Arboviruses are viruses mainly transmitted by arthropods, such as mosquitoes, flies, ticks, or fleas.
Unlike many other virus classifications, the grouping of arboviruses is based on an epidemiological criterion — the most common route of infection — instead of a phenotypic one. As a result, the arbovirus category includes very different types of viruses, making it impossible to characterize the group in terms of common clinical symptoms or shared virus traits (such as morphology, main host organisms, genome structure, etc.).
Nevertheless, their common infection route allows us to broadly characterize their geographic distribution, which is limited by the presence of suitable vectors. Most arthropods require warm and humid conditions to survive, and therefore arboviral diseases are mostly present in the tropics, where appropriate climatic conditions ensure the presence of vectors year-round or at least for entire seasons.
Most relevant arboviruses are naturally sustained by establishing strong anthroponotic transmission cycles. Essentially, this means that infected humans transmit the virus to biting arthropods, which then infect other human hosts when feeding again. At the same time, zoonotic transmission cycles, involving domestic or wild animals in the epidemiological cycle, are also common for most arboviral diseases, although they are usually less significant from an epidemiological standpoint.
In the first edition of ArboCat, we focus on three major arboviruses: dengue, chikungunya, and Zika.
Arboviral Disease in Southern Europe: Current Situation and Future Threats
Global warming is expanding the geographical range of suitable habitats for most vectors, which, together with increasing human movement and changes in land use and vegetation cover, may lead to the establishment of local arbovirus transmission cycles in temperate regions. In fact, autochthonous cases of dengue (France, Croatia, Madeira, Catalonia, and Murcia) and chikungunya (Italy, France) have already been observed in southern Europe. These two diseases, along with Zika and yellow fever, are transmitted by mosquitoes of the genus Aedes, the two most widespread species being Aedes aegypti and Aedes albopictus.
The former (Aedes aegypti) feeds exclusively on humans and is therefore the most effective transmitter of the disease. It is the main cause of the immense burden these diseases place on tropical regions. The latter (Aedes albopictus), on the other hand, feeds less frequently and on multiple host species, making it a less efficient transmitter. However, it spreads geographically much further than its sister species — which only lives in the tropics and is very sensitive to environmental conditions — and is thus responsible for the increasing arboviral colonization of temperate regions. In fact, the Invasive Species Specialist Group considers Aedes albopictus one of the top 100 invasive species. It was first introduced to Europe around 1979 and is now present throughout the Mediterranean basin, as shown in the following map:
Source: ECDC
In contrast, Ae. aegypti is only present in Madeira and has also been detected once in the Canary Islands.
Aedes mosquitoes are well adapted to urban environments, whether tropical or temperate — unlike, for example, other mosquitoes of the Anopheles genus, vectors of malaria. This poses an additional threat to our highly urbanized societies. In fact, current projections estimate that about two-thirds of the global population will live in urban areas by 2050 (https://www.un.org/development/desa/en/news/population/2018-revision-of-world-urbanization-prospects.html).
Overall, the presence of Aedes albopictus in Catalonia opens the door to the possibility of a disease outbreak caused by Aedes mosquitoes, which would necessarily have to begin with the arrival of an infected person from an endemic country (known as an imported case). As mentioned earlier, the diseases that these vectors can transmit are mainly yellow fever, dengue, chikungunya, and Zika. The global burden of yellow fever has been drastically reduced thanks to the development of a very effective vaccine. Currently, only occasional self-limited outbreaks occur (usually of zoonotic origin), and the probability of importation is very low. On the other hand, dengue and chikungunya have already demonstrated the potential threat they pose to southern Europe, as has Zika, evidenced during the 2015 pandemic in South America.
Although they are not the only arboviral threats looming over southern Europe due to global warming and globalization, ArboCat currently focuses on modeling the risk of a dengue, chikungunya, or Zika outbreak in Catalonia caused by the Aedes albopictus mosquito species, which is undoubtedly the most urgent and vivid emerging concern we currently face. The most relevant epidemiological elements of these diseases are described below:
Dengue
The dengue virus (DENV) is a +ssRNA virus (Group IV of the Baltimore Classification) belonging to the Flavivirus genus (Flavus means yellow in Latin, referring to the jaundice often caused by these viruses, such as the yellow fever virus).
There are five closely related serotypes (DENV1, DENV2, DENV3, DENV4, DENV5), and in endemic tropical regions, different combinations of the first four coexist. An infection with any of the serotypes is usually (about 80% of cases) asymptomatic or only causes very mild symptoms. Regarding the remaining 20% of cases, symptoms similar to the flu with varying severity appear. This clinical manifestation of dengue usually involves high fever, severe headaches, retro-orbital pain, myalgia, arthralgia, nausea, vomiting, gland swelling, or rashes (exanthems).
Symptoms typically last 2–7 days, after which the person recovers fully. However, about 1% of infected individuals develop complications and potentially life-threatening symptoms (which may appear as dengue hemorrhagic fever or dengue shock syndrome), involving plasma leakage, fluid accumulation, respiratory difficulty, severe bleeding, or organ dysfunction.
There is no specific treatment for dengue, and a vaccine is not yet available. Infection with a specific serotype provides lifelong immunity to that serotype and temporary cross-immunity to the others. However, subsequent infections with other serotypes (common in hyperendemic areas) increase the risk of developing severe dengue.
The global incidence of dengue has increased dramatically over the past few decades. Before 1970, only nine countries had experienced severe dengue epidemics. However, today dengue is endemic in more than 100 countries across Africa, the Americas, the Eastern Mediterranean, Southeast Asia, and the Western Pacific. Recent estimates indicate that 390 million dengue infections occur each year, of which about 96 million present clinical manifestations; currently, more than half of the world’s population is at risk of contracting this infectious disease.
More information can be found in the WHO Dengue Fact Sheet (https://www.who.int/news-room/fact-sheets/detail/dengue-and-severe-dengue), from which most of the data presented here is derived.
Chikungunya
The chikungunya virus (CHIKV) is a +ssRNA virus (Group IV of the Baltimore Classification) that belongs to the Alphavirus genus. The word “chikungunya” means “to become contorted” in the Kimakonde language, a reminder of the severe joint pain caused by the disease
Unlike dengue, chikungunya infection usually presents symptoms (it is estimated that only about 3–22% of cases are asymptomatic). It generally begins with a sudden and intense onset of high fever, accompanied by severe joint pain. Around 50% of patients develop rashes, along with other typical symptoms such as myalgia, headaches, nausea, and fatigue.
These clinical manifestations usually last a few days, but in some cases, joint pain can persist for months or even years. Occasional complications affecting the eyes, nervous system, and heart have been described, but serious complications are uncommon. Unlike dengue, chikungunya rarely causes hemorrhagic complications.
Chikungunya was first described in Tanzania in 1952 and spread rapidly. A significant outbreak occurred on several Indian Ocean islands in 2005, and it is now present in more than 60 countries.
Currently, there is no vaccine or antiviral drug for chikungunya. Treatment focuses exclusively on symptom relief. For more information: https://www.who.int/en/news-room/fact-sheets/detail/chikungunya
Zika
The Zika virus (ZIKAV) is a +ssRNA virus (Group IV of the Baltimore Classification) belonging to the Flavivirus genus, like dengue and the yellow fever virus. Zika infections are asymptomatic in 80% of cases, and in 20%, they manifest with mild symptoms such as fever, rashes, conjunctivitis, myalgia, arthralgia, general malaise, or headache, lasting between 2 and 7 days. ZIKV was first isolated in 1947 from a febrile macaque captured in the Zika Forest, Uganda.
Initially, it was suspected to be a mainly zoonotic disease, and it wasn’t until 2007, during an outbreak on Yap Island in Micronesia, that it was confirmed Zika could cause strong anthroponotic transmission. Despite its high morbidity (affecting three-quarters of the island’s population), it was described as a mild, self-limiting illness without significant health consequences.
However, during the global spread that began in 2013, two possible complications emerged as major public health concerns: first, a small percentage of infected individuals were later found to develop Guillain-Barré syndrome, a paralytic autoimmune disease, or other neurological problems. Second, infection during certain stages of pregnancy was found to cause irreversible neurological damage to the fetus (especially microcephaly).
There is currently no specific treatment or vaccine for Zika. Exposure to the virus has been observed to protect against future infections, but it is still unknown whether this immunity is lifelong or temporary. Moreover, immunity against the clinical manifestation of the disease may not protect fetuses of immune mothers from neurological damage.
Zika is one of the fastest geographically spreading viruses. As of today, a total of 84 countries and territories have documented mosquito-borne Zika virus transmission. More information can be found at https://www.who.int/news-room/fact-sheets/detail/zika-virus
