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“Spiking fever, searing muscle and joint pain, blood seeping through the skin, shock and possibly death—the severest form of dengue fever can inflict unspeakable misery. Once rare, dengue fever now threatens more than 2.5 billion people.” (1)
“Dengue is the most rapidly spreading mosquito-borne viral disease in the world. In the last 50 years, incidence has increased 30-fold with increasing geographic expansion to new countries and, in the present decade, from urban to rural settings…” (2)
History of Dengue Fever
People’s stories from hundreds and thousands of years ago tell the origins of ka-dinga pepo: the “disease of the devil” (in Swahili), a.k.a. Dengue Virus. Roughly 2000 years ago, one species of mosquito (Aedes aegypti) found new hosts. Those hosts lived in cities and villages along the Nile, and they recorded their experiences for us to read today. They told tales of a terrible illness that started with a fever and often a rash and easy-bruising. It caused unbelievable pain in the bones and joints, with a special pain emanating from behind the eyes. In its worst forms, the suffering included bleeding from the eyes and oozing blood from the pores. Some recovered, some died. (1)
These entrepreneurial mosquitoes spread across the tropical and sub-tropical world, eventually stowing away on ships with slave traders and rum-runners, and making their way to 17th century Boston and Philadelphia. By 1780, the famed Dr. Benjamin Rush (a signer of the Declaration of Independence) reported treating an outbreak of “Bone Break Fever” among Philadelphia residents, and the name stuck.(1)
Dengue Virus (DV) infections continued to plague much of the Tropical New World until hemisphere-wide mosquito eradication efforts in the 1950’s and 1960’s nearly exterminated it in the Western Hemisphere. 1945 marked the last Dengue epidemic in the United States. Unfortunately, Cuba did not eradicate their Aedes aegypti mosquitoes, although Castro maintained an official public façade that Cuba too should be certified as a Dengue-free nation. Cuba’s living population of Aedes aegypti mosquitoes and their accompanying Dengue Virus apparently provided an ongoing reservoir of Dengue Virus for re-infecting all the Caribbean and Central American and South American nations. (2,3)
Dengue in Mexico
Dengue virus re-appeared in Mexico in 1968 and has spread across almost of Mexico as Aedes aegypti mosquitos moved inland from the Gulf Coastal regions. Since that time, city-loving Aedes aegypti mosquitos and the Dengue Virus have basically been endemic to the Yucatan Peninsula, with confirmed Dengue Virus cases peaking and increasing the past 3 years in Yucatan State. (3)
This year, the Federal Government of Mexico has announced that overall cases of the milder, classic form of Dengue have fallen slightly in Mexico since 2009. "But the more serious hemorrhagic form has spiked to about 1,900 cases this year, compared with about 1,430 in the same period of 2009. Only 16 people have died this year from the hemorrhagic form, but the seriousness of the disease makes it a concern." For the first time in 64 years, there is evidence of Dengue in the United States as well (in Key West, Florida).
Dengue Fact Sheet
- Dengue infections in Merida and the Yucatan beach areas are 4 times higher this Spring 2010 than they were a year ago.
- Current rate may result in over 120,000 new Dengue Virus infections in Yucatan in 2010.
- The recent rates of Dengue Hemorraghic Fever as a percentage (%) of total confirmed Dengue virus infections have exceeded 50% across most of Mexico, which means that more than 50% of the confirmed Dengue virus cases proceeded to Dengue Hemorraghic Fever symptoms.
- 2.5 billion people – two fifths of the world’s population – are now at risk from Dengue.
- 50 million new Dengue Virus infections occur annually.
- Dengue Virus is transmitted only through being bitten by Dengue-infected female Aedes Aegypti (silent) mosquitoes in the Americas.
- Infection by 1 of 4 Dengue strains gives protection against only that strain for 4 months.
- Infection by the other 3 strains are possible during the meantime, and re-infection by the first strain is possible after 4 months.
- There is no such thing as "Dengue H".
- All four Dengue Virus strains (DV-1, … , DV-4) are circulating in and endemic to the Yucatan Peninsula.
- Prior Dengue infections seem to almost universally interfere with your subsequent immune responses to new Dengue infections, with each new infection producing more and more severe symptoms, increasing the likelyhood of the possibly fatal Dengue Hemorrhagic Fever (DHF) & Dengue Shock Syndrome (DSS).
- Dengue infections take several forms. Infections can present no symptoms, mild symptoms like just a rash, flu-like symptoms, distinctive pain behind the eyes, high fevers (104ºF or 40ºC), and sometimes fatal Dengue Hemorrhagic Fever with bleeding from the eyes, under the skin, and into the Gastro-Intestinal Tract.
- There is often a 3-week delay between the mosquito bite and onset of Dengue symptoms. There are anecdotal reports of as little as 4 days between being bitten and the onset of symptoms.
- The key days for getting Dengue testing are DAYS 4 – 6 after onset of fever/symptoms.
Different Types of Dengue Infections
It is important to understand the different types of infections so that you will know how to treat yourself and your loved ones.
- Primary Dengue Infections (First-Time Offenders): The commonly-used lab tests do not detect Dengue until Day 4 – Day 6 after the onset of symptoms. Waiting to get a tested during Days 4 – 6 will not change either your symptoms or medical treatments, unless your symptoms progress to DHF or DSS around Day 6.
- Secondary Dengue Infections (Infected people who have already had a prior Dengue Infection): The commonly-used lab tests do not detect Dengue until Day 10 after onset of symptoms, because prior Dengue infections heavily interfere with the patient’s immune response to the new infection.
How Do I Know if I Have Dengue?
Yucatan has only two labs that do Dengue testing: a Departmento de Salud lab and one at Universidad Autonoma de Yucatan (UADY). Turn-around times (TATs) for Dengue test results CAN be 1 day. Free Physician Evaluations and Free Testing are available at UADY’s Centro de Investigaciones "Hideyo Noguchi" research laboratory, across Calle 59 from the Merida Centennario Zoo, on the NE corner of Avenida Itzaes and Calle 59.
Park across the street at the park (Parque de la Paz) in front of the old jail/penitentiary. Walk into the building that is across from the Park on the northeast corner of Itzaes and Calle 59. Walk up to the front desk as you enter the Research Center, and explain that you need to be checked for Dengue.
"Ayudame, por favor." (Please help me)
"Yo penso que yo tener una infeccion de Dengue Virus." (I think I am infected with Dengue Virus)
"Aqui se realizo pruebas de diagnostico de Dengue." (I understand you can test for Dengue here)
The Mosquitos that Carry Dengue and How to Get Rid of Them
The Dengue carrying mosquitoes Aedes aegypti (A.ae.) mosquitoes need only a teaspoon of water that doesn’t evaporate for one week to convert eggs to free-flying adults at Yucatecan temperatures. With the kind of rain that this area has been experiencing over the past month, imagine all the teaspoons of water that might be lurking in your garden and your yard.
They (A. ae.) prefer clean water (or rain-water) residues, like those found in tinacos, flower pots, rubbish piles, old tires, old pipes, junk, rain water in unattended swimming pools, sink & floor drains, etc. The best methods for reducing Dengue risks are to kill and exclude mosquitoes from any areas where people live.
Other prevention ideas?
- When you are outside, wear long pants and socks or use a repellent.
- Put screens on your windows and doors to keep mosquitoes outside, especially at night.
- Seal or invert containers that might be outside in the rain, or which have water in them
- Put mosquito larvae-eating fish like gambusia or guppies into ponds or fountains
- Treat your fountains with chlorine to kill larvae at least weekly, plus use a copper algaecide to protect the chlorine. Since chlorine levels rapidly drop to ineffective levels when algae grows, especially after a rain, adding copper sulfate as an algaecide protects the chlorine.
- Typical larvicides (like Abate (Temephos) or copper salts : a.k.a. Paris Green) kill larvae for a long period of time, unless you drain the water. The old time larvicide Paris Green is a mix of Copper Arsenate and Copper Acetate, called cardenillo, verdigris, or aerugo in Spanish. It should be used as a dry powder at 1% – 3% levels that floats on the surface of the water. Various formulations of Temephos are sold under the trade-name “Abate” and are reported to work for at least 1 month and up to a year, depending on the application.
- Eliminate even small amounts of water that stand for a week or more
The adult A.ae. mosquitoes live roughly 1 month “in the wild” (normal conditions). Under better conditions, (like a nice laboratory with 3 hot meals and a cot), they can live for 6 months. Once an A.ae. adult mosquito gets Dengue Virus, they can transmit it for the rest of their little lives: 1 month – 6 months.
More About Larvicides
Larvicides are generally considered to be more effective at controlling mosquito populations than spraying to kill adult mosquitoes. Larvicides are also typically less toxic to humans than adult mosquito sprays, since they are usually applied to water that humans do not drink.
Maintaining sufficient free chlorine in a swimming pool or fountain also acts as a larvicide. When free chlorine levels fall in a pool or fountain, then mosquito larvae thrive. Copper Sulfate is one compound commonly used as an algaecide in swimming pools, because Copper Sulfate is nearly permanent (works until you drain the pool), and it protects the free chlorine from being chewed-up (consumed) by algae blooms that come after a rainfall(s). Rainfalls inject our swimming pools and fountains with algae-spore-laden dust, and the rainfall here is also generally basic (pH up to 9). Both the algae injection and pH increase reduces the amount of free chlorine very rapidly, reducing disinfecting power and reducing chlorine’s effect as a larvicide. (Now you know why your pool turns green faster after a rainfall!)
Since Aedes aegypti mosquitoes need one week for eggs to hatch and larvae to mature into free flying adults, if you maintain sufficient chlorine levels at least once a week, and suppress algae with an algaecide, then you effectively interrupt the mosquito’s breeding cycle by killing the larvae. Copper Sulfate (Tri-Hydrate) is available locally at places like Home Depot or the Baroso pool supply stores, and it only takes 34 grams (roughly 5 level tablespoons) of the pretty blue Copper Sulfate Tri-Hydrate salt to treat 50,000 liters of water (13,200 gal.).
A different form or type of larvicide, Paris Green (also using copper salts), is used for treating old tires, unused swimming pools, and other unused water features. In Spanish Paris Green is called cardenillo, verdigris, and aerugo. Paris Green is a toxic double salt of copper arsenate and copper acetate that floats on top of the water, which is why it is only typically used in old tires and other sources of unused standing water. It may also be worth considering using a synthetic organic pesticide larvicide like Abate (Temephos) .
Abate can be applied in various forms and concentrations, lasts a long time, and seems appropriate as a larvicide for non-drinking water supplies, based on USEPA reports. The USEPA reports that:
“Temephos, applied according to the label for mosquito control, does not pose unreasonable risks to human health.” (5)
“Temephos is not expected to have a direct impact on terrestrial animals. Risk quotients for freshwater fish only slightly exceed levels of concern; no acute toxicity data are available for marine fish species. Field monitoring data indicate little impact on birds. Aquatic invertebrates, particularly Daphnia magna, are extremely sensitive to temephos.” (5)
One final option of larvicides for mosquito control are natural pesticides that utilize bacteria spores to kill the larvae. Bacillus thuringiensis israelensis (called Bti for short) is a naturally occurring soil bacteria spore used to control mosquito larvae by releasing toxins into the mosquito’s gut, causing the larvae to stop eating and die. Since the larvae must eat the spores, Bti is only effective against actively feeding larvae, and does not affect mosquito pupae or adults. Because of its mosquito-larvae-only specific way of action, the USEPA reports that microbial larvicides (including Bti) are “essentially non-toxic to humans”, and that “microbial larvicides do not pose risks to wildlife, nontarget species, or the environment, when used according to label directions.“ (citation here)
There is one significant drawback to using Bti; it is reported to last only seven days in some applications. We hear Bti might be available here in Merida, but we cannot tell you where… yet!
What About the Neighbors?
If you think your neighbors are raising and herding mosquitoes in their backyard, there are a few measures you can take. First, do your best to contact them, and ask them to clean up rubbish & debris, and ask them to treat or drain standing water.
US and Canadian snowbird properties are some of the worst offenders, especially if there are no responsible and active property managers and the owners are absent. Snowbird-owned properties are notorious mosquito breeders because NOB folks love water features like pools, fountains, decorative troughs, hollowed-out stones and potted plants. These collect rainwater – and it only takes a tablespoon of residue that doesn’t dry to breed A.ae mosquitoes. The problem is aggravated by local property managers who are hesitant to do anything new or different, particularly if the owner has not directly ordered it.
Property managers are notorious for letting fountains and pools get super nasty for months, and then making a big effort the week before the owner returns to clean things up. Consequently, the owners suspect nothing, because they return to sparkling pools, while their neighbors suffered with months of mosquitoes.
In a case such as this, do your best to educate your neighbors directly and voice your concerns, and get their property manager’s contact information from them. (Send them this article!)
Finally, Mexican health authorities really have no authority to enter properties without the owner’s consent.
Dr. Fry had a friend in the centro who persistently tried to get various government agencies to help with their New York City neighbor’s algae/mosquito swamp-like swimming pool, with no success. In a case such as this, we suggest discreetly throwing a nylon stocking with granulated/powdered chlorine, which would work for a while, or with the aforementioned Paris Green, which would work nearly permanently.
Traps and Repellents
Mosquito traps (that emit CO2 from propane) or the UV light+fan ones work well at trapping just mosquitoes, but the ones that use a UV light attractant and electrical grids indiscriminately kill lots of many types of bugs, thereby reducing bat populations. This is not a good thing when you want to reduce the mosquito population.
There are, of course, many ways that mankind has devised to keep mosquitoes at bay. The easiest methods to come by here in the Yucatan are in the grocery stores: sprays and pumps of liquid repellant, mosquito coils that burn like incense and mosquito "cards" that are inserted into special holders that plug into the wall and emit mosquito hormones that keep them away. There are candles with citronella to burn on your patio. This year, we’ve also seen mosquito bracelets that emit citronella to keep the wearer mosquito-free.
There are devices that emit sounds that are supposed to repel mosquitoes, and there are bug lights, as well as "tennis rackets", nifty devices that allow you to swat at the mosquitoes and give them a deadly electrical shock when you get one (these, we hear, are available at the Chinese restaurant on Calle 47 at Parque Santa Ana).
Nothing is failsafe, and in our experience, using multiple methods is probably a good idea.
The Dengue Transmission Chain
Dengue Transmission occurs as a chain of events. Break just one link of the chain and Dengue transmission ends.
- Uninfected female A. ae. finds febrile Dengue infected Human.
- Female A. ae. bites the febrile Dengue Human.
- Female A. ae. mosquito hides and rests for 3-4 days.
- Dengue virus moves into the A. ae. mosquito’s salivary glands.
- Female A. ae. mosquito lays eggs in water.
- The now Dengue-infected female A. ae. mosquito finds human and bites human.
- Mosquito eggs hatch and develop into adults in 7 days at Yucatan temperatures.
- Newly hatched female mosquito has sex with male mosquitoes.
- Male mosquitoes feed on nectar and plants.
- Newly hatched A. ae. female mosquitoes search for their first blood meal, and if successful, they bite a human for their first blood meal, continuing the cycle.
If you break or interrupt any single link in this chain of mosquito activity, (1-10), Dengue transmission stops, which makes mosquito control our best proven way to control the spread of Dengue virus.
How to Eliminate Mosquito Breeding
Eliminating or treating mosquito breeding sites is highly effective and our best current solution. As you read through this list, you will undoubtedly find a few things to do that you haven’t thought of yet, as well as more than a few suggestions for some of your neighbors.
- Clean up rubbish piles & drain standing water
- Drill holes in the bottoms of all outdoor flower pots
- Cover household drains tightly, or treat with ammonia to kill larvae
- Maintain sufficient free-Chlorine levels in swimming pools to kill larvae
- Dispose of old tires, buckets, & plastic items that retain even small amounts of rain water
- Drain unused swimming pools to total dryness, and consider using a larvacide like Copper Sulfate to treat any rain water that accumulates during the owner’s absence.
- Treat fountains and pools with chlorine, copper, or add mosquito-eating fish, like Gambusia or guppies, mollies, and other live-breeding fish.
- Flush unused guest bedroom toilets weekly, or treat, or seal them with plastic wrap
- Flush all sink, shower or floor drains inside the house weekly, and cover or seal unused drains
Don’t Worry… Be Happy
And after all this, don’t worry about it too much. After living here for over eight years, we know a few people that have contracted Dengue while living here. We also know a lot of people that have not. Some of our friends who came down with Dengue suffered greatly, and some seemed to have a mild case. Take the precautions listed above and enjoy yourself. If you feel you might have Dengue, get yourself to a doctor as soon as possible and get tested. If you do have it, stay hydrated and stay in touch with your doctor. Luckily, the doctors here are not only competent, but they give you their cel phone numbers to call if you need them. Take advantage of the great healthcare here if you need it, and otherwise, don’t worry! Enjoy the Yucatan!
(1) “The Devil’s Disease, Dengue Fever”, P. McGuire, Johns Hopkins Public Health, Spring 2010 Ed., 2010, pp 16-21.
(2) Dengue: guidelines for diagnosis, treatment, prevention and control — New edition, WHO and the Special Programme for Research and Training in Tropical Diseases (TDR), 3rd Edition report, ISBN 978 92 4 154787 1, 2009.
(3) A Timeline for Dengue in the Americas to December 31, 2000 and Noted First Occurrences”. J Schneider, MPH and D Droll, Pan American Health Organization: Division of Disease Prevention and Control, June, 2001, 99 1-20.
(4) Mexico Worried by Rise in Hemorrageic Dengue by Mark Stevenson, Associated Press, July 21 2010.
(5) USEPA Pesticides: Reregistration: Temephos RED. http://www.epa.gov/oppsrrd1/REDs/temephos_red.htm
Links of Interest:
About Dengue in the United States by Ali Khan
Baroso Pool Supply is at Calle 51 #532-A x 74 y 76 in the centro
Editor’s Note: The facts of this article were painstakingly gathered by Dr. Steven Fry who lives in Merida, Yucatan with his wife. Dr. Fry is a doctor of Chemistry, and spent 27 years in Environmental and Public Health in the United States before coming to Merida. He was the first Western scientist invited into the former Soviet Union, (after its break-up) to evaluate public health & environmental issues in Ukraine, as a consultant to the US Agency for International Development. His wife is a laboratory director at UADY, specializing in Dengue and West Nile Viruses – and Dr. Fry developed a bit of a background from helping evaluate her data, and helping write her dissertation on Dengue and West Nile Viruses. She is a full-time UADY researcher working on various joint projects with the Dept. of Health and the Bill and Melinda Gates Foundation.