Sunday, December 13, 2015

The International Exchange Rate Of Infectious Diseases

Photo Credit- CDC


Although carried and spread primarily by short ranging mosquitoes, vector borne viruses - like dengue, chikungunya, and Zika - often make dramatic geographic jumps because they may be carried asymptomatically by travelers who can unsuspectingly deliver the virus to competent mosquito vectors anywhere in the world.

Every year the United States reports hundreds of `viremic visitors', who are diagnosed only after they arrive stateside.  How many go undetected is unknown, but the number undoubtedly runs into the thousands. 

Occasionally, these visitors manage to seed dengue, or malaria, or chikungunya into the local mosquito population, as happened with dengue in Key West in 2009.  This from the CDC's MMWR on Locally Acquired Dengue in 2010. 

Cases of dengue in returning U.S. travelers have increased steadily during the past 20 years (8). Dengue is now the leading cause of acute febrile illness in U.S. travelers returning from the Caribbean, South America, and Asia (9).

Many of these travelers are still viremic upon return to the United States and potentially capable of introducing dengue virus into a community with competent mosquito vectors.

Similarly, in 2007 Chikungunya was delivered to a small northern Italian village courtesy of a traveler who recently visited India (see It's A Smaller World After All) where it proceeded to spread to nearly 300 residents before control measures and winter temperatures ended the outbreak. 

In late 2013, one or more viremic travelers appear to have brought the Chikununya virus  to the French side of St. Martin in the Caribbean.  There - unlike in northern Italy - competent mosquito vectors thrive year round, and the virus began to spread uncontrolled across the Caribbean and into South America. 

CHKV, like Dengue, is now firmly entrenched in South and Central America, along with the Caribbean, and Zika seems poised to repeat the process.

While the exact circumstances of Zika's arrival is not known, the timing suggests it may have been carried in by travelers attending either the World Cup in Sao Paulo in 2014 or a canoeing event held in Rio the same year.  The Brazilian government estimates at least 500,000 local infections have now occurred.

Zika of arrival is linked to sports tournaments

Researchers investigating the virus may have entered the country in the World Cup or canoeing event

In The ECDC Risk Assessment On Brazil’s FIFA World Cup we looked at a 16-page PDF risk assessment on the World Cup, which not only included illnesses or infections that may be acquired while in Brazil, but of even greater concern, a much longer list of infections that might be imported into Brazil by international travelers.

Mass gathering events, like the Hajj, the Super Bowl, Mardi Gras, and the Chinese Lunar New Year are increasingly viewed as potential hot spots for virus transmission (see Lancet: Mass Gatherings And Health), and Zika's belated surge now suggests their impacts aren't always immediately apparent.

In the year ahead Brazil will host two significant interantional mass gathering events.  The first, is the year Carnival in Rio, held next year in early February. Fortunately, Rio isn't yet a hotbed of Zika transmission, although some cases have been reported.

A much bigger event is scheduled for next August when the 2016 Summer Olympic Games  are scheduled to run for 17 days in and around Rio de Janeiro. More than 10,500 athletes from more than 200 countries, are expected to compete - and more 500,000 foreign visitors are expected to fly in for the event. 

How much of a factor Zika will be in and around Rio by then is anyone's guess.

But large mass gatherings such as this are always a concern for public health offcials and of great interest to epidemiologists, and for reasons that go far beyond just mosqutio vectored viruses.  Everything from tuberculosis, to measles, to polio, to MERS are on the table.

In 2012, we looked at how the ECDC, along with local public health officials, geared up for three very high profile and well attended public events; The London Olympics and Para-Olympics Games, and the UEFA EURO 2012 football championship.

You can see their preparations in How The ECDC Will Spend Your Summer Vacation.

One of the ironies is that while North American travelers to Rio may worry they might pick up, and return home with Zika, they could just as easily deliver West Nile Virus to South America - potentially sparking yet another vector borne epidemic in Brazil. 


One of the realities of life in this highly mobile, interconnected, 21st century is that oceans and vast distances no longer afford protection against exotic diseases. 

An infected, but still asymptomatic traveler can board a plane in Beijing, or Cairo, or New York and be in any international city in less than 24 hours. 

Nearly two years ago, in The Global Reach Of Infectious Disease, we looked at rationale behind several national and international initiatives designed to deal with the growing threat of the international spread of infectious diseases. 

  • In WHO: IHR & Global Health Security, we looked at the large number of member states which have yet to meet the core surveillance and response requirements of the International Health Regulations that went into force in 2007.
  • In Be Prepared For Surprises (Redux), we looked at calls for further extensions on implementing the IHR core requirements.
  • An Assessment by the Director of National Security last year (see DNI: An Influenza Pandemic As A National Security Threat) found the global spread of infectious diseases – along with cyber attacks, terrorism, extreme weather events, WMDs, food and water insecurity, and global economic concerns.- constitutes a genuine threat to national security.
  • While in CIDRAP On The Global Health Security Agenda, we looked at a 26 nation initiative to improve global health surveillance & emergency response in this age of rising infectious diseases. 

Simply put, the international exchange rate of infectious diseases is increasing, and that's a trend that is unlikely to change anytime soon.

You'll find more on the CDC’s Global Health Website at:

Why Global Health Security Matters

Disease Threats Can Spread Faster and More Unpredictably Than Ever Before
A disease threat anywhere can mean a threat everywhere. It is defined by
  • the emergence and spread of new microbes;
  • globalization of travel and trade;
  • rise of drug resistance; and
  • potential use of laboratories to make and release—intentionally or not—dangerous microbes.
(Continue . . .)