Collection of free-living nymphs and adults of Amblyomma hebraeum (Acari: Ixodidae) with pheromone/carbon dioxide traps at 5 different ecological sites in heartwater endemic regions of South Africa

The capture of free-living adults and nymphs of Amblyomma hebraeum, the main vector of heartwater in domestic and wild ruminants in South Africa, by means of attraction-aggregation-attachment-pheromone/carbon dioxide traps at five endemic localities in South Africa is described. Although the traps were used successfully at each of the localities, no determination of their efficiency in relation to the actual abundance of ticks at a particular site was made. This study confirmed that the traps could be used in a variety of ecological areas to locate populations of free-living adult A. hebraeum.

Amino acid and protein depletion in medium of cell cultures infected with Cowdria ruminantium

Cowdria ruminantium (Rickettsiales) causes heartwater in ruminants of Africa, and some islands off Africa and in the Caribbean Sea. The in vitro culture method for the organism devised in 1985, which provided for the first time a means for production of adequate quantities of live organisms and their products, is erratic and requires improvement. We studied depletion of amino acids (AAs) and major proteins in culture medium taken daily from infected and uninfected ovine and bovine vascular endothelial cell cultures. AAs of these samples were analyzed by Pico Tag reversed phase HPLC precolumn derivatization, and major proteins determined by capillary electrophoresis using a 57 cm x 75 microns fused silica tube at high pH. In both ovine and bovine cell cultures, significant depletion of arginine and glutamine occurred over a 5-day observation period regardless of whether they were infected or uninfected. This indicates that supplementation of nutrient media with these AAs might improve conditions for growth of the organism. Both AAs are essential for survival of cultured cells, and probably for the rickettsia (although the metabolism of C. ruminantium is poorly understood). Concentrations of several AAs increased in infected cultures, implying de novo synthesis and/or proteolysis on the part of the organism. In fact, several protein fractions did decrease in culture medium throughout the course of infection, while increasing or remaining unchanged in uninfected control cultures. Proteolytic activity by C. ruminantium may be essential for nitrogen metabolism by the organism. It is suggested that studies such as these will facilitate the development of a specific medium for optimal in vitro growth of the heartwater organism, and may also lead to an understanding of the metabolic stratagem of C. ruminantium. This knowledge, in turn, could reveal the mechanism for pathogenesis of heartwater, with implications for control.

Heartwater in sheep and goats: a review

Heartwater (cowdriosis) is an important, often fatal, tick-borne disease of domestic and wild ruminants in sub-Saharan Africa and some Indian Ocean and Caribbean islands. The causal agent, Cowdria ruminantium (Cowdry 1925), is a rickettsia closely related to members of the genus Ehrlichia, and is probably a part of a complex of genomic species. Imported breeds of sheep and goats (especially Angoras) are highly susceptible, but indigenous populations of endemic areas may be resistant to infection. Very young stock (less than 9 d old) possess a natural resistance that is unrelated to the immune status of the dams. Symptoms of heartwater vary, but usually begin with fever and may involve neurological signs and respiratory distress. Clinical diagnosis is based on symptoms, history of tick-exposure and post-mortem findings, and is confirmed by demonstration of characteristic rickettsial organisms in vascular endothelial cells. Laboratory diagnosis is retrospective and includes fluorescent antibody and enzyme-linked immunosorbent assays. Serological tests are compromised by non-specific reactions with certain Ehrlichia spp. DNA and oligonucleotide probes have been developed, but are thus far unavailable in many countries affected by heartwater. Treatment with tetracyclines is effective if begun in the early stages of infection. Control is based on a knowledge of the disease cycle in nature, and is achieved through judicious tick control, vaccination or both. A virulent, blood-based vaccine is available. Existence of a carrier state in recovered animals, including wild ruminants, complicates control efforts, and eradication is feasible only in circumscribed foci. Problem areas in fundamental and applied research on heartwater, as it affects sheep and goats, are discussed.

Current status of in vitro cultivation of Cowdria ruminantium

Heartwater caused by Cowdria ruminantium infection is the most important tick-borne disease of ruminants in southern Africa. The in vitro culture system for this rickettsia, developed less than a decade ago, is responsible for the great majority of research accomplishments currently being reported in the areas of epidemiology, diagnosis and control of heartwater. Despite this progress, cultivation of C. ruminantium remains more of an art than a science, inasmuch as significant discrepancies exist, both among and within laboratories, in the ability to successfully and repeatedly produce cultured organisms. The current status of the in vitro system and its contributions are reviewed, and ongoing research in these areas by the Onderstepoort Veterinary Laboratory and collaborating institutions is discussed.

Spiroplasma ixodetis sp. nov., a new species from Ixodes pacificus ticks collected in Oregon

Eight strains of mollicutes were isolated from pooled suspensions prepared from western black-legged ticks (Ixodes pacificus) collected in Oregon. Morphologic examination by electron and dark-field microscopic techniques showed that each strain consisted of a mixture of motile, tightly coiled helical cells, small coccoid cells with diameters ranging from 300 to 500 nm, and pleomorphic, straight or branched filamentous forms. All cellular forms were surrounded by a single cytoplasmic membrane, and there was no evidence of a cell wall. The organisms were filterable and fastidious in their growth requirements. The optimum temperature for growth was 30 degrees C, but multiplication occurred at temperatures ranging from 23 to 32 degrees C. The strains catabolized glucose but did not hydrolyze arginine or urea. The genome size of strain Y32T (T = type strain) was 2,220 kbp, and the DNA base composition (guanine-plus-cytosine content) of this organism was 25 +/- 1 mol%. The eight isolates were serologically related to each other but were not related to 37 other type or representative strains belonging to the genus Spiroplasma. Strain Y32 (= ATCC 33835) is the type strain of Spiroplasma ixodetis sp. nov.

Pheromonal composition of two species of African Amblyomma ticks: similarities, differences and possible species specific components

Two species of bont ticks, Amblyomma hebraeum and Amblyomma variegatum, common to Africa were studied to compare types and quantities of compounds known or believed to serve as components of the attraction-aggregation-attachment pheromone (AAAP). A complex of attraction, aggregation and attachment stimulating pheromone components are used by these ticks to detect hosts, mates and, perhaps, minimize interspecific breeding. Solvent extraction of pheromone emitting ticks followed by gas chromatography and mass spectrometry revealed little qualitative difference in the composition of the AAAP in these two species. However, subtle differences in the relative proportions of the pheromonal blend are noted suggesting that such differences in concentration may facilitate species-specific discrimination during aggregation and attachment. Differences in the relative abundance of benzaldehyde and methyl salicylate in the males of the two species were especially noteworthy. Possible means by which such differences in phenolic compound composition may affect the behavior of these ticks are discussed.

Detection of Cowdria ruminantium by means of a DNA probe, pCS20 in infected bont ticks, Amblyomma hebraeum, the major vector of heartwater in southern Africa

A DNA probe, pCS20, previously described for use in detection of Cowdria ruminantium infections in Amblyomma variegatum (the principal vector of heartwater) hybridized with C. ruminantium DNA in organs of laboratory-infected A. hebraeum adult ticks (the major southern African vector of heartwater). The probe hybridized with C. ruminantium DNA in 46/49 midguts from male ticks and 26/29 from females, thus indicating infection. Corresponding salivary glands were less heavily infected, but infections were more numerous in glands from males. Infection in ticks was confirmed by transmission of the disease to susceptible goats. The probe did not hybridize with DNA from uninfected ticks or with DNA from a spotted fever group rickettsia commonly associated with A. hebraeum in Zimbabwe. The C. ruminantium specific pCS20 DNA probe can be applied to determine accurately the infection rates in the two major vectors of heartwater and the risk of exposure of ruminants in endemic areas.

Development and persistence of Cowdria ruminantium specific antibodies following experimental infection of cattle, as detected by the indirect fluorescent antibody test

Different breeds of cattle were experimentally infected with Palm River, a Zimbabwean isolate, or Ball-3, a South African isolate of Cowdria ruminantium, derived from tissue culture or tick or blood stabilates. C. ruminantium specific antibody responses were detected by an indirect fluorescent antibody test (IFAT) using C. ruminantium-infected bovine aortic endothelial (BAE) cell cultures as antigen. The first detection of antibodies to C. ruminantium generally coincided with the peak of the febrile reaction and the antibodies remained detectable for a period of 8-30 weeks in the Palm River infected group and 18-30 weeks in the Ball-3 infected group. Peak reciprocal antibody titres in both groups ranged from 64 to 2048 between 3 and 6 weeks post-infection. No apparent serological differences were observed among the various C. ruminantium isolates when tested in homologous and heterologous IFATs. Post-infection sera to Anaplasma marginale, Theileria parva parva, Babesia bigemina and Rickettsia conorii did not exhibit reactivity with the C. ruminantium antigen. These results indicate the possible use of C. ruminantium-infected cultures as antigen in IFATs to detect similar C. ruminantium-specific antibody responses in the field in clinically sick, recovered and carrier animals.

A cloned DNA probe identifies Cowdria ruminantium in Amblyomma variegatum ticks

Heartwater, caused by Cowdria ruminantium and transmitted by ticks of the genus Amblyomma, is a constraint to ruminant animal production in sub-Saharan Africa. This rickettsial disease could spread from endemically infected areas of sub-Saharan Africa and certain Caribbean islands to other countries, including the United States, in which Amblyomma ticks exist. To detect C. ruminantium in tick vectors and animals, we made DNA probes from C. ruminantium DNA isolated from endothelial cell cultures. Two clones were evaluated; pCS20 from Crystal Springs (Zimbabwe) strain DNA had a 1,306-bp insert, and pCR9 from Kiswani (Kenya) strain DNA had a 754-bp insert. Both DNA probes detected 1 ng of Crystal Springs DNA; however, the pCS20 probe had a 10-fold-greater ability to discriminate between C. ruminantium DNA and DNA from other organisms. Also, the pCS20 probe did not hybridize to 400 ng (highest amount tested) of DNA from bovine cells, 3 protozoa, 3 rickettsiae, and 12 bacteria. In all experiments, C. ruminantium DNA was detected in midguts from 99 of 160 Amblyomma variegatum nymphs infected as larvae and in midguts from 38 of 80 adult ticks infected as nymphs but not in midguts from control nymphs and adults. The presence of C. ruminantium in nymphs and adults was confirmed by transmission of heartwater to goats. The DNA sequences of both probes were determined; synthetic oligonucleotides from pCS20 are recommended as DNA probes for C. ruminantium.

Improved culture conditions for Cowdria ruminantium (Rickettsiales), the agent of heartwater disease of domestic ruminants

The causal agent of heartwater disease of domestic ruminants, Cowdria ruminantium, can, with difficulty, be isolated and passaged in lines of bovine endothelial cells grown in the presence of the Glasgow modification of Eagle's minimal essential medium. However, when Leibovitz's L-15 medium supplemented with 0.45% glucose at pH 6.0-6.5 is used as maintenance medium for these cells, isolation and serial passage may routinely be achieved.

In vitro isolation of Cowdria ruminantium from plasma of infected ruminants

A new and simple technique for isolation of C. ruminantium in bovine and ovine vascular endothelial cells (aorta, pulmonary artery) is described. Unlike previous studies, no efforts were made to retard cell growth by irradiation or chemicals. Instead, heparin-derived plasma samples obtained from only those animals exhibiting prolonged or extremely high temperatures were used. In this way, C. ruminantium was isolated from 27 of 37 samples (73%) and from 22 of 26 animals (85%). A total of six Zimbabwean stocks of C. ruminantium were isolated in cell culture.

Infection rates with Cowdria ruminantium of nymphs and adults of the bont tick Amblyomma hebraeum collected in the field in Zimbabwe

Cowdria ruminantium (heatwater) infection rates of field populations of the bont tick, Amblyomma hebraeum, were determined at two locations in the southern lowveld of Zimbabwe. At Mbizi Quarantine Station, unfed adult males and females, and nymphs were collected at intervals over a 2-year period using traps. At Lemco Ranch, engorged nymphs were collected on three occasions from weaner calves and allowed to moult to adults. The unfed ticks were fed in small pools on heartwater-susceptible sheep, some of which became infected. The infection rates of the ticks were then estimated statistically. Depending on the date of collection and locality, these rates were in the range 0.0-44.9% for males, 20.0-36.1% for females and 0.0-13.4% for nymphs. Most of these rates are considerably higher than those previously believed to occur.

Pheromone-mediation of host-selection in bont ticks (Amblyomma hebraeum koch)

The bont tick, Amblyomma hebraeum, is the principal vector to southern African ruminants of heartwater (Cowdria ruminantium infection). The role of feeding male ticks, which emit an aggregation-attachment pheromone, in attracting unfed ticks to cattle was investigated. Calves infested with feeding male ticks were more attractive to unfed adult ticks than were uninfested calves. The presence of the pheromone on previously infested cattle apparently allows unfed ticks to discriminate between hosts on which these parasites have fed successfully (suitable hosts) and those on which they have not (potentially unsuitable hosts). The use of acaricides is thus unlikely to reduce bont tick populations in areas where adequate numbers of alternate (wild) hosts are present. Also, cattle so treated may lose their resistance to heartwater through lessened exposure to infected ticks.

The pathogenicity and immunologic relationship of a virulent and a tissue-culture-adapted Babesia bovis

Babesia bovis grown in tissue culture was used to inoculate 12, 2-year-old Holstein steers. All 12 developed serological evidence of infection but only six had a febrile response of greater than or equal to 40 degrees C, and only one had a demonstrable B. bovis parasitemia. An average modest drop of 19% was observed in packed cell volume (PCV) during the period of reaction. All 12 steers were subsequently challenged with virulent B. bovis: seven on day 78 post inoculation (p.i.), two on day 106 p.i., and three on day 251 p.i. No demonstrable clinical response was observed in any of the 12 steers previously exposed to the tissue-culture organism, whereas severe signs of babesiosis occurred in seven 2-year-old, non-vaccinated control steers given a comparably virulent B. bovis challenge. All seven controls showed a febrile response, B. bovis parasitemias, with an average drop of 55% in PCV and a 28% mortality.

Growth of bluetongue and epizootic hemorrhagic disease of deer viruses in poikilothermic cell systems

Continuous cell lines from the ticks Dermacentor variabilis, D. parumapertus, D. nitens, Rhipicephalus sanguineus and R. appendiculatus, the mosquitoes Aedes albopictus and Culex quinquefasciatus and the African toad Xenopus laevis were tested for their ability to replicate bluetongue (BT) and epizootic hemorrhagic disease of deer (EHD) viruses, and for their sensitivity as potential isolation systems. BT serotype 17 grew to peak titers of 10(4.5)-10(7.5) TCID50 ml-1 in all except one of the tick cell lines, EHD 2 virus attained titers similar to that of BT 17 in the mosquito and toads cells, but failed to replicate in tick cells. Only Aedes albopictus and Xenopus laevis cells were as sensitive to infection with low-passage BT 11 and EHD 2 viruses as control cultures of Vero and BHK cells. At 27 degrees C, persistent infection of Xenopus laevis cells occurred, producing low yields of BT 17 and EHD 2. When shifted to 32 degrees C, these cultures expressed virus in exponential increments. No cytopathic effect (CPE) was seen in any of the tick-virus systems, but infected mosquito and toad cells detached from the monolayer within 3-6 days after inoculation with either virus. In the toad cells, this CPE was presaged by the development of plaques within 48 h after infection. Potential applications of poikilotherm systems in orbivirus research are discussed.

Distinctive staining of colonies of Cowdria ruminantium in midguts of Amblyomma hebraeum

Mallory's phloxine-methylene blue stain was used to differentiate colonies of Cowdria ruminantium in midgut epithelial cells of nymphal Amblyomma hebraeum that had been infected as larvae. Gut tissues were collected from nymphs that had fed on a susceptible sheep and were fixed in formol-saline on the day of repletion. Paraffin sections, 3-4 micron thick, were then stained and this rendered colonies and cell nuclei densely blue against a uniformly pink background of tick tissues. Colonies were easily distinguished from nuclei by their specific morphology. This method of parasite visualization may be adapted to field-collected ticks for rapid detection of C. ruminantium or to assays of susceptibility of tick populations to various strains of the organism.

Attenuation of Babesia bovis by in vitro cultivation

A virulent strain of Babesia bovis was adapted to grow in erythrocyte culture in the presence of equine serum and in lieu of bovine serum. Four splenectomized calves inoculated with the adapted strain, 429, developed hematologic signs of infection and a low grade fever, but remained free of central nervous system (CNS) signs and recovered. All of six control animals inoculated with a virulent strain reacted severely and five showed CNS signs and died. The calves injected with the attenuated strain were solidly immune when challenged with the virulent strain at 44 or 78 days after vaccination.

Babesia bovis: increased percentage parasitized erythrocytes in cultures and assessment of growth by incorporation of [3H]hypoxanthine

Babesia bovis was established in continuous in vitro erythrocyte cultures using a modification of techniques developed previously. Using optimal conditions for maintaining continuous exponential growth, a threshold number of infected erythrocytes was obtained after which exponential growth ceased. However, at this level, individual parasite development continued resulting in a higher proportion of mature merozoite infected erythrocytes. Additionally, the percentage parasitized erythrocytes could be increased by reducing the concentration of total erythrocytes. Growth was assessed by determining the percentage parasitized erythrocytes and level of incorporation of [3H]hypoxanthine. Uninfected erythrocytes did not incorporate the radiolabeled purine while infected cultures incorporated it in direct proportion to the increase in percentage parasitized erythrocytes. Pulse labeling experiments indicated that the trophozoite form (the developmental stage prior to division that results in paired merozoites) incorporated the majority of this purine into nucleic acids.

Isolation and characterization of Midway virus: a new tick-borne virus related to Nyamanini

Midway virus, a new tick-borne virus isolated from two species of Ornithodoros (Alectorobius) ticks of the capensis group (O capensis, O denmarki), is described from Midway, Kure, and Manana islands in the Central Pacific (Hawaiian Archipelago) and from northern Honshu (Japan). Midway virion is enveloped, unusually large, acid and temperature sensitive, and its type of nucleic acid is RNA. Complement-fixation (CF) tests show a close relation of Midway to Nyamanini virus, which has been isolated from ardeid birds and Argas ticks in Africa, the Indian subcontinent, and Southeastern Asia. However, cross-box tests (CF, mouse and tissue culture neutralization, immunofluorescence) show that these two viruses are quite distinct. Midway virus is lethal for newborn Swiss mice inoculated by intracerebral, but not intraperitoneal route. It fails to kill four-week-old mice by either route. Midway virus causes cytopathic effects in BHK-21 cells and titerable plaques in Vero cells. Antibodies to it were prevalent among nestlings of Larus crassirostris (Black-tailed Gull) on Aomatsushima I., but were scarce among those of Nycticorax nycticorax (Black-crowned Night Heron) of the same island.

Helical mycoplasmas (spiroplasmas) from Ixodes ticks

A new spiroplasma isolated from Ixodes pacificus collected in Oregon was serologically and morphologically distinct from known spiroplasmas. The new spiroplasma could also be isolated in tick cell cultures. Discovery of a new fastidious mycoplasma in ticks offers opportunities to explore the possible role of these agents in human and animal diseases.

Continuous cell lines from embryonic tissues of ticks (Acari: Ixodidae)

Six new cell lines were established in continuous culture from embryonic tissues of ixodid ticks. Four were from Dermacentor variabilis and two from D. parumapertus. The cells are mostly fibroblastic and diploid. Mosquito-borne viruses (Chikungunya, O'nyong, yellow fever, and St. Louis encephalitis) as well as tick-borne ones (Langat, Powassan, Colorado tick fever, Kemerovo, and Sawgrass) replicated in certain of these cell lines, but a nonvector-borne flavivirus, Modoc, did not. An underscribed virus from D. occidentalis ticks, which could not be isolated in Vero cells or newborn mice, was readily isolated in the D. variabilis cell line.

Clinical mange of the black bear, Ursus americanus, associated with Ursicoptes americanus (Acari: Audycoptidae)

Numerous specimens of a mite, Ursicoptes americanus Fain and Johnston, 1970 (Acari: Audycoptidae), were found in skin scrapings from an aged female black bear, Ursus americanus, afflicted with a severe generalized mange. The mite had not been found previously in association with clinical dermatitis. The male, nymph and larva of Ursicoptes americanus, heretofore unknown, are described and the generic diagnosis is amplified. Discovery of the male aids in resolving the questionable taxonomic status of the family Audycoptidae.

Establishment and characterization of a diploid cell line from the tick, Dermacentor parumapertus Neumann (Acarina: Ixodidae)

Establishment of a continuous cell line (RML-14) from embryonic tissues of the tick Dermacentor parumapertus Neumann is reported. The culture medium employed consisted of a combination (2:1) of Eagle's and L-15 (Leibovitz) media supplemented with 20% fetal bovine serum, 10% tryptose phosphate broth, and 0.1% bovine plasma albumin. At the 8th passage, 99% of dividing cells had the female chromosome complement, among which more than 70% had a diploid chromosome number of 22. At the 13th passage, cell population showed approximately a 3-fold increase during the first 8 days of culture. As of December 1976, had been subcultured 40 times.

Sunday Canyon virus, a new ungrouped agent from the tick Argas (A.) cooleyi in Texas

A new arbovirus was isolated from Texas, U.S.A., populations of the Cliff Swallow parasits Argas (Argas) cooleyi Kohls and Hoogstraal, 1960. The virus, named Sunday Canyon, is serological urelated to any of 185 arbovirus strains or 20 other viral agents with which it was compared. Morphologically it resembles Bunyamwera viruses and, in common with them, is sensitive to lipid solvents and acid pH, and apparently possesses RNA. Although considerably resistant to a temperature of 41.5 degrees C, it rapidly loses infectivity when incubated at 56 degrees C. It is lethal for newborn white mice and infective for the Vero and Antheraea eucalypti cell lines. Sundays Canyon virus is the second tick-associated, Bunyamwera virus-like agent known from North America and the third virus to be reported from A. cooleyi in Texas.

Isolation of a Soldado-like virus (Hughes group) from Ornithodorus maritimus ticks in Ireland

Three isolations of a virus of the Hughes group were obtained from seabird ectoparasites, Ornithodoros (Alectorobius) maritimus, on Great Saltee Island, Ireland. The agent is closely related to Soldado virsu, originally obtained from related ticks near Trinidad, West Indies, and represents the second recorded tickborne arbovirus in Ireland.

Viral contamination of a mosquito cell line, Aedes albopictus, associated with syncytium formation

Viral contamination associated with syncytium formation in two sbulines of Singh's Aedes albopictus cell cultures was investigated. Electron microscopy of the syncytia revealed the presence of five different types of virus-like particles, which morphologically resembled the parvo-, picorna-, toga-, and orbi-, and bacterial viruses. When a virus-free subline of the A. albopictus cells (SL3) was inoculated with extracts of the syncytium-forming A. albopictus cells, the parvo-, toga-, and orbi-type viral agents were consistently observed. Among these three agents, the togavirus-type agent is most likely responsible for the syncytium induction. Serological examination of the infected cell extract indicated that at least one of three virus-like agents, presumably the togavirus-type agent, was related to Chikungunya. O'nyong-nyong, and Western equine encephalomyelitis viruses (alphaviruses of the Togaviridae), but separable from these.

Tick-borne viruses associated with seabirds in North America and related islands

About 18 tickborne viruses associated with seabirds are known from Canada, Mexico, the Antilles, and the United States, including its oceanic islands. Antigenic groups to which these viruses belong are Hughes, B, Sakhalin, Uukuniemi, Kemerovo, Nyamani and Quaranfil. Members of the last two groups are known in the Western Hemisphere only from Central Pacific Islands and may reflect intrusion of elements of the Australian Region. In the areas under discussion, antibody surveys have been minimal, but there is some indication of human exposure to agents of the Kemerovo group. Members of the Hughes, Nyamanini, and Quaranfil groups are all argasid tick viruses, largely tropical or subtropical in distribution, and associated particularly with gulls and terns (Laridae). The remainder, with the exception of Mono Lake (Kemerovo group, Chenuda Subgroup), which may be a virus of inland birds, are ixodid tick viruses associated mostly with seabirds of the polar or subpolar regions. All of these viruses are seen as acted upon by a common pattern of geographic and biotic factors that influences antigenic diversity of the virus and favors development of strong insular variants.

Isolation of spotted fever group and Wolbachia-like agents from field-collected materials by means of plaque formation in mammalian and mosquito cells

Three isolations from ticks (Dermacentor occidentalis) of a rickettsia of the spotted fever group and 5 isolations from chipmunk (Eutamias rugicaudus) blood of a Wolbachia-like agent were obtained from plaques formed in Singh's Aedes albopictus (mosquito) and Vero (African green monkey kidney) cell cultures. These organisms could not be isolated by injection of the infected ticks or blood into embryonated chicken eggs, guinea pigs, or voles (Microtus pennsylvanicus), but fluid cultures of Grace's Antheraea eucalypti (moth1 and Singh's A. albopictus cells inoculated with the bloods yielded the Wolbachia-like agent.

Plaque production by arboviruses in Singh's Aedes albopictus cells

We report plaquing tests of 124 virus strains, mostly arboviruses of 21 serological groups, in Singh's line of Aedes albopictus cells. Thirty of these plaqued: all were arboviruses of six groups and were known or presumed to be mosquito borne. Failing to plaque were 86 strains of arboviruses, mostly tick borne, two strains of insect pathogens, and six animal viruses not classified as arboviruses. Among mosquito-borne agents, plaquing ability appeared related to serological classification. California group and most A-group viruses failed to plaque, but nearly all members of B and Bunyamwera groups readily plaqued. Within serological group B, 14 of 16 mosquito-borne agents plaqued, but none of 13 tick-borne or vector-unassociated viruses did so. Some implications of these results for recognition and classification of arboviruses are discussed.

Plaque assay of rickettsiae in a mammalian cell line

Clear-cut and repeatable plaque assays were obtained for three rickettsiae of the spotted fever group (Rickettsia rickettsi, R. conori, and R. montana) in Vero cells used in a manner similar to that for arboviruses. In addition, three typhus group agents (R. typhi, R. canada, R. prowazeki) induced plaques in these cells. In preliminary tests Coxiella burneti (Nine Mile strain) failed to produce plaques. Comparable results were obtained in plastic flasks and plastic culture trays incubated in ambient air with or without addition of N-2-hydroxyethyl-piperazine-N'-2-ethanesulfinic acid buffer. Larger and more well defined R. rickettsi plaques were produced when cultures were overlaid with Leibovitz (L15) medium than with either medium 199 or Eagle medium. Phosphate-buffered saline containing bovine plasma albumin (fraction V), in contrast to brain heart infusion broth, as a diluent for preparing inocula consistently permitted development of larger and more numerous plaques with three agents: R. rickettsi, R. conori, and R. montana. When R. rickettsi and R. typhi were assayed in parallel in primary chicken embryo cultures and Vero cells, comparable results were obtained, but with R. canada results in Vero cells were superior. In contrast, R. prowazeki produced inconsistent results in Vero cells.

Colorado tick fever virus: growth in a mosquito cell line

Two strains of Colorado tick fever virus grew in Singh's Aedes albopictus cells. In one of three experiments, virus growth continued for 7 weeks.