Rainfall, Temperature and Dengue Risk: New Insights from Rio

Aedes aegypti mosquito (cdc.gov)

Here is a study by researchers from Brazil that looked at the relationship between dengue activity and climate in Rio de Janeiro from 2001 to 2009. Using statistical modeling, they found that a 10-millimeter rise in rainfall led to a 6% increase in dengue cases in the subsequent month, and a 1 degree celsius rise in temperature led to a 45% increase.

Dengue is a viral infection transmitted by Aedes aegypti (and Aedes albopictus) mosquitoes; it is the most common mosquito-borne viral infection. It causes an acute febrile illness characterized by severe pain (giving it its colloquial name: "breakbone" fever). The disease is associated with significant morbidity.

The findings in the above study are not surprising: increased temperature allows the mosquitoes that carry dengue to breed and feed more efficiently. Increased rainfall can increase potential breeding sites for these mosquitoes (which breed in freestanding water). This study adds to the growing body of literature linking climate and infectious disease activity.

A warmer world is one in which the various vectors for tropical infections (especially mosquitoes) will enjoy a broader environment in which to breed and feed. More research into the relationship between climate change and emerging infectious diseases is warranted.

Hottest Year on Record for the Contiguous US: Implications for Emerging Infectious Diseases?

http://climate.ncdc.noaa.gov

Here is an article from the Huffington Post discussing National Oceanic and Atmospheric Administration (NOAA) data from 2012, noting that 2012 was the "hottest year on record for [the] lower 48 [United] states." The article is compelling and contains some nice images that outline how the average temperature has increased in the contiguous United States over the past century; it is believed that climate change has played a role in this mean temperature increase.

A byproduct of increasing global temperatures is the expansion of insect habitats that can harbor viral infections (such as dengue, chikungunya and yellow fever).  As I have previously noted, this phenomenon may have contributed to a dengue outbreak in southern Florida, and may lead to other viral infections (such as chikungunya) emerging in other parts of the United States, as well. A recent study from Mexico found the mosquitoes that can carry dengue (as well as the chikungunya and yellow fever viruses) at higher elevations than previously identified; this may represent expansion of the mosquitoes' habitat due to global warming.

If the world continues to grow warmer, the medical and public health communities need to be aware (and vigilant in surveillance) of the potential for traditionally "tropical" infectious diseases to emerge in (what were once) more temperate climates.

New Study Explores Relationship Between Biodiversity, Vector-borne Diseases and Economics

Aedes aegypti mosquito (one
of the mosquitoes that carries
the dengue virus; cdc.gov)

Here is an interesting article by Bonds and colleagues in PLOS Biology. Via creative statistical modeling techniques, these authors examined the relationship between the 'latitudinal gradient in income' (the fact that there are more poor people living in the tropics) and vector-borne and parasitic diseases (diseases like malaria that are carried by mosquitoes).

The authors found that vector-borne diseases (VBDs) have significantly affected economic development, and that VBDs are affected by underlying ecologic conditions, especially biodiversity. Interestingly, their model predicts that the burden of VBDs will rise (and local economies will suffer) if biodiversity falls.

Dengue risk map (for past 3 months); tropical areas are
disproportionately affected by many tropical infections;
www.healthmap.org/dengue/index.php

An example of a VBD is dengue, a viral disease carried by mosquitoes (the disease 'vectors'). Both the dengue virus and its mosquito vectors are influenced by local ecosystems. One example of how influencing an ecosystem can affect a VBD is the release of genetically altered mosquitoes into an area to help control the spread of dengue disease. This practice has been used in multiple locations to control dengue spread via limiting the local mosquito population's ability to reproduce.

An example of a VBD that has potentially emerged due to relatively poor biodiversity is Lyme disease; this is a bacterial infection carried by ticks; there were over 24,000 cases in the US in 2011.

The authors note that diverse, well-functioning ecosystems may have a positive effect on the health of a given population via decreasing the burden of vector-borne diseases. This, in turn, will positively affect a given area's economy. This research is intriguing and more research into the relationship between the burden of VBDs and ecology is warranted.

Dengue: Already Entrenched in Southern Florida?

Dengue activity over the past 3 months (blue areas = areas of ongoing
transmission risk); www.healthmap.org/dengue

Here is a nice article on the potential for dengue virus to re-emerge in the United States. The article links to a poster presentation by Shin and colleagues from the University of Florida that was presented at last month's American Society of Tropical Medicine and Hygiene annual meeting. These authors isolated a distinct strain of dengue virus from an outbreak of dengue fever in Key West in 2010 and note that this strain may circulate in the region. Only their abstract is available for review so it is not possible to comment on their research further at this time.

Dengue is a viral infection transmitted by Aedes aegypti (and Aedes albopictus) mosquitoes; it is the most common mosquito-borne viral infection. It causes an acute febrile illness characterized by severe pain (which gives the disease its colloquial name: "breakbone" fever). Although not associated with high mortality, the disease is nonetheless associated with significant morbidity.

Aedes albopictus mosquito (one of the mosquitoes
that carries dengue in the United States); cdc.gov

It is worth taking a second to orient yourself to the map that appears at the beginning of this post; this map shows areas of dengue activity over the past three months, and areas of sustained transmission appear in blue. The research by Shin and colleagues begs the question: should southern Florida also appear in blue on this map?

In recent years we have seen outbreaks of dengue in multiple areas of the United States, including Hawaii, Texas and Florida (as noted above). It is possible that climate change, with global increases in mean temperatures, is playing a role in expanding the disease by increasing the geographic range of the mosquitoes that carry the dengue virus.

Anything that can capture rainwater can become a breeding
ground for the mosquitoes that carry dengue (cdc.gov)

Here is an abc news story on dengue in Florida that provides a lot of nice background information on this problem. The article also discusses the use of genetically modified mosquitoes to help control the disease; see here for a previous discussion of this practice.

More examples of potential breeding grounds
for mosquitoes (cdc.gov)

The environment in southern Florida (and many other areas of the United States) can support endemic dengue transmission. To boot, we already have the mosquito vectors that carry this disease (as well as other diseases such as chikungunya). As both of these mosquito-borne viral illnesses have seen geographic expansion in recent years, the medical and public health communities in the US need to be especially vigilant in looking for cases of these diseases and controlling the mosquitoes that transmit them.

Genetically Altered Mosquitoes: Way to Control Dengue?

Aedes aegypti mosquito (cdc.gov)

Here is an interesting article published online in Time that outlines how genetically altered mosquitoes are being used in some parts of the world to control dengue. In a lab setting, mosquitoes are altered such that males become either infertile or carry genes that decrease longevity; the infertile males are then released into the environment and either fail to impregnate female mosquitoes or pass on genes that decrease the lifespan of the next generation of mosquitoes.

This approach was used in Malaysia in 2011 and is now being employed in Brazil. Although the long-term environmental effects of this approach are uncertain, it is clear that new technologies and approaches need to be deployed to combat dengue.

European Dengue Outbreak: Etiology & Implications

Here is a link to a Reuters story reporting on the large outbreak of dengue ("breakbone") fever in Madeira (a Portuguese-controlled series of islands in the Atlantic just west of Morocco).  Over 1,300 cases have been reported since the outbreak was first recognized in October; significantly, this is the first sustained European dengue outbreak since the 1920s.

Dengue fever is a viral infection transmitted by Aedes aegypti (and Aedes albopictus) mosquitoes; it is the most common mosquito-borne viral infection. This is a febrile illness associated with high morbidity and low mortality; the disease can be deadly, however, as it can cause shock and an illness characterized by widespread hemorrhaging.

It is believed that the virus crossed species from monkeys to humans in Southeast Asia or Africa sometime in the past 1,000 years, with rapid dissemination of the disease around the world in the 20th century. Per WHO estimates 50-100 million infections occur each year with 22,000 deaths; see the CDC's site for great information about the disease.

Dengue fever is an excellent example of a disease that has emerged and expanded due to globalization; it is believed that rural-urban migration, explosive population growth, increased international trade and poor urban infrastructure all have played roles in the rapid expansion of this disease.

Dengue infections have made a resurgence in the United States as well, with cases reported in Key West in 2009 and recent cases being reported in Hawaii and in Texas, also. There is concern that climate change and global warming are playing a role in the re-emergence of dengue in the US: see a thoughtful commentary on this in the Lancet.

http://www.who.int/csr/disease/dengue/impact/en/; Areas in red
represent areas where dengue has emerged since 1960

What are the implications of the outbreak of dengue in Madeira and the sporadic cases being seen in the United States? It is clear that "tropical" infections can emerge and expand rapidly, and that infectious diseases are affected by globalization and do not respect traditional borders. Any solution to the dengue problem must involve a trans-national approach that emphasizes collaboration and brings together professionals across disciplines.

cdc.gov; Aedes aegypti mosquito

It is also clear that this is not a static issue: the public health community must be vigilant with dengue surveillance and early intervention (such as improved mosquito control) when disease expansion is detected.

Finally, here is a nice CBS News piece with a general discussion of the scope of the dengue problem, as well as an interview with Dr. Anthony Faucci, director of the National Institutes of Allergy and Infectious Diseases.