Vectorborne diseases

Several important diseases are transmitted by vectors such as mosquitos, ticks or rodents. These vector organisms are sensitive to climatic conditions, especially temperature and humidity. Thus, the distribution of vector-borne diseases is restricted by the climatic tolerance limits of their vectors. Further, biological restrictions that limit the survival of the infective agent in the vector population also determine the absolute limits for disease transmission.

Climate plays a role in determining the distribution and abundance of insect species, either directly or indirectly through its effects on host plants and animals. Climate change is therefore expected to affect the geographical range and seasonal activity of many vector species (9). This sensitivity is reduced, however, if the vector is adapted to an urban or domestic environment. In addition, land-use change is also likely to be the major factor in future changes in vector distribution and abundance in Europe.

The effect of climate change on actual human cases of disease is much harder to forecast than changes in the distribution of the vectors. The life-cycle stages of the infecting parasite within the vector are also limited by temperature. A minimum temperature threshold is required to complete the extrinsic incubation period. These limits will expand northwards with climate change.

The current main vector-borne diseases in Europe can be classified as:

• formerly widespread, such as malaria, which is currently epidemic in Armenia, Azerbaijan, Tajikistan, Turkey and Turkmenistan;

• locally endemic, such as leishmaniasis in southern France, Italy, Portugal and Spain, and tick-borne encephalitis in southern Scandinavia and central and eastern Europe; and

• emerging diseases, such as Lyme disease, which is prevalent over much of Europe.

These diseases are addressed in more detail below. Malaria

Malaria is the most important vector-borne disease worldwide, and has also become a growing problem in Europe in recent years (Fig. 8 and 9, Table 3). Five of the 51 countries in the WHO European Region currently have epidemics: Armenia, Azerbaijan, Tajikistan, Turkey and Turkmenistan. In 1994, the population of these countries was estimated at 82 million, and 88 313 cases of malaria (passive case detection) were reported. A recent assessment in Tajikistan found that the incidence of malaria was 22 per 1000 population, and in one district about 15% of these cases were caused by Plasmodium falciparum (110). This incidence is 10 times that reported using routine or passive surveillance.

Control measures, both before and during the global effort by WHO to eradicate malaria, sharply reduced the incidence of the disease in Turkey. By 1971, P. falciparum had been eradicated and only 2046 cases of P. vivax were recorded, most of which were found in a small area in southeastern Anatolia. From the late 1960s, however, vigorous expansion of irrigation in the Adana-^ukorova plain allowed the main vector, Anopheles sacharovi, to proliferate. Extensive

Fig. 8. Cases of malaria in the WHO European Region in 1997

Fig. 8. Cases of malaria in the WHO European Region in 1997

Source: WHO Regional Office for Europe.

Fig. 9. Imported malaria cases per 100 000 population in nonendemic countries in Europe in 1997

Source: WHO Regional Office for Europe.

Table 3. Malaria outbreaks in the European Region

No. of

Total

cases with

Country

Year

no. of

confirmed

Main vector

Parasite

cases

local vector

transmission

Azerbaijan

1996

13 135

13 135

An. sacharovi

P. vivax

Tajikistan

1997

30 054

30 054

An. superpictus

P. vivax

An. pulcherrimus

P. falciparum

Turkmenistan

1998

137

129

An. superpictus

P. vivax

Ural Mountains

(Russian Federation) 1996

12

2

An. messae

P. vivax

Uzbekistan

1983-1992

755

39

An. superpictus

P. vivax

An. pulcherrimus

Turkey

1997

35 456

35 446

An. sacharovi

P. vivax

Source: WHO Regional Office for Europe.

Source: WHO Regional Office for Europe.

agricultural development also attracted a steady flow of migrant labour from the areas of southeastern Anatolia that had malaria. Inevitably, malaria transmission quickly increased and, by 1977, more than 100 000 cases of P. vivax malaria were reported from Adana and the adjacent provinces of Hatay and Igel (88.1% of all cases). Concentrated efforts entailing considerable cost succeeded in reducing the number of cases countrywide to 15 000 by 1989. This could not be sustained, however, and the malaria situation deteriorated once more, the vast majority of cases being reported from southeastern Anatolia.

One of the largest development projects in the eastern Mediterranean area is under way in this region. The Southeastern Anatolia Project involves the construction of 13 dams, 19 hydroelectric power plants and an irrigation network for 1.7 million hectares of land. This irrigation project and social changes in the region have contributed to the increased risk of malaria now facing Turkey. In 1990, only 8886 cases were reported from the entire country versus 12 218 in 1991, 18 676 in 1992, 47 210 in 1993 and 84 345 in 1994. In recent years, the Government has renewed its efforts to fight malaria, incorporating them into the Southeastern Anatolia Project with support from the United Nations

Development Programme and WHO. In 1998, 36 461 cases were reported, 87.1% from southeastern Anatolia, 8.7% from the Adana area and 4.2% from other areas of Turkey.

Local transmission has been reported recently in Turkmenistan, Uzbekistan and the Ural Mountains (Russian Federation) and is thought to have originated from cases imported from nearby Afghanistan, Azerbaijan or Tajikistan (111). Malaria was probably imported into the countries of the former USSR as a result of war in Afghanistan and Azerbaijan and the associated population movements across national borders.

Governments were unable to afford insecticides or to manage appropriate vector control programmes. Consequently, mosquito densities increased and enhanced the probability of local malaria transmission of imported parasite strains. There is now a risk that malaria may be introduced to surrounding countries where potential malaria vectors are present. The importation of cases into Bulgaria and Romania from the countries of the former USSR is now increasing, and this increases the risk of local transmission. This risk of the reintroduction of malaria to the eastern part of the European Region could be increased by climate change.

Concomitant with increases in the volume of international travel, the number of imported cases of malaria has increased steadily in all countries in Europe. It has been estimated that, in the period 1985-1989,

16 000 people living in Europe were infected with malaria while travelling, but this number is likely to be an underestimate (112). In 1994,

17 deaths from malaria were recorded among European travellers.

Data from the United Kingdom (113) show that cases of imported malaria rose from 66 in 1966 to 2500 in 1996 - nearly a 40-fold increase, some of which is explained by the increase in the volume of international travel. The majority of imported cases originated in Africa. Most imported cases are caused by P. vivax, but the prevalence of imported falciparum malaria increased by some 15% during 1977-1986 in the United Kingdom (114). Since 1988, P. falciparum has accounted for more than half the cases (113). In 1997, a rise in cases imported from East Africa was noted from the malaria epidemics triggered by heavy rains in Kenya and Uganda. Other European countries with imported malaria are France, Germany and Italy.

Airport malaria occurs when vectors that have arrived on aircraft transmit the disease to people. People who work and live in or near airports are most at risk. Since 1969, 63 cases of airport malaria have been reported in western Europe, and most were caused by P. falciparum. Some tropical vectors can survive in the low temperatures of a luggage compartment and are therefore ideal for overseas transportation. For example, An. arabiensis from Madagascar overwinters in altitudes above 1500 m (115). Six cases of airport malaria were described in and around Charles de Gaulle Airport in Paris during the very hot summer of 1994 (116). Consequently, aircraft are now sprayed regularly with residual pyrethroids.

The six major vectors of European malaria - An. atroparvus, An. labranchiae, An. maculipennis, An. messeae, An. sacharovi and An. superpictus - are distributed throughout the continent. Jetten & Takken (114) have reviewed the published data on vector distribution, but many of these are probably out of date. The current distribution of malaria vectors in Europe needs to be mapped.

Transmission has been reported from Greece, Italy, Portugal and Spain (114). The population of An. labranchiae has recently increased in Italy because of large-scale rice cultivation. The prevalence of An. messeae has increased in the Russian Federation (Russian Plains, lower Volga, Crimea and the Ural Mountains) as a result of environmental changes such as eutrophication of lakes and ponds and warmer springs. Consequently, local transmission of P. vivax malaria has occurred in these regions (111).

Introduced, indigenous or autochthonous malaria is defined as an infection transmitted by a local Anopheles mosquito in a country that has achieved eradication (117). This is only possible when the following conditions are fulfilled:

• a sufficient density of local Anopheles mosquitos;

• a sufficient incidence of imported malaria;

• compatibility between the local vectors and the imported Plasmodium strain; and

• optimal climatic factors allowing a complete sporogonic cycle in the vector.

In western Europe, several cases of local transmission have also been reported. In Corsica in 1970-1971, P. vivax caused an autochthonous outbreak of malaria that infected both tourists and local residents (117). In 1997, a woman with no history of travel or blood transfusion, living far from the nearest airport, was diagnosed with P. vivax infection in Maremma, Italy. Investigations revealed that the parasite had been transmitted by An. labranchiae, the previous vector of malaria in Italy. The vector had acquired the parasite from a neighbour infected with P. vivax during a trip to India (118). This case illustrates the ease with which malaria can be transmitted when the above conditions are fulfilled.

There is concern that imported cases may lead to the reintroduction of P. falciparum malaria in Europe. Some local mosquitos were clearly once vectors of the European strain of P. falciparum, but it is not known whether they also are capable of transmitting its tropical strains. It has been shown that An. atroparvus, An. messeae, An. sacharovi and An. labranchiae are refractory to strains of P. falciparum from India, Kenya and the Malayan peninsula (119-122). However, the suspected vector of malaria in the United Kingdom, An. plumbeus, has been infected with P. falciparum, although it is not yet clear whether the mosquito is able to transmit the parasite (121). These studies therefore suggest that, in general, European vectors of malaria are not able to transmit tropical P. falciparum malaria. Only after a long period of selection would the tropical parasites become adapted to transmission by Anopheles spp. in Europe.

The risk of reintroduction of vivax malaria in western and central Europe under conditions of climate warming must be addressed. P. vivax is present in the eastern part of the European Region and has been responsible for recent local cases (described above). The vectors An. atroparvus, An. sacharovi and An. messeae are susceptible to P. vivax from Africa, Asia and South America (119). There is a risk that P. vivax is homogeneous in its adaptation to vectors. This would mean that the parasite could be imported from any endemic country.

Although climate warming is predicted to continue, it is unlikely to have more than a very limited role in the countries of the European Region. Milder winters could increase the potential for malaria transmission in a season, but they would negatively affect the geographic distribution of most palaearctic malaria vectors by reducing the extent and presence of their breeding places. Nevertheless, more research and accurate models are needed to predict the effects on malaria of climate change.

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  • Clarence
    Does depleted ozone layer cause increase vector borne disease?
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    How ozone depletion cause malaria?
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