Other mosquito-borne viral infections that cause encephalitides

Public Health Significance of Urban Pests Unlike Europe, the United States was free of WNV before 1999. Then, between 1999 and 2003, the virus spread rapidly in the United States, from the east coast to the west coast Gould Fikrig, 2004. There have been thousands of equine and human infec- tions with hundreds of deaths and, unlike in Europe, birds, predominantly crows Corvus spp., also have been affected badly. The CDC human case counts total 19,655 infections for the years 1999–2005, including 782 fatalities CDC, 2006. At present, WNV has been detected in most of Canada and Central America and in the Caribbean. Most likely, the virus was brought to North America by illegally imported birds. Molecular analysis has shown that the North American virus strain is related most closely to a strain isolated in Israel, which is much more virulent than the European strains Lanciotti et al., 1999; Brault et al., 2004. According to Higgs, Snow Gould 2004, 73 mosquito species have been found natu- rally infected with WNV, could be experimentally infected or have even been shown to transmit the virus after experimental infection. At present, more than 15 mosquito spe- cies in Europe Higgs, Snow Gould, 2004; Medlock, Snow Leach, 2005 and at least 60 species in numerous genera in the United States are suspected to be vectors of WNV CDC, 2005. Most important among them are species that feed on different types of hosts, such as those of the Culex pipienscomplex, which were involved in several outbreaks, par- ticularly in urban areas where they breed peridomestically in small man-made water containers. As with other indiscriminate biters, the northern house mosquito Cx. pipiens is both ornithophilic and mammophilic, thus serving as a bridge vector between birds and mammals, including human beings Fonseca et al., 2004; Kilpatrick et al., 2005.

11.4.2. Other mosquito-borne viral infections that cause encephalitides

For Europe, six mosquito-borne viruses have been described as active during the last 50- year period: Inkoo, Lednice, Batai Calovo, Tahyna, SindbisOckelbo and West Nile viru- ses. The Tahyna, SindbisOckelbo and West Nile viruses are pathogenic in people Lundström, 1994. A seventh virus Usutu virus, whose pathogenic potential for people is unknown, was detected in 2002 in Austria, where it was responsible for mass mortality in birds, mainly common blackbirds T. merula Weissenböck et al., 2003. Antibodies to Usutu virus were also recently found in birds in the United Kingdom Buckley et al., 2003. The course of most European mosquito-borne virus infections in people is considered to proceed asymptomatically or to present as mild summer flu-like symptoms. Only occa- sionally do these infections result in meningitis or encephalitis, the etiologies of which usually are not pursued. A study conducted in English hospitals revealed that about 60 of all registered viral encephalitides are of unknown etiological origin Davison et al., 2003. Diagnosis usually is based on the symptoms and some clinical parameters; virus isolation or the detection of specific viral RNA or DNA is seldom attempted. To date, comprehensive scientific studies on the activities of mosquito-borne viruses in Europe are almost completely lacking. In the United States, in addition to WNV, four other mosquito-borne encephalitis viru- ses are of epidemiological interest: St. Louis, eastern equine, western equine and La Mosquitoes 353 352 Table 11.1. Pathogen transmission by mosquitoes in Europe and North America Infection Pathogen Clinical Countries Important vectors manifestations Inkoo virus Bunyavirus Bunyaviridae No visible signs Northern Europe Aedes communis Inkoo virus Aedes punctor Lednice virus Bunyavirus Bunyaviridae No visible signs Central Europe Culex modestus Lednice virus Batai Calovo virus Bunyavirus Bunyaviridae No visible signs Northern and central Europe Anopheles maculipennis Batai Calovo virus sensu lato Tahyna virus Bunyavirus Bunyaviridae Sometimes mild febrile illness; Germany and eastern Ae. vexans Tahyna virus rarely meningitis Europe Aedes caspius Aedes dorsalis Sindbis virus Alphavirus Togaviridae Severe headache, muscle ache, Mediterranean Basin Culex spp. Sindbis virus dengue-like symptoms Aedes spp. Ockelbo virus Alphavirus Togaviridae Febrile disease with rash Finland, Norway, Russian Cx. pipiens Ockelbo type and polyarthralgia Federation and Sweden Culex torrentium Aedes cinereus WNV Flavivirus Flaviviridae Mild, flu-like illness; Europe and Cx. pipiens WNV sometimes meningitis North America Cx. modestus Culex quinquefasciatus Culex restuans Culex tarsalis St Louis encephalitis Flavivirus Flaviviridae Usually mild, febrile illness; Throughout Cx. pipiens SLEV rarely meningitis the United States Culex nigripalpus Cx. tarsalis Eastern equine Alphavirus Usually mild, flu-like illness; Eastern United States Ae. vexans encephalitis Togaviridae sometimes encephalitis, Aedes canadensis EEEV coma, death Aedes sollicitans Culex salinarius Culiseta melanura Coquillettidia perturbans Western equine Alphavirus Togaviridae Usually mild, flu-like illness; Western and central Cx. tarsalis encephalitis WEEV sometimes encephalitis, United States Aedes melanconion coma, death La Crosse encephalitis Bunyavirus Bunyaviridae Usually mild, febrile illness; Upper mid-western Aedes triseriatus La Crosse encephalitis virus rarely seizures, coma and mid-Atlantic United States Malaria Plasmodium ovale Fever attacks, chills; Southern Europe, An. sacharovi Plasmodium malariae in P. falciparum infection, southern North America An. atroparvus Plasmodium falciparum often renal failure, coma, death An. labranchiae An. freeborni An. pseudopunctipennis An. quadrimaculatus Filariasis D. immitis Dogs: cardiovascular filariasis; European Mediterranean Ae. caspius D. repens human beings: pulmonary countries, throughout Ae. detritus D. tenuis or subcutaneous lesions the United States Ae. vexans Cx. pipiens An. maculipennis s.l. Mansonia spp. Sources: Schaffner 2003; CDC 2004b. Public Health Significance of Urban Pests Mosquitoes 355 354 et al., 1969a,b; Moore et al., 1974. It is a febrile, but usually not fatal disease transmitted by Aedes spp., especially the subgenera Aedimorphus, Diceromyia and Stegomyia Jupp McIntosh, 1988; Diallo et al., 1999. Recently, epidemics of chikungunya fever have occur- red on several islands in the Indian Ocean, such as the Union of the Comoros, Madagascar, Mayotte, Mauritius, Seychelles and Réunion, as well as in southern Asia India and Malaysia Institut de Veille Sanitaire, 2006; WHO, 2006a. As of 29 March 2006, the Réunion outbreak has affected more than a fourth of the total population and has brought about extraordinarily severe cases, including meningoencephalitis and multi- organ failure, and even 155 deaths associated directly or indirectly with the infection ProMED Mail, 2006. As the virus vector in the Indian Ocean is the Asian tiger mos- quito, which had been introduced at least a century ago Fontenille Rodhain, 1989; Salvan Mouchet, 1994, there is great concern that the virus will be imported by tra- vellers and be transmitted and established in Europe by the Asian tiger mosquito. In fact, 160 cases of chikungunya disease were imported to France between April 2005 and 31 January 2006 Institut de Veille Sanitaire, 2006, and several others were imported to Germany, Italy, Norway and Switzerland WHO, 2006b. As a consequence, the ECDC, together with a group of experts, is at present conducting a risk assessment for Europe.

11.4.5. Malaria