06 October 2013

Co-morbidities: Poverty and the re-emergence of tropical infectious diseases in the US

Dengue fever virus structure, showing its herringbone
 pattern. Image by Visual Science

The image to the left shows the structure of one of the four related RNA viruses of the family Falviviridae that cause dengue fever and dengue hemorrhagic fever. The tight herringbone structure suggests both its complexity and its evolutionarily-derived stability. 

Indeed, according to the World Health Organization (WHO), the worldwide incidence of dengue has increased by 30 times in the last 50 years, with somewhere between 50 and 100 million new infections every year.1 And, not only have raw numbers increased, but dengue’s spread has widened, from Africa and South Asia, into the Caribbean, Europe, and North America.1

Dengue fever is a disease transmitted most often by the Aedes aegyptae mosquito – although other Aedes species are implicated.2  Aedes bites mainly during the day. Its malaria-carrying cousin, Anopheles, bites mainly at night. In both cases, only the female bites – their bodies use human blood to mature their eggs. It is in part due to the widening geographic range of Aedes aegyptae and albopictus mosquitos into North America and Europe, that the dengue virus finds its new victims. The spread of these vector insects appears to be mainly due to international trade in such commodities as used tires and bamboo.1

Symptoms of dengue range from mild to acute and include fever, headache, and severe muscle/joint pain. When I contracted dengue on a pediatric research trip to Jamaica, I remembered the term "breakbone fever," as it is sometimes called. The best treatment is analgesics to relieve the pain and rest. Dengue hemorrhagic fever is considerably more severe, which can result in death.

Typical household storage containers in rural India. Water
left standing in vessels like this attracts mosquitos who lay
eggs. Photo, Anne Wallis, 2012.
We think of dengue – and its kissing cousins, malaria, yellow fever, Japanese encephalitis, and chikangunya – as tropical diseases of the developing world, and, inherently of poverty. While dengue is certainly thriving right now in India (see my prior blog post on India’s dengue outbreak), outbreaks are common in the the rest of Asia, the Caribbean, South America, and Africa. In recent years, cases have been reported in the US  and there was a documented outbreak of chikangunya in Italy in 2007 and in 2010, dengue outbreaks in France and Croatia.1

What? The US? Europe? Didn’t we eliminate these tropical diseases? Surely, we don’t have the conditions that breed dengue? Or, do we? 

Malaria, which is caused by a bacteria carried by the Anopheles mosquito, was endemic in the US until about 1950. In the 1930s, President Franklin Roosevelt invested the new Tennessee Valley Authority (TVA) with the mandate of eradicating malaria in the Tennessee River Valley. Prior to this effort, malaria affected as much as 30% of the local population. In the 1940s, the US Public Health Service controlled malaria near military posts in the southern US. The National Malaria Program, constructed by the Centers for Disease Control and Prevention (CDC) in cooperation with state and local health programs, began in 1947. By the late 1940s, these coordinated efforts resulted in the elimination of the disease in the US. Regular malaria control activities continued into the 1970s.

In 2013, there have been 19 locally-acquired (i.e., the virus was not acquired while traveling to a part of the world with endemic dengue) cases in Florida (http://www.doh.state.fl.us/Environment/medicine/arboviral/surveillance.htm) and in 2012, dengue-carrying mosquitos were found along the Texas-Mexico border. (Click on http://www.healthmap.org/dengue/index.php for a very cool interactive map showing dengue cases in the US between 2010 and the present.)

In August 2012, Dr. Peter Hotez, founding dean of Baylor College of Medicine's National School of Tropical Medicine, penned an important opinion piece for the New York Times (“Tropical Diseases: The New Plague of Poverty”). He noted that rising rates of poverty in the US, along with increased inequality, have brought with them a natural sequelae: rising rates of the so-called “neglected tropical diseases.” Hotez noted, correctly, that in Louisiana, Mississippi, and Alabama, poverty rates top 20% (using US Federal poverty line definitions) and in some counties in Texas, poverty rates reach close to 30%. Moreover, the ratio of rich-to-poor, expressed as the Gini coefficient, is as high in some parts of the US as in parts of the developing world.

Rural Cary, Mississippi. Running water and electricity
are available, but drainage and sanitation remain
sub-standard. Source: Huffington Post
Along with dengue fever cases in south Florida this year and infected mosquitoes discovered on the Texas-Mexico border, public health officials have also reported cases of cysticercosis, an infection caused by a larval cysts of a tapeworm found in pigs – this is not the tapeworm acquired by eating undercooked pork. Rather, people acquire cysticercosis when they ingest eggs excreted by a person with an intestinal tapeworm. Dr. Hotez has noted in recent talks and interviews that he has been seeing an increasing number of tapeworm patients in the Houston area.

These infections are typically found in parts the world where there is both poor sanitation and free-ranging pigs. The CDC has reported that between 1990 and 2002, there were at least 221 deaths from cysticercosis in the US.3 The fact that most of these deaths were among foreign-born immigrants does not remove the importance of this disease as an emerging poverty-related disease in the US; it has been declared by the CDC as one of the five “neglected parasitic infections” in the US (http://www.cdc.gov/parasites/npi.html). 

Other emerging “neglected” infections reported among poor populations in this US in recent years are toxocariasis, another parasitic infection; and Chagas disease. Chagas is transmitted by an insect and is a leading causes of heart failure in Latin America; it is also transmitted from pregnant mother to fetus. Nearly 3 million African-Americans are infected with roundworms (toxocariasis, another CDC-designated neglected parasitic infection) and 300,000 or more people, mostly Hispanic Americans, with Chagas disease.

Another tropical disease, murine typhus, is a bacterial infection transmitted by fleas and linked to rodent infestations. In the US, there are about 200 cases of murine typhus annually (http://www.cdph.ca.gov). It is endemic in parts of southern California, Hawaii, and Texas. A 2008 CDC investigation of an outbreak of murine typhus in Texas found that in 60% of cases, there was direct evidence of rodents living outside patients’ homes; others had outside water sources or stored unsecured garbage out of doors.4

These tropical diseases thrive in areas that are poor and warm and where housing is substandard, drainage and plumbing are poor, and lacking in hygienic sanitation

A nurse in south India counting
larvae in a household water container
 as part of Dr. Erin Reynolds'
study of chikangunya by  Photo: Anne Wallis, 2012
In a time in the US where some legislators want to “drown government in the bathtub,” and are shunning expanded healthcare to the extent that they have shut down the Federal government, we should be worried. The job of state health departments, with the assistance of the (FEDERAL!) CDC, is to investigate and report on outbreaks of all infectious diseases. We need the Public Health Service and state health departments to revive and sustain vector control programs. Further, the CDC and the National Institutes of Health, are mandated with the task of researching these often neglected diseases.

As I have noted elsewhere and will continue to make note of, poverty is an epidemiologic issue. Disease is its natural mate. We must change the conditions of poverty in order to reduce rates of disease and to halt the emergence of previously rare or controlled diseases.

I think it’s time to reopen the government and get back to work.


1. World Health Organization. Global strategy for dengue prevention and control, 2010-2020. Geneva, Switzerland: WHO; 2010.
2. Halstead SB. Dengue. The Lancet. 2007;370(9599):1644-1652. doi: http://dx.doi.org/10.1016/S0140-6736(07)61687-0.
3. Sorvillo F, DeGiorgio C, Waterman S. Deaths from cysticercosis, united states. Emerg Infect Dis. 2007;Feb.

4. Adjemian J, Parks S, McElroy K, Campbell J, Eremeeva M, Nicholson W. Murine typhus in austin, texas, USA. Emerg Infect Dis. 2010;Mar.

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