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Emerging Infectious Diseases and the International Traveler
In 1992 a landmark report by the Institute of Medicine titled “Emerging Infections: Microbial Threats to Health in the United States” highlighted the importance of the often underappreciated concept of emerging infectious diseases (EIDs). This report brought EIDs back into scientific discourse; however, the awareness that diseases emerge and periodically reemerge goes back millenia, as highlight in their historical review.
The term “emerging infectious diseases” is broad. It covers newly recognized human diseases caused by pathogens that have recently jumped species as well as older pathogens that have emerged in new populations due to changes in human behavior or modifications to natural habitats. It also encompasses older pathogens that are reemerging in areas once brought under control, often due to microbiological adaptation or breakdown of public health measures. An overview of recent travel-related EIDs in the 21st century is given in Table 3.1 .
TABLE 3.1
Examples of Travel-Associated Emerging and Reemerging Infections in the 21st Century
| Year | Travel-Related Emerging Diseases in the 21st Century |
|---|---|
| 2003 | Global outbreak of severe acute respiratory syndrome (SARS) caused by a novel coronavirus. SARS was spread by travelers to 30 countries on five continents. |
| 2005-2007 | Reemergence of chikungunya fever leads to a large outbreak, affecting the islands in the Indian Ocean and large parts of South and Southeast Asia (1) . The spread is facilitated by adaptation of the chikungunya virus (CHIKV) to a new vector, Aedes albopictus . Imported cases in Italy and France prompt autochthonous infections in both countries (2) . CHIKV infections are confirmed in travelers returning to Europe, Australia, the United Kingdom, and the United States. |
| 2008 | First outbreak of Lujo hemorrhagic fever. The novel arenavirus is named after the origin of the index patient, a travel agent from Lusaka who was airlifted to Johannesburg for treatment, causing several further cases through nosocomial spread. |
| 2009-2010 | “Swine flu pandemic” caused by influenza A (H1N1). A virulent variant of H1N1 had also caused the Spanish flu in 1918-1919, killing tens of millions of people worldwide. |
| 2011-2012 | Outbreak of acute muscular sarcocystosis among international travelers returning from Tioman Island, Malaysia (3) . |
| 2012-2013 | First recorded outbreak of Middle Eastern respiratory syndrome caused by a novel coronavirus (MERS-CoV). |
| 2012-2014 | Outbreak of schistosomiasis in German and Canadian travelers returning from Corsica (France). |
| 2013-2015 | First recorded chikungunya outbreak in the Western hemisphere starts from the Caribbean Islands, spreading to the United States, Latin America, and French Polynesia. |
| 2013-2014 | Zika virus outbreaks in the Western Pacific and Southeast Asia lead to a number of imported infections in international travelers. |
| 2014 | Fatal case of influenza (H5N1) in a Canadian traveler returning from China. |
| 2014 | Chikungunya virus imported by a traveler from Cameroon into France leads to 14 autochthonous cases (4) . |
| 2014-2015 | Large Ebola Zaire outbreak in Guinea, Liberia, and Sierra Leone. A Liberian traveler visiting friends and relatives imports Ebola into the United States, causing two consecutive cases among healthcare workers. |
| 2015 | First case of influenza A (H7N9) outside China found in a Chinese traveler to Malaysia. |
| 2015 | Zika virus cases documented in South America for the first time. |
Sources
1 Burt, F.J., Rolph, M.S., Rulli, N.E., Mahalingam, S., Heise, M.T., 2012. Chikungunya: a re-emerging virus. Lancet 379 (9816), 662–671.
2 Tomasello, D., Schlagenhauf, P., 2013. Chikungunya and dengue autochthonous cases in Europe, 2007-2012. Travel Med. Infect. Dis. 11 (5), 274–284.
3 Esposito, D.H., Stich, A., Epelboin, L., Malvy, D., Han, P.V., Bottieau, E., et al., 2014. Acute muscular sarcocystosis: an international investigation among ill travelers returning from Tioman Island, Malaysia, 2011–2012. Clin. Infect. Dis. 59 (10), 1401–1410.
4 Delisle, E., Rousseau, C., Broche, B., Leparc-Goffart, I., L'Ambert, G., Cochet, A., et al. 2015. Chikungunya outbreak in Montpellier, France, September to October 2014. Euro. Surveill. 20 (17), pii:21108.
According to a recent comprehensive literature review by , more than 1400 species of infectious organisms known to be pathogenic to humans were identified. Around 175 pathogenic species were associated with diseases considered to be emerging, and of these, 75% were zoonotic. Causal factors contributing to the emergence of new pathogens include human population growth resulting in human encroachment on wildlife habitats, increased human contact with domestic and wild animals, changes in agricultural practices, and globalization of food markets.
Travelers are an important factor in the global dissemination of EIDs due to the increased frequency and speed of both local and international travel. International travelers may have been in direct or indirect contact with previously isolated, remote populations and ecosystems. The challenge is that travelers returning home may harbor exotic infections that are still in the incubation stage. During the acute stage of illness, nonspecific flu-like clinical signs and symptoms may not suggest the correct diagnosis to local healthcare providers. Thus, infections acquired during travel may be transmitted to others in the community by returned travelers before the diagnosis can be made. In addition to international travelers, imported animals, birds, foods, and insects from abroad, especially from tropical developing countries, can also pose a significant threat to the public health of receiving countries by serving as means of transportation for pathogens into new geographic areas.
Approach to International Travelers
The burden of detection of imported infectious diseases among returning travelers is most likely to fall on primary healthcare providers who initially see the ill traveler, and the public health officials to whom they report. In the 21st century, all persons presenting for diagnosis and treatment of an acute illness should be asked, “Where have you traveled?” as part of the routine medical history, since so many individuals in our global society have traveled to or may have originated in tropical developing areas.
The travel history should be as specific as possible in terms of the cities and areas of each country visited. Activities and exposures, such as swimming in freshwater lakes or rivers, walking barefoot on beaches or muddy trails, receiving many insect bites, eating raw or exotic foods, drinking unsafe water or beverages with ice cubes in countries with low sanitation, and close/intimate contact with new partners should also be subject to inquiry. Since most of the EIDs are specifically discussed in other chapters of this book, the purpose of this chapter is to provide a conceptual framework for their consideration and recognition.
Emerging and Reemerging Zoonoses
Many factors are responsible for emergence of infectious pathogens that originate in wild animals. These include travel into previously uninhabited areas, changes in land use and demographic patterns with disruption of stable ecosystems, greater contact with previously isolated animal populations, changing agricultural practices that allow transfer of pathogens between wild and domestic animals, and food customs that involve hunting, butchering, and ingesting wild game including nonhuman primates (bush meat).
Two transmission patterns have been described for transmission of pathogens from wild animals to humans. One pattern consists of rare events when direct animal-to-human transmission of an animal pathogen occurs, but then direct human-to-human transmission maintains the infection in the human population for a limited time or permanently. Examples of diseases with this transmission pattern are human immunodeficiency virus (HIV), influenza A, Ebola virus, and severe acute respiratory syndrome (SARS).
The second pattern of transmission is where human infections with animal pathogens result from repeated episodes of direct animal-to-human transmission or repeated vector-mediated animal-to-human transmission, and the infections are not usually propagated by human-to-human transmission. Examples of diseases with this transmission pattern are rabies and other lyssa viruses, Nipah virus, West Nile virus, Hantavirus, Lyme borreliosis, tularemia, leptospirosis, and ehrlichiosis.
Vector-Borne Diseases
Many emerging infections are vector-borne diseases. When competent vectors such as mosquitoes, ticks, and fleas preexist in a geographic environment, movement of infected human or animal hosts into that area can lead to rapid expansion of transmission.
West Nile Virus
A prime example of an emerging vector-borne disease was the spread of West Nile virus in North America, transmitted from birds to humans and horses by Culex mosquito vectors. Following the initial 1999 detection of the agent in Queens, New York, the new pathogen (thought to have been introduced by an infected human traveler or migrant bird) spread rapidly across the continental United States from coast-to-coast within 5 years—affecting human activities, veterinary practices, and blood-banking guidelines in the wake of its spread ( Fig. 3.1 ).
Chikungunya Virus
One of the most important emerging vector-borne viruses in the recent years has been chikungunya virus (CHIKV). The alphavirus is transmitted to humans by the bite of infected daytime-biting Aedes mosquitos, mainly Aedes aegypti and, since more recently, Ae. albopictus . The disease has been reported among travelers returning from endemic areas as a cause of acute illness characterized by sudden onset of fever, chills, severe joint pain with or without swelling, lower back pain, and a maculopapular rash, similar to the symptoms of dengue fever. In some CHIKV patients, residual joint pain and impairment persists for months and even years after the acute illness, leading to considerable morbidity.
As reviewed by a large outbreak of chikungunya emerged in 2004 in coastal Kenya; in 2005 it spread to the Indian Ocean islands and subsequently to India and many countries in Southeast Asia, causing several million cases. In 2007 cases of chikungunya were first reported from Europe: infected air travelers imported the disease into France and Italy, which led to local autochthonous transmission. It turned out that the virus had adapted to a new vector, Aedes albopictus , facilitating viral spread to an even wider geographic region. Further factors contributing to the magnitude of the outbreak were an increase in air travel to and from affected areas, previous lack of exposure to the virus of the newly involved populations, and spread of Ae. albopictus from its native Asia to islands of the Indian Ocean basin and to Southern Europe. In 2013 a further large outbreak originated on the Caribbean island of St. Martin. It rapidly spread through the Caribbean, to the United States, to Latin America, and to French Polynesia, causing several million cases. This was the first time a chikungunya outbreak had officially been recorded in the Western hemisphere.
Zika Virus
Another emerging virus transmitted by Aedes spp. is Zika virus (ZIKV). Zika is a flavivirus that causes a clinical picture indistinguishable from chikungunya or dengue. Even though it usually causes a mild illness, the development of subsequent Guillain Barré syndrome has been reported in a number of cases. Zika was known to be endemic in Africa and Southeast Asia. In 2007, an outbreak of Zika virus infection occured on Yap island (Micronesia). These were the first recognized cases outside Africa and Asia. The emergence of ZIKV on the isolated island community showed the potential of the virus to spread through commerce and travel across long distances. An even larger outbreak occurred in 2013-2014 in French Polynesia, causing an estimated 32,000 cases and spreading within the region. In the context of this outbreak, cases related to international travel were reported by in two Japanese, one Norwegian, and two Italian travelers. A number of further travel-related cases of Zika virus were reported. Interestingly, one scientist returning to the United States from Cameroon with ZIKV later infected his wife, probably by sexual transmission, as discussed by . In 2015, Zika was first detected in Brazil, leading to autochthonous transmission of the virus.
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