Travelers’ Health Stories

CDC Yellow Book Celebrates 50 Years of Protecting International Travelers

From the late 1960s to today, the CDC Yellow Book has evolved to become the gold standard resource for US travel medicine providers.
17 different CDC Yellow Book editions fanned out in two different stacks with the older editions in the top stack and the newer editions in the bottom stack.

CDC Yellow Book: 50 years old and still going strong

Since its first publication 50 years ago, the CDC Health Information for International Travel(a.k.a. the CDC Yellow Book) has evolved from a small pamphlet to a major medical reference, providing the most current and comprehensive travel health guidelines.

Increasing International Travel Makes Travel Medicine a Critical Public Health Pursuit

In 2016, international tourist arrivals reached over 1.2 billion for the seventh year in a row, according to the United Nations World Tourism OrganizationExternal (UNWTO) and that trend does not seem to be waning. As people continue traveling, they are also seeking destinations less commonly visited than in the past. The most recent UNWTO annual report found that African, Asian, and Pacific regions were experiencing the strongest growth in tourism. With US travelers taking more than 73 million international trips each year, the association between travel and destination-specific risks continues to be a critical area of study for CDC’s Travelers’ Health program.

Cover of the “CDC Yellow Book 2018: Health Information for International Travel.”

2018 Edition Updates

  • New cholera vaccine recommendations
  • Revised guidance on the use of antibiotics in the treatment of travelers’ diarrhea
  • Special considerations for wilderness and expedition trips and work-related travel
  • New sections on healthy travel to Cuba and Burma

“Risks that travelers face are dynamic—some travel destinations have become safer, while in other areas new diseases have emerged or other diseases have reemerged,” says CDC Yellow Book’s Chief Medical Editor, Phyllis Kozarsky, MD.

The risk of travelers becoming ill or injured during an international trip depends on many factors, such as region of the world visited, health status and age of the traveler, length of travel, and the types of activities abroad.

Kozarsky observes, “the importance of protecting the health of individual travelers, as well as safeguarding the health of the communities these travelers are returning to, can’t be overstated.”

CDC Yellow Book Helps Practitioners and Travelers Stay Abreast of International Travel Trends and Ways to Minimize Health and Safety Risks

The CDC Yellow Book is published every two years in collaboration with Oxford University PressExternalas a hard-copy medical reference so health professionals can remain up-to-date with international travel trends and the shifting target of infectious disease risks. It provides authoritative travel health recommendations to help international travelers prepare for trips, stay healthy while abroad, and monitor their health upon return.

Program Impact

The award-winning CDC Yellow Book serves as the foundation for all content on the Travelers’ Health site, which ranks among the 10 most visited of all CDC websites. The most popularly viewed travel health pages are consistently the CDC Yellow Book’s list of yellow fever and malaria recommendations by destination country.

Gold Olympic rings

January 2018 – Breathtaking snowy peaks, brutally cold winds, and frost-covered temples and pagodas await travelers and athletes who make the trek to the remote Taebaek Mountain resort town of PyeongChang for the 2018 Olympic (February 9–25) and Paralympic Games (March 8–18). South Koreans’ goodwill and warm hospitality will be prized come February, now that the country has geared up to host:

CDC’s Division of Global Migration and Quarantine’s (DGMQ) Travelers’ Health Branch urges US athletes,  delegation staff, and spectators to get recommended vaccines and stay safe, warm, and healthy during their “road to gold” in South Korea.

Mass gatherings pose distinct health, safety, and security concerns

Colorful Buddhist temple surrounded by snow-covered trees and forest

Olympic spectators are encouraged to refresh their minds and bodies with a Gangwondo “Temple Stay” or visit. The Woljeongsa Temple is located in a thick forest in the eastern valley of Odaesan Mountain.

Globalization of travel and trade increase the probability and speed of many threats, which is why DGMQ continues studying ways to lessen the risks that intensify during mass gatherings. A mass gathering means a large number of people come together in one place for a specific purpose, such as a global sporting event like the Olympics.

“As was the case with a measles outbreak during the 2006 World Cup in Germany, the implications of mass gatherings are not only that people might get sick while they are at an event,  but also that a traveler attending the mass gathering could spread an infection further after returning home,” says Travelers’ Health Branch’s Dr. Joanna Gaines. She further explains that injury and safety risks also increase in a crowd and that mass gatherings put a strain on local healthcare resources, security, and infrastructure.

Gaines recommends that travelers check CDC’s travel notice for the 2018 Winter Olympics, get their flu shot, and pack for bitterly cold weather. “It is still flu season here in North America and in Korea. It’s also critical to wear warm layers of clothing to prevent hypothermia or frostbite at both outdoor and indoor events.” 

Protecting people attending mass gatherings like the Olympics

Detailed, transparent-style illustration with the words “Pyeongchang” and silhouettes of its’ mountains and city skyline

PyeongChang Skyline

DGMQ conducts risk assessments to identify health threats that US travelers going abroad may face. Based on data from around the world and expert analysis, CDC provides updated travel notices and guidance for event and travel planners, travelers, and medical providers. Dr. Gary Brunette, Travelers’ Health Branch Chief, notes that the most cost-effective way to protect Americans from health threats beyond our borders is prevention and early detection. “We don’t want to scare people from going, because it’s fun to participate in sporting and other enjoyable large events. However, someone needs to be concerned about the health-related risks, and that job falls on us. We look at mass gatherings from a risk perspective, and it’s our job to constantly work toward reducing those risks.”

Given the importance of minimizing risks and detecting infectious diseases early, Brunette and Gaines have been spearheading an interdisciplinary Mass Gatherings Working Group to harness expertise from across CDC. The working group provides comprehensive risk assessment, surveillance, and preparedness support for mass gathering planners. As part of that work, Dr. Brunette recently traveled to Seoul to support a planning symposium for the Winter Olympics.

Program impact

Offering host countries (like South Korea) the opportunity to call on public health expertise to help mitigate travel health risks improves their chances for leaving a positive Olympic legacy behind.

Remember: Before packing for a chilly yet thrilling Olympic experience in PyeongChang, travelers should review the State DepartmentExternal and CDC Travel notices and prepare well to ensure their trip ends up a winner.

GeoSentinel Uncovers Rare Disease

View of Tioman Island, Malaysia

View of Tioman Island, Malaysia, when arriving by ferry. (Photo courtesy of Douglas Esposito)

Tioman Island is a short ferry ride away from the east coast of peninsular Malaysia in the South China Sea. Shaped like a giant sleeping dragon, the island has lush tropical forests, secluded beaches, and emerald waters teeming with vibrant aquatic species and coral reefs that attract diving enthusiasts and other summer travelers from around the world. In 2011, however, dozens of summer tourists returned home with something far less desirable than sandy shoes, spoiled sunset snapshots, and senseless souvenirs.

In October, two travel medicine specialists in Munich and Würzburg, Germany, treated a handful of patients with high fever, severe muscle pain, and abnormal levels of eosinophils, a specific kind of disease-fighting white blood cell. The abnormal white blood cells, combined with inflamed muscles and fever, indicated that the doctors were dealing with something unusual. However, another clue to the mysterious illness was that all of these patients recently traveled to Tioman Island.

A muscle biopsy identified their disease as muscular sarcocystosis, a parasitic infection rarely reported in humans. Upon diagnosis, the patients were immediately reported to the GeoSentinel Global Surveillance Network, which generated an inquiry across its membership to see if anyone else was finding similar symptoms in their patients.

Standing guard for disease threats in travelers

GeoSentinel is a worldwide communication and data collection network of travel and tropical medicine clinics, created in 1997 by a partnership between the Centers for Disease Control and Prevention (CDC)’s Division of Global Migration and Quarantine (DGMQ) and the International Society of Travel Medicine. Since its creation, the GeoSentinel network has steadily expanded from the initial nine U.S. sites to more than 60 travel and tropical medicine clinics in 29 countries on six continents. These sentinel clinics routinely and rapidly share patient observations and build the crucial science around geographic- and migration-based illness trends. This information helps CDC stay on top of changing disease risks, update pre-travel recommendations to keep people healthy during international trips, and provide medical evaluation advice for travelers who become ill.

Sarcocystis parasite moves beyond Malaysia

Sarcocystosis is a disease caused by a parasite called Sarcocystis. Two forms of the disease can occur: One form causes an intestinal infection and the second causes muscle pain, fevers, and other symptoms. The second form, muscular sarcocystosis, is spread through food, water, or soil that has been contaminated with feces from an infected animal. Muscular sarcocystosis occurs in tropical and subtropical regions, including countries in Southeast Asia. The disease is common among wild and domestic animals but is reported only rarely in humans.

The unusual illness and specific travel history of the patients diagnosed with muscular sarcocystosis struck a chord with other physicians in the network who also treated patients recently returned from holiday on Tioman Island with similar signs and symptoms.

Within days of the original reports, Douglas Esposito, MD, a DGMQ medical officer in the Travelers’ Health Branch, helped launch an international outbreak investigation, in collaboration with GeoSentinel. During the investigation, GeoSentinel and other physicians around the globe were notified to be on the lookout for patients with symptoms compatible with muscular sarcocystosis and a history of recent travel to Tioman Island. Data were collected and submitted to Esposito at CDC for analysis. Eventually, more than 30 patients suspected to be infected with the Sarcocystis parasite with travel during 2011 were identified from Germany, France, the Netherlands, Switzerland, Italy, and Canada.

Map of Tioman Island, Malaysia.

Map of Tioman Island, Malaysia, in the South China Sea. (Map by Emily W. Lankau, DVM)

Communication with travel medicine specialists across the globe, in the patients’ home countries, was fundamental to initiating the Tioman Island investigation. Without the GeoSentinel network, it is possible the outbreak would never have been identified or investigated. During the investigation, the network also yielded unexpected benefits. “Because GeoSentinel got the word out, an affected but undiagnosed person struggling with muscle pain and fevers who was living in Singapore noticed the outbreak report in the journal EuroSurveillance during a Google search, and contacted me. Within one working day, the network arranged for a travel medicine specialist from Tan Tock Seng Hospital to evaluate the patient and make the diagnosis,” states Esposito. This example highlights the importance of global collaboration for sharing and broadcasting up-to-the-minute travel medicine information to help someone who might otherwise have gone undiagnosed and improperly treated.

After many months of careful monitoring with no new cases, 66 new patients were identified and reported to GeoSentinel in the summer of 2012. By the end of 2012, close to 100 suspected cases were identified, with nearly three-fourths meeting the case definition for muscular sarcocystosis. CDC and European public health authorities issued travel notices. Additionally, Ministry of Health Malaysia posted notices around the island reminding people to practice good handwashing and of the importance of careful food and water hygiene and avoiding contact with certain animals. Throughout 2013, the number of cases identified dropped significantly, causing investigators to breathe a temporary sigh of relief. Perhaps the outbreak had slipped away, quietly and as mysteriously as it arrived.

Water storage barrels.

Water is captured during the rainy season and stored in these vats. (Photo courtesy of Douglas Esposito)

However, in 2014, while many of the world’s top disease detectives were busily managing the challenges of more notorious zoonotic disease outbreaks, like Middle East Respiratory Syndrome (MERS) in the Arabian Peninsula and Ebola in West Africa, a couple dozen more possible cases of sarcosystosis were identified as, yet again, coming from Tioman Island.

While the first two waves of infections were tied to travel to Tioman Island primarily during the summer months, the 2014 cluster of infections initially appeared to be occurring along with travel in early spring.

“There seems to be a seasonal trend related to the rainy and dry seasons. Depending on how rainy the rainy season gets may affect the timing of when we start seeing tourist infections,” says Esposito. Somehow, he explained, the tourists are getting infected, possibly through contaminated drinking water, by drinking while showering or brushing their teeth, or even swallowing water near the beaches. While Esposito is concerned about the unsanitary water supply on the island, the locals are not. “It isn’t the locals getting sick; it is only the tourists so far. This makes it hard to make a case for changing the way they collect and store their drinking water because it would be a significant economic burden for those living on the island.”

Water Monitors

Predatory water monitors roam freely around the island's fresh water pools near a popular beach in one of the tourist villages. (Photo courtesy of Douglas Esposito)

Esposito and his fellow collaborators continue watching the situation and investigating when cases are reported. “There is definitely something unusual happening on Tioman Island with this insufficiently understood disease,” says Esposito. Water systems on the island were known to be deficient. Esposito continues, “Several patients who came back from their vacation on Tioman Island became sick for months and even years after contracting sarcosystosis. Because there is no treatment and the disease can be severe, we began investigating this outbreak for clues about how it is spread so we could interrupt transmission.”

While Esposito and the investigation team could not pinpoint the exact source of infection on the island, based on what was known about how the Sarcocystis parasite infects animals, they put cats, macaques, snakes, and water monitors, all plentiful on the island, high on the list of suspect hosts potentially causing the disease to spread to humans. The fact that transmission remains persistent and seems to come back on a seasonal basis yielded some important additional leads. Another important clue was the recovery of Sarcocystis nesbitti from two of the infected travelers to the island. Although the natural host of this specific organism is not yet known, knowledge of its genetics and the complicated lifecycle of Sarcocystis in general further implicates reptiles like snakes and, possibly, water monitors.

During a technical assistance visit to Tioman Island in 2014, Esposito and the investigation team honed in on the water monitor: a prolific, ravenous, and cannibalistic omnivore that enjoys free rein, leaving its droppings all about the villages, near the streams, and around the beaches. It is possible that this hypothesis could end up being a breakthrough discovery; however, at this point, Sarcocystis has been identified only in the muscle and not from the gut of some captured water monitors.

Increasing GeoSentinel’s capability and impact

Esposito and one of his Malaysian colleagues

Esposito and one of his Malaysian colleagues discover bradyzoites released from sarcocysts in processed muscle from water monitors captured on Tioman Island. (Photo courtesy of Douglas Esposito)

With U.S. residents making more than 60 million international trips in 2014, for everything from tourism, to business, to mission work, to visiting friends and relatives, Travelers’ Health staff must remain vigilant for health threats resulting from increased globalization. Gary Brunette, MD, DGMQ’s branch chief for Travelers’ Health, echoes these sentiments, “International travel is a part of our nation’s cultural, leisure, and business pursuits. We understand that, which is why we invest so much in the tools and networks we trust to aid us in preventing, detecting, and responding to disease threats around the globe. The GeoSentinel network is an important part of our arsenal for identifying and monitoring disease threats so we can continue preventing travel-imported cases of diseases like Ebola, MERS, dengue, and chikungunya in the United States. We know that uncommon tropical diseases, like sarcosystosis and these others, could be just a plane ride away.”

For Esposito, the Tioman Island outbreak investigation raises new possibilities. “Expanding the capabilities of GeoSentinel to include doing more collecting, testing, and banking of patient samples can assist us with better pinpointing the source of diseases, detecting where travelers acquire them, and sharing the most effective treatment options or therapies,” he says.

For more information about international travel health risks and recommendations, see: