Assessing the risks of West Nile virus-infected mosquitoes from transatlantic aircraft: implications for disease emergence in the United Kingdom

Abstract

The number of West Nile virus (WNV)-infected mosquitoes aboard aircraft from the United States that arrive in the United Kingdom each summer was determined using a quantitative risk assessment. In the worst-case scenario, when WNV levels in mosquitoes are high (at epidemic levels) the probability of at least one WNV-infected mosquito being introduced into the United Kingdom was predicted to be 0.99. During these periods, a mean of 5.2 infected mosquitoes were estimated to be aboard flights from the United States to the United Kingdom during May to October, with 90% certainty that the exact value lies between one and ten mosquitoes. Heathrow airport was predicted to receive the majority of the infected mosquitoes (72.1%). Spatial analysis revealed the region surrounding Heathrow satisfies the criteria for potential WNV exposure as both WNV-competent mosquitoes and susceptible wild bird species are present. This region is, therefore, recommended for targeted, risk-based surveillance of WNV-infected mosquitoes in addition to an increased awareness of the risks to horses, birds and humans.

FIG. 1.

Model framework used to determine the number of West Nile virus (WNV)–infected mosquitoes introduced to the United Kingdom on aircraft from the United States. All values correspond to the vector season (May to October). Region i refers to the three regions of the United States (western, central, and eastern; see Table 1) and airport j refers to the UK destination airport (Table 3).

FIG. 2.

Relative frequency graph of the number of WNV-infected mosquitoes entering the UK aboard flights from the United States, per season (May to October). The number of iterations was 50,000 and the minimum and maximum values of N were 0 and 26, respectively.

FIG. 3.

Graph showing the mean number of WNV-infected mosquitoes that enter the United Kingdom from the United States per vector season (N) and the probability that at least one WNV-infected mosquito arrives in the United Kingdom per season (P_N>0) with (a) a percentage change in the prevalence of WNV in mosquitoes in all regions (P_i) (where 0% is the baseline (P_w=0.00205; P_c=0.00443; P_e=0.00136)); (b) a percentage change in the baseline number of mosquitoes on aircraft (S_M) (where 0% is the baseline (14 mosquitoes)); (c) a change to the sensitivity of the flashlight search method (Se), which in the baseline model is assumed to vary from 0.239 to 0.398. Results shown correspond to WNV-infection in mosquitoes at epidemic levels.

FIG. 4.

Scenario analysis for the prevalence of WNV in mosquitoes in region i (P_i). Graph showing the mean number of WNV-infected mosquitoes that enter the United Kingdom from the United States per vector season (N) and the probability that at least one WNV-infected mosquito arrives in the United Kingdom per season (P_N>0). Results shown correspond to WNV-infection in mosquitoes at epidemic levels.

FIG. 5.

Box-plot comparing the log of the mean number of WNV-infected mosquitoes that enter the United Kingdom from the United States per season (Log(N)) if alternative studies are used to estimate the number of mosquitoes present on an aircraft from the United States. Study 1: Takahashi et al. (1984); Study 2: Russell et al. (1984); Study 3: Hutchinson et al. (2005); Study 4: Le Maitre and Chadee (1983); Study 5: Haseyama et al. (2007).

FIG. 6.

Map of the United Kingdom showing airports with their share of the number of WNV-infected mosquitoes from the United States (N) estimated for the vector season (May to October), with the distribution and relative density of WNV-susceptible wild bird populations and the records of WNV-competent mosquitoes. The lowest mosquito density (0–0.02/km²) has no fill color enabling the bird densities to be seen underneath this layer. The bird density data do not give an absolute density figure, but rather a relative one, relative to the other species of birds present. Bird data presented include the barn owl (Tyto alba); buzzard (Buteo buteo); carrion crow (Corvus corone); hooded crow (Corvus cornix); house sparrow (Passer domesticus); jay (Garrulus garrulus); jackdaw (Corvus monedula); kestrel (Falco tinnunculus); little owl (Athene noctua); magpie (Pica pica); raven (Corvus corax); rook (Corvus frugilegus); sparrowhawk (Accipiter nisus); tawny owl (Strix aluco); and tree sparrow (Passer montanus). Data for Charadriiformes were not available. (Color images available online at www.liebertonline.com/vbz).

FIG. 7.

Map of the United Kingdom (as Fig. 6) with relative mosquito and bird abundance scores: these are the ratio of the records of WNV-competent mosquitoes per km² and the relative WNV-susceptible bird density. (Color images available online at www.liebertonline.com/vbz).