Vital facts about vector-borne diseases

Vector-borne diseases have typically been considered a threat in warm climates. But during the ACP/Annals of Internal Medicine virtual forum “Emerging and Resurging Vector-Borne Diseases,” Christine Laine, MD, FACP, pointed out that the conventional wisdom is changing.

“International travel and climate change are enabling vectors to cross borders and emerge in seasons and geographic regions that extend well beyond their historical patterns,” said Dr. Laine, who is Editor-in-Chief of Annals and a Senior Vice President at ACP. She noted that more than 700,000 people die of vector-borne diseases annually, according to data from the World Health Organization.

To review recent developments about these diseases, forum moderator Vidya Sundareshan, MD, MPH, FACP, professor and chief of infectious diseases at Southern Illinois University School of Medicine in Springfield, Ill., led three expert panelists in a case-based discussion on Dec. 4, 2024. A summary article and a full recording were published Dec. 10, 2024, and are available to ACP members and Annals subscribers.

The first case featured a healthy 34-year-old woman who is 14 weeks pregnant and considering canceling her planned travel to the Caribbean because of viruses, such as Zika, chikungunya, and Oropouche, that could be dangerous to her pregnancy.

On the matter of Zika, at least, she should be reassured, said panelist Davidson H. Hamer, MD, FACP. “The reality is that Zika sort of disappeared,” he said. “There's very little Zika transmission in the Caribbean in recent years. ... We've only had a few cases in the last five years, and they've all been probable cases, so only confirmed by serology and not PCR [polymerase chain reaction]. I don't think there's much of a risk right now of Zika.”

Similarly, chikungunya has been detected worldwide and caused recent outbreaks in Paraguay and Brazil but hasn't been seen much in the Caribbean, said Dr. Hamer, who is a professor of global health and medicine at Boston University Schools of Public Health and Medicine.

He described Oropouche virus as a “new player” that has been reported in Brazil and other countries along the Amazon Basin.

“What's novel is the outbreak was identified in Cuba, and with a Cuban outbreak, there have been a number of Americans, people that visited Cuba, traveling back to the U.S. that have been infected, as well as a pretty good number of Europeans,” Dr. Hamer said. “This, as we're learning more about Oropouche virus, is increasingly concerning.”

The emerging evidence suggests that Oropouche is a neurotropic virus that can be transmitted from a pregnant woman to her fetus and can lead to early miscarriage and later stillbirth, he said. It has also been associated with microcephaly in infants and Guillain-Barré syndrome in infected adults. “The challenge is we don't know how widespread it is,” Dr. Hamer said.

Other unanswered questions about the Oropouche virus include its enzootic cycle, its primary hosts, and its primary vectors, the experts noted. Lyle Petersen, MD, MPH, director of the CDC's Division of Vector-Borne Diseases, pointed out that the currently circulating strain is a reassortment virus, or a combination of two Oropouche viruses. “Whenever viruses reassort, they can change vectors, they can change hosts, so there's just a whole lot of unknowns,” he said.

Oropouche is believed to be transmitted primarily by midges, tiny blood-feeding insects that are almost invisible to the naked eye, but there's a possibility that it might be transmitted sexually as well, since live virus has been grown from semen samples of infected patients, Dr. Hamer said. (The CDC offers recommendations on its website for avoiding transmission, including through sex.)

Regarding the panel's case, Dr. Hamer said while it's probably safe for the patient to travel to the Caribbean, she should look at the CDC website before choosing which island to visit and use antivector measures during her trip. “There are a number of antivector measures that the CDC recommends on their website, and all of those are safe in pregnancy if used appropriately,” he said.

The second case asked how to manage a 47-year-old man with well-controlled type 2 diabetes who presents with fever, headache, and myalgias that began several days after he returned from a trip to Massachusetts, where he spent a lot of time outdoors. His legs and arms have evidence of healing insect bites.

Forty-eight percent of forum attendees said they would arrange testing for West Nile virus and eastern equine encephalitis. Panelist Roger Bedimo, MD, FACP, a professor of medicine at the University of Texas Southwestern Medical Center and chief of infectious diseases at the VA North Texas Health Care System in Dallas, agreed but noted that it's important to have a high index of suspicion for other diseases, since the patient's symptoms are nonspecific, and to consider ticks as well as mosquitoes.

“This is an area where we have a number of tick-borne illnesses that could also present similarly, and the one that I would single out mostly is the black-legged tick or Ixodes scapularis, because in that region, we have anywhere from Lyme disease to anaplasmosis to babesiosis to ehrlichiosis; they all can be transmitted by the same tick,” he said.

Dr. Petersen stressed that just because somebody doesn't report a tick bite, it doesn't mean they didn't have one. “Ixodes scapularis ticks in particular are very, very small. They tend to bite in places like the back of the knee, where people aren't normally looking,” he said.

The third clinical vignette featured a 35-year-old man with well-controlled type 1 diabetes who requests the dengue vaccine before an extended business trip to Brazil. Dr. Petersen explained that although there is one dengue vaccine currently available in the United States, it is being phased out, and this patient does not meet the narrow criteria for vaccination.

“The name of the game right now [with dengue] is to prevent mosquito bites,” Dr. Petersen said. He pointed out that dengue is the only arbovirus that doesn't have a significant animal reservoir and that it is spread almost exclusively by the Aedes aegypti mosquito.

“Now why is this mosquito bad? This mosquito is bad because it likes humans,” he said. He explained that it has adapted itself to urban environments and bites multiple people in a single blood meal, often infecting an entire household at the same time; it also prefers to bite in daytime and likes to remain indoors, in warm environments.

To avoid bites, travelers can be counseled to stay in air-conditioned locations or rooms with screened windows. Wearing long sleeves and pants can help, and there are several mosquito repellents that are effective, Dr. Petersen said.

“Personally, I use DEET, and that's my own preference, simply because it's been around for 60, 70 years. It's extremely safe,” he said. He recommended sticking to 30% DEET or less, as the percentage relates to duration of action rather than efficacy and there's not much additional benefit with higher concentrations.

While severe dengue is more common in younger and older patients, the main factor that affects mortality is timeliness of treatment, Dr. Petersen said. About 1% to 5% of people who develop symptomatic dengue go on to develop severe disease, and there is a 24- to 48-hour period where they are at risk for dengue hemorrhagic fever and shock syndrome. Prompt recognition and treatment with isotonic fluids are required during that time to prevent life-threatening shock, he said.

Alarm symptoms to watch for, especially in areas where dengue is endemic, include abdominal pain, vomiting, hepatomegaly, and increase in hematocrit, Dr. Petersen said. He also noted that severe symptoms are more common in patients experiencing a second dengue infection. The experts pointed out that NSAIDs are contraindicated in patients with dengue due to the potential for hemorrhagic complications.

Dr. Sundareshan closed the forum by asking the panelists to offer any lessons learned from COVID-19 that could help in fighting vector-borne diseases. Dr. Hamer stressed the importance of developing readily available diagnostics and making clinicians aware of the need for testing, while Dr. Bedimo noted that it's vital to keep global channels of communication open and to share information. Dr. Petersen advised that thorough preparation is key.

“No one would have ever predicted West Nile virus invading the U.S., no one would have ever predicted Oropouche suddenly becoming a problem this year, no one would have ever predicted chikungunya spreading around the planet,” he said. “It's just very hard to predict what's going to happen next, so we just need to have the capacity… to diagnose these things rapidly when they do occur, and also find good ways of dealing with them when they do occur.”