Wednesday, January 26, 2011

Dengue in the United States


I am ending this summary series on dengue fever with a description of the situation in the United States, where it is now an emerging disease.

First let's discuss a bit about the Aedes mosquitoes in the US. If you remember from last week's description of Aedes aegypti ecology, the range of this mosquito, which is primarily constrained by climate, extends north and south from the equator to the tropical and subtropical lines of latitude. Here are some maps to refresh:


This map depicts regions at risk for dengue transmission in blue. We can see much of Texas is included here. Indeed, autochthonous cases of dengue do occur in the southern extent of Texas, close to the border with Mexico. There have been seven outbreaks along the border region since 1980. However, in general, cases in Texas are still rare. Nonetheless, it is important to note from this map those regions in the US that are well within the range of Aedes aegypti, which includes a large section of the South. Florida in particular is well within the range of this mosquito and is indeed the locus of new emergent dengue fever in the US.

Autochthonous dengue cases (i.e. acquired by infected mosquitoes in the Florida Keys) began popping up in Key West, Florida in the summer of 2009. The Centers for Disease Control and Prevention (CDC) published an excellent report of this early emergence in its Morbidity and Mortality Weekly Report. You can find this report here. It is well worth the read. The three initial Key West cases identified in the latter part of the summer of 2009 (August) as well as the most recent reported case in April 2010 (this was the most recent case as of the time of the report, which was May 2010) are all described in the report with detailed symptoms and history.

The most interesting aspect of the report, however, is the description of the follow-up field survey carried out to identify the extent of human infection in the area. With only 240 community participants tested in the sero-survey, the study was quite small, but 5.4% (13 people) of these demonstrated evidence of recent dengue infection. Clinical follow-up was also encouraged. Physicians in the Key West area were contacted by local health department officials and asked to send specimens for all patients exhibiting dengue-like symptoms.Of the 21 samples sent to the CDC for analysis, 42.9% (9 patients) were infected with dengue virus. The authors of the report point out that the identification of these cases from 2009 and 2010 are the first dengue cases acquired in the continental United States outside the Texas-Mexico border since 1945, and the first cases acquired in Florida since 1934.

During the period 2009 to 2010 a total of 57 dengue infections were locally acquired in Key West.

If you are interested in tracking dengue virus around the world or in the United States, the CDC maintains a very good interactive map that has recent updates of reported dengue cases in all countries. Check it out here.

The Florida Keys Mosquito Control District has done an outstanding job of controlling A. aegypti in the Keys, and particularly in Key West. A fascinating discussion between the people behind this mosquito control program and Vincent Racaniello, Alan Dove and Rich Condit of This Week in Virology described the unique aspects of mosquito ecology in the Keys, and the specific control efforts employed there to hold this dengue vector in check. I highly recommend viewing this excellent video podcast below.

One of the important messages to take away from the work going on with mosquito control in the Keys is that comparable efforts are largely nonexistent in much larger Florida urban centers (e.g. Miami) with similar presence of A. aegypti mosquitoes. As such, the potential for large significant outbreaks of dengue in places like Miami are greatly increased now that dengue virus has been locally identified in southern Florida.

This fear was, indeed, realized when the first locally acquired case of dengue in many decades was identified in Miami on November 11, 2010. If you remember from last week's discussion of A. aegypti ecology one of the most important control efforts for the mosquito is the removal of containers that can collect water around the home. After the identification of this dengue case in Miami, field workers from the Miami-Dade county health department were dispatched to go house to house eliminating these mosquito breeding sites, while also spraying insecticides. Unfortunately, the level of baseline mosquito control is much less in Miami than in Key West, and so future outbreaks remain a real threat, especially given the much larger urban population in Miami.

What about the rest of the United States? Does dengue infection represent a genuine public health concern for more northern climes? After all, A. aegypti may be well established in the southern-most part of the United States, but the range of the mosquito is quite limited with respect to the rest of the continental US. Isn't it?? Well, there are two important points that need to be made in order to answer this. First, we need to return to the Aedes mosquito genus and quickly discuss another species that can transmit dengue virus: Aedes albopictus, also known as the Asian tiger mosquito:


This mosquito is native to Southeast Asia and is similar to Aedes aegypti in that it chooses small water containers for laying its eggs, it prefers to live in human friendly environments, and it bites during the day. It differs in that it can thrive in more temperate climates. Like A. aegypti, it is native to tropical and subtropical climates, but A. albopictus has also adapted well to cooler climates. Relative to A. aegypti, this mosquito was somewhat limited in its geographic distribution until it was able to exploit a uniquely modern ecological niche. During the last 25 years, the global tire industry has established a truly massive export/import shipping trade between Southeast Asia and Japan, and countries all over the world, but especially, the United States. Prior to shipping, tires are stacked in the open and collect rainwater that A. albopictus finds ideal for laying eggs. Soon these tires are removed and shipped around the world. Tires are shipped by large boats, where they are, again, stacked in the open and available to collect rainwater during the trip. When the eggs become re-submerged in water, the eggs can begin their life cycle development through larvae and pupae on to adults, and arrive ready to bite and lay new eggs in their new country. In this way, beginning in the 1980s A. albopictus became redistributed across the globe, with particular success in the US, as depicted in the maps below:


Global range of Aedes albopictus as of 2007


So, with the introduction of Aedes albopictus into much of the United States, is dengue fever also destined to become well established? Not necessarily. One caveat is that A. albopictus is not as an efficient vector for dengue virus as A. aegypti. The future is not entirely certain, and as more dengue occurs in the southeastern US, it will be important to carefully monitor whether the virus is able to be transmitted further north (i.e. out of Florida) by the established alternative vector, A. albopictus.

But there is another important consideration for the transmission of dengue virus in the US: A. albopictus may not be necessary for transmission further north. Climate change may be enough. As regional temperatures continue to rise, the habitat suitable for A. aegypti, the efficient dengue vector, will continue to expand north. As this habitat expands, so too does the potential range for dengue fever if the A. aegypti mosquitoes that head northward into warmer weather are carrying the virus. And since pools of these mosquitoes are now known to carry the virus in the far south, this scenario is not difficult to imagine.

Only time will tell, of course. We simply do not know how dengue will develop across the United States. It may never become endemic. However, increased funding to mosquito ecology research and mosquito control efforts are likely to be wise investments in many areas across the US.


21 comments:

  1. Why did dengue suddenly appear in Florida in 2009-2010 for the first time since 1934? Did something notable happen with the climate or the ecology that was not present for the past 70 or so years?

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  2. Max,
    Maybe in 2009 the cases of dengue were properly identified as dengue verses something else. The symptoms of dengue (fever, headache, muscle and joint pain) seem similar to flu like symptoms, so maybe there have been cases but they were not being properly diagnosed. Also the climate is getting warmer so I think that may be another reason.

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  3. This makes the discussion on global warming even more important as that could be one of the factors that could help spread the dengue fever all over the world. The rise in temperatures is just what A.aegypty needs to be able to thrive in the northern hemisphere.

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    1. I agree. I think the discussion on global warming is even more important when we are talking about the potential for a greater range of potential dengue fever. As this vectors move northward they are creating a much larger range of where outbreaks of dengue fever can occur. The fact that we don't know how dengue fever will outbreaks will proceed is a bit worrisome. But I think the efforts made in the Keys shows our ability to control such an outbreak.

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  4. The Florida dengue situation is back in the news. There is an article in the Guardian UK that discusses the resistance of local people in the Florida Keys to the release of genetically modified mosquitoes that are resistant to dengue infection:

    http://www.guardian.co.uk/environment/2012/jul/10/florida-resist-insect-release?newsfeed=true

    I thought this was interesting because it shows the interplay of politics and ID control. It's so much more complicated, on a political and policy level, than it may appear on the surface. The public health allies on the one hand are the environmentalists who are working to prevent the progress of global warming that leads to mosquito spread, but on the other hand, they oppose public health interventions that have a perceived environmental risk.

    I think this shows the importance of involving the community from the outset when deciding on public health interventions. Education and widespread community seminars might be helpful to show the population exactly how the proposed intervention works, and why it poses minimal environmental risk. While genetic modification may not sit well with people because it is tinkering with delicate ecosystems, it could also allow for a dramatic reduction in pesticide use (as the article mentions...) and thus have a net positive impact on the ecosystem compared to if it were not implemented at all.

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  5. Dengue is a huge concern worldwide. When I was in South Asia a few years ago, I noticed an increase in public service announcements and notices on local media sources and in hospitals and schools, as a result of a spike in dengue cases, on ways to prevent dengue and stop the growth of mosquitoes especially during the summer/ monsoon seasons. The focus was similar to the PAHO program which was to eliminate standing sources of water in places inhabited by people. The continuous announcements and fear-inducing news reports of dengue-related deaths did result in community members taking care to eliminate sources of standing water without much government intervention (of course, not that the governments, local and central, were prompt or vigilant or probably even concerned enough to send health inspectors to enforce these measures). I would have to look into how successful those methods have been in decreasing infections or deaths and whether communities are still taking these safety measures without governmental intervention.

    The Aedes mosquitoes are fascinating in the way they have adapted to their environments and continue to cause disease. One thing I am curious about is the use of Toxorhynchites and any programs that may have used these to control the vector as you mentioned in the previous post.

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  6. Previously the week, there was news of a man on Long Island who had been hospitalized on September with the dengue virus. The infection was thought to have been contracted locally which would make it the first known case of dengue in New York State Aedes albopictus has been spotted a lot more frequently over the past couple of summers, and their appearance may be due to the changes in climate.

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  7. In this post, we have learnt that climate change has expanded the regions at risk for dengue transmission in the U.S. In another post about the West Nile virus, the climate is an explanatory factor for the annual epidemic peaks of West Nile virus cases in the drier western U.S. Climate change clears plays a crucial role in the transmission of infectious diseases nowadays in addition to the environmental consequences. Moreover, states like Texas are at increased risk of outbreaks for dengue and West Nile virus as temperature increases. Although mosquitos are the vectors to transmit both diseases, the mosquito control strategies could vary due to the different ecology of different species.

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    1. You make a great point, Wen. I think that a major point here is that any effort to control dengue as well as West Nile incidence will require great interdisciplinary effort and cooperation. Any effort would require thorough knowledge of the mechanisms of and patterns in transmission (as well as the impact that climate change, seasonal changes, etc. have, as you mentioned). Knowledge of the impact of variability in strains, especially with geographic considerations, is very important. So also the knowledge of local health policies, since public health efforts would have to be administered within the frame of those policies (just look at areas of northern Pakistan where immunization activities had to be suspended because the governing bodies banned those activities).

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  8. Nicole MastrogiovanniJuly 22, 2014 at 9:42 AM

    After reading this section on Dengue it is clear that climate change plays a major role in the increase and spread of the disease due to the mosquitos ability to live in the warmer temperatures. However, climate change is bound to happen so the need to spread awareness about open containers and putting covers on them so rain does not collect and create a perfect environment for the mosquitos. It seems as though as though it would be time consuming but not costly but if this disease becomes endemic or even epidemic it would become extremely costly and obviously life threatening to many people. I also think personally that dengue may have been around before and we were not actively looking for it since we were not aware it was in the U.S. yet but now that we are aware of the signs and sometimes symptoms that occur, we can identify cases as dengue versus writing it off as something else. As of now, in the Northeast, we do not have to worry about dengue but we should start creating advertisments in newspapers, billboards, or television for covering open containers so people can get into the habit and possibly prevent dengue from spreading in the first place.

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    1. In other countries, it is not easy to bring awareness to the community, when when awareness did reach the people, it may be difficult to treat it when it is costly and some may not have easy access to hospitals or to proper care.

      how would you deal with the problem of Dengue in other countries that may not have access to advertisements in newspapers, billboards, or televisions? what program should be conducted to help create awareness of dengue and decrease the incidence rate.

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    2. My mother is of caribbean descent and she has plants all over the place, some with out soil and just water. Recently, my boyfriend and I saw worm like specimens floating in the water, it was also very cloudy. We told my mother that is a breeding ground for mosquitos but this was early in the summer. The weather is still warm although we are in the middle of October, she still has the pot filled with water and her plants in it along with a few others that recently sprouted. How would I bring awareness to her about mosquitoes and dengue fever when she associated mosquitoes with the summer season? She is a stubborn woman and I don't want her to have to get rid of her plants.

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    3. I recently traveled to South Carolina and to my excitement found numerous posters throughout the southern airports about Dengue including how to protect yourself if you are going into an area where there has been documented dengue cases and how to properly check your baggage for any mosquitos that may attempt to stowaway. This is a great start but as Difaa stated, there may be countries where there is limited access to advertisements of any kind, this is where public health workers would need to travel to the areas and explain in person the dangers of dengue and how to protect themselves. Although this would take various resources such as funding for protective nets and time for gaining the trust of the population, it is a realistic approach if and when dengue ever becomes a serious concern.

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    4. Nicole M., you have made a good initial point, that is climate change is bound to happen so we should start spreading awareness and prevention methods now. The interactive CDC map of reported dengue cases shows suspected reports of dengue near the Sacramento, CA area in March 2016. In many regions in the U.S. (Northeast, Midwest etc.) we do not really worry about dengue, we may not even know what dengue is thus it is important to start creating awareness through public health social marketing strategies. We learn that behavior is one of the hardest things to change, it could take time for people who are not currently use to the idea of mosquito control through simple methods of "drain and cover" to start implementing such small but effective tasks in everyday lives.

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  9. Dengue prevention in rural environments like Key West in contrast to preventative measures in urban centers like Miami brings up an interesting point. The urban nature of cities like Miami creates an attitude that projects civilization and rejects wilderness. As a result, diseases such as Dengue which are transmitted by a mosquito vector seem like a less obvious threat in cities. In reality, however, open water sources and tropical clients provide sufficient breeding grounds for mosquitoes. Combined with the dense population, Dengue outbreaks could affect many more people than in a more rural environment like Key West. The perception of a more "wild" environment in Key West leads to more proactive action when in fact it is just as necessary in urban environments like Miami.

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  10. It'd be interesting to learn if Southern California region identified any Dengue cases in the recent past since it shares similar weather condition as Florida and Texas-Mexico border. If Southern California had success in containing aedes egypti or aedes alcobictus then, both Florida and Texas may find it helpful to learn from Southern California's experiences.

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    1. For the past four years, Southern California has fallen victim to intense drought (http://www.cbsnews.com/pictures/californias-drought/). Thus, I suspect that the likelihood of an outbreak/infection would be decreased with a lesser amount of viable small standing water containers where A. aegypti or A. albopictus could lay eggs. Reproduction seems much more difficult in this region of the U.S. Florida, on the other hand, maintains a much more humid environment than Southern California.

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  11. Whole reading this I came across an article where a woman was misdiagnosed with West Nile Virus, ended up actually contracting dengue. The misdiagnosis led to her death. The woman was traveling from a tropical place where dengue can be endemic.

    http://www.pediatricnews.com/home/article/fatal-us-dengue-fever-case-misdiagnosed-as-west-nile-virus/beed9e6422eba8017a62231feff19b4d.html

    I think another thing to consider when it comes to dengue is the influx of immigrants that come into America from the tropics. However, do immigrants get added to the case tally or are the counted separately? Also with the recent trends in climate change around the country, how does that affect the spread of Dengue and the mosquitos that carry it.

    As we all travel around the world, it may have helped contribute to the spread of dengue.
    http://www.npr.org/blogs/health/2013/03/12/174142169/dengue-fever-no-longer-just-a-visitor-to-florida-keys

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  12. http://www.newyorker.com/magazine/2012/07/09/the-mosquito-solution

    Great article about mosquitoes. Little fact mentioned, researchers estimate that mosquitoes have been responsible for half the deaths in human history! and how globalization has been good to mosquitoes, particularly species like Aedes aegypti, which travel easily and can lie dormant in containers for months.

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  13. I think that it is so interesting and unfortunate how climate change is slowly shifting the typical geographic regions that can be affected by Dengue Fever. I know that a new Dengue Fever vaccine was recently approved for clinical trials in December of 2015, but this is only advised to be used in endemic regions. I wonder will the if Dengue Fever will eventually become endemic in the United States to the point where we will need to utilize this vaccine.

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  14. As this post mentions, Global Climate change is a significant contributor to the possibility of rising rates of dengue in the US, specifically the northern region. As climate changes, the increased amount of precipitation results in increased humidity. Along with higher average temperatures these environmental factors may replicate just enough of a tropical climate for A. aegypti to successfully migrate south. Even for those that deny the rapid advance of global warming, it is feasible that even a small change in weather can contribute to the migration of the mosquito to the North. As the weather changes ever so slightly, shorting the intensity and length of the winter, overwintering may become easier for the mosquito to achieve. While previously a simple matter of a degree or two or perhaps an extra week or so of cold weather was preventing a successful overwinter, those obstacles may now be eliminated for A. aegypti. Thus, migration of A. aegypti is a serious concern and can happen spontaneously resulting a realization that may be a few million infections to late.

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