANTS

Arbovirus cases notified to the Northern Territory Department of Health and Community Services by general practitioners and local laboratories between 1 July 1990 and 30 June 1991.

MAIN OUTCOME MEASURES

Date and place of infection, age, sex and symptoms.

RESULTS

Doctors in the Northern Territory notified 368 cases; another 14 were infected interstate. The epidemic started in September, peaked in January and tailed off in April. The highest attack rates occurred in the rural areas of Jabiru, Litchfield Shire and Katherine. Those most affected were 30-34 year olds. Children, the elderly and Aboriginal people were under-represented.

CONCLUSIONS

Epidemic polyarthritis is a wet season problem in the Northern Territory, affecting the rural towns and districts more than the cities. Pre-planned mosquito control measures (effective water drainage and larval control) limited the extent of the 1990-1991 epidemic in Darwin City and Palmerston. The low attack rate in children reflects asymptomatic and less clinically severe infections. The under-representation of Aboriginal people may be the result of infection occurring earlier in life. A related cross-sectional seroprevalence survey has shown that rural Aboriginal people across all age groups have a significantly higher seropositive rate than urban non-Aboriginal residents.

Experimental infection of Aedes aegypti (Diptera: Culicidae) by the oral route with Sindbis virus

The infectivity, dissemination, and transmissibility of wild-type Sindbis (SIN) virus were studied in Aedes aegypti (L). There was an initial decline in the viral titer of whole mosquitoes for 3 d after ingestion of virus, followed by a gradual increase to a maximal level by day 6. Immunoperoxidase staining of Ae. aegypti for viral antigen showed infection of midgut epithelial cells on day 1, of the fat body by day 3, and of the brain by day 4. By day 5, there was infection of the foregut, hindgut, Malpighian tubules, ovariole sheaths, Johnston's organ, thoracic ganglia, ventral nerve cord, and salivary glands. Viral antigen was not detected in the flight muscles and was found only in ovariole sheaths of the ovaries; germinal tissue was not infected. The transmission rate from SIN-infected Ae. aegypti to neonatal mice was 40%. A comparison of Ae. aegypti infected with SIN and with a neuroadapted strain of Sindbis virus (NSIN), which is more neurovirulent than SIN to mice after intracerebral inoculation, did not reveal significant differences in infectivity, dissemination, or transmissibility. The important differences between SIN and NSIN in a mouse model were not reflected in the infection of Ae. aegypti by the oral route.

Insect-transmitted vertebrate viruses: flaviviridae

The Flaviviridae include almost 70 viruses, nearly half of which have been associated with human disease. These viruses are among the most important arthropod-borne viruses worldwide and include dengue, yellow fever, and Japanese encephalitis viruses. Morbidity and mortality caused by these viruses vary, but collectively they account for millions of encephalitis, hemorrhagic fever, arthralgia, rash, and fever cases per year. Most of the members of this family are transmitted between vertebrate hosts by arthropod vectors, most commonly mosquitoes or ticks. Transmission cycles can be simple or complex depending on the hosts, vectors, the virus, and the environmental factors affecting both hosts and viruses. Replication of virus in invertebrate hosts does not seem to result in any significant pathology, which suggests a close evolutionary relationship between virus and vector. Another example of this relationship is the ability of these viruses to grow in invertebrate cell culture, where replication usually results in a steady state, persistent infection, often without cytopathic effect. Yields of virus from insect cell culture vary but are generally similar to yields in vertebrate cells. Replication kinetics are comparable between insect and vertebrate cell lines, despite differences in incubation temperature. Both vertebrate and insect cell culture systems continue to play a significant role in flavivirus isolation and the diagnosis of disease caused by these agents. Additionally, these culture systems permit the study of flavivirus attachment, penetration, replication, and release from cells and have been instrumental in the production and characterization of live-attenuated vaccines. Both vertebrate and insect cell culture systems will continue to play a significant role in basic and applied flavivirus research in the future.

Isolation of Getah virus from mosquitos collected on Hainan Island, China, and results of a serosurvey

An isolate of Getah virus was obtained from Culex mosquitos collected in Mao'an Village, Baoting County, Hainan Province, China, in 1964. The virus (strain M-1) replicated in laboratory-bred Aedes aegypti and Cx. fatigans (= quinquefasciatus), and was transmitted by laboratory-bred Ae. albopictus to healthy newborn albino mice. Skeletal muscles of newborn albino mice experimentally infected with the virus showed degeneration, atrophy, necrosis, and inflammatory changes of muscle fibers. Antibody prevalence in humans and animals ranged from 10.3% by neutralization tests of samples from healthy people in 1979 to 26.4% by CF tests of samples from people with febrile illnesses in 1982. The high prevalence of antibody in pigs, horses, and goats (17.6% to 37.5%) indicated that infection with Getah or a closely related virus is relatively common in domestic animals.

Limited potential for mosquito transmission of a live, attenuated chikungunya virus vaccine

Studies were conducted to determine the potential for transmission of a live, attenuated chikungunya (CHIK) virus vaccine by orally exposed or virus-inoculated mosquitoes. The vaccine (CHIK 181/clone 25) replicated in and was transmitted by female Aedes albopictus and Ae. aegypti after intrathoracic inoculation. Mosquitoes also became infected with the vaccine after ingesting virus from either a blood-soaked cotton pledget or a viremic monkey. However, because of the low viremias produced in inoculated humans, it is unlikely that mosquitoes would become infected by feeding on a person inoculated with the live, attenuated CHIK vaccine. Although the vaccine was transmitted by mosquitoes after intrathoracic inoculation, there was no evidence of reversion to a virulent phenotype.

Intranasal infection of Getah virus in experimental horses

Aerosol transmission in equine Getah virus (GV) infection was examined by intranasal inoculation with 10(3.0) to 10(7.0) TCID50 of the MI-110 strain in 7 experimental horses. The establishment of intranasal infection of GV was confirmed in all these horses by detecting serum neutralizing antibody against the MI-110 strain. Horses inoculated with more than 10(4.0) TCID50 of the virus manifested mild pyrexia, eruptions, serous nasal discharge, lymphopenia or monocytosis. Viremia ranging from 10(1.0) to 10(3.5) TCID50/0.2 ml occurred in horses inoculated with 10(5.0) TCID50, or more. Virus recovery from the nasal cavity was observed only in horses inoculated with 10(7.0) TCID50, and the viral titers recorded were 10(3.0) TICD50/ml or less. From these results, it is assumed that GV disseminated from the nasal cavity of naturally infected horses, except for intranasal infection with a lot of the virus, is probably very low in titer. So it seems to be rare that GV in natural cycles is spread from horse to horse by aerosol transmission.

Effect of environmental temperature on the vector competence of Culex pipiens and Cx. torrentium for Ockelbo virus

The effects of environmental temperature on Ockelbo virus infection, dissemination, and transmission were studied in Culex torrentium and in the Uppsala and the El Gabal strains of Cx. pipiens. Temperatures tested included 10 degrees C, 17 degrees C, 24 degrees C, and a cyclic (10-24 degrees C, mean = 17 degrees C) regimen designed to mimic typical hourly temperatures in an area endemic for Ockelbo disease in Sweden during the transmission season. The vector competence of both the Uppsala and the El Gabal strains of Cx. pipiens was directly related to environmental temperature. Mosquitoes held at 10 degrees C had a reduced ability to transmit virus as compared to those held at 17 degrees C, 24 degrees C, or in the cyclic temperature regimen. A rapid increase in dissemination rates was observed in mosquitoes exposed to a shift in temperature from 10 degrees C to 24 degrees C. After 4 days at 24 degrees C, these mosquitoes had vector competence similar to those held at 24 degrees C for the entire incubation time. In contrast, virus dissemination in Cx. torrentium was rapid at all temperatures tested and appeared unaffected by environmental temperature; infection and dissemination rates were consistently higher in Cx. torrentium than in either the Uppsala or the El Gabal strains of Cx. pipiens. It seems the transmission of Ockelbo virus by Cx. pipiens might be interrupted by a prolonged period of cold weather, while transmission by Cx. torrentium would continue.

[Transmission cycles of the West-Nile virus in Madagascar, Indian Ocean]

Virological, serological and entomological research conducted in Madagascar since 1975, reveal the wide-spread presence of West-Nile virus on the island. This arbovirus has been isolated from humans, parrots and egrets. Vectors belong to the genus Culex (e.g. Cx. decens, Cx. quinquefasciatus, Cx. antennatus, Cx. univittatus), however the virus has also been isolated from Aedes and Anopheles. Serological tests carried out on over 1,600 human and almost 1,000 animal sera, revealed that human beings could be infected throughout the island. Other potential vertebrate hosts, apart from birds, are oxen and bats. Insectivores, rodents and lemurs are probably involved in the transmission cycles only to a very small extent.

Ecologic studies of mosquitoes and birds as hosts of Ockelbo virus in Sweden and isolation of Inkoo and Batai viruses from mosquitoes

Field studies were conducted in central Sweden from 1983 through 1985 to obtain information on the etiologic agent of Ockelbo disease, described in Sweden in the 1960s and probably identical to Pogosta disease in Finland and to Karelian fever in the western USSR. Mosquitoes (63,644) collected during this 3 year period yielded 21 virus strains. Ockelbo virus isolations were from Culiseta morsitans (5 strains), Culex pipiens and/or Cx. torrentium (6 strains), and Aedes cinereus (3 strains). Inkoo (6 strains) and Batai (1 strain) viruses were recovered from Ae. communis. Blood samples collected March-May from migrating birds on the southeast and est coast of Sweden and in July and August from resident birds in east-central Sweden were tested for neutralizing antibody to Ockelbo virus. Antibody was not detected in 328 birds sampled during spring migrations. Two of 58 (3.4%) birds bled in July and 8 of 78 birds (10%) bled in August had antibody to Ockelbo virus. Ockelbo virus circulates in a mosquito-bird-mosquito cycle, with Cs. morsitans and Cx. pipiens and/or Cx. torrentium as enzootic vectors. Antibody was detected in passerine birds. Other classes of birds or other vertebrates were not sampled. Aedes cinereus may serve primarily to transmit virus to people. The role of other mosquito species as vectors for people is unknown.

Transplacental infection in mice inoculated with Getah virus

Transplacental transmission was demonstrated in pregnant mice subcutaneously inoculated with Getah virus. Viremia was shown in the infected dams, and high-titered virus was detected in the placenta and later in the fetus, suggesting virus invasion of the fetus through hematogenous infection of the placenta. High-titered virus was shown in the fetal brain and muscle and in the brain of the young dying soon after birth. Intrauterine infection resulted in a reduction of the litter size, number of young born alive and survival rate to 1 week of age. These results were further corroborated by necropsy performed several days after virus inoculation. The stage of gestation at the time of virus inoculation greatly influenced these results. Dams inoculated at 12 days of gestation delivered all dead babies, whereas virus inoculation at 5 days of gestation had no effect on the number of young born alive. The dams inoculated at 8 days of gestation had reduced litter sizes and those inoculated at 16 days of gestation produced slightly fewer live babies. Gestational stage at the time of virus inoculation also influenced viral growth in fetuses and placentas. The infection rate was low in dams inoculated at 5 days of gestation, high in dams inoculated at 8 or 16 days of gestation and 100% in dams inoculated at 12 days of gestation. High-titered virus was shown in placentas and fetuses of the dams inoculated at 8, 12 or 16 days of gestation. These results suggest that Getah virus may readily cross the placental barrier through hematogenous infection of the placenta in mice.

Experimental infection and horizontal transmission of Modoc virus in deer mice (Peromyscus maniculatus)

Deer mice (Peromyscus maniculatus) were inoculated with a sublethal dose of a field strain of Modoc virus to determine patterns of viral persistence, shedding, and transmission. Blood, serum, urine, fecal, and oral swab samples were collected at selected intervals until 63 days postinoculation (PI) after which lung, liver, spleen, kidney, and salivary glands were explanted. Viral assays were conducted by intracranial inoculations of suckling mice and antibody titers were determined by the micro-complement-fixation test. Viremias lasted for up to 4 days PI. Antibody titers were present by day 8 PI, peaked at day 13-20 PI, and persisted until day 63 PI. There was no evidence of viral shedding in urine, fecal, or oral swab samples. Virus was detected in explanted lungs only. In a separate experiment, deer mice were inoculated with virus and lungs were removed from five mice per wk for 10 wk. Indirect fluorescent antibody (IFA) techniques were used to determine the location of virus in lung tissue and to examine fixed tissue for lesions. IFA showed virus in lung parenchymal cells beginning 42 days PI and persisting at least 70 days PI. No histopathologic changes were seen. Horizontal transmission of the virus was studied by placing uninoculated mice with inoculated mice for 42 days and determining if the test animals developed antibodies or had virus in their lungs. Fifty-percent of the uninoculated mice developed antibody. One of these animals had virus in its lungs. Therefore, Modoc virus may be transmitted by direct contact.

An investigation of the potential of Aedes camptorhynchus (Thom.) as a vector of Ross River virus

Aedes camptorhynchus (Thom.) collected on the mid-south coast of New South Wales during the winter of 1982 were highly susceptible to infection (ID50 = 10(2.4) VERO pfu/mosquito) when fed on rat tail skins containing blood and serial dilutions of the T48 strain of Ross River (RR) virus. After 2 d, when no virus was detectable, rapid proliferation allowed transmission from 5 d post ingestion. A maximum transmission rate occurred 9 d post-feeding when 4 of 4 infected mosquitoes transmitted virus. The susceptibility of Ae camptorhynchus to RR virus infection was compared with that of a laboratory colony of Ae aegypti (L.) (ID50 = 10(3.8) VERO pfu/mosquito).

Experimental assessment of the vector competence of Culex (Culex) neavei Theobald with West Nile and Sindbis viruses in South Africa

Culex univittatus is the maintenance vector of West Nile (WN) and Sindbis (SIN) viruses on the inland plateau of South Africa and also transfers infection to man. Both viruses have frequently been isolated from this mosquito and human immunity is high on the plateau. In the Natal lowlands where Cx univittatus is replaced by Cx neavei, apparently the main vector there, immune rates are low and few isolations have been made from this species. Vector competence experiments were carried out with Cx neavei to compare with those previously done with Cx univittatus. It was thought that a difference in such competence might explain the contrasting epidemiological situation in the two regions. After infective feeds on chicks with high viraemias of each virus, Cx neavei showed a high transmission rate. However, the 50% infection thresholds determined by exposing birds with differing viraemias to mosquitoes of this species, were 4.4 logs per ml (WN) and 5.8 logs (SIN). These thresholds are both higher than those for Cx univittatus. It is concluded that Cx neavei is a poorer vector of both viruses which accounts for the lower viral activity, including the lower incidence of human infection, in the Natal lowlands.

Experimental infection with Murray Valley encephalitis virus: galahs, sulphur-crested cockatoos, corellas, black ducks and wild mice

Orally infected Culex annulirostris or intravenous injections were used to infect 10 Galahs, 15 Sulphur-crested cockatoos, 12 Corellas, 4 Black ducks and 10 wild mice with Murray Valley encephalitis (MVE) virus. The birds produced moderate viraemias of titres of log 10(2.0)-10(6.0) suckling mouse intracerebral (SMIC) LD50/ml for durations of 1-9 d. However, the wild mice developed low grade viraemia for 1-4 d. Recipient Cx annulirostris feeding on viraemic birds sustained infection rates of 0-10% but, by extrapolation from vector competence data, some titres were capable of infecting greater than or equal to 50% of recipient Cx annulirostris. HI antibody responses were moderately high for most birds (reciprocal titres of 1 in 640-2560) by 14 d and usually persisted at detectable levels for 140 d and, in some individuals, for 6 months. However, 6 of 12 Corellas and 1 of 4 Black ducks became seronegative after 42 and 56 d, respectively. As with previously tested animals, any IgM response was absent 4 wks post-infection. The viraemias attained indicate that these bird species may be considered as potential hosts of MVE, but the limited viraemic response of wild mice suggests that they might play only a minor role.

Togaviridae

The family Togaviridae comprises four genera: Alphavirus (with 26 species), Rubivirus (one species), Pestivirus (three species), and Arterivirus (one species). The main characteristics of the member viruses are: (i) the virus particles are spherical, 50-70 nm in diameter, including an envelope with surface projections that incorporate two or three polypeptides, usually glycosylated; (ii) the nucleocapsid comprises a core protein and a single strand of positive-sense RNA, molecular weight about 4 X 10(6); where characterized, the RNA has an m7G 'cap' at the 5' end and is polyadenylated at the 3' end; (iii) maturation occurs by budding of spherical nucleocapsids 30-35 nm in diameter, with proven or presumed icosahedral symmetry, through cytoplasmic membranes. Where characterized, translation of structural proteins occurs on subgenomic messenger RNA(s); these appear to represent the 3' end of the genome. Nearly all alphavirus species are transmitted by mosquitoes. Transmission also occurs transovarially (Alphavirus) or transplacentally (Rubivirus and Pestivirus). Members of a genus are serologically related, but are not related to members of other genera.

Arbovirus infections in humans in New South Wales. Seroepidemiology of the alphavirus group of togaviruses

A seroepidemiological study of the prevalence of antibodies to alphaviruses (Ross River [RRV], Sindbis [SIN] and Getah [GET] viruses) was carried out on 16 842 specimens of sera collected during 1981 and 1982 from individuals of all ages living in all health regions of New South Wales. Prevalence rates were found to be low in the major coastal cities of Sydney, Newcastle and Wollongong, and in the whole of the Tablelands. In coastal populations, the rates were highest in the far and mid North Coast zones (about 16%), and slightly lower in the south. The ratio of RRV:SIN antibodies was 62:1 in coastal populations. Antibody prevalence rates varied from 10%-20% in the Western Slopes, rising to 30%-40% in centres in the Western Plains; the RRV:SIN antibody ratio here was 11:1. The unexpectedly high infection rates, especially in the Western Plains, suggest that Ross River virus, in particular, may constitute a public health problem of hitherto unsuspected significance in New South Wales.

Association of a Sindbis-like virus with Ockelbo disease in Sweden

An alphavirus isolated from Culiseta mosquitoes has been associated with Ockelbo disease, an exanthema arthralgia syndrome occurring in Sweden. The isolate was made from mosquitoes collected in Edsbyn (central Sweden), an area with considerable Ockelbo disease morbidity. This isolate proved to be indistinguishable from Sindbis virus by complement-fixation and hemagglutination-inhibition tests, and was antigenically related to Sindbis in plaque reduction neutralization tests. Patients with Ockelbo disease developed neutralizing antibodies to the virus in their convalescent sera, suggesting that it is the etiologic agent of the disease.

The vector competence of Australian Culex annulirostris with Murray Valley encephalitis and Kunjin viruses

Australian populations of Culex annulirostris Skuse from 10 localities, Brisbane, Townsville, Cairns, Charleville, Kowanyama (Queensland), Darwin (Northern Territory), Mildura (Victoria) and Port Hedland, Karratha and Mt. Tom Price (Western Australia) showed considerable heterogeneity in response to oral infection with low passage level Murray Valley encephalitis (MVE) and Kunjin (KUN) viruses. However, there was no consistent pattern of either high or low vector efficiency for populations collected from different regions. Overall, Cx annulirostris was a more competent vector of MVE virus than KUN virus, both in oral susceptibility (suckling mouse intracerebral ID50 of 10(1.7)-10(3.9) compared to 10(2.7)-10(4.8)/mosquito, respectively) and in ability to transmit virus to chicks after 10 d extrinsic incubation (50-89% compared to 0-55%). High or low susceptibility with one virus did not correlate with susceptibility for the other. Although these date were probably subject to some seasonal variation, they generally support the view that Cx annulirostris is the principal vector of MVE in Australia. Poor to moderate susceptibility with KUN suggests that (1) it is only a minor vector in some localities, (2) vertebrate hosts have high viraemias or (3) the strain used may have been atypical.

A field study on the effects of Fort Morgan virus, an arbovirus transmitted by swallow bugs, on the reproductive success of cliff swallows and symbiotic house sparrows in Morgan County, Colorado, 1976

We studied the transmission of Fort Morgan (FM) virus within colonies of nesting Cliff Swallows and House Sparrows under three bridges in Morgan County, Colorado during 1976. Nests were examined, and blood or brain specimens were collected from nestlings once or twice a week. Flying birds and small mammals were also studied. We analyzed nesting activity, virus isolations from nestlings of both species, fledging success, multiple infections within a brood of nestlings, infection frequency by age of nestlings, nestling mortality, and infection frequencies by avian species and bridge site. Fort Morgan virus was isolated from 7% (80/1, 156) of the blood and brain samples collected from nestlings. The duration of viremia for nestling House Sparrows was at least 3-4 days based on virus isolation from sequential blood samples. Viremia of nestling Cliff Swallows and House Sparrows did not reduce fledging success, nor were young nestling sparrows viremic more frequently than older nestling sparrows. Nest destruction (by falling down) was a more important cause of nestling mortality than FM virus infection. All age groups of nestling sparrows were viremic at equal rates, but younger nestlings (less than or equal to 7 days old) were more likely than older nestlings (greater than 7 days old) to develop an encephalitic infection. Among nestling House Sparrows, FM virus infections were clustered in time and space. Nestling House Sparrows with FM virus-infected nest-mates were infected more often than conspecifics whose nest-mates were not infected. We concluded that nestling Cliff Swallows and symbiotic House Sparrows that reside in swallow nesting colonies are the principal vertebrate hosts for the maintenance and amplification of FM virus.

Getah virus in Aedes vexans nipponii and Culex tritaeniorhynchus: vector susceptibility and ability to transmit

Vector competences of Aedes (Ae.) vexans nipponii (nip.) and Culex (Cx.) tritaeniorhynchus to Getah virus were assessed by using a membrane feeding technique. The Getah virus was present at high titer in both species of mosquitoes after 21 days of extrinsic incubation at 28 degrees C. Infection rates on 21 post-feeding were 100 per cent (4/4) for Ae. vexans nip. at a virus dosage of 10(5.3) PFU/ml and 60 per cent (3/5) for Cx. tritaeniorhynchus at similar virus dosage. More than 10(3.5) PFU of virus was detected in salivary glands of both species of mosquitoes on day 21 of extrinsic incubation. Forty percent (2/5) of Ae. vexans nip. transmitted the virus into serum-agar after ingesting 10(4.3) PFU/ml of virus blood mixture. In experiments with Cx. tritaeniorhynchus ingesting 10(7.5) PFU/ml of virus blood mixture, 57 per cent (4/7) were able to transmit the virus to suckling mice and 59 per cent (10/17) transmitted the virus into serum-agar.

Clinical variations of border disease in sheep according to the source of the inoculum

Non-cytopathogenic pestivirus obtained from lambs with border disease, with or without nervous signs, was inoculated into pregnant ewes at 57 to 65 days of gestation. Live lambs born to inoculated ewes were clinically identical to the lambs from which virus was obtained, ie, either a hairy birth coat with central nervous system disturbance or a hairy birth coat without central nervous system disturbance.

Spondweni virus infection in a foreign resident of Upper Volta

Spondweni virus is a mosquito-borne flavivirus previously reported to cause human disease in Southern and West Africa. A serologically confirmed case of Spondweni virus infection in a U.S. citizen residing in Upper Volta is reported. Symptoms included fever, chills, headache, myalgia, nausea, and rash. A greyish mucoid lining was present on the posterior pharynx. The differential diagnosis included rickettsial infection, leptospirosis, typhoid fever, and numerous viral illnesses including Lassa fever. Evidence of Spondweni virus infection was also found in two other U.S. citizens residing in Gabon and Cameroon. Spondweni virus might be a cause of acute febrile illness throughout West Africa, and its presence should be considered in the differential diagnosis of febrile illness and in antibody surveys in that region.

Antigenic relationships of flaviviruses with undetermined arthropod-borne status

In contrast to most of the arthropod-borne flaviviruses, the flaviviruses with undetermined arthropod-borne status are probably disseminated only by direct contact with excreta (saliva, urine, feces, etc.); however, as yet undescribed arthropod transmission cycles may be found for some of them. Twenty-two of these flaviviruses, including prototype and recently isolated strains, were compared. Biologic properties were defined by infectivity titrations in suckling mice and Vero, LLC-MK2, and primary Pekin duck embryo cells, and antigenic relationships were defined by complement-fixation and plaque reduction neutralization tests. An antigenic classification scheme is proposed. Antigenic and biologic properties delimit two large clusters. The first, comprising a single antigenic complex, includes those which have yet to be isolated from arthropods, but are likely to be so (Israel turkey meningoencephalitis, Koutango, Negishi and Aroa viruses). The second, encompassing five antigenic complexes, is comprised of viruses which have been isolated exclusively from rodents or bats (Saboya, Carey Island, Dakar bat, Sokuluk, Bukalasa bat, Entebbe bat, Phnom Penh bat, Modoc, Sal Vieja, Jutiapa, San Perlita, Cowbone Ridge, Rio Bravo, Apoi, Tamana bat and Montana Myotis leucoencephalitis viruses) but includes three viruses (Saboya, Sokuluk and Entebbe bat viruses) which may be arthropod-borne, as indicated by replication in mosquito cells in vitro.

Vector competence of Culex pipiens quinquefasciatus for Murray Valley encephalitis, Kunjin, and Ross River viruses from Australia

Australian populations of Culex pipiens quinquefasciatus Say from Brisbane, Charleville, Cairns, Kowanyama (Queensland), Darwin (Northern Territory), Mildura (Victoria), and Port Hedland (Western Australia) proved to be either poorly susceptible or refractory to oral infection with low passage level Murray Valley encephalitis, Kunjin, and Ross River viruses. With respect to past or future epidemics of Murray Valley encephalitis or Kunjin (both known to cause clinical encephalitis in man), and of epidemic polyarthritis, this mosquito is viewed as an unlikely vector.

Three modes of transmission of Ross River virus by Aedes vigilax (Skuse)

Some 15% of the saltmarsh mosquito, Aedes (Ochlerotatus) vigilax (Skuse) mechanically transmitted the T48 strain of Ross River virus from viraemic donors to uninfected recipient suckling mice. The population tested from near Brisbane was moderately susceptible to infection (ID50=10(4X5+/-0X1) suckling mouse LD50/mosquito) when fed virus diluted in blood on a cotton pledget. Rapid proliferation of the virus in the mosquito allowed transmission from 4 days after ingestion with maximum transmission of 50-57% occurring from 10 days. From assay of Ae vigilax ovaries and of progeny reared from parentally-infected females, preliminary data suggest transovarial transmission as a potential survival method for this virus.

An outbreak of Mayaro virus disease in Belterra, Brazil. III. Entomological and ecological studies

Results in entomological and vertebrate host investigations made during dual outbreaks of Mayaro (MAY) and yellow fever (YF) viruses in Belterra, Pará, Brazil in 1978 are reported. Over 9,000 insects representing 26 species were assayed in 396 pools for the presence of arboviruses. Pools of Haemagogus janthinomys Dyar yielded the only isolates of either MAY or YF virus. The minimum field infection rate for nine isolates of MAY virus from Hg. janthinomys was 1:82, and for two isolates of YF virus was 1:368. Analysis of collection data showed Hg. janthinomys to be attracted to man as a blood source and present in all habitats sampled, although most abundant in the forest canopy. Twelve hundred bird sera and 584 mammal sera were tested by hemagglutination-inhibition (HI) tests for antibody to MAY virus. Highest MAY antibody prevalence rates were found among marmosets (Calithrix argentata, 32 positive of 119 tested, 27%). Mayaro virus was also isolated from the blood of a sylvan marmoset captured at the peak of the MAY virus outbreak. Experimental infection of marmosets with MAY virus confirmed that a substantial viremia follows infection with this virus. Marmosets were also found with HI antibody to YF virus (5/119, 4%). The results presented indicate that Hg. janthinomys was the principal vector of both MAY and YF viruses and that marmosets were the main amplifying hosts for MAY virus, and perhaps for YF virus as well.

Border disease: virus persistence, antibody response and transmission studies

Three groups of five and one group of four oestrus-synchronised sheep were inoculated with Border disease (BD) virus at 52 +/- 2 days after their first service. Transmission of virus to offspring as demonstrated by virus isolation, detection of viral antigen and, or antibody response occurred in 12 of 19 sheep and probably in four others which aborted or produced stillborn lambs. Both apparently normal and clinically affected animals excreted virus in saliva, urine and faeces, and excretion and contact transmission to sheep and pigs persisted for up to two and a half years. Most of the tissues of infected sheep contained virus titres between 10(3.5) and 10(5.5) TCID50 per g. The immune response in the lambs varied, in some it began before birth, in others a transient or low level response was observed in the first or second year, while others remained serologically negative for two and a half years.

[Arboviral diseases in South-West Pacific islands (author's transl)]

Islands of the south-west Pacific area belong to the melanesian group, excepted Niue, Tonga, Wallis and Futuna which are polynesian. Through New Guinea, there is a geographic relation to the eastern part of Australia, rich of 42 arbovirus types. Dengue and Ross River fever are the most important arboviral diseases in the region; both affect islanders after introduction of virus by travellers to localities where efficient vectors are present. Dengue types 1, 2 and 4 were isolated from man and from mosquitoes in this area. Successive outbreaks, transmitted by Aedes aegypti or by Ae. polynesiensis, resulted in thousands of cases with a few fatal hemorrhagic forms. Ross River virus, responsible of epidemic polyarthritis, evaded in 1979, from Australia to Fidji, Wallis, Futuna and New Caledonia. Suspected vectors are Aedes vigilax and Culex annulirostris. Aedes aegypti and Ae. polynesiensis are also possible carriers. Murray Valley encephalitis virus caused severe outbreaks in Australia and fatal cases in New Guinea; it is a possible invader when Culex annulirostris is abundant. Expansion of arboviral diseases is a major epidemiological problem in south-west Pacific islands. Research work on pathogeny and vectors control must be intensified in this area.

The use of Toxorhynchites mosquitoes to detect and propagate dengue and other arboviruses

Both sexes of Toxorhynchites amboinensis, an unusually large, non-blood sucking mosquito, were found to be as susceptible to infection with each of the four types of dengue virus by intrathoracic inoculation as Aedes albopictus. Tx. amboinensis infected with dengue virus could be identified easily by fluorescent antibody staining of head squashes and had advantages of size, hardiness, and safety (for females) as compared with Ae. albopictus. Tx. amboinensis also were more susceptible to infection with Japanese and St. Louis encephalitis viruses than were Vero cell cultures and appeared useful for detection and propagation of other flaviviruses and arthropod-borne viruses of other taxonomic groups.