A research by Instituto Oswaldo Cruz (IOC/Fiocruz) in Brazil and the Pasteur Institute in France has assessed the possibility of establishment of the wild cycle of chikungunya in Brazil, which may make it harder to control the virus and impossible to erradicate it in the territory. The disease is currently found in urban centers in the country and Aedes aegypti is its main vector. Published in the science magazine Plos Neglected Tropical Diseases, the work shows that, in laboratory tests, wild mosquitoes of the Haemagogus leucocelaenus and Aedes terrens species collected in the state of Rio de Janeiro (but found in a large part of the forests of the American continent) are capable of transmitting the virus. Further study is required to verify whether Brazilian monkeys are capable of working as chikungunya reservoirs, similarly to what happens in the wild transmission cycle of yellow fever, another originally African virus.
In the African continent, where the chikungunya virus was first identified in 1952, the wild and urban cycles of the disease may occur simultaneously. In urban areas, A. aegypti mosquitoes get infected when taking a bloodmeal from a sick person, and then transmit the infection to other individuals. In African forests, different species of wild mosquitoes acquire the virus as they take a bloodmeal from sick monkeys, and then spread the infection to the other animals. In this case, human infection may occur accidentally, when humans enter the jungle and are bitten by infected vectors.
“Today, the transmission cycle of chikungunya in Brazil takes place in urban areas and involves the A. aegypti mosquito. Our results indicate that the wild mosquitoes we studied do possess the conditions necessary for the virus to establish a cycle of wild transmission in the Americas. This scenario would represent a serious public health issue, as the infection would become harder to control”, states enthomologist Ricardo Lourenço de Oliveira, head of the Laboratory of Hematozoa-Transmitting Mosquitoes of Instituto Oswaldo Cruz and leader of the study. “It is fundamental to include the chikungunya virus in a surveillance routine that involves the wild environment”, the researcher emphasizes.
Oliveira points out that there are no scientific data published regarding the susceptibility of American monkeys to the chikungunya virus. “We know that in Africa monkeys are an integral part of the wild cycle of the virus. However, there are significant differences between the species of primates that inhabit the two continents. Complementary investigations are needed to test the susceptibility to infection of primates in our continent and their potential as virus amplifiers to infect mosquitoes”, he sums up. The researcher states that it is not possible to rule out the possibility that other groups of vertebrates may also be a part of the wild cycle of transmission of the disease.
Results prove transmission potential
To assess the possibility of wild transmission, researchers tested H. leucocelaenus and A. terrens mosquitoes collected in the Brazilian state of Rio de Janeiro. The mosquitoes were fed blood samples containing the two lineages of the chikungunya virus that circulate in Brazil, called East-Central South African and Asian lineages.
The essays revealed these mosquitoes have a high potential for disseminating the disease: seven days after ingesting the blood infected with the African lineage, more than 60% of the insects presented infected viral particles (capable of causing infection) in their saliva. For some H. leucocelaenus mosquitoes, the presence of the infective virus in the saliva occurred even faster, only three days after ingesting the infected blood. As for the Asian lineage, researchers offered them blood with a lower virus count. Even then, approximately 40% of H. leucocelaenus and 5% of A. terrens presented infective viral particles in their saliva after seven days.
“The fact that these mosquitoes are competent regarding chikungunya transmission tells us that they might transmit the virus from infected people to animals in the forest, making it possible to begin a wild cycle of the disease”, states researcher Anna-Bella Failloux, head of the Arboviruses and Insect Vector Unit of the Pasteur Institute and co-author of the study. She points out that, although it involves a series of challenges, research on the wild transmission of viruses is key to fighting emerging diseases. “It is necessary to stimulate the investigation of these cycles. However, wild mosquitoes do not develop well in laboratory colonies. Another frequent difficulty is the fact that many wild viruses do not grow well in in vitro cultures. This is why we know so little about them”, she says.
Yellow fever, an example of the urban-wild shift
According to scientists, a virus’ shift from urban to wild cycle is a phenomenon observed with yellow fever in the past. Brought to the Americas from Africa, the disease initially circulated in the cities, causing great epidemics. In the early 20th century, successful campaigns for the erradication of A. aegypti stopped the disease from being transmitted in the urban areas of the continent. However, once viral circulation had already been established in the forests, with dissemination among wild mosquitoes (including H. leucocelaenus) and monkeys, periodical epidemics continued to occur in areas near forests.
“In the regions where the wild cycle occurs, vaccination has become the only effective tool to prevent cases of yellow fever. But this tool is not yet available for chikungunya”, warns Ricardo, pointing out that the steps taken to control A. aegypti do not apply to vectors in the forests. “Wild mosquitoes, such as H. leucocelaenus and A. terrens, lay their eggs in tree hollows where water accumulates during rainy periods. This kind of breeding places cannot be eliminated or sealed as we do with tires, plant pot saucers, water tanks and other examples of urban breeding grounds for A. aegypti. If a wild cycle begins, chikungunya may become very difficult to control in our country”, he adds.
Surveillance routine is the main recommendation
The authors of the study defend the organization of surveillance programs for chikungunya that include an assessment routine for monkeys and mosquitoes similar to the one recommended for yellow fever. This would make it possible to determine whether the transmission of the virus in wild environments is already happening and to monitor this possibility in the future. In areas close to forests, researchers advise doubling the attention regarding human cases of chikungunya, as the study showed that wild mosquitoes are capable of transmitting the virus. “It is crucial to fight this disease by intensifying control measures for A. aegypti, especially in areas near forests, to prevent the virus from establishing itself in forest areas”, Roberto emphasizes.
According to the scientists, surveillance actions must include the collection of samples of monkeys and mosquitoes to assess the occurrence of natural infection. In addition, it is necessary to search for antibodies in primates and other vertebrates. “In the case of yellow fever, some monkey species, such as baboons, are more susceptible to infection. Monkeys falling sick are an alarming sign that the disease is circulating - this is why they are known as ‘sentinels’. As for chikungunya, we have yet to identify which animals are the most important in terms of monitoring”, the researcher ponders.
Prior studies
In previous studies, published in April 2014, the same group of scientists pointed out for the first time the possibility of chikungunya transmission in urban areas of Brazil and other countries in the Americas. The study that confirmed that A. aegypti and A. albopictus of ten American countries have high vectorial competence was published in the Journal of Virology. The first cases of chikungunya in Brazil were confirmed by the Ministry of Health after the study was published, in September 2014.
According to the Ministry, from the beginning of the year to March 16, almost 13,000 cases of chikungunya were reported in the country. This number is 44% lower than the rates observed in the same period of the previous year, when more than 23,000 cases were reported. This drop in the national average does not mean the disease has backtracked throughout the country. In Rio de Janeiro there have been 6.7 thousand occurrances of chikungunya in 2019, against 5.8 thousand in 2018 - a 14% increase. With the highest rate in the country, the state of Rio de Janeiro has reported twice as many cases of chikungunya as dengue fever this year. High infection rates have also been observed in the states of Tocantins, Pará and Acre.
The symptoms of chikungunya are similar to those of dengue fever and include fever, red spots on the skin, intense joint pain, often accompanied by swelling, and others. In addition to the possibility of developing into severe forms, chikungunya causes concern due to the risk of developing chronic issues: in some patients, joint pain remains for months or even years, causing mobility problems.