The potential of Aedes triseriatus (Diptera: Culicidae) as an enzootic vector of West Nile virus

Vector competence of Virginia mosquitoes for Zika and Cache Valley viruses

Background

Vector-borne diseases are a major public health concern and cause significant morbidity and mortality. Zika virus (ZIKV) is the etiologic agent of a massive outbreak in the Americas that originated in Brazil in 2015 and shows a strong association with congenital ZIKV syndrome in newborns. Cache Valley virus (CVV) is a bunyavirus that causes mild to severe illness in humans and ruminants. In this study, we investigated the vector competence of Virginia mosquitoes for ZIKV and CVV to explore their abilities to contribute to potential outbreaks.

Methods

To determine vector competence, mosquitoes were fed a blood meal comprised of defibrinated sheep blood and virus. The presence of midgut or salivary gland barriers to ZIKV infection were determined by intrathoracic inoculation vs oral infection. After 14-days post-exposure, individual mosquitoes were separated into bodies, legs and wings, and saliva expectorant. Virus presence was detected by plaque assay to determine midgut infection, dissemination, and transmission rates.

Results

Transmission rates for Ae. albopictus orally infected (24%) and intrathoracically inoculated (63%) with ZIKV was similar to Ae. aegypti (48% and 71%, respectively). Transmission rates of ZIKV in Ae. japonicus were low, and showed evidence of a midgut infection barrier demonstrated by low midgut infection and dissemination rates from oral infection (3%), but increased transmission rates after intrathoracic inoculation (19%). Aedes triseriatus was unable to transmit ZIKV following oral infection or intrathoracic inoculation. CVV transmission was dose-dependent where mosquitoes fed high titer (ht) virus blood meals developed higher rates of midgut infection, dissemination, and transmission compared to low titer (lt) virus blood meals. CVV was detected in the saliva of Ae. albopictus (ht: 68%, lt: 24%), Ae. triseriatus (ht: 52%, lt: 7%), Ae. japonicus (ht: 22%, lt: 0%) and Ae. aegypti (ht: 10%; lt: 7%). Culex pipiens and Cx. restuans were not competent for ZIKV or CVV.

Conclusions

This laboratory transmission study provided further understanding of potential ZIKV and CVV transmission cycles with Aedes mosquitoes from Virginia. The ability for these mosquitoes to transmit ZIKV and CVV make them a public health concern and suggest targeted control programs by mosquito and vector abatement districts.

Vector-borne diseases

After a request from the European Commission, EFSA's Panel on Animal Health and Welfare summarised the main characteristics of 36 vector‐borne diseases (VBDs) in https://efsa.maps.arcgis.com/apps/PublicGallery/index.html?appid=dfbeac92aea944599ed1eb754aa5e6d1. The risk of introduction in the EU through movement of livestock or pets was assessed for each of the 36 VBDs individually, using a semiquantitative Method to INTegrate all relevant RISK aspects (MINTRISK model), which was further modified to a European scale into the http://www3.lei.wur.nl/mintrisk/ModelMgt.aspx. Only eight of the 36 VBD‐agents had an overall rate of introduction in the EU (being the combination of the rate of entry, vector transmission and establishment) which was estimated to be above 0.001 introductions per year. These were Crimean‐Congo haemorrhagic fever virus, bluetongue virus, West Nile virus, Schmallenberg virus, Hepatozoon canis, Leishmania infantum, Bunyamwera virus and Highlands J. virus. For these eight diseases, the annual extent of spread was assessed, assuming the implementation of available, authorised prevention and control measures in the EU. Further, the probability of overwintering was assessed, as well as the possible impact of the VBDs on public health, animal health and farm production. For the other 28 VBD‐agents for which the rate of introduction was estimated to be very low, no further assessments were made. Due to the uncertainty related to some parameters used for the risk assessment or the instable or unpredictability disease situation in some of the source regions, it is recommended to update the assessment when new information becomes available. Since this risk assessment was carried out for large regions in the EU for many VBD‐agents, it should be considered as a first screening. If a more detailed risk assessment for a specific VBD is wished for on a national or subnational level, the EFSA‐VBD‐RISK‐model is freely available for this purpose.

A rapid identification guide for larvae of the most common North American container-inhabiting Aedes species of medical importance

Mosquitoes are the single most important taxon of arthropods affecting human health globally, and container-inhabiting Aedes are important vectors of arthropod-borne viruses. Desiccation-resistant eggs of container Aedes have facilitated their invasion into new areas, primarily through transportation via the international trade in used tires. The public health threat from an introduced exotic species into a new area is imminent, and proactive measures are needed to identify significant vectors before onset of epidemic disease. In many cases, vector control is the only means to combat exotic diseases. Accurate identification of vectors is crucial to initiate aggressive control measures; however, many vector control personnel are not properly trained to identify introduced species in new geographic areas. We provide updated geographical ranges and a rapid identification guide with detailed larval photographs of the most common container-inhabiting Aedes in North America. Our key includes 5 native species (Aedes atropalpus, Ae. epactius, Ae. hendersoni, Ae. sierrensis, Ae. triseriatus) and 3 invasive species (Ae. aegypti, Ae. albopictus, Ae. japonicus).

Infection, dissemination, and transmission of a West Nile virus green fluorescent protein infectious clone by Culex pipiens quinquefasciatus mosquitoes

We report the construction and comparative characterization of a full-length West Nile virus (WNV) cDNA infectious clone (ic) that contains a green fluorescent protein (GFP) expression cassette fused within the viral open reading frame. Virus derived from WNV-GFP ic stably infected Culex pipiens quinquefasciatus mosquitoes at comparable rates to virus derived from the parental (non-GFP) ic. However, insertion of this GFP cassette resulted in a temporal delay in in vivo replication kinetics and significantly decreased dissemination to head tissue. Consistent with previous reports of WNV-infected mosquito midguts, focal GFP expression was observed at 3 days post-infection (dpi), with the majority of posterior midgut epithelial cells being positive by 7 dpi. GFP foci were observed in one pair of salivary glands (1/15) dissected 14 dpi. Mice exposed to WNV-GFP-infected mosquitoes developed viremia, and GFP was detected in lymph node homogenates. These data demonstrate the effectiveness of our strategy to generate a replication competent construct with increased reporter gene stability that may be used to study early events in infection.

Susceptibility of fox squirrels (Sciurus niger) to West Nile virus by oral exposure

Fox squirrels (Sciurus niger) (five of eight) were infected with West Nile virus (WNV) when challenged by the oral route with 10(2.3) or 10(3.4) plaque forming units (PFU). The mean maximum serum WNV titer of infected fox squirrels was 10(5.1) PFU/mL and ranged from 10(4.6) to 10(5.6) PFU/mL. These levels of viremia are infectious for several mosquito vectors of WNV. This virus was also isolated from swabs of the oral and rectal cavities, and urine swabs between day 5 and 9 postexposure (p.e.) in amounts as high as 10(2.0), 10(2.8), and 10(2) PFU, respectively. WNV RNA was detected in salivary gland and/or kidney tissue of three squirrels between day 65 and 72 p.e. in the presence of WNV neutralizing antibody, suggesting that long-term persistent infection occurs in fox squirrels. These observations justify further studies to determine if nonarthropod transmission and long-term persistent infection occur naturally in fox squirrels and contribute to trans-seasonal maintenance of WNV.

A metapopulation model to simulate West Nile virus circulation in Western Africa, Southern Europe and the Mediterranean basin

In Europe, virological and epidemiological data collected in wild birds and horses suggest that a recurrent circulation of West Nile virus (WNV) could exist in some areas. Whether this circulation is permanent (due to overwintering mechanisms) or not remains unknown. The current conception of WNV epidemiology suggests that it is not: this conception combines an enzootic WNV circulation in tropical Africa with seasonal introductions of the virus in Europe by migratory birds. The objectives of this work were to (i) model this conception of WNV global circulation; and (ii) evaluate whether the model could reproduce data and patterns observed in Europe and Africa in vectors, horses, and birds. The model was calibrated using published seroprevalence data obtained from African (Senegal) and European (Spain) wild birds, and validated using independent, published data: seroprevalence rates in migratory and resident wild birds, minimal infection rates in vectors, as well as seroprevalence and incidence rates in horses. According to this model, overwintering mechanisms are not needed to reproduce the observed data. However, the existence of such mechanisms cannot be ruled out.

Fox squirrels (Sciurus niger) develop West Nile virus viremias sufficient for infecting select mosquito species

The West Nile virus (WNV) viremia and shedding profiles of 11 adult fox squirrels (Sciurus niger) infected by needle inoculation or mosquito bite were characterized. Daily mean titers (95% confidence intervals) for all squirrels on days 1 through 6 postexposure (p.e.) were: 10(1.7 (1.32.1)), 10(4.4 (4.04.8)), 10(5.3 (5.05.6)), 10(4.4 (3.94.9)), 10(2.7 (2.03.4)), and 10(1.1 (0.52.1)) plaque-forming units (PFU)/mL. The highest WNV serum titers of individual squirrels infected by needle inoculation or mosquito bite ranged from 10(4.5) to 10(6.1) and from 10(5.1) to 10(5.3) PFU/mL, respectively. Nine (82%) squirrels, including all 4 squirrels infected by mosquito bite, had WNV serum titers > or =10(5.1) PFU/mL that persisted on average for 1.6 +/- 0.3 days. Infection and dissemination rates of Culex pipiens (L.) that fed on squirrels with serum titers of 10(4.4 +/- 0.1) PFU/mL were 56% and 13%, respectively. Both of these rates increased to over 80% when fed on squirrels with a mean WNV titer of 10(5.5 +/- 0.1) PFU/mL. Infection and dissemination also occurred in Aedes triseriatus (Say) but at a much lower rate. WNV was isolated from the oral and rectal cavities of all squirrels and from urine that was opportunistically collected from 5 squirrels. The largest quantity of WNV recovered from swabs of the oral cavity and urine was 10(3.1) PFU. The longest periods after exposure that WNV was isolated from the oral cavity and urine from a squirrel were 22 and 17 days p.e., respectively. WNV RNA was also detected in kidney tissue in 1 squirrel 29 days p.e., suggesting that fox squirrels can be persistently infected. Collectively these observations provide further evidence that squirrels can contribute to the natural history and epidemiology of WNV, especially in peridomestic environments.

West Nile virus viremia in eastern chipmunks (Tamias striatus) sufficient for infecting different mosquitoes

Chipmunks might play a role in enzootic WNV cycles and be an amplifying host for mosquitoes that infect humans.

In eastern chipmunks (Tamias striatus) inoculated intramuscularly with 10^1.5 to 10^5.7 PFU of West Nile virus (WNV), serum titers developed sufficient to infect Aedes triseriatus (Say), Ae. vexans (Meigen), and Culex pipiens (L.). Mean titers (95% confidence interval) of 8 chipmunks were 10^{3.9(3.3–4.5)}, 10^{6.7(6.4–7.0)}, and 10^{5.8(4.1–7.5)} PFU/mL on days 1–3 postinoculation (p.i.) and 10^5.8 PFU/mL in 1 chipmunk on day 4 p.i. Mean estimated days that WNV titers were >10^4.8 and >10^5.6 were 1.7 (1.1–2.3) and 1.4 (1.0–1.6). The longest period of viremia >10^4.8 PFU/mL was 3–4 days. WNV antigen was detected in the small intestine of 2 chipmunks and the kidneys of 4 chipmunks by immunohistochemistry. WNV also was detected in urine, saliva, and feces of some chipmunks. These data suggest chipmunks might play a role in enzootic WNV cycles and be an amplifying host for mosquitoes that could infect humans.