A Tale of Two Vectors

David McCormick

David McCormick

On opposite sides of the world in 2009, two vector-borne diseases that we thought were under control – dengue fever in the Florida Keys and human African trypanosomiasis (AHT, more commonly known as African sleeping sickness) in northern Malawi – returned.

AHT is one of the most dreaded tropical diseases, with good reason – it is invariably fatal in the absence of proper treatment. Over 60 million people in sub-Saharan Africa are at risk and 50,000 die each year. The main vector of AHT is the tsetse fly, and control efforts have focused on eliminating this insect from endemic regions. During the 1980’s and 1990’s, a successful vector control program virtually eliminated the tsetse fly from Malawi, but as the disease burden decreased, funding stopped and now it appears that they fly has returned to northern Malawi.

Distribution of the Tsetse Fly in Africa. Malawi is outlined in dark blue. (Courtesy: IAEA)

The return of the tsetse fly isn’t just bad for humans – the fly can also carry a trypanosome that fatally infects cattle, which has a severe impact on the local economy. The economic impacts can be just as devastating – a Malawian villager reported, “Our colleague last year lost almost all his cattle totaling 30 head and remained with only two. We are worried because everybody is losing livestock. The flies are bringing poverty here.

Dengue fever – also known as “breakbone fever” for the severe pain caused by the first infection – is the most common vector-borne viral disease worldwide. The first infection is rarely fatal, but there are four different dengue serotypes. Infection with one serotype confers immunity to that serotype, but if someone who has already been infected with one dengue serotype is infected by a different serotype, dengue hemorrhagic fever, a life-threatening illness, can result.

In May 2010, the Centers for Disease Control and Prevention (CDC) announced the first locally acquired case of dengue Fever in the United States since 1946 – and that there had likely been an ongoing outbreak of dengue in the Florida Keys since 2009. A serosurvey found that up to 5% of all Florida Keys residents have been exposed to dengue. Yes, you read the first part of that sentence correctly – we eliminated dengue from the United States when we eliminated malaria (both using vector control strategies) in the mid-40’s, and now it’s back.

The Florida Keys are a major tourist destination, and the main fear is that dengue will spread from the Keys to the rest of the United States when travelers return home. Many of the mosquitoes in the United States are capable of transmitting the virus, so the possibility of spread is a real concern. The map below shows the distribution of Aedes albopictus, one of the main vectors of dengue, in the United States. It is present in all of the counties in red, and absent from those in blue. Gray counties represent those that the Division of Vector-Borne Infectious Diseases (DVBID) didn’t have the necessary funding or manpower to survey.

Map Showing the Distribution of Aedes albopictus in the United States (courtesy of: CDC DVBID)

These two outbreaks follow the same pattern – successful vector control programs resulted in the elimination of a deadly disease, and the success of these programs led many to conclude that they were no longer necessary. The problems posed by these two outbreaks are similar, but the official responses couldn’t be more different. In Malawi, the return of the tsetse fly was front page news for the Sunday edition of the national paper, while news of the dengue outbreak in the United States was somewhat more difficult to find (the New York Times ran a nice article on the subject). The Malawian government and local officials have discussed the outbreak openly, asked for help, and are proposing to fund a control initiative. In contrast, the United States has imposed severe budget cuts on the CDC, forcing the closure of the DVBID (the same division that warned us of the problem), and health officials in the Keys are denying that dengue is a problem. Which country’s policies seem more sensible?

More on the dengue outbreak can be found at:

White Coat Underground

TIME

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