**2.1.2 Infectivity**

Infectivity is defined as the degree of susceptibility of *Anopheles* mosquitoes to different *Plasmodium* species, i.e. refers to the possibilities that the sporogonic cycle of parasite could be completed within a concrete vector species. It is well known that mosquito populations of the same species but different geographic areas can differ drastically at infectivity level due to genetic reasons (Frizzi et al., 1975).

Infectivity tests carried out on European populations of species of the *An. maculipennis* complex showed that *An. atroparvus* can transmit Asian strains of *P. vivax* and African strains of *P. ovale* but is refractory to African strains of *P. falciparum* (James et al., 1932; Garnham et al., 1954; Ramsdale & Coluzzi, 1975; Teodorescu, 1983; Ribeiro et al., 1989). However, more recent studies have shown the ability of *An. atroparvus* to generate oocysts of *P. falciparum* (Marchant et al., 1998), but not to complete sporogony. Information about *An. labranchiae* is quite confusing due to the scanty and old infectivity tests conducted. Moreover laboratory studies have revealed that *An. labranchiae* can transmit *P. ovale* (Constantinescu & Negulici, 1967) but populations of the vector collected in Italy were refractory to African strains of *P. falciparum* (Ramsdale & Coluzzi, 1975; Zulueta de et al., 1975). Nevertheless recent researches with populations from Corsica have indicated that *P. falciparum* cycle can be successfully completed in *An. labranchiae* (Toty et al., 2010). Furthermore *An. labranchiae* has been involved in transmission of autochthonous vivax malaria cases and in Corsica (France), Greece and Italy (Sautet & Quilici, 1971; Zahar, 1987; Baldari et al., 1998) and even several outbreaks of *P. falciparum*, *P. malariae* and *P. vivax* in Morocco (Houel & Donadille, 1953). Under laboratory conditions, *An. sacharovi* has been demonstrated as an excellent vector of *P. vivax* (Kasap, 1990) and *An. messeae* was reported, not only as being the main vector of malaria over a large part of European Russia several decades ago (Detinova, 1953), but also the responsible of disease resurgence in Russia and Ukraine more recently (Nikolaeva, 1996). With regard to *An. maculipennis* it is known that in certain coastal areas in the Balkans, Asia Minor and Northern Iran (Postiglione et al., 1973; Zaim, 1987; Manouchehri et al., 1992), the species has participated actively in malaria transmission cycles. Due to its recent description, *An. daciae* yet must be tested on its susceptibility to *Plasmodium* species

Outside the species of the *An. maculipennis* complex is remarkable that European populations of *An. plumbeus* can produce sporozoites of tropical strains of *P. falciparum* (Marchant et al., 1998; Eling et al., 2003), as well as also Eurasiatic strains of *P. vivax* (Shute & Maryon, 1974). Even some authors suggest that *An. plumbeus* is capable of transmitting the four Plasmodium species (Shute & Maryon, 1969). However this hypothesis should be confirmed with modern molecular techniques. Respect to *An. algeriensis* and *An. claviger*, it is important to note that in natural populations it has been shown the presence of oocysts of *P. vivax* at intestinal level (Blacklock & Carter, 1920; Horsfall, 1972). In the case of *An. algeriensis*, even has been successfully tested the transmission of *P. falciparum* in laboratory conditions (Becker et al., 2010). *An. superpictus* can transmit *P. vivax* (Kasap, 1990) but its susceptibility to *P. falciparum* has not been tested, although this anopheline is probably sensitive, as it belongs to the subgenus *Cellia*, to which the principal African malaria vectors also belong. Another species of the subgenus *Cellia* poorly represented in Europe, such as *Anopheles multicolor* and *Anopheles sergentii*, have been also found parasitized by *P. vivax* and *P. falciparum* in natural conditions (Kenawy et al., 1990). Finally, there is no infectivity information about *An. marteri, An. cinereus* and *An. petragnani*. Anyway the epidemiological role of these species it seems secondary due to their zoophylic behaviour and rural distribution.

#### **2.1.3 Vulnerability**

492 Current Topics in Tropical Medicine

Infectivity is defined as the degree of susceptibility of *Anopheles* mosquitoes to different *Plasmodium* species, i.e. refers to the possibilities that the sporogonic cycle of parasite could be completed within a concrete vector species. It is well known that mosquito populations of the same species but different geographic areas can differ drastically at infectivity level due

Infectivity tests carried out on European populations of species of the *An. maculipennis* complex showed that *An. atroparvus* can transmit Asian strains of *P. vivax* and African strains of *P. ovale* but is refractory to African strains of *P. falciparum* (James et al., 1932; Garnham et al., 1954; Ramsdale & Coluzzi, 1975; Teodorescu, 1983; Ribeiro et al., 1989). However, more recent studies have shown the ability of *An. atroparvus* to generate oocysts of *P. falciparum* (Marchant et al., 1998), but not to complete sporogony. Information about *An. labranchiae* is quite confusing due to the scanty and old infectivity tests conducted. Moreover laboratory studies have revealed that *An. labranchiae* can transmit *P. ovale* (Constantinescu & Negulici, 1967) but populations of the vector collected in Italy were refractory to African strains of *P. falciparum* (Ramsdale & Coluzzi, 1975; Zulueta de et al., 1975). Nevertheless recent researches with populations from Corsica have indicated that *P. falciparum* cycle can be successfully completed in *An. labranchiae* (Toty et al., 2010). Furthermore *An. labranchiae* has been involved in transmission of autochthonous vivax malaria cases and in Corsica (France), Greece and Italy (Sautet & Quilici, 1971; Zahar, 1987; Baldari et al., 1998) and even several outbreaks of *P. falciparum*, *P. malariae* and *P. vivax* in Morocco (Houel & Donadille, 1953). Under laboratory conditions, *An. sacharovi* has been demonstrated as an excellent vector of *P. vivax* (Kasap, 1990) and *An. messeae* was reported, not only as being the main vector of malaria over a large part of European Russia several decades ago (Detinova, 1953), but also the responsible of disease resurgence in Russia and Ukraine more recently (Nikolaeva, 1996). With regard to *An. maculipennis* it is known that in certain coastal areas in the Balkans, Asia Minor and Northern Iran (Postiglione et al., 1973; Zaim, 1987; Manouchehri et al., 1992), the species has participated actively in malaria transmission cycles. Due to its recent description, *An. daciae* yet must be tested on its susceptibility to

Outside the species of the *An. maculipennis* complex is remarkable that European populations of *An. plumbeus* can produce sporozoites of tropical strains of *P. falciparum* (Marchant et al., 1998; Eling et al., 2003), as well as also Eurasiatic strains of *P. vivax* (Shute & Maryon, 1974). Even some authors suggest that *An. plumbeus* is capable of transmitting the four Plasmodium species (Shute & Maryon, 1969). However this hypothesis should be confirmed with modern molecular techniques. Respect to *An. algeriensis* and *An. claviger*, it is important to note that in natural populations it has been shown the presence of oocysts of *P. vivax* at intestinal level (Blacklock & Carter, 1920; Horsfall, 1972). In the case of *An. algeriensis*, even has been successfully tested the transmission of *P. falciparum* in laboratory conditions (Becker et al., 2010). *An. superpictus* can transmit *P. vivax* (Kasap, 1990) but its susceptibility to *P. falciparum* has not been tested, although this anopheline is probably sensitive, as it belongs to the subgenus *Cellia*, to which the principal African malaria vectors also belong. Another species of the subgenus *Cellia* poorly represented in Europe, such as *Anopheles multicolor* and *Anopheles sergentii*, have been also found parasitized by *P. vivax* and *P. falciparum* in natural conditions (Kenawy et al., 1990). Finally, there is no infectivity information about *An. marteri, An. cinereus* and *An. petragnani*. Anyway the epidemiological role of these species it seems secondary due

**2.1.2 Infectivity** 

*Plasmodium* species

to their zoophylic behaviour and rural distribution.

to genetic reasons (Frizzi et al., 1975).

Vulnerability is determined by the number of gametocyte carriers (malaria patients) during the suitable period for malaria transmission. If we analyze the data about imported malaria in Europe in recent years we can extract several conclusions. Malaria represents about 77% of tropical diseases imported in Europe (TropNetEurop, 2010). A total of 65.596 cases were reported in Europe between 2000 and 2009 (Table 3). However this number is clearly underestimated, since in last years the number of malaria reporting sites in Europe has increased significantly. Most of these cases are referred to immigrants (48.5%), and *P. falciparum* (81%) was the dominant species in analytic results. A high percentage of malaria cases in immigrants correspond to Visiting Friends and Relatives (VFR). This group of special epidemiological significance refers to those people who, once are established in their host countries, often travel to their origin countries to visit family or friends. Travels that these people can do to their origin countries exponentially increase the chances of disease contracting, since usually these areas are endemic regions and the stay within resident population and their customs is often long and intense (Gascón, 2006). Therefore this is an important collective to promote the need to take appropriate prophylactic measures during travels to endemic areas. Several studies have revealed that only 16% of VFR search for medical advice pre-travel, being malaria prophylaxis practically nonexistent in this collective (Leder et al., 2006). The European countries with higher number of imported malaria cases reported yearly are France and Germany, usually followed by other like Spain, Italy or Belgium. As it was shown before, malaria receptivity is remarkable in concrete regions of these countries.


*Note 1*: Unkn./Coinf. (Plasmodium species unknown or coinfection of various species), Imm./Refu. (Immigrants/Refugees), For. Vis. (Foreign Visitors), Eur. E.C. (Europeans living in EC), Eur. Exp. (European Expatriates).

Table 3. Imported malaria in Europe between 2000-2009 (TropNetEurop, 2010).

Re-Emergence of Malaria and Dengue in Europe 495

quantitative terms in Old continent. Of the hundreds of dengue imported cases reported yearly en Europe (Table 4), the vast majority are represented by tourists (about 84%). A difference to what happens with malaria, immigration (9%) seems to have comparatively little influence on dengue importation. This could be explained, of course by distinct perspectives and approaches of European tourists (e.g. travels to urbanized areas) and immigrants who come to Europe (e.g. Africa, where malaria is much prevalent than dengue, is the main origin from immigrants who arrive to Europe), but also by differences between incubation periods and existing prophylactic measures in both diseases. All dengue cases reported have shown the typical symptomatology of disease, including febrile symptoms in more than 90% of cases (TropNetEurop, 2010). However, it is important to note that the majority of imported dengue infections remain undiagnosed, with a ratio between symptomatic and asymptomatic travelers estimated in 1/3.3 (Cobelens et al., 2002). In general terms, it is estimated that about 80% of all dengue infections are asymptomatic (Farrar, 2008). This high asymptomatic, added to the fact that dengue is not a notifiable disease in much of European countries (Bueno Marí & Jiménez Peydró, 2010c; 2010d), allow

us to consider that the knowledge of dengue virus circulation is very limited.

852 (48)

742 (46)

2001 2002 2003 2004 2005 2006 2007 2008 2009 Total

1167 (50)

1273 (53)

1419 (57)

1553 (61)

9170

1023 (51)

Imm./Refu. 10% 5.5% 8.2% 12.9% 6.8% 10.5% 9% 6.8% 11.3% 9%

For. Vis. 0.8% 0.5% 1% 0% 1.2% 4.8% 2.2% 0.8% 2.4% 1.5%

Eur. E.C. 86.7% 91.3% 79.6% 81.9% 87.7% 77.1% 81.3% 87.3% 83.9% 84.1%

Eur. Exp. 2.5% 2.7% 11.2% 5.2% 4.3% 7.6% 7.5% 5.1% 2.4% 5.4%

*Note 1*: Imm./Refu. (Immigrants/Refugees), For. Vis . (Foreign Visitors), Eur. E.C. (Europeans living in

*Aedes aegypti* is the primary urban vector of dengue and yellow fever basically because it exist a 'domesticated' form of the species that is rarely found more than 100 m from human habitation and feeds almost exclusively on human blood (Reiter, 2010). Both factors allow that *Ae. aegypti* will be considered as an excellent urban vector of viruses. Its distribution was traditionally limited by latitude between 45º N and 35º S according to the existence of January and July 10° C isotherms. Although records out of this latitude range are very rarely, it must be pointed that European northernmost collection of the species occurred in Brest (France) at 48º N (Christopher, 1960). Moreover recent studies have demonstrated that *Ae. aegypti* larvae can withstand temperatures of 2.5° C (Chang et al., 2007). In Eastern Europe it was also seen at its temperature limit at Odessa (Ukraina) at 46º N. (Korovitzkyi and Artemenko, 1933). Despite the species was relatively common in Mediterranean countries, it disappeared from the entire region in the mid-20th century, for reasons that currently are not clear but probably related with thermic tolerance and intensive mosquito

Table 4. Imported dengue in Europe between 2001-2009 (TropNetEurop, 2010).

Cases (sites reporting)

477 (37)

EC), Eur. Exp. (European Expatriates).

664 (47)

The temporal distribution analysis of imported malaria cases indicates that high-risk months for disease transmission (between July and September) also coincides with the period of the most cases reported in Europe. Therefore most of cases occur during the epoch theoretically favorable for malaria transmission. In regard to the diagnostic delay, i.e. the average time between appearance of symptoms and malaria diagnosis (when therapy began), it shows disparate values according to each country. For example, in Eastern Spain the diagnostic delay of imported malaria was estimated in 13.7 days (Bueno Marí & Jiménez Peydró, 2012), while in other European countries like Sweden, France or Italy values are clearly lower, ranging from 3 to 8.2 days (Romi et al., 2001; Askling et al., 2005; Chalumeau et al., 2006). From an epidemiological point of view it is very important to reduce the diagnostic delay, because this is the period when malaria patients could be a source of infection for *Anopheles* females. Additionally, from an exclusively clinical perspective, delay to diagnosis leads of course to high parasitemia, which itself leads to severe forms of malaria.
