Urban pests

As far as mosquitoes are concerned there are two types of people: those they love and those they leave alone. If you are unfortunate enough to be one of those people that mosquitoes love, the future may look particularly nasty—Spain is one of the latest countries to be colonised by the Asian ‘tiger’ mosquito. Its sting is more painful than the indigenous European strain and it bites during the day, while the European variety comes out at night. Last year, the tiger mosquito was the cause of a viral epidemic in Italy. Could Barcelona be next?

The tiger mosquito (Aedes albopictus)—named for its distinctive black and white stripes, and no doubt in part too for its painful bite—was first detected in Spain in August 2004 in Sant Cugat. It spread fast, and by 2007 a total of 55 Catalan municipalities had been colonised by the insect, including Barcelona, more than double the 23 of 2006.


The mosquito is of Asian origin, but is thought to have hitched a ride to other countries by laying its eggs in the water collected in used rubber tyres. As used tyres were transported all over the world, these eggs were dispatched far and wide and the tiger mosquito proliferated in Europe. It is now established in Albania, Italy, France, Belgium, Montenegro, Israel and Switzerland. Although it hasn’t been spotted in the UK yet, health authorities are aware of the potential risk.


Servei de Control de Mosquits (SCM) is a public organisation dedicated to monitoring and controlling the mosquito population in the Baix de Llobregat area, around the delta del Llobregat. SCM keep traditional mosquito populations in check by spraying their breeding sites: water holes or ponds created by rain, and marshland. They use biological pesticides made of naturally occurring bacteria; these kill mosquito larvae while having no ill effect on the environment or on people.


Since 2005 SCM has also been trying to control the tiger mosquito. This species can be more difficult to deal with because its eggs can withstand dry periods and it can breed in a very small quantity of standing water—for instance, the amount found in a plant pot—so that a person’s home can become a haven. Dr. Roger Eritja of SCM is responsible for controlling tiger mosquito populations. “We undertake public information campaigns so that people avoid breeding them in their homes. It’s quite simple—just don’t leave even small quantities of stagnant water in vases or recipients in fresh air.”


The Ajuntament’s Agencia de Salud Pública de Barcelona (ASPB), the municipal public health authority, has also taken steps to inform the public and lay to rest any fears of health risks, according to a health agency spokesperson, who asked not to be named. “We draw up annual action plans [to combat them], the same as with cockroaches or rats. This year, 10 specialists have been informing people door to door, house to house, about the risks of the tiger mosquito.”


The spokesperson also stressed that the tiger mosquito does not pose a direct health risk at this moment. “Although the bites are very annoying, the tiger mosquito, in our environment, doesn’t carry disease.”
This is not the first time Spain has faced an invasion of mosquitoes. Aedes aegypti, or the ‘yellow fever mosquito’, was also unwittingly transported throughout the Mediterranean by sea trade as early as the 1700s. At that time the insect aided in the spread of yellow fever and dengue fever, which claimed thousands of lives up to the 19th century. In 1821 yellow fever hit Barcelona hard, killing a sixth of its population, and during the 18th and 19th centuries the disease re-occured across Europe and the Americas.


Yellow fever is just one of the many diseases that mosquitoes can pass on—given the right circumstances. Diseases can either be carried horizontally, from human to human via a mosquito, or ‘vertically’, in which the female mosquito passes the virus on to her eggs. In vertical transmission, the infected eggs lie dormant through the dry (or cold) season, and when they hatch they carry the virus, thus maintaining it from one year to the next.


Due to better sanitary conditions and knowledge about disease, we are not used to seeing mosquito-borne epidemics in Europe. That’s why last year’s outbreak in Italy was such a shock. In August 2007, chikungunya—a mosquito-borne disease related to dengue fever—swept through the small town of Castiglione di Cervia in northern Italy. The disease was traced back to someone who had recently travelled to India and brought the virus back in his blood. It was then passed on to other people via tiger mosquitoes, which were present in a high density.


Although Italy has long had a problem with its smallest foreign residents and spends €3.5 million a year to combat tiger mosquitoes, nobody predicted this outbreak. With rising temperatures, a growing population of virus-vectors and the frequency at which people now travel to and from tropical climates, could Barcelona have the potential for a similar epidemic?


“It’s true that certain strains of disease that until now weren’t seen in Europe are arriving, but the causes are multiple—the rise in temperature for one,” said the health agency’s spokesperson.


Dr Eritja went a step further, conceding that there is a very small possibility of the same happening here. “It could occur somewhere where there are the same variables of humans and mosquitoes and pathogen [disease], but these variables are very hard to unite all at once. We have to think of it in terms of probability…everything is possible to some extent. We are talking about a very low probability, although it cannot be a zero. It’s very unlikely that the situation [in Italy, last year] will happen again somewhere else.”


It is not the tiger mosquito, itself, but the changing environmental and social conditions that may lead to the mosquito becoming a vector for disease. In fact, we have been living with this potential for longer than the tiger mosquitoes have been around. “Viruses can be transmitted by many indigenous species of mosquitoes much more easily than by the tiger mosquito, so the presence of this new species is not actually an added risk,” Dr. Eritja pointed out.


Although tiger mosquitoes are not necessarily an added health threat, they are a pest that may be heading to a plant pot near you. They cannot breed during the winter months here because it is too cold, so we have yet to see which new zones will be blighted this year, but if other countries are anything to go by, experts warn that a spread is to be expected. “The species will rapidly populate most of the country, with higher concentrations in coastal areas,” Eritja predicted.


For those lucky people who never experience the bites of these tropical visitors, there is nothing to worry about, but for mosquito-magnets everywhere—watch out.