Could Dengue Spread in a Warming World?

Guest Essay by Kip Hansen

A new paper in nature microbiology makes the following statement:

“Dengue is a mosquito-borne viral infection that has spread throughout the tropical world over the past 60 years and now affects over half the world’s population. The geographical range of dengue is expected to further expand due to ongoing global phenomena including climate change and urbanization.”

 The paper, “The current and future global distribution and population at risk of dengue” [Messina2019 ] is open source and available as a .pdf here.  As required by the Editorial Narrative of the NY Times regarding all things climatic, our ever-hard-working NY Times journalist, Kendra Pierre-Louis, was tasked with spreading the gloom.

If you don’t know what Dengue Fever is you obviously have not traveled to the Caribbean or South or Central America recently, where dengue is endemic and epidemic.

“Dengue fever is a mosquito-borne tropical disease caused by the dengue virus. Symptoms typically begin three to fourteen days after infection. This may include a high fever, headache, vomiting, muscle and joint pains, and a characteristic skin rash.  Recovery generally takes two to seven days. In a small proportion of cases, the disease develops into severe dengue, also known as dengue hemorrhagic fever, resulting in bleeding, low levels of blood platelets and blood plasma leakage, or into dengue shock syndrome, where dangerously low blood pressure occurs.” —  Wiki

Like malaria, dengue is transmitted by mosquitoes, but the mosquitoes each individually need to obtain the virus by biting an infected human before they can pass dengue on to an uninfected human by biting them.   Thus, two things are necessary for the occurrence of dengue fever in a human population:  a widespread population of mosquitoes (typically  Aedes aegypti) and infected humans.  Note that Aedes mosquitoes can only fly about 100 meters in the wild, so suitable breeding sites have to be ubiquitous in the environment and many infected human hosts have to be present and live close together.  This is why dengue has such prevalence in the tropical third world.

Further, Aedes aegypti prefer to breed in very small bodies of water — such as the rain water collected in a discarded car tire or a tin can  and often in the rain barrels common in communities that do not have reliable municipal water systems.

In the Domincan Republic, where my wife and I recently worked for a humanitarian NGO, dengue is rampant — both endemic (said of a disease or condition that is regularly found among particular people or in a certain area) and epidemic (an instance of widespread occurrence of an infectious disease in a community at a particular time). (see maps below)   This just means that it is commonly found among the people there, and that, sometimes, some city or region will experience an outbreak that effects a significant percentage of the population.

Personal Experience:  While we were serving there, the largest city on the northern coast had a dengue epidemic which had been ongoing for a couple of months.  The regional health minister contacted us with a plea for help (we had worked with him on some local health clinic projects).  When we asked what they had done in the past for these outbreaks, he explained that the health department would go to the local “all-inclusive resort” (owned and operated by international corporations) and borrow their anti-mosquito spraying equipment.  In this case though, the epidemic was so wide-spread that the resort had its equipment in almost full-time use to ensure that none of their American or European guests contracted dengue.  We arranged for the organization that we worked for to purchase a powerful aerosol  sprayer that could be mounted in the back of a small pickup (that could easily pass through the narrow streets) and two backpack sprayers (think Ghost Busters) to go into the narrow alleyways between homes.  Spraying along with community efforts to remove all mosquito breeding sites (trash, tires, tin cans) and to cover water barrels with cloth tops stopped the epidemic within a week.  The availability of community-owned vector control equipment (sprayers) resulted in a long term improvement in dengue control in this city.    Dengue was and is still present there, due to the mobility of infected individuals and the mobility of Aedes aegypti due to transportation of goods and materials from around the country, resulting in their re-introduction to the city. But its incidence has been reduced below epidemic levels.

Backpack sprayers, which look oddly like those featured in Ghost Busters, are very effective in knocking down mosquito populations in crowded cities with their narrow alleyways between and behind houses.

Dengue needs lots of people crammed into small areas with plentiful breeding sites — small warm bodies of water.  This describes almost all the poorer sections of all Developing World (3^rd World) countries in the tropical regions of the planet.

This map shows malaria in the top half — we can see that malaria has been more or less confined to central Africa, with lower risk areas in the north of South America and throughout India and Southeast Asia.  Malaria has been more or less eliminated from North America and Europe – and has never been a real problem in dry, desert areas.  Dengue has a different, though similar, distribution,   although again Central Africa, India and SE Asia are potential hot spots as this map is of “suitable” environment.

This next map gives a bit  more informative view:

from Bhatt el al. 2013, in Nature “The global distribution and burden of dengue”.Click here for full sized image.

(Note:  the bottom global map in the image was distorted in the original.)

The three global views in this image are of different things and the colors do not represent the same ideas.  The top map shows “evidence consensus” (how sure are we that dengue exists in this nation or does not exist).  The second show how probably it is that dengue exists (0 to 1) in 5 km grids sections.  The bottom (distorted) map shows “number of infections” which depends on reliability of reporting.

Now that we have some idea of where dengue is found now, and WHY it is found there, let’s see what Messina et al. (2019) are projecting:

Now, let’s see what they are trying to show in this map. “Under a moderate warming scenario, 2.5 billion more people could be at risk for dengue fever by 2080.” The darker colors represent “higher dengue risk” than in 2015.   The authors claim to have “modeled”  the future of dengue as follows (serious readers can skip their description):

Dengue future modelling ensemble approach. Our final aim was to produce nine maps, a prediction for dengue suitability in the years 2020, 2050 and 2080 under three different emissions scenarios (RCPs). Each of these nine maps were composed of 100 ensemble predictions that randomly sampled (with replacement) the  following aspects of the analysis:

1. The fitted dengue BRT [ensemble boosted regression tree] model (from a choice of 100 BRT models fitted to 2015 data).
2. The predicted future distribution of Ae. aegypti (from a choice of 100 modelpredictions).
3. The predicted future distribution of Ae. albopictus (from a choice of 100 model predictions).
4. The predicted temperature suitability for dengue transmission (from a choiceof 17 GCMs).
5. The predicted minimum monthly precipitation (from a choice of 17 GCMs)
6. The predicted relative humidity (from a choice of 17 GCMs).
7. The predicted maximum monthly precipitation (from a choice of 17 GCMs).

This approach sought to fully propagate the uncertainty in the climate, Aedes and dengue models through to the final prediction (see maps of uncertainty estimates in Supplementary Fig. 5). These 100 predictions were then summarized by mean and 95% credible intervals to give the final prediction for each year RCP combination.

This approach is so fraught with problems that I don’t know where to start.  But biological and historical plausibility is a good place to start.  Is the distribution of Ae. Aegypti mosquitoes, and thus dengue,  primarily temperature dependent?

The answer to that is NO and YES.  Ae. Aegypt cannot survive in deserts with extremely low humidity, they cannot survive where temperatures get too cold in winter, thus there is a lower temperature climatic limit, but according to the CDC, they already  can and do survive in much of the United States, as of 2017:

But when we look at the map of where dengue is found today, first map in this essay, we find it is not simply found where Ae. Aegypti are currently found, as the map of the United States shows.  Why not?  Because the existence of dengue in any particular place depends on much more than simply the (possible) existence of its primary insect vector.  Note that the same is true for malaria — the mosquitoes are here (in reduced numbers due to controls) but malaria is not.

Ae Aegypti  mosquitoes are the primary insect vector for both dengue and yellow fever — by looking at the historical records for yellow fever we have a proxy for dengue (which was confined to SE Asia before World War II).  Even in the depths of the Little Ice Age,  yellow fever was present and killing people as far north as Boston in the 1600s-1800s.  Should we expect a modern epidemic of dengue in Boston?  Of course not.

Why? Because:

“….the mosquito vectors capable of transmitting malaria, yellow fever, and dengue have been present throughout much of the United States since the 1600s. What has clearly changed in the United States from the 18th and 19th centuries to the present is the availability of potable water, sanitation, and social lifestyles. These developments have essentially eliminated the need to store water in indoor containers and reduced contact with mosquitoes. After World War II, and particularly during the 1950s, a boom in the US economy increased the standard of living and aided the widespread use of television and air conditioning. In addition, the use of screened terraces and windows increased.”…”Thus, diseases such as malaria, yellow fe­ver, and dengue have all but disappeared.”

The Messina et al. projections of dengue risk in the American south and in the desert areas of southern Arizona and New Mexico are not biologically plausible when one takes into account current living standards and modern vector control efforts already extant in these areas.  Northern Australia is another area that is unlikely to be plagued by mosquito borne disease.

As for Africa, India, China, and SE Asia, as population increases and concentrates in cities, the poor will live in slums, rife for dengue, and incidence of dengue will rise proportionally.   But as  these developing countries advance and standards rise to include screened windows, air conditioning, and sanitation — there will be less risk of mosquito-spread diseases.  As health standards rise, there will be fewer infected individuals being bitten by mosquitoes thus less transmission of these diseases.

The Bottom Line:

 1.  Dengue is a mosquito borne disease that depends on poor sanitation and non-existent vector control to remain a problem in areas where it is endemic. It can be eliminated, just like yellow fever and malaria were eliminated in the United States.

 2.  Raising standards of living high enough to allow homes to be protected with screened windows, screen doors, municipal water (with pipes) and to have reliable functioning trash collection will eliminate much of the dengue load in a country.

 3.  Public health programs that inform the population of risks and instruct them to eliminate breeding spots for mosquitoes, along with serious vector control (spraying)  reduces risk.

 4.  Where dengue is endemic and epidemic, it is a real ongoing public health problem and should not be ignored or brushed off, even if this new paper exaggerates the increasing risks — in some countries it is just a fact of life taking children and the elderly to their graves.

 5.  It is unreasonable to maintain that as these developing countries advance that they will not achieve the two items (2 and 3) above….international aid programs can help in this regard and will do much more to protect the health of people  than any of the climate change initiatives being pushed by political activists.

6.  Recommended reading for those interested in this topic: Lessons from malaria control to help meet the rising challenge of dengue

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Author’s Comment Policy:

 Please try to stay on topic — I know it is a real temptation to simply rail against models and modelling but that is not the real problem with this paper or its use as climate change propaganda by the NY Times.

I think that there has been a failure to review the results of their models against biological and sociological/political plausibility.

I’d like to read your views on the subject. Address your comment to “Kip…” if you are ‘speaking’ to me.

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