The diffusion of the chikungunya virus across Mexico

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Nov 162015

It is now just over a year since the first locally locally transmitted cases of chikungunya were reported in Mexico. (Local transmission means that mosquitoes in Mexico have been infected with chikungunya and are spreading it to people).

Distribution of Chicungunya, as of October 2015

Distribution of Chikungunya, as of October 2015 (WHO)

The virus, first isolated in Tanzania in 1952, began to spread rapidly after an outbreak that originated in Kenya in 2004. By 2006, the virus had reached India, and by 2007 northern Italy. Imported cases were identified in Taiwan, France and the USA in 2010. In these stages, chikungunya was spread to new areas primarily by travelers infected with the virus.

The first locally-transmitted cases in the Americas were on the Caribbean island of St. Martin in 2013.

The chikungunya virus was first recorded in Mexico in October 2014 and has since spread rapidly from Chiapas through much of the country (map).

Incidence of Chikungunya, 2015, up to 24 October

Incidence of Chikungunya, 2015, up to 24 October. Data: SINAVE/DGE. Cartography: Geo-Mexico

As of 24 October 2015, 9423 cases have been reported in Mexico in total. The states most affected (in terms of number of cases) are Guerrero (1620 cases since October 2014), Michoacán (1548), Veracruz (1203), Oaxaca (1151) and Yucatán (1131).

Two mosquito vectors (Aedes aegypti and Aedes albopictus) are involved in the spread of chikungunya. Aedes aegypti is exclusively tropical in distribution, but Aedes albopictus also thrives in temperate climes, giving chikungunya the potential to spread virtually throughout the Americas.

The number of cases spikes with the rainy season since mosquitoes breed in stagnant water. Curiously, about two-thirds of the cases in Mexico have been in females, and only one-third in males. Presumably, this is due to differences in work and/or living habits, with females having greater exposure than males to the mosquitoes involved.

There is no vaccine to prevent chikungunya or medicine to treat the viral infection.

The most common symptoms of chikungunya virus infection are fever and joint pain. Other symptoms may include headache, muscle pain, joint swelling, or rash. Joint pains can last months or even years. The disease is debilitating, but rarely fatal. Someone who has contracted chikungunya develops lifelong immunity (unlike dengue fever, where no such immunity exists).

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The geography of dengue fever in Mexico

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Dec 112014

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.

Monthly incidence of dengue cases in Mexico, 2000-2006

Monthly incidence of dengue cases in Mexico, 2000-2006. Source: San Martín, Brathwaite et al (2014).

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).

Rates of dengue by state, 2007 and 2014

Rates of dengue by state, 2007 and 2014. Rates are cases / 100,000 population

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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Sep 172012

The 1991-96 cholera epidemic in Mexico originated in Peru in January 1991. It quickly spread northwards, reaching Central America by March and Mexico by July (see map). The cholera epidemic then spread slowly across Mexico before abating.

The spread of cholera in Mexico, 1991-1996

The spread of cholera in Mexico, 1991-1996 (Geo-Mexico Fig 18-6)

The incidence of cholera was much higher in the Gulf coast states than either inland or along the Pacific coast. By the time the epidemic was over in 1996, more than 43,500 cases had been reported in Mexico and 524 people had died.

Main Source:

PAHO 1997 (Pan American Health Organization) Cholera Situation in the Americas 1996, Epidemiological Bulletin, vol 18 (1)

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Aug 082011

Durango has long been considered the scorpion capital of Mexico (even the local soccer team became known as Los Alacranes, the Scorpions). At one point in the past, the city paid a bounty for each scorpion killed. Some historical accounts suggest that the scorpion catch rose dramatically, until the local authorities realized that some families had started their own financially lucrative scorpion-breeding programs.

These days, few scorpion stings are reported in Durango, partly because Durango’s scorpion hunters (alacraneros) catch and kill thousands each rainy season; prime specimens are encased in souvenir key rings and wall clocks sold in the local market. They also supply medical research labs.  Research in one lab at the University of California has isolated several peptides that appear to suppress the immune system, promising another way to prevent transplant rejection.

Lourival Possani, and his colleagues at Mexico’s National University (UNAM) have discovered a toxin (named scorpine) in scorpion venom that slows down the growth of malaria parasites in fruit flies; if similar techniques work in malarial mosquitoes, it may be possible to dramatically reduce the spread of malaria.

About 250,000 people in Mexico are stung by scorpions each year—more people than in any other country. Several dozen people die each year. Indeed, for the past 20 years, scorpion stings have been the leading reason in Mexico for  deaths due to adverse reactions and poisoning caused by venomous plants and animals. There are more than 200 different species of scorpions in Mexico, of which only 8, all belonging to the genus Centruroides are a significant public health risk. The map shows the areas defined by Mexico’s Health Secretariat as being of High, Medium and Low risk for dangerous scorpions.

Mortality remains higher in the smallest settlements, and is greatly reduced in mid-sized and large settlements. This is a function of both the reduced proximity of medical care in small settlements and of the higher numbers of scorpions/10,000 people in less urbanized settings. The highest mortality rates by age occur in the 0-1 years group (7 deaths/million), followed by the 1-4 age group (3.8/million) and the 60+ years group (0.8/million) (all data from

Scorpion risk in Mexico

Scorpion risk in Mexico (Secretaria de Salud)

Fortunately, progress is being made. The number of recorded deaths from scorpion stings [1] has fallen from more than 1,000/year in the 1950s to 285 in 1995, about 80 in 2003, and 57 in 2005. This improvement is the result of public health campaigns stressing the importance of seeking emergency treatment and of the development of antivenin serum (known as Alacramyn in Mexico and Anascorp in the USA). Mexico’s antivenin industry, led by the Bioclon Institute, is world class, exporting serum to the USA and Australia as well as throughout Latin America. The biggest threat from scorpions comes from central and northern states in Mexico, including several along the Pacific Coast: Nayarit, Jalisco, Colima, Michoacán and Guerrero.

According to UNAM’s Biomedical Investigation Institute, 277,977 people in Mexico reported scorpion stings in 2010. In the first five and a half months of 2011, 98,818 people in Mexico have been stung. The five states with the highest incidence of reported scorpion stings are: Jalisco (19,995), Guerrero (15,769), Morelos (13,123), Guanajuato (12,326) and Michoacán (10,597).

The incidence of scorpion stings rises sharply in summer when higher temperatures encourage scorpions to leave their lairs and go exploring.

Q. What other factors, besides the ones mentioned in this post, might help explain the pattern of risk shown on the map? Hint – can you think of things that the states shown as “high risk” — or the “low risk” ones — have in common?

– – – – –

[1] A Google search using the terms “scorpion”, “deaths” and “Mexico” finds dozens of websites all claiming that “In Mexico, 1000 deaths from scorpion stings occur per year.” This includes the two highest ranking sites in the results here and here, for articles dated 14 April 2011 and 20 August 2009 respectively. Given that 1000 deaths/year from scorpions has not been true for 20+ years, perhaps it’s time for these sites  to update their data by referring to Geo-Mexico!

The geography of cholera in Mexico

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Feb 282010

When Mexico braced herself for the imminent arrival of cholera from South America in 1991, many people believed that the disease had never previously been known here. However, during the 19th century, there were several outbreaks, including the epidemic of 1833 in which more than 3,000 people died in the city of Guadalajara alone.

A meeting of public health officials in January 1833 stressed the need for public areas to be regularly cleaned. This meeting called for the construction of six carts, to be used each night for removing the excrement left on street corners, since not all the houses had “accesorios” (toilets). This proposal reveals the unsanitary conditions which prevailed in Guadalajara at that time.

The epidemic struck in July, and peaked in August, when more than 200 people died of the disease each day. Sporadic cases dragged on into early 1834.

The shout of “Aguas!” (“Waters!”) as a warning of imminent danger, still used in many contexts today, actually dates back to when there was no sewage collection or provision. It was used to warn passers-by in the street below that the contents of “night buckets” were about to be emptied onto their heads…

In 1849 the city of Guadalajara feared a second epidemic. The authorities published a list of precautions that they considered essential, and a list of the “curative methods for Asiatic cholera.” At that time, the only major hospital in the city was the Hospital Belén. Its rival, the San Juan de Díos hospital, was “small and poorly constructed, insufficiently clean, and careless in waste disposal.”

The situation was made worse because  the  San Juan De Díos River was little more than an open sewer running through the center of the city. This river is now entirely enclosed and runs directly beneath the major avenue of “Calzada Independencia.”

Only two methods of sewage disposal were in use in 1850. Some houses took their sewage to the nearest street corner, where it was collected by the nightly cart for subsequent removal from the city. Other (higher class) houses deposited their sewage in open holes in the ground which allowed the wastes to separate, with the liquids permeating into the subsoil and the solids accumulating. Not exactly ideal in terms of public health!

The town council called for the construction of more of these latrines and for the activities of the night carts to be reduced.

The council also advocated increasing the air circulation in the city and simultaneously fumigating it. Authorities in Cuba had tried something similar in 1840, when they had spread resin, and fired batteries of cannons simultaneously, all over Havana! It was believed that the air housed cholera and other diseases and that it could directly affect the organism, through its “miasmas.”

The “Cuban solution” is tried in Guadalajara

In 1850, the epidemic began and the Guadalajara council voted to try the Cuban solution. On August 7th , at the height of the epidemic, fireworks, artillery and everything else were ignited – even the church bells were rung – in order to stimulate air movement and purification , “to increase the electricity in the air and reduce the epidemic.”

During the 1833 epidemic, various industrial plants, including ones making soap, starch and leather, had been closed, though no regulations were ever passed for their subsequent improvement. This time, in 1850, more drastic measures were taken. Tanneries had to construct their own watercourses, and their water was not allowed to collect and stagnate by bridges. Soap works were transferred out of the city completely. Despite these efforts, many stagnant pools of water would have lain on the city’s poorly constructed cobblestone streets: pools of water just waiting for an outbreak of cholera.

The police force was given the power to supervise everyone’s adherence to the regulations. Inspectors were appointed for each district or barrio to see that all “night activities” (carts included) terminated before 8:00 a.m., that sewage water was not used for the irrigation of plants, that gatherings were not too large, and that billiard, lottery and society halls all closed at the start of evening prayers.

A group of doctors was obliged to give its services free to anyone who needed medical help. The doctors apportioned the city among themselves and were told by the town council that they would be paid for their services as soon as council funds permitted. The main idea, of course, was to help the poor, perhaps not so much from any altruistic motives but to avoid any inconvenience to the rich!

Fortunately, any new outbreak of the disease in modern Guadalajara will be handled very differently to these 19th century epidemics. The excellent modern medical facilities in the city, and the large number of qualified doctors, mean that anyone unlucky enough to contract the disease should be able to get adequate treatment ensuring a full and speedy recovery.

This is an edited version of an article originally published on MexConnect

Click here for the complete article

Note: The diffusion of cholera in Mexico during the 1991-1996 epidemic is discussed, alongside a map showing the incidence of the disease, in chapter18 of Geo-Mexico: the geography and dynamics of modern Mexico.

Geo-Mexico also includes an analysis of the pattern of HIV-AIDS in Mexico, and of the significance of diabetes in Mexico.