In my last blog piece, I wrote that we have seen a number of diseases that are not endemic to the U.S. pop up and cause outbreaks. Some of these are infections that we have not previously seen people acquire in the continental U.S., but rather were involving international travelers who acquired the infection elsewhere, then traveled to the U.S., became ill, and were diagnosed with the illness here.
Dengue virus is endemic to over 100 countries in the southern hemisphere, predominantly those in tropical and subtropical regions of the world. While we do occasionally see outbreaks of dengue fever in the U.S. territories of American Samoa, Puerto Rico, and the U.S. Virgin Islands, and the freely associated states, including the Federated States of Micronesia, the Republic of Marshall Islands, and the Republic of Palau, cases in the continental U.S. have historically been in international travelers or travelers from these territories.
In 2020, there was an outbreak of locally-acquired dengue fever in Florida (72 cases), which might be less of a concern if it were not for the pattern of emerging outbreaks of other diseases for which we generally have not seen locally acquired infections in the past, but now have resulted in local outbreaks, especially in Florida – e.g., zika, chikungunya prior to the dengue fever outbreak and most recently malaria (while we have historically had local transmission of malaria, we had eliminated malaria from the U.S. back in 1951). It is interesting to note that the common denominator for these four infections is that they are all transmitted through mosquito bites.
Dengue virus is a flavivirus and flaviviruses cause vector-borne diseases in humans. Vector-borne means that the virus is transmitted to humans through the bite of an insect – typically a mosquito, flea or tick. Dengue virus is transmitted by mosquitoes. Other mosquito-transmitted flaviviruses include the yellow fever virus, Japanese encephalitis virus, the West Nile viruses, and Zika virus.
The mosquitoes that are capable of transmitting dengue virus are of the Aedes species. Modelling studies show that Aedes aegypti and Aedes albopictus mosquitoes could potentially extend their natural habitats in the southern hemisphere up to some areas of the U.S., particularly the southeastern parts of the country.
We may miss cases of dengue fever in the U.S., because in some people (estimated to be about 75% of cases), the illness is mild or even asymptomatic. Those with mild symptoms may attribute them to a cold or the flu, or be seen, but not tested for dengue fever given that this is not something most U.S. doctors have seen before.
Dengue fever can manifest as a severe disease, in fact, it is estimated that 22,000 people die from dengue annually across the globe. Classic dengue fever is characterized by high fever and intense muscle, joint and bone aching, and even muscle spasms, giving rise to its colloquial name, “breakbone fever.”
In classic dengue fever, the first phase of the illness lasts 2 – 7 days. In addition to fever and the muscle, joint and bone pain, some patients will complain of headache, nausea and vomiting and some may experience bruising, bloody nose, or bleeding from their gums. On or about the third day of the illness, most people who are symptomatic with dengue will develop a red rash, that may be smooth, may have tiny bumps, or both. In most cases, the rash does not itch. Some patients will get a rash all over their body, while others will have patchy rashes. When the rash is generalized, it generally begins over the tops of the feet and the back of the hands and then spreads to the arms, legs, abdomen and chest. There is much variation in the possible rashes and in some cases, the particular manifestation of rash tells us what the major underlying pathological process is (e.g., immune reaction versus disruption to the proper functioning of the blood and blood clotting system).
While most people recover within about a week, some people do deteriorate and require hospitalization (about 1 out of every 20 cases). Those at highest risk for severe disease are infants, pregnant moms and those with a reinfection of the virus. They can experience profound fluid losses or maldistribution of these bodily fluids, internal bleeding (sometimes called dengue hemorrhagic fever) and potentially can develop shock (sometimes referred to as dengue shock syndrome).
While many lay people are under the false impression that the initial infection of any illness is the worst, and that subsequent infections will always be milder, this is not always the case, and dengue fever is the classic exception. In the case of dengue reinfections, a phenomenon called antibody-dependent enhancement (ADE) can occur. In essence, what takes place is that the initial infection triggers an immune response. Part of that response is the production of antibodies against the virus. When non-neutralizing antibodies are produced (e.g., antibodies that attach or bind to the virus, but do not interfere with or prevent the virus from entering and infecting cells), there are some cases in which these weak antibodies can actually promote the viral cell entry and infection, often also stimulating an intense inflammatory process, a condition we refer to as ADE.
There is no specific treatment available for treating dengue fever. The efforts to develop a vaccine against this infection has been hampered by the potential for ADE. However, there is a vaccine, though it is not licensed in all countries where dengue virus is endemic.
For now, Americans need not take any specific precautions unless you are planning travel to countries where dengue is endemic. However, we need to be mindful of our own changing and evolving public health situation in the U.S.
With climate change, we may see more and more diseases that are not endemic in the U.S. migrating northward. Further, if we continue to exercise a great deal of complacency against other disease outbreaks in the U.S. such as SARS-CoV-2 and monkeypox, we risk allowing these diseases to become endemic, especially, as we allow new variants to emerge and allow infections to transmit among our wildlife and domesticated animals. We often have an opportunity to eliminate infectious diseases, but that window does not stay open for indefinite periods of time. Further concerning is the fact that after going to the efforts to eliminate a number of vaccine-preventable illnesses in the U.S., we have allowed these diseases to reemerge in outbreaks due to the uncontrolled misinformation, disinformation and anti-vax campaigns that are undermining vaccine confidence in the U.S. and globally.
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