Aedes aegypti mosquito. [CDC]
With autumn winding down across North America and the morning frost lingering longer into the day, it’s difficult to think about those balmy summer nights where the world’s deadliest animal silently buzzes around, looking for its next target to take its essential blood meal. Yet, much of the globe is starting its summer cycles or lives in regions where mosquitoes are a relentless threat. Mosquitoes carry some of the world’s deadliest diseases—malaria, yellow fever, and dengue to name a few—and with the rise in Zika virus transmission, researchers are starting to look at the real possibility that in multidisease endemic regions, mosquitoes could carry Zika, dengue, and chikungunya viruses concurrently, transmitting them all at once.
Previous work has shown that Aedes mosquitoes were able to carry and transmit dengue and chikungunya simultaneously. Now, researchers at Colorado State University (CSU) are presenting their recent findings at the 2016 Annual Meeting of the American Society of Tropical Medicine and Hygiene (ASTMH) that the same mosquito family is capable of carrying Zika and chikungunya viruses concurrently.
Scientists are investigating whether antibodies to dengue can either intensify or, conversely, neutralize a Zika virus infection. Zika and dengue are closely related and come from the same family of viruses, known as flaviviruses. Very little is known about Zika's possible interactions with chikungunya, however, in humans or mosquitoes.
Because of this critical information gap, the scientists at CSU began to test what happens when Aedes mosquitoes, responsible for transmitting the viruses, encounter more than one virus at a time. In a contained experiment, the scientists allowed mosquitoes to feed on blood that contained dengue, chikungunya, or Zika, either alone or in combination. The investigators found substantial evidence that mosquitoes can pick up and transmit Zika and chikungunya simultaneously—something that had not been confirmed before, given the recent arrival of Zika in the Americas.
“Their saliva is clearly testing positive for both, which could mean that people bitten by this type of mosquito could be infected by both viruses at once,” remarked lead study presenter Claudia Rückert, Ph.D., postdoctoral researcher at CSU.
Currently, it is unclear if mosquitoes can carry all three viruses at the same time, although it would seem that real-world opportunity for mosquitoes to encounter more than one of these viruses appears to be increasing, unfortunately. A recent study in Nicaragua revealed that about one in five patients who tested positive for dengue, chikungunya, or Zika also had a co-infection with at least one of the other two diseases. Some even tested positive for all three.
“We need to understand more about what happens in both mosquitoes and people when all of these viruses are circulating in close proximity,” Dr. Rückert said.
A second study released at the annual meeting, researchers from Brazil presented new insights from their efforts to work in real time to untangle a web of neurological complications encountered in a group of 21 patients at the Roberto Santos General Hospital in the northeastern city of Salvador-Bahia over a 4-month period in 2015 during an “unprecedented and concurrent” outbreak of Zika, dengue, and chikungunya.
So far, their evidence indicates that each disease may be linked to severe neurological problems, including a rare disorder seen in two patients called opsoclonus-myoclonus-ataxia syndrome or OMS—colloquially referred to as “dancing eyes–dancing feet syndrome” for the chaotic eye movements and muscle contractions it causes. One of those patients tested positive for Zika and dengue. But the other patient tested positive for chikungunya and dengue, though it's been difficult to determine whether this “co-infection” was to blame.
“Our analysis shows that each of these viruses may have the potential to cause a range of neurological complications, some very severe, and patients should be monitored for symptoms,” explained Isadora Siqueira, M.D., Ph.D., a scientist with Brazil's Oswaldo Cruz Foundation. “What's very difficult to determine is whether having a co-infection with two of these viruses increases the risk of neurological problems. We are still looking closely at the case of the patient who was infected with both chikungunya and dengue.”
Dr. Siqueira and her colleagues were particularly surprised to encounter OMS, which globally occurs in about one in a million people. Seeing it around the same time in two unrelated patients in the same hospital is considered unusual. Dr. Siqueira said she and her colleagues are now investigating whether the problem was the result of an autoimmune disorder sparked by one of the viruses that caused the immune system to attack nerve cells, similar to what occurs in Zika patients who develop Guillain-Barré syndrome.
“This kind of clinical research is essential so that we can better understand the true public health threat of these mosquito-borne viruses,” said Stephen Higgs, Ph.D., president of ASTMH. “Tropical medicine scientists must continue to be vigilant on the front lines of these outbreaks and provide new and beneficial insights for physicians around the world. It is great to have so many of our members involved, and to see the impact of their work.”