Certain emerging vector-borne diseases are entering high income countries’ attention in an unprecedented way.
Two years ago we wrote about chikungunya, a disease that most Spaniards—including doctors—had not even heard of, but which has already become far more familiar. Now Zika virus disease is gaining broad interest in the wake of its impact on foetal development and in the detection of imported cases, including in the United States and Europe. Without dismissing Zika as a public health priority, we may need a broader perspective on a problem that certainly will not stop growing during the next decade.
Let’s start with a summary of the main factors favouring the transmission of emerging vector-borne diseases in our environment: 1) globalisation, which allows fast international transit and the possibility of introducing foreign agents into new areas; 2) climate change, which can increase the density of vectors and permit their spread in previously uninhabitable zones; 3) the urbanisation of natural environments, which brings households closer to vectors and possible intermediate hosts (domestic animals); 4) the high susceptibility of the population, as it has not been previously exposed to those agents; 5) the ineffectiveness of vector eradication programmes.
The convergence of these factors is producing simultaneous phenomena, including the introduction of vectors and emerging diseases into new territories, and increases in the incidence of indigenous diseases that traditionally produced only sporadic cases. Nevertheless, the scope of these diseases is often underestimated due to the lack of specific surveillance systems and the unspecificity of their clinical manifestations.
As an example, West Nile fever, principally transmitted by Culex mosquitoes, was reported in more than 200 humans each year in the EU in both 2012 and 2013.
Other vectors, such as Phlebotomus sandflies, have colonised a large part of the European continent. The increased density of these vectors, coupled with the emergence of new intermediate reservoirs (Leporidae) and the proximity of the population to this animal reservoir due to changes in land use, has produced one of the biggest outbreaks of leishmaniasis in recent years in the region of Madrid (Spain). Toscana virus, transmitted by the same vector, is estimated to be one of the main causes of viral meningitis and meningoencephalitis during the summer months in countries such as Italy. Even if surveillance of the Toscana virus is not standardised, several seroprevalence surveys have pointed out that a considerable proportion of residents of the Mediterranean coast have been exposed to the virus.
Among the Aedes vectors, Aedes albopictus—the tiger mosquito—is widespread and continuously adapting to new territories and areas in Europe. In Spain alone it is present in 14 of the 45 provinces and it has been newly detected in two of them during the past six months. Aedes Aegypti—a vector that has proven more efficient than Aedes Albopictus for the transmission of arbovirus such as dengue, chikungunya, or Zika—is already present across the Black Sea and Madeira Islands (Portugal) and could potentially adapt to the Mediterranean coast. In France and Croatia local transmission of dengue was reported for the first time in 2010, as well as a considerable outbreak in Madeira. Additionally, thousands of imported cases of chikungunya have been recorded in Europe over the past two years and hundreds of imported cases of Zika in recent months.
In addition to this, we must take into account the threat posed by the reintroduction of eradicated diseases such as malaria through Anopheles vectors. In Europe there are more than 5000 imported cases of malaria each year because of the large number of travellers returning from countries where malaria is endemic. This infectious disease, once eradicated from our territories due to the effectiveness of control campaigns during the 1950s and 1960s, is now back in the spotlight.
Vector-borne diseases are a reflection of the complex world in which we live and constitute a global challenge that requires global solutions, including: the reinforcement of national and international coordination mechanisms; the development of preparedness and response plans at points of origin and entry; and firm commitment from governments, intergovernmental agencies, and NGOs.
Laura Reques Sastre is a medical doctor and specialist in preventive medicine and public health. She is on Twitter @requeslaura
Aser García Rada is a paediatrician and a freelance journalist. He is on Twitter @AserGRada
Competing interests: Laura Reques Sastre has no competing interests to declare. Full disclosure on Aser’s biography.