Eurosurveillance, Volume
15, Issue
2,
14 January 2010
Epidemiological analysis of mosquito-borne Pogosta disease in Finland, 2009
Pogosta disease is a viral disease caused by a mosquito-borne alphavirus, Sindbis virus (SINV), and large human outbreaks of SINV infection have emerged in Finland every seven years. After a major outbreak in 2002 an epidemic was expected to take place in 2009. Data from the National Infectious Disease Registry showed a small outbreak in humans in 2009 with a total of 105 reported cases but the seven-year cycle did not recur as anticipated.
Introduction
Sindbis virus (SINV) is a mosquito-borne alphavirus (of the family Togaviridae), present in Eurasia, Africa and Oceania [1,2]. Antibodies to SINV are detected in humans in various geographical areas but clinical infections caused by SINV are reported mostly from Finland where SINV is associated with fever, rash and arthritis, known as Pogosta disease [3]. The treatment is symptomatic. Clinically similar diseases are found in Sweden (Ockelbo disease) and in Russia (Karelian fever) [4,5]. The majority of clinical cases occur in Finland during August and September when the primary vectors, the ornitophilic late summer mosquito species Culex and Culiseta, are abundant. The incidence of Pogosta disease has been highest in the eastern parts of Finland in recent decades.[3,6].
Remarkably, outbreaks of Pogosta disease have thus far emerged every seven years since the first outbreak was noted in 1974, and the cause for this phenomenon is yet to be discovered. Tetraonid birds such as grouse, might contribute to this pattern [3]. Grouse have previously shown population cycles with population “crashes” coinciding with SINV outbreaks [7]. Antibodies to SINV have been detected in grouse and migratory birds [3,6]
The last major epidemic in Finland took place in 2002 with almost 600 reported cases and it was anticipated that an outbreak would occur again in 2009. This paper describes the characteristics of the Pogosta disease in Finland from June through October 2009 and discusses the findings in relation with the previous epidemic.
Methods
Since 1995, all confirmed diagnoses of SINV infection have been reported to the National Infectious Disease Registry (NIDR) at the National Institute for Health and Welfare (THL). Notifications include information on date of sample collection, date of birth, sex, and on place of treatment. Multiple notifications of persons with the same date of birth, sex and place of treatment received within a 12-month period were combined as one case. The place of treatment refers to the health care center or hospital (in particular hospital district) where the diagnosis has been made. Data were analysed by sex, age, week and month of disease onset and by hospital district of treatment. Finland has a population of 5.3 million and is divided into 20 hospital districts. Laboratory diagnosis is based on enzyme immunoassays (EIA) and/or in some cases a haemagglutination inhibition test (HI) (5).
Results
From June through October 2009, a total of 105 laboratory confirmed cases were reported to the NIDR and the incidence of SINV infection was two cases per 100,000 inhabitants per year (Figure 1). Most of the cases occurred in September (n=60) followed by August (n=33). Sixty percent (n=63) of the cases were females. The highest incidence (4.6/100,000/year) was among persons aged 50-59 years. Only two of the cases were aged under 18 years.
Figure 1. Number and incidence rates of laboratory confirmed Sindbis virus cases, Finland, 1995-2009 (n=3,041)
The incidence was highest in north Karelia, followed by east Savo, central Ostrobothnia and central Finland together with southern Ostrobothnia (Figure 2). The incidence rates in 2009 were considerably lower than those in 2002.
Figure 2. Number and incidence rates of laboratory confirmed Sindbis virus cases by health care districts, Finland, 2002 (n=597) and 2009 (n=105)
The number of cases was highest in central Finland (n=15). The majority of the cases (n=10) in central Finland occurred in July-August whereas only one case was reported from north Karelia during this time period (Figure 3). On the contrary, 13 cases were reported from north Karelia and five from central Finland during the months of September-October. In 2002, cases peaked in September in both of the hospital districts.
Figure 3. Sindbis virus cases in north Karelia and central Finland, 2009 (n=29) and 2002 (n=212)
Discussion
A major Pogosta disease outbreak has occurred every seven years in Finland since 1974, with hundreds or even thousands of patients. Following this pattern, another outbreak was expected for 2009. However, the number of cases was substantially lower than in previous epidemics in 1995 and 2002 when 1301 and 597 cases were reported respectively [6]. The 105 cases reported in 2009 exceed the average number of cases (n=57) in the non-epidemic years during 1995-2009. However,in some intermediate years, 1997-1998, 2000 and 2003, the number of cases exceeded the number reported in 2009. In comparison, five SINV infections were reported in Sweden in 2009 (Sirkka Vene, personal communication, 1 December 2009).
The factors behind the puzzling cycles in the epidemiology of Pogosta disease are unclear but recently attention has focused on tetraonid birds. In the epidemic years of 1974 and 1981, grouse population crashed in north Karelia [4]. Further, detection of SINV antibodies in one quarter of grouse examinated in the year following the 2002 epidemic, indicated vast exposure of grouse to SINV, and suggested that the virus may have an endemic cycle in tetraonid birds [6]. The density of grouse was above average in 2007 but the population crashed in 2008. The recovery of grouse population can be rapid and was anticipated to occur in 2009. However, the density of their population continued to decline to an all time low (since the measurements from 1980s) [8]. Hence, it is plausible that grouse play a significant role in the human epidemiology of SINV. It is possible that the continuing decline in the grouse population in 2009 diminished the role of grouse as amplifying hosts and that therefore, a milder outbreak than expected was observed.
Similar to previous findings, the incidence of SINV was highest in the hospital district of north Karelia. The difference in incidence of this hyperendemic region with other hospital districts was not, however, as prominent as previously. In 2009 many of the hospital districts with high incidence were located in central and northwestern parts of Finland. These observations may point towards a geographical shift in the incidence of SINV virus infection. The relatively low incidence in north Karelia compared to the epidemic in 2002 may also reflect the increased human seroprevalence towards SINV which indicates immunity. The district of Kainuu had no cases, which was surprising since previous studies showed high seroprevalence in that area [6]. This could be attributable to significant underdiagnosis or acquired immunity, which may result from high number of cases during the 1970s and 1980s.
Most cases in central Finland occurred in August whereas in north Karelia the number of cases peaked in September. This could reflect the variations in mosquito activity and population size due to differences in weather conditions in these areas. The month of May was drier than normally in Joensuu (the largest city in north Karelia) but the rainfall in June-July was considerably higher than on average [9]. Perhaps the dry May in Joensuu contributed to fewer cases in July-August but the high rainfalls in June and July created better environmental conditions for mosquito development and thus, more human cases of SINV occurred in September-October. Weather conditions are also likely to influence human outdoor activities, and thereby exposure to SINV.
In summary, a limited outbreak of SINV in humans took place in Finland in late summer and autumn of 2009 but the expected seven-year cycle did not recur. The data suggest a geographical shift in disease incidence. It is likely that fluctuations in grouse populations play a major role in the occurrence of SINV epidemics. To further elucidate the role of tetraonid birds and other factors, such as weather and climate variations, more epidemiological studies and proper mathematical modeling of SINV epidemics is needed.
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