Thursday, February 13th marked the Emerging Pathogens Institute’s thirteenth annual Research Day, a celebration of academic investigations into novel and reemerging infectious diseases.
David Nelson, MD, Senior Vice President for Health Affairs with the UF Health Sciences Center offered opening remarks which praised the institute’s ability to unite researchers from various disciplines. “EPI has always been the epitome of what I believe the University of Florida stands for, which is collaborative interdisciplinary research,” Nelson said.
One hundred and forty-three posters were presented in the Reitz Union’s grand ballroom, and even more people were in attendance.
Keynote talks followed a lively poster session and focused upon the global public health threat posed by dengue. Peruse a summary of the hour-long talks below, or watch a recording of the full-length presentations on the University of Florida mediasite.
Kathryn Hanley, Regents’ Professor at New Mexico State University’s department of biology, opened her talk with a look-back at how much has changed in the past two decades regarding arbovirus research. “Twenty years ago, people were really mostly talking about dengue,” she said. There was an awareness of yellow fever virus and of chikungunya and only a very few people interested in Zika virus back then, she said. But times have changed, and dengue is now frequently discussed as a growing threat to global public health.
Hanley recounted that as she became more deeply involved in development of a dengue vaccine (TV003), her background in ecology and evolution began prompting questions as to what would happen next if the vaccine worked. What if dengue could be eradicated locally or globally?
Her thoughts turned to the ancestor of dengue, a virus that spilled over into people likely from a sylvatic cycle between forest-living mosquitoes and primates in Africa or Southeast Asia which historically did not involve people. “The type infecting people was now ecologically and evolutionarily distinct [from its ancestor], so my concern was if we eradicate this human endemic cycle in people, how easy or difficult would it be for sylvatic dengue to reemerge and reestablish in people?”
Hanley next researched the evolutionary relationship of sylvatic and human-endemic dengue viruses and found that as of a decade ago, it had jumped at least four separate times from the forest animal-endemic cycle to the human-endemic cycle. Her research found that there are no evolutionary barriers, such as mutations, that allow sylvatic dengue to spillover into human populations. In other words, it is primed for zoonotic spillover events. In the time since, researchers have noted a number of spillover events from sylvatic dengue strains to people, Hanley said.
Her work in West Africa then delved into examining dynamics between both mosquitoes and people to look for cues as to what drives outbreaks. By sampling mosquitoes using human sentinels, her team found time cycles punctuated by huge amplification events of sylvatic dengue in mosquitoes. They also found similar time cycles of amplification events in school children.
Her team then looked for characterizations of dynamics within the three main affected primate species,
Patas monkeys, baboons, and African green monkeys. One year they found markers in the blood of infant monkeys indicative of a recent dengue infection, even though the study turned up no evidence of dengue in primate-feeding mosquitoes for four years. “We think there are more hosts of sylvatic dengue and chikungunya than previously considered, there are more to be found in the host system,” Hanley said.
In other words, non-human primates may be spillover hosts, just like humans, and they may even be that primate hosts amplify the virus or be members of a larger reservoir community. Dengue also causes low levels of disease in individually infected monkeys; sick animals had low levels of viral replication and mild symptoms, perhaps too low to be maintained by mosquito transmission alone.
This finding led Hanley and others to investigate what level of viral replication maximizes dengue transmission. Modeling studies revealed that a low-level replication strategy actually maximizes dengue transmission, counter to classical virulence models which predict that higher viral replication levels equate to higher transmission levels.
Hanley’s research has also zoomed out to the wider ecological perspective of asking how changes in land cover and land use at broad scales potentially affects spillover and spillback of forest-cycling arboviruses. “There has a been a lot of thinking in the ecological community about how land cover disturbance might impact the emergence of vector-born infectious diseases,” she said.
Using Borneo as an example of decades of deforestation and ecological disturbance, Hanley discussed how disturbances in biodiversity due to degraded and disturbed systems make arboviruses more prevalent or set the stage for spillover or spillback events.
Hanley’s own recent work in Borneo showed that highly disturbed palm plantation habitats had markedly fewer mosquitoes that intact forest, but that the diversity narrowed to just Aedes species. Especially abundant was Ae. albopictus, a species happy to spend time in both palm plantations and forests, Hanley said. “It’s happy to span the distance between the two.”
Her data from Borneo support that idea of amplification, that the more distributed an ecosystem is and the more reduced its biodiversity becomes, then the more likely it is that arbovirus diseases become endemic and the risks increase of spillover to people. In terms of timing, she noted the greatest risk for sylvatic dengue spillover occurs when people are in the forest or during forest clearing operations.
Hanley closed her talk with comments on recent work in Manaus, Brazil where her team is investigating whether the recently-emerged Zika virus may establish a sylvatic cycle with spillback potential. Hanley’s ongoing studies of mosquitoes at urban-forest interfaces seek to answer the how arboviruses flow between forests and cities in tropical urban-forest mosaics landscapes.
Derek Cummings, a UF preeminence professor in the College of Liberal Arts and Sciences, segued from Hanley’s talk to focus more narrowly upon patterns of dengue infections in human-endemic cycles. “Dengue has been one of the big winners of globalization,” said Cummings, who is also an EPI faculty member. “It is one of the most dramatically reemerging diseases in the last century.”
In human populations, even where it is endemic, there is large variability of dengue year to year. Dengue presents in people with four different serotypes, its range covers the planet’s tropical regions. “It’s limited only by immunity, and it’s increasingly a disease of the young,” because the youngest people are the ones with the least immunity, Cumming said.
Cummings recent work has reanalyzed existing range maps which show the global distribution of dengue to also account for immunity. Prior work by others estimates about 390 million dengue infections per year of which about 96 million people seek care. But when Cummings reanalyzed these models to incorporate feedback from immunity, his team lowered the estimate to 105 million infections per year with about 51 million clinical illnesses. This work was recently published in Science Translational Medicine.
Places with high transmission intensity, high temperatures and a lot of mosquitoes to support transmission, might not actually result in measurably more infections compared to a place with lower transmission because of the feedback provided by immunity, Cummings noted. This is because immunity is acquired more quickly in places that experience higher transmission.
The four dengue serotypes that infect people today all originated at different times from the sylvatic dengue reservoir. The four types differ from each other genetically. The first time people are infected, they generally do not experience severe illness and they typically acquire long term immunity to that specific serotype and short term immunity to the three other dengue serotypes. But after the short term immunity degrades over a year or two they are left only with the long term immunity to the serotype that caused their initial infections.
The problem with dengue kicks in when someone experiences a secondary infection., which tends to be more severe. “These are the ones that are more likely to show up at a hospital, and are more likely to have hemorrhagic manifestations,” Cummings said. It’s thought that once someone has a secondary infection, lifelong immunity to all serotypes results, researchers are not exactly sure what drives the severity of secondary infections.
Cummings next pivoted to presenting his recent research examining immunity interactions between Zika virus and dengue virus. The two pathogens are genetically related. His interest was peaked in 2017 when dengue seemed to practically disappear from the Americas a few years after the Zika epidemic. “2017 was a really different year,” he remarked. “We wanted to find out if this was a statistical aberration or if it was in line with the normal year to year variation.” Prior years had seen large numbers of dengue cases, partially due to increased surveillance activity, he noted.
Using publicly available data from Brazil and Colombia, he and a former postdoctoral fellow (Rebecca Borchering, now with the Univ. of Georgia) analyzed dengue, Zika and chikungunya and found that 2017 was aberrantly low.
The project was informed by a prior study he’d done looking at Zika attack rates. That study found that people with higher levels of dengue immunity were at lower risk of becoming Zika positive. In other words, having experienced prior dengue infections protected people, at least partially, from Zika infection.
After confirming that 2017 was an unusually low year for dengue cases in the Americas, Borchering and Cummings created simulations of dengue and Zika transmission following the Zika outbreak. They found that the low numbers of dengue incidence in 2017 and 2018 were best explained by simulations that had varying levels of cross-immunity built into them. The models also suggested that periods of reduced dengue could be followed by very large epidemics of dengue, once cross-protection waned and dengue resurged.
This prediction from the models has unfortunately been seen in real life; many countries of the Americas have experienced record numbers of dengue cases in 2019 including 1.9 million cases reported from Brazil. Climate change likely also plays a role in this developing story.
“Zika was eating dengue’s lunch, but then people were being born into the population that had no immunity,” Cummings said. “And if dengue is suppressed because of cross-immunity from Zika that is temporary, then it makes sense that once that temporary immunity is lost, dengue will come roaring back. All our simulations predicted large dengue outbreaks three or four years after the Zika outbreak.”
Cummings also discussed problematic outcomes of the only licensed dengue vaccine, due to adverse immune reactions. Despite its failings, it is still the most powerful tool deployed against dengue in the past twenty years, he said. “Even with its warts, it’s the one vaccine we have, and there is pervasive risk to so many people.”
Cummings other work seeks to better understand the nature of protection offered from natural infections. By studying titer levels in school-aged children via blood samples taken before and after dengue seasons, his research team is building a model of immune responses to dengue. This work detects asymptomatic dengue infections and is examining the cross-reactive immune responses between all four dengue serotypes.
Written by: DeLene Beeland