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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:
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.
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
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.