The geography of dengue fever in Mexico

Preventing, diagnosing and treating dengue fever is a major public health issue in many parts of the world, including central America and Mexico. The disease is transmitted by mosquitoes. Infected patients develop a sudden high fever, usually accompanied by generalized body pain and a skin rash. The pain can be very severe, hence the disease’s common name of breakbone fever.

Several species of mosquito can transmit dengue, but female Aedes aegypti mosquitoes are the main transmitter of dengue in Mexico. These mosquitoes bite during the day, mostly in the period two hours either side of dawn and dusk. The mosquito bites an infected person and ingests blood with the dengue virus, which incubates in the mosquito for a period of 8 to 12 days. After that the mosquito can begin to transmit the virus by biting other people. From 5-7 days later any newly infected person is likely to have symptoms.

The graph below shows the monthly number of dengue cases in Mexico from 2000 to 2006. It is clear that most cases are reported between July and November, with very few cases occurring between December and May. This can be explained by Mexico’s climate. Almost of all of Mexico receives most of its rainfall between June and October. The mosquitoes that spread dengue need stagnant water to breed. There are far more small pools of water available for mosquito breeding in and immediately after the annual rainy season. Eliminating potential locations where water can collect and stagnate is an important element of dengue prevention programs.

The graph also shows that the number of cases of dengue was increasing rapidly between 2000 and 2006. Indeed, numbers continued to rise until 2013 when more than 50,000 cases were reported for the year (an average of more than 4000/month). At first sight, this suggests that dengue prevention programs have not been very successful, but in fact the rise echoes what was happening worldwide. One possible, at least partial, explanation may be that changes in climate have allowed dengue mosquitoes to thrive in environments where they were previously scarce. People in such areas are unprepared for dengue; they may not have instituted prevention programs, and may have been slow to receive correct diagnosis. The migration of people affected by dengue from one region to another may also have helped the disease spread, provided there were host mosquitoes in the destination region.

The good news is that the number of dengue cases in Mexico in 2014 has fallen from its 2013 level by about 50%, so the dengue epidemic may finally be on the wane. As of 24 November 2014, 28,109 cases had been reported for the year, an average nationwide rate of 23.47 cases/100,000 people. Dengue is fatal in a relatively small number of cases, with 33 deaths reported in Mexico so far this year..

The total number of cases may finally be on the decline, but the figures for 2014, when looked at state by state, suggest that the spatial pattern of dengue cases in Mexico is changing. The two maps below compare the rate of cases per 100,000 people on a state-by-state basis for 2007 and 2014. (The color-coded key is identical for both maps).

In 2007, the highest rates of dengue were found in the states of Veracruz and Quintana Roo, with Oaxaca, Guerrero and Colima comprising the next category. (Those five states are the ones colored red on the 2007 map). At the other extreme, no cases were recorded in 2007 in the state of Baja California, or in several tiny states including Aguascalientes.

The pattern shown on the 2014 map is quite different. In general, rates of dengue at the state level have not increased in Mexico, but decreased. However, there is a clear shift in emphasis towards the north-west, where several states had much higher rates in 2014 than in 2007. The extreme example is Baja California Sur, where the rate for 2014 (up to 24 November) was a whopping 549.9 cases / 100,000 people, more than five times the rate registered in any other state. Equally apparent is the belt of low-rate states (from Chihuahua to the State of México) down the center of the country from the U.S. border to Mexico City. These states are at relatively high elevation where fewer mosquitoes are found.

The states of Baja California Sur, Veracruz, Sinaloa, Sonora and Guerrero account for 55% of the 28,109 confirmed cases of dengue fever reported in Mexico as of 24 November 2014.

Mexico’s Health Secretariat publishes maps of each state showing which municipalities have reported cases of dengue. These maps are updated weekly. The link is to a pdf document with maps for 2014 up to 24 November.

There is some good news. In 2015, Mexico will be the first nation in the world to get a new dengue vaccine, developed by French company Sanofi Pasteur. The company hopes to have manufactured more than 40 million doses by the first half of 2015, and has decided to introduce it first in Mexico, with the first vaccinations likely to be offered to the public late next year or early in 2016. In trials, the vaccine proved 60.8% efficient in preventing the disease.

Reference for graph:

José Luis San Martín, J.L., Brathwaite, O., Zambrano B, et al (2014): “The Epidemiology of Dengue in the Americas Over the Last Three Decades: A Worrisome Reality”; Am. J. Trop. Med. Hyg., 82(1), 2010, pp. 128–135

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