Introduction and Purpose
Period of the epidemiological cycle | Mechanisms | Definition and predicted patterns | Code | References | Findings from this studya
| |
---|---|---|---|---|---|---|
Introduction | Seasonal introduction by migratory birds | Migratory birds may introduce WNV in spring from endemic areas (North or sub-Saharan Africa) or in summer from epidemic areas (Eastern Europe) | Southern spring migrants |
I
1a
| ++ | |
Eastern summer migrants |
I
1b
| ++ | ||||
WNV persistence in overwintering infected mosquitoes | Vectorial transmission stops during winter with mosquito’s diapause and may restart in spring when infected mosquitoes become active. WNV may persist in overwintering ornithophilic mosquitoes. Only two abundant ornithophilic mosquito species with a vector competence for WNV are found in the study area (Culex modestus and Cx. pipiens) |
Culex modestus only |
I
2a
| − | ||
Culex pipiens only |
I
2b
| − | ||||
Both species |
I
2c
| + | ||||
Amplification | Transmission to birds involves a single or few species of vectors | WNV amplification may involve one or both competent ornithophilic mosquito species according to differences in their feeding behaviour, competence and habitat preferences. |
Culex modestus only |
A
1xx
| ++ | |
Culex pipiens only |
A
2xx
| − | ||||
Both species |
A
3xx
| + | ||||
Heterogeneity in avian host competence | Field and experimental studies suggest great differences between bird species in attractiveness to vectors and WNV infectiousness. In the study area, WNV have been isolated in only two bird species (House sparrows, Passer domesticus; and black-billed magpies, Pica pica). | House sparrows and black-billed magpies only |
A
x1x
| (Jourdain et al. 2007) | - | |
All bird species involved, heterogeneous competences among bird species |
A
x2x
| ++ | ||||
All bird species involved, homogenous competences among bird species |
A
x3x
| + | ||||
Avian host species diversity | In a heterogeneously competent bird community, high-species diversity may reduce transmission through a dilution effect because a lower proportion of mosquito would bite the most competent host species | Absence of ‘dilution effect’ |
A
xx1
| ++ | ||
‘Dilution effect’: diversity reduces the probability of transmission |
A
xx2
| − | ||||
Dispersal | WNV-infected wild birds may disperse the virus over long distances as they move across the study area in accordance with their flying ability and propensity. | (Jourdain et al. 2007) | ||||
Spillover | Only some vector species are likely to act as ‘bridge vectors’ | One or both mosquito species may be responsible for transmitting WNV from birds to incidental hosts (horses in the study area). |
Culex modestus only |
S
1
| − | |
Culex pipiens only |
S
2
| − | ||||
Both species |
S
3
| + | ||||
All steps | Density-dependent transmission process | Transmission rate should increase with the local abundance of avian hosts and mosquito vectors through an increased host–vector contact rate |
Methods
Ecological Database
Spatial Analysis Procedures to Predict Areas of WNV Transmission
Scenarios
Mapping Potential Sites for WNV Introduction
Mapping Potential Sites for WNV Amplification/Dispersal
Mapping Potential Sites for WNV Spillover
Calculation of a WNV Circulation Index
Calculation of a WNV Spillover Index
Evaluation of Scenario Predictions
Wild Bird Serological Data
Equine Serological Data
Statistical Analysis
Results
Prediction of WNV Circulation in Wild Bird Populations
Model | Scenario | Component mechanisms | AIC |
w
AIC
| Coefficients [95% CI] | |||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
Introduction | Amplification | Spillover | Intercept | Age class: adult | WNV circulation index | WNV spillover index | ||||||
Vector | Host | ‘Dilution effect’ | ||||||||||
Seroprevalence in magpies versus WNV circulation indexa
| I1bA321
| Eastern summer migrants |
Culex modestus and Culex pipiens
| All bird species, heterogeneous competences | Absence of ‘dilution effect’ | 171.3 | 0.30 | −15.23 [−15.47; −14.99] | 2.51 [2.47; 2.55] (p < 10−7) | 0.12 [0.12; 0.13] (p < 10−4) | ||
I1aA121
| Southern spring migrants |
Culex modestus only | All bird species, heterogeneous competences | Absence of ‘dilution effect’ | 173.0 | 0.13 | −14.05 [−14.29; −13.80] | 2.48 [2.45; 2.52] (p < 10−7) | 0.13 [0.129; 0.134] (p < 10−3) | |||
Seroprevalence in horses versus WNV spillover indexb
| I1aA121S3
| Southern spring migrants |
Culex modestus only | All bird species, heterogeneous competences | Absence of ‘dilution effect’ |
Culex modestus and Culex pipiens
| 725.5 | 0.57 | −5.17 [−5.20; −5.14] | 0.074 [0.073; 0.074] (p < 10−6) | 0.064 [0.064; 0.065] (p < 10−7) |
Step | Hypothesis | Code | Magpies seroprevalence data Σw
AIC (n) | Horses seroprevalence data Σw
AIC (n) | |||
---|---|---|---|---|---|---|---|
Introduction |
Introduction by migratory birds
|
Southern spring migrants
|
I
1a
|
0.64 (30)
|
0.25 (15)
|
0.95
|
0.662 (21)
|
Eastern summer migrants |
I
1b
|
0.39 (15)
|
0.288 (18)
| ||||
Virus overwintering |
Culex modestus only
|
I
2a
| 0.36 (45) | <10−3 (15) | 0.053 | 0.046 (27) | |
Culex pipiens only
|
I
2b
| 0.13 (15) | 0.002 (12) | ||||
Both species |
I
2c
| 0.23 (15) | 0.002 (15) | ||||
Amplification | Vector amplification |
Culex modestus only
|
A
1xx
|
0.37 (25)
|
0.85 (42)
| ||
Culex pipiens only
|
A
2xx
| 0.13 (25) | <10−4 (27) | ||||
Both species |
A
3xx
|
0.50 (25)
| 0.15 (24) | ||||
Host amplification | Magpies and sparrows only
|
A
x1x
| <10−5 (30) | <10−3 (0) | |||
All bird species, heterogeneous competences
|
A
x2x
|
1 (30)
|
0.63 (69)
| ||||
All bird species, homogenous competences |
A
x3x
| <10−3 (15) | 0.37 (24) | ||||
Diversity effects |
Absence of ‘dilution effect’
|
A
xx1
|
0.81 (45)
|
0.97 (60)
| |||
‘Dilution effect’ |
A
xx2
| 0.19 (30) | 0.03 (33) | ||||
Spillover | Spillover |
Culex modestus only
|
S
1
| - | 0.01 (31) | ||
Culex pipiens only
|
S
2
| - | 0.01 (31) | ||||
Both species
|
S
3
|
-
|
0.98 (31)
|