It has been asserted that WNV urban outbreaks are dangerous when associated with Cx.
p. molestus (Hubálek, 2000). It is still not clear if the outbreaks referred to and that re-emerged in Europe recently were caused or exacerbated by hybrids. The ability of British Cx. p. molestus to bite both humans and birds means that it could be a bridge
-vector for the WNV, and that it needs to be monitored as part of a program of control of West Nile fever. However, high vectorial capacity would require equal preference for biting humans and birds. Our data so far shows Cx. p. molestus biting birds for only 5.5
% of its meals. Therefore with current data we cannot consider Cx. p. molestus a major threat in case of WNV arrival in Britain, especially if we consider the fact that its distribution is rather patchy in Britain, although generally very abundant when present.
A recent paper shows that WNV epidemics in North America are likely to be driven also by a shift in Culex pipiens biting behaviour from avian to mammalian. This study was based on the observation that North American human infections with WNV peaked during late summer and early fall, and this, it was demonstrated, was associated with a decrease in American robins the preferred host of Culex pipiens, and other WNV competent avian hosts after these birds disperse and migrate following breeding. This change in biting behaviour may reflect an increase in Culex pipiens hybridization in North America, although it has yet to be tested and a shift due to robin decrease offers a more parsimonious alternative (Kilpatrick et al., 2006). In addition, US WNV outbreaks are thought to be caused by a more virulent virus strain as compared to that present in Europe, as well as birds of America being naïve to infections therefore prone to develop a high viremia. A shift in feeding behaviour of British Culex pipiens s.l. seems less likely to happen, since no hybrids have been detected in this and previous studies, although it cannot be ruled out yet.
In addition, collections of Cx. p. molestus at the farm showed a dramatic increase towards the end of the summer (first three weeks of September, year 2005) and greatly outnumbered Cx. p. pipiens that peaked in July-first week of August (Curtotti and Malcolm unpublished).
This seems to be explained by the fact that Cx. p. molestus towards the end of the summer has had enough time to expand its population and Cx. p. pipiens numbers has already dramatically decreased.
These preliminary data, tell us that Cx. p. molestus may be able to transmit WNV to humans in Britain at the end of the summer, after Cx. p. pipiens has acted as an enzootic vector. Furthermore, pigeons were the bird hosts which we studied and which are relevant especially in urban settings where pigeons are known to be widely diffused.
However, assessment of risk of WNV transmission from mosquitoes to humans have to take account of abundance of the mosquito vector, WNV infection prevalence, biting behaviour, and vector competence (Kilpatrick et al., 2006). Vector competence of WNV in Britain has not been investigated to date, and although British migratory and non migratory birds have been found carrying WNV antibodies, no live WNV has been detected (Buckley et al., 2003) and no humans or mosquitoes have been found infected (Hulme et al., 2002)
5.5.6 Conclusions
No hybrids of the two Culex pipiens forms have been detected in Britain. This finding may partly explain why WNV outbreaks have not yet hit this country and may never do, especially because US Culex pipiens hybrids are thought to have favoured the rapid spread of this arthropod borne disease. Characteristics of hybrids will need to be further studied, but their increased ability to act as bridge vectors seems to be attributed to their dual avian-mammalian biting capacity (Fonseca et al., 2004). In addition, our field study shows that British Cx. p. molestus, although having a very strong human-mammalian preference, will occasionally bite birds. On the other hand, Cx. p. pipiens has been found to be exclusively ornithophilic.
Further information is needed in order to have a clear picture of the dynamics of WNV transmission to humans (e.g. vector competence in Britain), and the present data suggest that Cx. p. molestus in Britain should be monitored as part of a program of control of WNV, although considering its low density and the fact that we have found Cx. p.
molestus taking only 5.5 % of its meals from birds, its potential as WNV vector remains low.
-Fig. 5.1 PCR identification of Cx. p. molestus collected from the Thameside city farm (East-London) as blood-fed adult females in a house occupied by humans and dogs.
Lane 1 to 5: CQ11F2 and MolCQ11R primers amplifying Cx. p. molestus CQ11 microsatellite allele. Lane 7: Cx. p. molestus positive control. Lane 8: Cx. p. pipiens positive control. Lane 9: negative control.
Fig. 5.2 PCR identification of adult female Cx. p. pipiens reared from natural larval breeding sites in the vicinity of the Thameside city farm (East London). Lane 1 to 14 pipCQ11R and CQ11F2 primers amplifying Cx. p. pipiens CQ11 microsatellite allele.
Lane 15: Cx. p. molestus positive control. Lane 16: Cx. p. pipiens positive control. Lane 17: negative control.
0.29 0.22 0.20 1
K b
M 1 2 3 4 6 7 8 9 10 11 12 13 14 15 16 17
1 2 3 4 5 7 8 9 M
M
KbKb Kb
0.298 0.220 0.201
Location Place of collection Cx. p. molestus CQ11 allele Cx. p. pipiens CQ11 allele Total
Barking, London Beckton, sewage works * 57 0 57
Barking, London trap 0 959 959
Barking, London natural breeding sites 4 86 90
Barking, London farm 130 121 251
Barking, London house 6 0 6
Hounslow, London Mogden, sewage works ** 5 41 46
Durham Durham 1 72 73
203 1279 1482
Total
* samples collected under ground **samples collected on the surface and under ground
Table 5.1. PCR identification of Culex pipiens s.l. samples collected from London borough of Barking&Dagenham (Barking), London borough of Hounslow (Hounslow), city of Durham (Durham). No specimens were found in which both Cx.p. pipiens and Cx. p. molestus CQ11 microsatellite allele were amplified. For information on mosquito collection types and origin see appendix.
Status of blood feeding Mosquito biotype Caravan and farmer's house Pigeon hut
Table 5.2. PCR identification of Cx. p. pipiens and Cx. p. molestus collected in structures with human or avian hosts at Thameside city farm (East London).
Mosquito
Table 5.3. Blood meal identification of Cx. p. pipiens and Cx. p. molestus by means of precipitin test. For information on mosquito collection types and origin and on type of antiserum employed see appendix and material and methods section, respectively.
Site Year
Culex pipiens
sample Nearest airport Beckton Sewage treatment works
( Barking, East London) 2003-2004-2005 larvae London City < 4 Km Beckton Sewage treatment works
(Barking, East London) 2003-2004 egg rafts London City < 4 Km Surroundings of Beckton Sewage treatment
works (Barking, East London) 2004-2005 larvae London City < 4 Km Thameside city farm
(Barking, East London) 2005-2006 adult females London City < 4 Km House in Barking
(Barking, East London) 2003 adult females London City < 4 Km Mogden sewage treatment works
(Hounslow, West London) 2005 larvae Heathrow < 8 km
Durham
(County of Durham) 2005 genomic DNA Newcastle < 30 km
Table 5.4. Information on mosquito collection types and origin.