This study evaluated and described malaria cases diagnosed at the ISPTM of the Medical University of Vienna between 1 January 2010 and 31 December 2020. In total, 122 cases were recorded at our institution, which relates to about one fifth of all malaria cases reported in Austria during this time period (595 cases) [21, 22]. Results suggest an increasing trend in case numbers. The majority of patients were male (71.1%) and the median age was 34.5 years. With 55.1% P. falciparum was the most common species, followed by P. vivax (19.5%).

Although malaria case numbers worldwide have, for the most part, been declining since the turn of the millennium [20], this is not the case in non-endemic regions. At our institution, annual case numbers were consistently higher in the years 2016–2020 than from 2010–2015, with a peak in case numbers in the year 2019; however, as there was an increase in patient samples that our institution received over time, these results could rather reflect an increase in conducted tests than an actual increase in cases. In most European countries, including Austria, malaria is a notifiable disease, thus official national data are available. To better assess the overall development of case numbers within the entire country, in the following we provide absolute and relative frequencies of malaria cases using data from Austria’s national reporting system [21].

In 2010, there were 48 (8.0%, 48/595) cases, followed by 7 (1.2%, 7/595) cases in 2011, 28 (4.7%, 28/595) cases in 2012 and 42 (7.1%, 42/595) cases in 2013. During the next years case numbers increased, from 68 (11.4%, 68/595) cases in 2014 to 81 (13.6%, 81/595) cases in 2015 and a maximum of 82 (13.8, 82/595) cases in 2016. Numbers remained relatively high with 78 (13.1%, 78/595), 62 (10.4%, 62/595) and 72 (12.1%, 72/595) cases in the years 2017, 2018 and 2019, respectively, before finally dropping to 27 (4.5%, 27/595) cases in 2020. Official data therefore showed a similar pattern on case numbers as data from our institution, with the number of cases increasing in the second part of the decade, reaching a peak in 2015 and 2016 [21]. This trend was also seen in the entire European Union, where the peak was reached in 2019 with 8638 total cases (in 2020: 2321 cases) [22].

Apart from confounding factors, such as increased testing, also growing international mobility, altered travel behavior and migration movements might be responsible for the overall increase in case numbers. All of these developments were also taking place in Austria, with the past decade holding a significant influx of migrants and refugees, reaching a maximum of 88,340 asylum applications in the year 2015 [23]. Moreover, both outbound and inbound tourism increased from 2010 to 2019, as data from the World Tourism Organization (UNWTO) suggests [24]. A factor contributing to a rise in malaria infections among tourists could be an increased incidence of more “careless travelling”, as malaria often occurs in travellers who do not take chemoprophylaxis or use it inadequately [25, 26]. In contrast to the early years of tropical tourism, today, many travellers do not seek pretravel consultation at all, most often because of low risk perception [27], which is already known from the VFR group, as described above. Vygen-Bonnet and Stark reported a decline of chemoprophylaxis usage among imported malaria cases in Germany between 2001 and 2016 [10]. The majority of travellers make their bookings online [28], which, while being convenient and quick, might be counterproductive when it comes to encouraging individuals to inform themselves about health concerns prior to travelling. Even if chemoprophylaxis is prescribed, rates of non-compliance between 11% and 38% have been reported [29], with common reasons for non-adherence being “not seeing mosquitoes”, forgetfulness and medication side effects [30].

When analyzing seasonality, it was shown that most cases were diagnosed during summer and early autumn, especially in the months of August (15.6%, 19/122) and September (13.1%, 16/122). These findings are in accordance with data from the rest of Europe [22]. Seasonality is most likely related to the summer holiday season, which comes hand in hand with more people travelling abroad; however, due to the COVID-19 pandemic and its accompanying travel restrictions, international tourism came to a rather abrupt and involuntary halt in the year 2020. International arrivals dropped by 74%, with one billion fewer arrivals than in the previous year [31]. When taking a closer look at the year 2020, it becomes apparent that the majority of cases (73.3%, 11/15) were observed between January and March. With Austria’s first lockdown starting on 16 March 2020, most patients were therefore diagnosed before and at the beginning of the first coronavirus wave. During the rest of the year, with travel restrictions now largely implemented, only four more cases were diagnosed, one each in July, September, October and December. The aforementioned seasonal patterns could therefore not be observed in the year 2020.

Interestingly, the majority of cases were male (71.1%, 86/121). This finding is also in accordance with data from other European countries and European surveillance [10, 30, 32]. The male predominance might be explained by Austrian travel patterns, with potentially more men travelling to the tropics. Moreover, the majority of migrants and refugees were male [23]. Some studies have also suggested gender-specific differences in seeking pretravel consultation and adhering to chemoprophylaxis [33, 34]. Men are likely to take more risks while travelling (e.g. engage in outdoor activities, sleep in tents, etc.) and to travel to more remote, rural areas with a higher risk of acquiring malaria [35]; however, malaria was not restricted to a certain age class. Patients were aged between 1 and 84 years, with a median age of 35 years. A 2015 analysis of European surveillance data reported the same median age for patients presenting with any travel-associated illnesses in general [36]. Therefore, this result is probably a reflection of the age group in the Austrian population that is most likely to travel to tropical regions, particularly with a travel behavior that implies a higher risk to acquire vector-borne infections. It is also in accordance with the young age of many migrants and refugees [37]. Of note, most cases were adults, which is in stark contrast to endemic countries, where the main burden of malaria morbidity and mortality is carried by children under the age of 5 years [38].

With 55.1% (65/118), P. falciparum was by far the most common species. This was of little surprise, as P. falciparum has the highest prevalence of all malaria parasites worldwide [38] and is also the most likely species to cause severe disease [39], thus rarely remaining undiagnosed; however, P. falciparum was less common than one would have assumed when looking at European surveillance data, for example the ECDC reported a frequency of 88.2% for this species in the year 2019 [22]. In the current study 38.1% (45/118) of patients had tertian malaria, with P. vivax being a little more common than P. ovale. PCR was used to discriminate between P. ovale wallikeri and P. ovale curtisi infections and it was shown that P. ovale curtisi was more common (60.0%, 9/15) than P. ovale wallikeri; however, conclusions are limited by the very small case number (n = 15). Nevertheless, they are in accordance with data from returning travellers in Italy [40] and China [41], which also showed higher numbers of P. ovale curtisi cases. Additionally, a 2021 meta-analysis comparing the two types, found a significantly higher proportion of P. ovale curtisi cases [42]. Other forms of malaria were rare: only seven (5.9%, 7/118) individuals were diagnosed with P. malariae. The species is believed to have a somewhat patchy geographic distribution and to only occur infrequently in areas of endemicity [43]. It seems likely that tourists simply are only very rarely infected with this parasite. Of note, all cases infected with P. malariae were male. The cause for this remains unclear, but it might just be coincidence and attributed to the small case number (n = 7). The rarity of P. malariae cases was exceeded only by P. knowlesi infections of which there were none. This can probably be attributed to the infrequency of P. knowlesi infections in humans and a very limited geographic distribution of the species [44]. Apart from that, P. knowlesi is also difficult to diagnose by light microscopy and probably suffers from underreporting [45]; however, since the introduction of PCR into standard routine malaria diagnostics at the ISPTM in 2014, P. knowlesi cases would likely have been identified, if there had been any [19]. It was revealed that 82.6% (19/23) of individuals with a P. vivax infection were male, a higher percentage than for P. falciparum (70.3%, 46/64) and P. ovale (60.0%, 12/20). Furthermore, those infected with P. vivax were on average younger than those infected with other Plasmodium spp.: the median age was just 20.0 years, while the median age for patients infected with other species ranged from 31.0 to 36.0 years. The demographic data of P. vivax cases thus had apparent similarities to those of migrants and refugees, of which the majority were also young and male [23]. Therefore, it seems plausible that a significant proportion of these infections were imported by this population group. This hypothesis is also supported by the origin of migrants and refugees arriving in Austria: from 2014 to 2017, approximately half originated either from Syria or Afghanistan [23]. According to the WHO, Syria has not reported any indigenous malaria cases since 2004 [38], but due to the ongoing civil war, data available on malaria transmission in the country are limited; however, malaria is endemic to Afghanistan, with the predominant species being P. vivax [38, 46]. While in the current study, geographic data was only available for less than a third of all cases, Afghanistan was the most frequently named non-African country.