22 November 2007
Chikungunya in north-eastern Italy: a summing up of the outbreak
R Angelini1, AC Finarelli2, P Angelini2, C Po2, K Petropulacos3 G Silvi1, P Macini2, C Fortuna4, G Venturi4, F Magurano4, C Fiorentini4, A Marchi4, E Benedetti4, P Bucci4, S Boros4, R Romi4, G Majori4, MG Ciufolini4, L Nicoletti4, G Rezza4, A Cassone (firstname.lastname@example.org)4
1. Dipartimento Sanità Pubblica, Azienda Unità Sanitaria Locale (Department of Public Health, Local Health Unit), Ravenna, Italy
2. Servizio di Sanità Pubblica, Regione Emilia-Romagna, Bologna, Italy
3. Servizio Presidi Ospedalieri, Regione Emilia-Romagna, Bologna, Italy
4. Department of Infectious, Parasitic and Immunomediated Diseases, Istituto Superiore di Sanità, Rome, Italy
The first outbreak of autochthonously transmitted Chikungunya virus (CHIKV) in Europe, which recently occurred in the province of Ravenna in north-eastern Italy [1,2], has been completely controlled: the last case onset occurred on 28 September in the town of Rimini, and in October no cases were confirmed. Of the 334 suspected or probable CHIKV cases involved in the outbreak, samples were examined of 281 and 204 were laboratory-confirmed by PCR, Hemagglutination-inhibition or both. Reasonably, the number of laboratory-confirmed cases most likely constitutes an underestimate of the extent of the outbreak, since blood or serum samples were not available for all of the individuals who fulfilled the clinical and/or epidemiological criteria of the case-definition.
Geographical distribution of cases
Most cases were reported among persons living in or visiting the initially affected villages of Castiglione di Cervia and Castiglione di Ravenna. Four smaller clusters of local transmission were also detected in four towns in the same region (i.e., Cervia, Cesena, Ravenna, and Rimini) which are located 9 to 49 km from the initially affected villages. For at least three of the four clusters, population movement (i.e., persons who visited the area that was primarily affected or persons from the primarily affected area who visited one of the four towns) can be reasonably assumed to have been the main determinant of local transmission. However, if this was the case, the question arises as to why no previous outbreaks of CHIKV occurred in other Italian regions in 2005-2006 (after the epidemic in Reunion), when at least 30 infected travellers returned to locations infested by mosquito vector populations ; the same question arises for several hundred cases reported among travellers returning from affected areas to a number of European countries in the same period [3,4,5]. Possible explanations include: i) high concentration of vectors in the affected towns; ii) highly viremic persons exposing themselves to aggressive Aedes albopictus populations as a consequence of the structures of houses and/or behavioural factors (e.g., spending time outdoors in houses’ surroundings).
Overall, the epidemic in Italy can be said to be the result of the combined effect of the globalisation of vectors and humans, which occurred through a two-step process: i) the introduction and adaptation of the vector Ae. albopictus to a new environment (i.e., a temperate climate); and ii) the introduction of CHIKV in a previously infection-free country, with totally susceptible subjects, as the result of population movement (i.e., travelling human hosts, acting as a sort of Trojan Horse). However, the epidemic was limited in space and time, with a marked decay rate since the adoption of appropriate control measures (albeit they were taken at different times in different locations). In addition, there is probably a time-limited capacity of the vector to sustain infection transmission beyond the hot season in a country with a temperate climate.
What did we learn from the Italian epidemic?
- Vector-borne diseases, historically confined to tropical environments, can be introduced within Europe if the conditions are appropriate (i.e., the presence of vectors). The major determinant of the outbreak in Italy was probably the high vector density at the time of arrival of the index case, which could be explained by the lack of preventive vector-control measures in an area that was considered to be 'infection-free';
- The vectorial capacity of Ae. albopictus for CHIKV is high . A few hours spent in a highly vector-dense village by only one feverish patient caused a large outbreak in a naïve population;
- As seen for other infectious diseases causing international crises (i.e., SARS), population movement and vector colonisation of new areas are important determinants of disease globalisation.
Nonetheless, there are still some questions that need to be investigated further:
- Why did a single case result in an outbreak while none of the many CHIVK-infected travellers have caused local transmission upon returning home to Italy or other European countries in roughly similar vector-dense areas;
- Has the infection been eradicated in the affected area or could it reappear at the beginning of the next hot season due to overwintering?
- What is the probability that other similar events will occur in Italy or other European countries where Ae. albopictus is present, and could other infections, such as Dengue, cause similar outbreaks in Europe?
These questions need to be adequately answered so as to strengthen activities for the surveillance and control of Ae. albopictus and other vectors of exotic infectious diseases (i.e., Chikungunya and others) and to perform early diagnosis of viral agents that can be imported and transmitted in Europe. Furthermore, more intense research efforts should be promoted in Italy and in Europe on the mosquito-virus relationship, as well as in other critical areas concerning vaccine and specific antiviral drugs. Experimental infections in vector populations, virus and vector genotyping are some of the specific investigations already planned in Italy.
- Angelini R, Finarelli AC, Angelini P, Po C, Petropulacos K, Macini P, Fiorentini C, Fortuna C, Venturi G, Romi R, Majori G, Nicoletti L, Rezza G, Cassone A. An outbreak of chikungunya fever in the province of Ravenna, Italy. Euro Surveill 2007;12(9):E070906.1. Available from: http://www.eurosurveillance.org/ew/2007/070906.asp#1
- G Rezza, L Nicoletti, R Angelini, R Romi, AC Finarelli, M Panning, P Cordioli, C Fortuna, S Boros, F Magurano, G Silvi, P Angelini, M Dottori, MG Ciufolini, GC Majori, A Cassone Infection with Chikungunya virus in Italy : An outbreak a temperate region. Lancet, 2007, in press.
- Nicoletti L, Ciccozzi M, Marchi A, Fiorentini C, Martucci P, D’Ancona F, Ciofi degli Atti M, Pompa MG, Rezza G, Ciufolini MG. Correlates of Chikungunya and Dengue in travellers. Emerging Infectious Diseases, in press.
- Depoortere E, Coulombier D, ECDC Chikungunya risk assessment group. Chikungunya risk assessment for Europe: recommendations for action. Euro Surveill 2006;11(5):E060511.2. Available from: http://www.eurosurveillance.org/ew/2006/060511.asp#2
- Joint ECDC/WHO European Risk Assessment on Chikungunya in Italy: a mission report. Available from: http://www.ecdc.europa.eu/pdf/071030CHK_mission_ITA.pdf
- Vazeille M, Jeannin C, Martin E, Schaffner F, Failloux AB. Chikungunya: A risk for Mediterranean countries? Acta Trop. 2007 Oct 12; [Epub ahead of print].
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