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By Jeffrey R. Powell
There are reasons to suspect that the serious threat that Zika can pose to human health has been exaggerated. The topic could disappear from the headlines pretty soon. But there are other mosquito-borne pathogens that we really should be concerned about, especially those that live in high latitudes.
There are two species of mosquitoes (called "vectors") that are capable of transmitting viruses that cause human diseases such as yellow fever, dengue fever, chikungunya fever or Zika. One of them is Aedes aegypti, endemic to sub-Saharan Africa and limited to hot climates, which is why these diseases have always been considered tropical. This also explains why they have not received much attention: mainly, they take place in third world countries, where there would not be much economic benefit from developing medicines or vaccines.
The second mosquito is Aedes albopictus. In 1960, he escaped from his native environment in Asia and began to conquer the world… literally. It earned its common name: the Asian tiger mosquito. It is capable of transmitting the same viruses as Ae. aegypti. Like its relative, it resides in human habitats and is an aggressive human biter. The most important thing is that it can survive for a whole year in less warm climates, it is capable of inhabiting both tropical and temperate environments.
The populations of Ae. albopictus that live in higher latitudes lay eggs in the fall that can delay their development ("diapause" in scientific jargon) until a warmer time such as spring and then hatch. Currently, permanent breeding populations are found in much of Europe and up to New Jersey, or perhaps Connecticut, in the US.
Let's not lose sight of the tiger
Why could Ae. albopictus become a pest in the US and Europe? One reason is that, while this species continues to expand, it has also been adapting over the past 30-50 years, which means we can expect larger populations. For an epidemic caused by a mosquito-borne disease to persist, a certain density of the mosquito vector is necessary for the pathogen to continue to move from one carrier to another while immunity develops. It is likely that Ae. albopictus is reaching its critical density in some areas of its expanded area.
Which virus is most likely to become a serious problem as a result of increasing populations of Ae. albopictus? Generally, Ae. albopictus does not have the same capacity as Ae. aegypti to transmit human disease viruses. However, dengue fever is endemic to areas of China where Ae. aegypti, but only Ae. albopictus, which shows that it is capable of causing epidemics of dengue fever if its density is high enough.
Even more disturbing is what happened on the small island of Reunion, in the Indian Ocean, during the years 2005 and 2006. There were around 250,000 infections (in a total population of approximately 800,000 people) of the chikungunya virus, another pathogen. transmitted by mosquitoes. However, on the island of Reunion, Ae. aegypti, only Ae. albopictus, which should not have been able to sustain a chikungunya epidemic. How did this 'weak' vector manage to cause such an outbreak? The mystery was solved thanks to subsequent research: the virus had mutated! The new strain of the chikungunya virus, responsible for the epidemic, is quite good at reproducing in Ae. albopictus. The evolution of the virus turned a weak mosquito vector into a much stronger vector.
The chikungunya is going to arrive
People affected by the chikungunya virus had symptoms similar to those of dengue: high fever, rashes and joint pain. The name comes from an African language, Makonde, and means "that bends", alluding to the posture adopted by infected people. The majority, between 72% and 97% of humans infected with chikungunya develop these symptoms, compared to a minority of between 20% and 25% of people infected with Zika or dengue.
Chikungunya fever lasts longer and has more severe symptoms than the relatively mild and short-lived Zika. Even after eliminating the chikungunya virus, symptoms can persist. One year after the appearance of an outbreak in Italy in 2007 (caused by Ae. Albopictus), 66% of the patients still had symptoms.
The chikungunya virus had not been registered in the New World until 2013, the year in which it was detected on the island of San Martín, in the Caribbean. Since then, it has spread throughout the New World tropics, probably transmitted by Ae. aegypti, the tropical mosquito. It is important to mention that the current strains of chikungunya in the New World are not the same as those in Reunion, which reproduced very well in Ae. Albopictus, the tiger mosquito.
It is not unreasonable to think that it is only a matter of time before the chikungunya strains that can be transmitted by the tiger mosquito appear in the New World, either by introducing themselves or by creating new mutations that can reproduce in Ae. albopictus even better than the mutant strain from Reunion Island. If this were to happen, Zika could be considered just a curious anecdote in the history of mosquito-borne epidemics.
Jeffrey R. Powell, Professor of Ecology and Evolutionary Biology at Yale University, is passionate about Aedes aegypti, which he has studied for 50 years. This opinion piece was published in English on the STATS website.