Culex pipiens affected by joint infection of a mosquito iridescent virus and Strelkovimermis spiculatus

Dual infections with a mosquito iridescent virus (MIV) and the mermithid nematode, Strelkovimermis spiculatus were recorded in natural Culex pipiens populations around La Plata city, Argentina. S. spiculatus was detected in 82% of samples that were positive for MIV infection. Dissected larvae of Cx. pipiens with patent MIV infection presented 42% infection with S. spiculatus. Larvae of Cx. pipiens exposed to MIV and S. spiculatus under laboratory conditions produced a high joint infection rate (82.5%) while no infection was recorded on larvae exposed to virus suspension only. Field and laboratory results suggest a strong association between S. spiculatus and MIV in natural populations of Cx. pipiens, in which S. spiculatus could be a mode of entry for the virus into the mosquito hemocele.     

Adverse ffects of covert iridovirus infection on life history and demographic parameters of Aedes aegypti

Abstract Sublethal viral infections can cause changes in the body size and demography of insect vectors, with important consequences for population dynamics and the probability that individual mosquitoes will transmit disease. This study examined the effects of covert (sublethal) infection by Invertebrate iridescent virus 6 (IIV‐6) on the demography of female Aedes aegypti and the relationship between key life history parameters in covertly infected female insects compared with healthy (control) insects or non‐infected mosquitoes that had survived exposure to virus inoculum without becoming infected. Of the female mosquitoes that emerged following exposure to virus inoculum and were offered blood meals, 29% (43/150) proved positive for covert IIV‐6 infection. The net reproductive rate (R₀) of covertly infected females was 50% lower for infected females compared to control mosquitoes, whereas non‐infected exposed females had an R₀ approximately 15% lower than that of controls. Reproduction caused a significant decrease of about 13 days in mosquito longevity compared to females that did not reproduce (P < 0.001). Infected females lived 5–8 days less than non‐infected exposed females or controls, respectively (P = 0.028). Infected females and non‐infected exposed females both had significantly shorter wings than control insects (P < 0.001). There was a significant positive correlation between wing length and longevity in covertly infected female mosquitoes but not in control or non‐infected exposed mosquitoes. Longer lived females produced more eggs in all treatments. There were no significant correlations between body size and fecundity or the production of offspring. There was also no correlation between fecundity and fertility, suggesting that sperm inactivation was a more likely cause of decreased fertility in older mosquitoes than sperm depletion. We conclude that covert infection by iridescent virus is likely to reduce the vectorial capacity of this mosquito.     

Effects of an optical brightener and an abrasive on iridescent virus infection and development of Aedes aegypti

The insecticidal properties of certain entomopathogenic viruses can be greatly improved in mixtures with substances that affect the integrity of the insect peritrophic membrane, particularly optical brighteners. We aimed to determine the effect of an optical brightener, Blankophor BBH, and an abrasive compound, silicon carbide, alone and in mixtures, on the prevalence of patent and covert infection of Aedes aegypti (L.) (Diptera: Culicidae) by Invertebrate iridescent virus 6 (IIV‐6) (Iridoviridae). The prevalence of patent infection by IIV‐6 was < 1.5% in all treatments involving virus. Contrary to predictions, there were significantly fewer patent infections in virus treatments involving Blankophor with or without silicon carbide compared with controls. Covert infection of adults detected by insect bioassay was between 6.7 and 12.2%, although no significant differences were observed between treatments. Exposure to IIV‐6 alone or silicon carbide alone did not significantly increase larval mortality compared to the controls, whereas exposure to Blankophor alone, or in any combination with IIV‐6 or silicon carbide, clearly increased larval mortality. These effects did not carry‐over to the pupal stage. Adult females emerged ～1.5 days later than males. Compared to control insects, female development rate was extended by 11.4 and 12.6% in the treatments involving IIV‐6 alone and silicon carbide alone, respectively. The sex ratio at adult emergence did not differ significantly between control insects and those of other treatments. These results support the hypothesis that the gut is unlikely to represent the principal point of infection of mosquito larvae by iridescent viruses.     

Iridoviruses associated with epizootic haematopoietic necrosis (EHN) in aquaculture

Systemic infections of teleost fishes caused by iridoviruses have recently been recognized in Australia, Asia, Europe and the USA. These iridoviruses are different from those of the established genera Lymphocystivirus and Goldfish Virus 1-like Viruses of the family Iridoviridae. The agents exhibit similar physicochemical properties, are antigenically related and prove to be of high virulence to different teleost fishes in aquaculture. The first iridovirus, epizootic haematopoietic necrosis virus, responsible for an epizootic outbreak of haematopoietic necrosis in redfin perch, was reported in Australia. Some years later, similar iridovirus epizootics occurred in sheatfish and catfish in Europe. The Australian and the European isolates proved to be antigenically related and showed properties in common with frog virus 3, the type species of the genus Ranavirus of the Iridoviridae. Further iridovirus isolates from fish, amphibians and reptiles exhibited a close relationship with each other and with frog virus 3. It is important to note that the Australian amphibian iridovirus, Bohle iridovirus, was experimentally transmitted to teleost fish inducing high mortalities. The occurrence of similar viruses in different host species in the aquatic environment and their inter-species transmission emphasize the importance of health control in aquaculture.     

Persistence of Invertebrate iridescent virus 6 in soil

Soil represents an important reservoir for mostentomopathogenic viruses. Invertebrateiridescent viruses (IIVs) (Iridoviridae) arenon-occluded DNA viruses that infectagriculturally and medically important insectspecies, especially in damp or aquatichabitats. We used virus extraction and insectbioassay techniques to determine the effect ofsoil moisture and soil sterility on thepersistence of Invertebrate iridescentvirus 6 (IIV-6) in a soil over a 90 day periodin the laboratory. Loss of activity of IIV-6in dry soil (6.4% moisture, –1000 kPa matricpotential) was very rapid and was not studiedbeyond 24 h. Soil moisture did not affect therate of inactivation of virus in damp (17%moisture, –114 kPa matric potential) or wetsoil (37% moisture, –9.0 kPa matricpotential). In contrast, soil sterilizationsignificantly improved the persistence of IIV-6activity, both in damp and wet soil. Controlvirus suspensions retained 0.72–0.87% oforiginal activity after 90 days, which wassignificantly more than the activity retainedin soil. These figures represent half lives of4.9 days for IIV-6 in non-sterile soil, 6.3days in sterilized soil (data pooled formoisture treatments), and 12.9 days for thecontrol virus suspension. We conclude thatextra-host persistence in soil habitats may bean important aspect of the ecology of IIVs.     

Geographically variable selection in Ambystoma tigrinum virus (Iridoviridae) throughout the western USA

We investigated spatially variable selection in Ambystoma tigrinum virus (ATV) which causes frequent and geographically widespread epizootics of the tiger salamander, Ambystoma tigrinum. To test for evidence of selection, we sequenced several coding and noncoding regions from virus strains isolated from epizootics throughout western North America. Three of the sequenced regions contained homologues for genes putatively involved in host immune evasion and virulence: eIF‐2α, caspase activation and recruitment domain (CARD) and β‐OH‐steroid oxidoreductase. Selection analysis showed evidence of very strong purifying selection on eIF‐2α, purifying selection within certain viral clades on CARD and positive selection on β‐OH‐steroid oxidoreductase within certain clades. Analysis using multidivtime and Tajima’s relative rate tests indicate accelerated rates of evolution within clades associated with anthropogenic movement. These clades also demonstrate greater spatial variability in selection, suggesting a lack of local adaptation (i.e. locally adapted populations should exhibit little to no selection because of absent or reduced variation in fitness once a fitness optimum is reached). Increased transfer of non‐native viral strains to naïve salamander populations, in conjunction with local maladaptation as a result of local selection pressures, may explain the spread and emergence of ATV epizootics in A. tigrinum in western North America.