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    More bugs are coming, spreading more disease. Are doctors ready to respond?

    As the climate changes, vector-borne diseases like dengue, Zika, and Lyme are expanding into more areas. That challenges physicians to recognize the symptoms.

    A mosquito, that is silhouetted against the moon, bites a human arm

    “I was on fire.”

    That is how Deborah Heaney, MD, explains the fever she developed toward the end of her vacation on the Caribbean island of Curaçao earlier this year. Her muscles and joints ached to the point that her partner had to just about carry her from the water.

    After Heaney returned home to Ann Arbor, Michigan, her symptoms grew worse. “My whole body started itching, everywhere,” she says. “I could not sleep because I could not stop scratching.”

    Red blood spots spread across her torso. Severe diarrhea left her feeling dehydrated. Visits to an emergency room and her doctor’s office produced puzzled looks, orders for blood tests, and speculations that she had an infectious disease, an autoimmune disorder, or cancer.

    “I was convinced I had leukemia,” she says.

    Online searches steered her to other possibilities, including dengue, a disease that is spread by mosquitoes and is widespread in tropical climates. Heaney asked her doctor to add dengue to the analysis of her blood samples. Two days later, the test came back positive.

    Heaney had experienced, as both a patient and a physician, a growing medical dilemma: Vector-borne diseases (VBDs) — especially dengue, Zika, and chikungunya (spread primarily by Aedes aegypti mosquitoes), and Lyme disease and Rocky Mountain spotted fever (spread by several types of ticks) — are expanding into areas of the country where people have little experience with them. As a result, many physicians are slow to recognize symptoms and provide treatments.

    “Your average doctors are not usually thinking of those [diseases] first,” says Susan McLellan, MD, MPH, director of Biosafety for Research-related Infectious Pathogens at the University of Texas Medical Branch in Galveston. “Awareness is low because the incidence is low.”

    That’s changing. In Puerto Rico, Texas, and other southern U.S. states, physicians are seeing more cases of these illnesses than before. There are several reasons: Climate change has brought warmer temperatures and extended spring seasons to many states, expanding the territory and timeline for insects to thrive. Travelers who get infected in countries that have the diseases return home and get bitten by another mosquito, which spreads more infections by biting more people. And changing land-use patterns, like housing developments near woodlands and regrowth of forest areas, increase the breeding of insects and animals (such as deer) that carry them.

    In the face of those developments, infectious disease experts are urging more vector control, physician training, and research into vaccines and treatments.

    “If we don't change what we are doing, the situation will only get worse,” says Thomas W. Scott, PhD, MS, an epidemiologist and distinguished professor at the University of California, Davis.

    More disease

    “In the last 20 years, we’ve had more than one million cases of vector-borne diseases that have been officially reported to us,” says Benjamin Beard, MD, deputy director of the division of Vector-Borne Diseases at the U.S. Centers for Disease Control and Prevention (CDC). “The number of cases each year has approximately doubled over that period.”

    In late March, Puerto Rico’s Department of Health declared a public health emergency after 549 cases of dengue were confirmed in the first three months of the year. That compares with 1,293 for all of 2023. Last summer, a total of nine locally transmitted cases of malaria — a mosquito-borne disease that had not been locally transmitted in the United States for decades — were reported in Texas, Florida, and Maryland. Mosquito-borne diseases (such as West Nile virus and chikungunya) have been extending their reach northward in the United States, and reported cases of Lyme disease in the United States have nearly doubled since 1991.

    “The threat of existing and emerging vector-borne diseases continues to grow,” notes an assessment from the U.S. Department of Health and Human Services. “As geographic ranges of vectors expand, the number of pathogens spread by vectors continues to climb.”

    “If you look at our Lyme disease maps, you’ll see Lyme disease has now spread across the entire state of Pennsylvania, the entire state of Massachusetts, all the way up in the Hudson River Valley in New York,” Beard adds. “People are being exposed to a disease they had not seen before.”

    Dengue lessons

    Dengue exemplifies the growing challenge. The World Health Organization reports that the global incidence of dengue grew tenfold from 2000 to 2019, for a total in the latter year of 5.2 million reports from 129 countries. In the United States, dengue fever reports increased by 26% from 2007 to 2017. Texas and Florida are among the hardest hit.

    “We have seen an increase of locally acquired dengue here [in recent years], and also Zika,” says Paola Natalia Lichtenberger, MD, an infectious disease physician at the University of Miami Miller School of Medicine.

    Dengue presents several challenges for doctors and epidemiologists:

    It’s usually asymptomatic.

    The lack of severe illness in most infected people is good for them, but not good for doctors who see only the minority of more severe cases, like Heaney’s, and for epidemiologists trying to get a handle on the number of cases in a region.

    “It’s easy to miss, because the severe cases are the tip of the iceberg,” McLellan says.

    Even in Puerto Rico, the health emergency this year is based on an undercount of infections.

    “We think for every case that is reported, we have six to eight cases that were not reported,” says Inés Esquilín-Rivera, MD, director of the Pediatric Infectious Diseases Division at the University of Puerto Rico School of Medicine.

    It looks like other ailments.

    When doctors don’t recognize vector-borne illnesses, they might not initiate the correct treatments, which vary among the VBDs. For example, while Lyme can be treated with antibiotics, the main medical response to dengue is supportive care to reduce certain symptoms, such as pain and so-called “break-bone fever.”

    Dengue should be suspected in patients whose symptoms include fever, headache, nausea, and muscle and joint pain, and who live in or recently traveled someplace where the disease is endemic, Esquilín-Rivera says.

    One common misstep for doctors is to pump dengue patients with fluids because they seem to be suffering from the kind of dehydration that is common with many viral illnesses. But “dengue has some clinical manifestations that are very different from other viral diseases,” Esquilín-Rivera says.

    When Heaney went to the emergency room in Ann Arbor, she was given two IV bags of fluids because of her diarrhea and lack of urination. In Puerto Rico, doctors learned that dengue patients often suffer from capillary leak syndrome, in which fluids leak from small blood vessels into such body cavities as the lung and abdomen.

    “People get low blood pressure and go into shock” as a result of the leakage, Esquilín-Rivera says. “Most people who die of dengue die because of shock.”

    Hydration has to be administered especially carefully. “Intravenous fluids should be given to patients with capillary leak syndrome in a stepwise manner, with constant re-evaluation of their condition and adjustments to the IV flow” as the syndrome subsides, she advises.

    It gets worse with subsequent infections.

    There are four serotypes of dengue. When someone gets infected by the virus for the first time, “they usually do not get very sick,” Esquilín-Rivera says. But if that person subsequently gets infected with another serotype, they will get more ill than the first time, through the phenomenon of antibody-dependent enhancement. If the patient doesn’t realize they’ve been infected before, because the symptoms were so mild, the doctor won’t be alert to the increased danger of a subsequent infection.

    “If you get infected a third time, the risk is even higher of being hospitalized or even dying,” Esquilín-Rivera says.

    Tests are not fast.

    Tests for dengue are not widely and quickly available in most areas. Heaney was fortunate that her blood test was able to include dengue. In Puerto Rico, Esquilín-Rivera says, molecular and antibody tests are sent to the Health Department surveillance laboratory and can take days to be processed and reported.

    Education and research

    Efforts are growing to increase physicians’ awareness of VBDs. In Puerto Rico, public health officials and medical institutions employ everything from email alerts and webinars to in-person training and testing to educate doctors about how to recognize and treat the diseases. In order to get or renew their medical licenses, doctors there have to complete education courses about dengue. Texas has a similar requirement regarding tick-borne diseases for certain doctors.

    By and large, however, the educational opportunities are voluntary, although public health agencies, medical schools, and hospitals push those opportunities hard in areas where there are (or have been) outbreaks. In Florida, the Miller School and Jackson Health System are among those that continuously train students, residents, and practicing physicians about VBDs, Lichtenberger says.

    She notes that Florida health providers need to be on guard because the state has experienced outbreaks of dengue, Zika, and chikungunya in recent years, and it routinely gets visitors and new residents from all over the world, including the Caribbean. When there was an increase in dengue cases around Miami several months ago, the state health department sent “provider alerts” directly to licensed practitioners in that area about recognizing and responding to the disease, says Florida Department of Health press secretary Jae Williams.

    But with all that doctors have to continually learn about throughout their medical careers, it can be difficult for them to devote much time to fully understanding diseases that they have not seen and that are often asymptomatic. “By and large, they aren’t paying attention to this unless there’s an outbreak,” Scott says.

    Scott and Beard believe the keys to containing the spread of these diseases are better on-the-ground control of insect populations (which is largely up to state and county agencies, and individuals); more widespread protection of people in vulnerable areas (which includes basic measures like solid waste removal and providing people with window screens); and the development of more effective vaccines.

    In February, the CDC released the National Public Health Strategy for the Prevention and Control of Vector-Borne Diseases in People, which outlines cross-sector public/private strategies, including with academic institutions and hospitals, for research, insect control, and public education. The strategy sets targets to reduce cases of dengue, West Nile virus, Lyme disease, and Rocky Mountain spotted fever.

    Among the challenges noted in the report is that although “vaccinations are a mainstay of the prevention of many infectious diseases,” there is only one licensed vaccine available to protect against domestic transmission of VBDs. That vaccine is limited to children ages 9-16 who previously had dengue. Vaccine experiments and trials are underway but none of the products appear to be on the brink of federal approval for public use.

    That leaves it up to physicians and patients to be alert, especially in areas with significant mosquito or tick populations. Heaney, who took three weeks to fully recover from dengue, said travelers need to look up online State Department summaries of disease risks in the countries they are traveling to. (She recounted her experience and concerns about awareness in The New York Times.) Doctors also need to ask patients about recent travel history — “I’m starting to ask that of just about every patient,” she says — especially if the patient has symptoms that are not attributable to common ailments like the flu, COVID-19, or pneumonia.

    “Everybody needs to be more educated,” about VBDs, Heaney says.