The Chikungunya epidemic in Italy and its repercussion on the blood system

Abstract

Background

The Chikungunya virus (CHIKV) is transmitted by Aedes mosquitoes and recently caused a massive epidemic on La Réunion Island, in the Indian Ocean. Between July and September 2007 it caused the first autochthonous epidemic outbreak in Europe, in the Region of Emilia-Romagna in the north-east of Italy.

Materials and Methods

After the first reports of an unusually high number of patients with a febrile illness of unknown origin in two contiguous villages, an outbreak investigation was carried out to identify the primary source of infection, the modes of transmission and the dynamics of the epidemic. An active surveillance system was also implemented. Laboratory diagnosis was performed through serology and polymerase chain reaction (PCR) analysis. Blood donation was discontinued in the areas involved from September to October 2007 and specific precautionary blood safety and self-sufficiency measures were adopted by the regional health and blood authorities and the National Blood Centre. An estimate method to early assess the risk of viraemic blood donations by asymptomatic donors was developed, as a tool for "pragmatic" risk assessment and management, aiming at providing a reliable order of magnitude of the mean risk of CHIKV transmission through blood transfusion.

Results

Two hundred and seventeen cases of CHIKV infection were identified between 4^th July and 28^th September. The disease was fairly mild in most of the cases. The precautionary measures adopted in the blood system caused a considerable reduction of the collection of blood components and of the delivery of plasma to the pharmaceutical industry for contract manufacturing. The estimated risk of CHIKV transmission through blood transfusion peaked in the third week of August.

Conclusion

A CHIKV epidemic poses considerable problems for public health authorities, who not only need good routine programmes of vector control and epidemiological surveillance but also local and national emergency plans to sustain the blood supply, so as to promptly deal with the potentially severe effects of an epidemic outbreak, especially when affected areas locally require a significant blood inventory and at the same time represent a critical resource for other areas depending on external supplies of blood components.

Introduction

Chikungunya fever is an acute arthropod-borne viral illness reported in many parts of Africa, Southeast Asia, the Western Pacific, and India¹. The causative agent - the Chikungunya virus (CHIKV) - is a 70 nm diameter single-stranded positive RNA-enveloped virus that is a member of the genus Alphavirus of the Togaviridae family^1,2.

Chikungunya is a specifically tropical disease that is relatively uncommon and poorly documented³. Epidemics of fever, rash and arthritis, resembling Chikungunya fever were recorded as early as 1824 in India and elsewhere^4,5. However, CHIKV was isolated for the first time during a Tanzanian outbreak in 1952⁶ and since then, it has caused many outbreaks in both Africa and Asia, where its re-emergence has been unpredictable with intervals of 7–8 years to 20 years between consecutive epidemics⁷. In Africa a sylvatic transmission cycle between wild primates and mosquitoes, such as Aedes luteocephalus, Aedes furcifer, and Aedes taylori, is thought to maintain the virus whereas in Asia it is transmitted from human to human through an urban transmission cycle, mainly by infected Aedes aegypti and - to a lesser extent - by Aedes albopictus⁸. Human beings are the virus reservoir during epidemic periods whereas outside these periods, monkeys, rodents, birds, and other unidentified vertebrates serve as the main reservoirs of CHIKV³.

CHIKV is responsible for an acute infection of abrupt onset which is characterised by high fever, arthralgia, myalgia, headache and oedema, but other symptoms such as rash, epistaxis, gingivorrhagia, nausea, vomiting, flushed face and photophobia have also been described. The symptoms generally start 4–7 days after the mosquito bite occurred, are usually of short duration, and recovery is most often complete^3,4. The most typical clinical sign is poly-arthralgia, which is generally very painful, as suggested by the word "Chikungunya" which means "to walk bent over" in the Swahili or Makonde African dialects, and refers to the effect of the incapacitating arthralgia^4,9. The latter distinguishes a CHIKV infection from dengue which otherwise shares the same vectors, symptoms, and geographical distribution^3,10,11. The diagnosis is made from evocative symptoms in a patient living in or coming back from an area where there is a known outbreak of Chikungunya fever, and laboratory confirmation is made by polymerase chain reaction (PCR) or serology^3,12.

Common haematological abnormalities in the acute phase include lymphopenia and thrombocytopenia, which may be associated with bleeding^13,14.

Neurological complications such as meningoencephalitis have been reported - even recently - in a small proportion of patients^15,16. CHIKV infection can also be associated with ocular involvement which can be complicated by granulomatous and non-granulomatous anterior uveitis, optic neuritis, retrobulbar neuritis, and dendritic lesions^17–19. Mother to child transmission of CHIKV had never been reported previously until the 38 biologically documented cases of neonatal CHIKV infection recorded –with a high rate of morbidity - during the recent massive outbreak in French La Réunion island^16,20,21. The surveillance system there estimated that almost 266,000 people (about 35% of the population) had a clinical form of Chikungunya infection between March 2005 and June 2006²².

CHIKV has been imported to Europe and to the USA by infected travellers returning from areas with high incidence rates, and A. albopictus - one of the main vectors of CHIKV - has been introduced into several European countries and also to Central America, Brazil, and the USA^3,23–25. In Italy -where A. albopictus was first recorded in 1990 - surveillance for cases of CHIKV imported by travellers is mandatory²⁶. The mosquito is now present in almost all regions and can be active throughout the year thanks to its ability to colonise new areas and its adaptability to favourable climatic conditions^26,27. Travellers from areas affected by Chikungunya have recently been diagnosed with Chikungunya fever in Italy too²⁸. During the month of August 2007 local transmission involving A. albopictus caused the first autochthonous CHIKV outbreak in Europe, in the province of Ravenna, Italy²⁹.

This study is aimed at reporting information on the impact of the CHIKV epidemic (July–September, 2007) on blood supply in the Region of Emilia-Romagna and at describing the precautionary measures adopted to prevent the transmission of CHIKV through blood transfusion.

Materials and Methods

Patients, chronology and laboratory diagnosis

In the months of July and August 2007 the local health authority of the province of Ravenna, in the Region of Emilia-Romagna in north-east Italy, detected an unusually high number of patients with a febrile illness and concomitant symptoms such as myalgia, severe back and joint pain, headache, and skin rash, in two small contiguous villages: Castiglione di Cervia and Castiglione di Ravenna. In the second week of August, the local health trust implemented an active surveillance system with the aim of identifying all the persons with the above-mentioned symptoms through a standardised questionnaire including personal data, recent travel and date of symptom onset³⁰. In the second half of the month an outbreak investigation was carried out to identify the primary source of the infection through the positioning of insect traps for subsequent entomological study, modes of transmission and dynamics of the epidemic. Soon after (on August 18^th) the first mosquito control measures were implemented in public and private areas. Infection with CHIKV was suspected because of the clinical symptoms and due to the fact that A. albopictus - whose presence in the area was already known - was the predominant insect to be found in the traps. Moreover, on 28^th August, the first patient with febrile illness coming from a country affected by an outbreak was identified. At the end of the month, clinical and epidemiological investigations carried out by the local health trusts suggested an arbovirus as the possible cause of the outbreak²⁹. Investigations were performed in collaboration with the regional microbiology reference laboratory of Bologna University and the Department of Infectious and Parasitic Diseases of the National Institute of Health in Rome

As from 29^th August - in order to monitor the epidemiological situation closely – the health authorities of Emilia-Romagna developed the following case definition for active surveillance purposes³¹, based on general practitioners and hospital emergency units.

Clinical criteria: acute onset of fever (>38.5 °C) and severe arthralgia not explained by other medical conditions.

Epidemiological criteria: residing in or having visited epidemic areas, having reported transmission within 15 days prior to the onset of symptoms.

Laboratory criteria: at least one of the following tests in the acute phase: virus isolation, presence of viral RNA by reverse transcriptase-PCR (RT-PCR), seroconversion to virus-specific antibodies in samples collected at least 1 to 3 weeks apart, or presence of virus-specific IgM antibodies in a single serum sample collected.

On this basis the following categories of cases were reported: a) possible case: a patient meeting clinical criteria; b) probable case: a patient meeting both the clinical and epidemiological criteria; c) confirmed case: a patient meeting the laboratory criteria, irrespective of the clinical presentation. The term suspected case was used for both possible and probable cases awaiting laboratory confirmation. A suspected case was classified negative if RT-PCR was negative on samples collected within the first 5 days following the onset of symptoms or if the serology test was negative on samples collected 6 days or more after the onset of symptoms.

Patients' samples were stored at –80 °C before being examined at the regional microbiology reference laboratory of Bologna University and at the Department of Infectious and Parasitic Diseases of the National Institute of Health in Rome. Laboratory diagnosis included testing for antibodies to CHIKV by haemagglutination inhibition and real-time RT-PCR which targets the nsp1 gene of CHIKV, as detailed in the article by Rezza and colleagues³⁰. PCR was also used to detect CHIKV in specimens of A. albopictus captured locally during the outbreak.

Blood safety and self-sufficiency measures

The Emilia-Romagna blood system network comprises 13 Blood Transfusion Services (BTSs), co-ordinated by a Regional Blood Coordinating Centre (RBCC). In the late evening of August 29^th the laboratory confirmation of CHIKV infection led to the thorough activation of the procedures against the outbreak of CHIKV disease.

At that moment a laboratory test for routine biological qualification of blood components through systematic screening for CHIKV genome by nucleic acid amplification testing was not available; hence, on 1^st September blood donations from people living in the municipality of Ravenna and Cervia were discontinued "till new order" through a ministerial circular and - according to the Italian Ministry of Health Decree of March 3^rd 2005³² - a 21-day deferral policy was introduced nationwide for blood donors who had visited the affected areas, even for a few hours. In addition, an immediate specific revision of pre-donation questionnaire was introduced, aimed at the early detection of CHIKV infection's sign and symptoms. On 2^nd September, blood collection suspension measures were extended to Cesena and to the nearby town of Cesenatico and on 23^rd September to Rimini. As from 1^st September, all stocked blood components collected – starting from 1^st August -from donors living in the affected were eliminated. The same provision was adopted for plasma delivered to the pharmaceutical industry for contract manufacturing and for blood components delivered by involved BTSs in the Emilia-Romagna Region to any other BTS in the nation.

The RBCC of Emilia-Romagna, in co-operation with the National Blood Centre (NBC), implemented an emergency blood supply plan including: 1) reduction of plasmapheresis in favour of whole blood donations and reduction of standard blood stocks in the BTSs of the region operating outside the areas interested by the outbreak, in order to increase the availability of blood components for intra-regional supply of affected areas; 2) indication to all regional hospital transfusion committees to further promote optimal use of blood components.

Moreover, the NBC promptly set up a national plan for the southern regions lacking Emilia-Romagna red blood cell supply, through an interregional alert.

When the epidemic began to decline, discontinuation of blood collection in the affected areas appeared to be too strict as precautionary measure, considering the consequent loss of blood components, but the decision to restart blood collection had to be made on evidence-based balance between residual risk of CHIKV transfusion transmission and loss of blood donations. Hence, new precautionary measures aimed at restarting blood collection were adopted including: 1) assessment of the risk of viraemic blood donations to enter the inventory; 2) quarantine of blood components for 5 days (subsequently reduced to 2 days); 3) release of blood components after active and documented feed-back from blood donors, about absence of symptoms during the post-donation quarantine period; 4) pathogen inactivation of platelet concentrates.

Assessment of the risk of viraemic blood donations

In the second half of September 2007 the incidence of new cases of CHIKV disease decreased dramatically and around 19^th September the epidemic began to fade away. At that moment an estimate method to early assess the risk of CHIK viraemic blood donations in real time was developed by a working group of epidemiologists of the Region of Emilia-Romagna, who performed the estimates. The method was informally shared with the epidemiologists of the Joint Research Centre of the European Commission (Ispra, Varese, Italy) and approved by the NBC.

The estimates of the mean risk of viraemic blood donations were aimed at identifying a specific risk threshold, below which the existing local epidemiological scenario could be regarded as equivalent to that of other transfusion-transmitted infections for which donated blood is currently tested. In fact, the risk of these latter infections cannot be further reduced by the safety and preventive measures for blood-borne diseases which are currently adopted. The assessment of the magnitude of the above risk was deemed necessary for the decision-making process of restarting the collection of blood donations in the affected areas.

A risk of one viraemic blood donation per 380,000 was considered acceptable as in Italy this is the current estimate of the risk of post-transfusion hepatitis B virus transmission in the serological window period (Velati C. and the Italian group for the study of blood-borne diseases of the Italian Society for Transfusion Medicine and Immunohaematology – Research and Development Division, unpublished data).

The method had to be quickly adaptable to the regional information system in use during the epidemic and capable of providing weekly updates of the estimates.

General approach

The first estimates were made in the second half of September 2007; the calculations were refined weekly and updated with consolidated incidence data which encompassed all the currently confirmed and probable cases of CHIKV infection. A proportion of asymptomatic cases, undetected by the surveillance system, was also included in the estimate. Indeed, serological surveys implemented during prior outbreaks showed that CHIKV infection can be asymptomatic³³. The retrospective and prospective analyses were carried out on a weekly basis for each town involved and for the whole regional area affected by the epidemic.

Statistical approach

The methodology developed by Biggerstaff and Petersen in 2002^34,35 to estimate the mean risk of transfusion-transmitted West Nile virus in the United States was used to carry out the calculation of the risk of CHIK viraemic blood donations. Their formula³⁴ (see the cited reference for the formal development of the method) $V(t)=\sum _{i-1}^n{I}_{(x_i-v_{0i},x_i-v_{0i}+v_{li})}(t)$, where for each case i = 1, 2, …., n, X_i − V_0i represents the time viraemia starts, and X_i − V_0i +V_li represents the time viraemia ends, was used to count the number of viraemic cases at any given time t during the outbreak. All the currently confirmed and suspected cases were included in the calculation.

The estimate of the duration of CHIK viremia was based on the more extensively documented viraemia of dengue viruses^36–38 and on data from the CHIKV epidemic that occurred in La Réunion Island between 2005 and 2007³⁹. We calculated 8 days for the total duration of CHIKV viraemia: 2 days before the first symptoms and 6 days after them, including the day of symptom onset. Moreover, we assumed that the total duration of viraemia was similar in symptomatic and asymptomatic infections. A seroprevalence study conducted among the general population of La Réunion Island⁴⁰ suggested that 15 percent of infected individuals were asymptomatic. In order to calculate the number of asymptomatic viraemic patients we, therefore, hypothesised that 85 out of 100 cases of CHIKV disease were symptomatic, while 15 out of 100 were asymptomatic. We also assumed that the asymptomatic infection ratio was constant over the course of the epidemic, and that any blood donor had the same risk of CHIKV infection as the general population⁴⁰. Moreover, transfused blood components from potentially viraemic blood donors were supposed to transmit infection to recipients with 100-percent efficiency. The information about how viraemia relates to symptom onset and the above assumptions were used to develop a model scenario in which the daily number of symptomatic and asymptomatic cases (potentially undetected by the surveillance system) and the viraemic periods around both of them were computed (figure 1). We computed only 2 viraemic days before symptoms for symptomatic cases as the probability of blood donation in the symptomatic phase was regarded as negligible and then "pragmatically" chose to adopt the scenario with asymptomatic cases (figure 2) in which a viraemic period of 8 days for each asymptomatic patient was computed. The estimate of the risk of CHIKV transmission through blood donation was thus based only on asymptomatic cases. This model scenario was validated relying on two basic assumptions: a) the precautionary measure of quarantining blood components had to be in place, and b) any symptomatic blood donor was expected to self-defer or be excluded from donation by the pre-donation medical examination. We thus estimated the current risk of CHIKV transmission from a unit of blood donated by an asymptomatic viremic donor at time t during the epidemic. The risk was finally calculated - on a weekly basis - relating the number of individuals infected who remain asymptomatic to the resident population (calculated on 1^st January, 2007) and assigning to each week the risk of the day with the estimated highest number of viraemic cases.

Figure 1.

Model scenario of the daily number (included in the broken bar) of symptomatic (S) and asymptomatic (A) cases and computed viraemic period around A (8 days of viraemia) and S cases (for which only 2 viraemic days before symptoms were considered)

Figure 2.

Model scenario of the daily number (included in the broken bar) of asymptomatic (A) cases and computed viraemic period around them (8 days of viraemia)

Results

The epidemic of CHIKV disease occurred during the summer season of 2007 in the provinces of Ravenna, Forlì-Cesena, Bologna and Rimini. The individual assumed to be the index case was a man of Indian origin from Kerala, a region of India affected by the CHIKV epidemic. He arrived in Italy on June 21^st and developed a fever 2 days later. A high titre of anti-CHIKV antibodies (> 1:1280) was detected in a serum sample collected in early September from this man, who was excluded from further data analyses. The first autochthonous case was identified to be occurred on July 4^th in Castiglione di Cervia and was a relative of the index case who had met him on June 23^rd. The epidemic began to fade away around 19^th September 2007 with the incidence of new cases dramatically decreasing until 28^th September 2007 when the last case of CHIKV was detected. Three hundred and thirty-seven suspected cases were reported; 217 met the laboratory criteria and proved to be positive, 30 were classified as probable cases because refusal to give blood samples made a certain diagnosis impossible, and the remaining 90 samples proved to be negative. Figure 3 shows the distribution of the dates of the onset of symptoms for positive and probable CHIKV cases; the epidemic curve was last updated on 16^th January, 2008.

Figure 3.

Epidemic curve of Chikungunya in Italy (July–September 2007). Distribution of dates of onset of symptoms for positive and probable CHIKV cases (last update: 16^th January 2008)

The centre of infection was in Castiglione di Cervia and Castiglione di Ravenna where 142 confirmed cases of CHIKV infection were reported; subsequently, the epidemic spread and brought about some small secondary outbreaks in Cervia (19 cases), Cesena (15 cases), Ravenna (9 cases), Rimini (6 cases) and Bologna (5 cases). Moreover, further several sporadic cases were scattered throughout different places in the same districts.

The distribution of cases by age was relatively homogeneous (male 45.6 per cent, female 54.4 per cent). The mean age of patients was 57, and 42 per cent were older than 65. In the large majority of the patients the disease was mild and self-limiting. Fever lasted for a few days in most patients and a macular rash appeared in more than 50 percent of cases; however, arthralgia was intense and often persistent even after the abatement of fever. Table I reports the per cent distribution of symptoms. Only one death occurred, in an 83-year-old man with severe underlying conditions.

Table I.

Per cent distribution of symptoms in patients affected by Chikungunya disease in Italy, 2007

Symptom	Cases %
Fever	94.5
Asthenia	94.5
Arthralgia	93.6
Skin rash	53.5
Headache	50.2
Myalgia	49.8
Diarrhoea	23
Itching	20
Vomiting	19
Photophobia	15
Conjunctivitis	3

The distribution of the 154 days of suspension of blood collection by BTSs in affected areas is reported in figure 4. The BTSs of Cervia and Cesenatico had, respectively, the longest (44 days) and the shortest (12 days) period of discontinuation of blood donations.

Figure 4.

Periods of suspension of blood donations in the blood transfusion centres in the areas affected by the Chikungunya epidemic

The 21-day deferral for blood donors who had visited the affected areas was maintained for the whole month of September. In October it was decided to restart blood collection and the 21-day deferral was replaced by quarantine of blood components for 5 days until 19^th October, subsequently reduced to 2 days until 20^th November, when all precautionary measures - including the pre-donation specific questionnaire – were discontinued. Pathogen inactivation of platelet concentrates was not carried out, although it had been included in the measures to be adopted after the resumption of blood collection, as this blood component was supplied through intra-regional alternative supply.

The precautionary measures adopted caused a considerable loss of blood components in September and October 2007 in comparison to the same period of the previous year and the elimination of stocked/delivered RBC and plasma units donated from 1^st August by donors living in the affected areas, as detailed in table II. In contrast, thanks to the BTSs not involved in the epidemic, in the month of October, 1911 extra RBC units (+ 2.1%) were collected in the region in comparison to October, 2006; the BTSs involved in the outbreak, instead, entered a reduction of 657 RBC units (−12% in comparison to October, 2006).

Table II.

Impact on regional blood supply of precautionary measures adopted in the Region of Emilia-Romagna as from 1^st September, 2007

	RBC (units)	FFP°(litres)
August	725*	613.73*
September	3748§	1809.08§
October	657#	448.37#
Total	5130	2871.18

^°includes apheresis and whole blood fresh-frozen plasma (FFP).

^*eliminated by blood transfusion centres (BTC) or plasma manufacturer.

^§uncollected by all the regional BTC (compared to blood collection data of the same month of 2006).

^#uncollected by the BTC involved in the epidemic (compared to blood collection data of the same month of 2006).

Plasma units uncollected, eliminated or undelivered (as detailed in table II) to the licensed private pharmaceutical company for contract manufacturing caused a loss of gross proceeds from derived medicinal products (albumin, intravenous aspecific immunoglobulins, factor VIII, factor IX, prothrombin complex, antithrombin) quantified at € 944, 600. Deducing the costs not sustained for plasma contract manufacturing, the calculated loss of net proceeds totalled € 574,000. The complete calculation of the regional economic loss must also include € 101,000 due to the elimination of 725 RBC units and €47,000 for 304 RBC units purchased through extra-regional supply. To this sum, a further € 614,000 loss of gross proceeds from the remaining 4405 RBC units not collected should be added.

The reduction of blood component usage stimulated by regional health authorities allowed the saving of 368 RBC units in September and 58 in October, producing a reduction of 1.95 and 0.28 per cent, respectively, in comparison to the same months in 2006.

The NBC set up a national plan, involving other regions of northern and central Italy, to supply the southern regions lacking their usual red blood cell delivery from Emilia-Romagna.

Figure 5 illustrates the curve of the total distribution of asymptomatic cases of CHIKV infection per week (June 10^th 2007, through October 14^th 2007) in all the affected areas; local distribution curves were produced for the towns of Cesena, Cervia, Ravenna and Rimini.

Figure 5.

Total distribution of estimated number of asymptomatic cases of CHIKV infection per week, in the Region of Emilia-Romagna, Italy, June 10^th, 2007, through October 14^th, 2007

In table III the numbers of donations necessary to yield one viraemic blood unit in all the towns involved in the CHIKV epidemic are detailed per week (June 10^th 2007, through October 14^th, 2007). The same data were calculated for each of the above mentioned towns.

Table III.

Distribution of the estimated numbers of asymptomatic cases of CHIK viraemia and of blood donations necessary to yield one viraemic unit in all the towns of the Emilia-Romagna region involved in the CHIKV epidemic, June 10^th, 2007, through October 14^th, 2007

All the Emilia-Romagna towns involved in the CHIKV epidemic
Week No.	Beginning date of the week	Estimate of the maximum number of asymptomatic viraemic cases	Rate per 100,000 of the maximum estimated number of asymptomatic viraemic cases	Estimated number of donations necessary to yield one viraemic blood unit
1	06/10/07	0.1764	0.02	5,927,149
2	06/17/07	0.1764	0.02	5,927,149
3	06/24/07	0.0000	0.00	-
4	07/01/07	0.1764	0.02	5,927,149
5	07/08/07	1.2348	0.12	846,736
6	07/15/07	1.2348	0.12	846,736
7	07/22/07	1.9404	0.19	538,832
8	07/29/07	5.8212	0.56	179,611
9	08/05/07	8.8200	0.84	118,543
10	08/12/07	9.1728	0.88	113,948
11	08/19/07	10.9368	1.05	95,599
12	08/26/07	6.8796	0.66	151,978
13	09/02/07	7.2324	0.69	144,565
14	09/09/07	1.9404	0.19	538,832
15	09/16/07	2.1168	0.20	493,929
16	09/23/07	1.9404	0.19	538,832
17	09/30/07	0.5292	0.05	1,975,716
18	10/07/07	0.0000	0.00	-
19	10/14/07	0.0000	0.00	-

The highest weekly estimated risk of yielding one viraemic unit from an asymptomatic viraemic donor was 1:3801 and was calculated in the third week of August for the municipality of Cervia, the district of the village of Castiglione di Cervia, where the largest CHIKV outbreak occurred. In the same week the highest number of asymptomatic patients (1.05) per 100,000 inhabitants was also calculated for all the areas involved in the CHIKV epidemic, when 95,599 donations were estimated necessary for one donation to be viraemic. In the other towns involved, the highest weekly estimated risk of yielding one viraemic unit from an asymptomatic viraemic donor was: (i) 1:45070 in Ravenna, during the third week of August; (ii) 1:66665 in Cesena, in the first week of September; and (iii) 1:259870 in Rimini, over the second and third weeks of September.

Conclusions

The first autochthonous CHIKV outbreak in Europe took place in the Region of Emilia-Romagna, Italy. It was fairly limited in comparison to the first and massive outbreak which occurred on La Réunion Island in the Indian Ocean, between 2005 and 2007. Indeed the total number of CHIKV patients - including the confirmed and probable cases -was three orders of magnitude lower in comparison to the estimated 312,500 out of 757,000 inhabitants of La Réunion who were affected by this mosquito-borne disease³⁹.

The Italian outbreak most probably began with a man from India who developed a febrile syndrome 2 days after his arrival in Italy, was presumably highly viraemic when visiting some relatives of his and at a later examination showed high titres of antibodies against CHIKV. The clinical course of the disease was fairly mild and the case–fatality rate (< 0.5%) was similar to that reported in La Réunion¹³. Similar to previous findings, arthralgia - which was often severe and persistent - was one of the prevailing symptoms⁴¹, and more than half of the patients presented with skin rash³.

Sequence analysis of the viral genome revealed that this outbreak was caused by a strain similar to those detected in the Indian subcontinent^30,42 and that this new variant contained the same mutation found in the Indian Ocean variant of the African genotype of CHIKV^7,13,23,30, which is currently hypothesised to have modified the virus' ability to infect A. Albopictus or, perhaps, even the severity of the disease associated with human infection¹³. The successful introduction and rapid spread of the infection -together with the further scattering throughout different localities and far from the two villages initially affected -seem to confirm this hypothesis^13,30. Moreover, such changes are common in viruses that have a positive-stranded RNA genome and might facilitate the adaptation of CHIKV to a new mosquito vector, highlighting alarming scenarios of globalisation of vector-borne diseases¹³.

Chikungunya poses considerable problems for public health authorities once an outbreak has commenced because the major environmental measures used to reduce sources of mosquito breeding may not be fully implemented within the timescale of an outbreak. Good routine programmes of vector surveillance and control are needed for a country to be able to mitigate the effects of an outbreak.

In addition, reliable and tested mechanisms of extraordinary blood supply are necessary for a national blood system to sustain the impact of the outbreak on the blood inventory. Indeed, the precautionary measures adopted - justified by the unavailability of a laboratory test for routine biological qualification of blood components -produced a considerable impact on the blood supply of Emilia-Romagna that changed its role from being an exporter of blood components to being an importer. In fact other Italian regions balanced Emilia-Romagna's lack of extra-regional output of RBC units.

The need for a safe blood supply and the precautionary principle had to be balanced against regional self-sufficiency of blood components and derivatives, especially during the phase when the epidemic was decreasing.

The mathematical model developed allowed a dynamic estimate of the risk during the decreasing phase of the epidemic and a retrospective weekly estimate of its highest level, thus providing a reasonable approximation to the mean risk of CHIKV transmission through blood transfusion. It also provided the right order of magnitude of the risk and proved to be a useful tool for "pragmatic" risk assessment and management when precautionary measures had to be modified or interrupted and blood collection was restarted.

A possible limitation of this approach could be the lack of precise knowledge of the duration of asymptomatic viremia for which further studies are needed to better document the kinetics of CHIK viraemia³⁹.

A considerable advantage deriving from this experience is that in the future the same model could be used to assess whether or not the risk of new blood-borne diseases threatens the blood supply to the same degree as other regularly screened pathogens.

Strategies such as quarantining the donated blood for 5 to 2 days or applying a mathematical model to assess the risk of blood donations from asymptomatic donors, can be applied with reasonable confidence only in the presence of an efficient and reliable case detection and reporting system together with a specific pre-donation medical evaluation of donors³¹. Indeed, these systems must ensure that the clinical evaluation of the donor is checked through at least an active and documented post-donation feed-back of absence of symptoms and that the probability of blood donation from symptomatic donors remains substantially negligible.

Despite the absence of documented cases, blood transfusion-related CHIKV transmission is plausible and the risk of viraemic donations can be considered substantial in an outbreak for several reasons³⁹: (i) the high viral load during the acute phase of the infection^23,43; (ii) the fact that some cases of CHIKV transmission were shown to be occurred among laboratory personnel handling infected blood⁴⁴; (iii) the fact that CHIKV has been transmitted to a health care worker drawing blood from an infected patient²³; (iv) mother to child transmission, very recently documented⁴⁵.

In conclusion, the comprehensive documentation³¹ of this event is of paramount importance to learn how to deal with possible future analogous emergencies through efficient co-operative interventions, vector control, active epidemiological surveillance and prompt management of blood safety and self-sufficiency. Benefits from this experience will become evident in the case of the occurrence of other new or emergent diseases which could threaten the safety and sufficiency of blood and blood product supply, which is one of the goals of any local and national blood system⁴⁶.