Most travel infections result from visiting countries in tropical areas. The most common identified GI pathogens in returned travellers are Giardia, Salmonella, Shigella and Campylobacter, the most common febrile conditions include malaria, dengue, typhoid, chikungunya and so on, and the most common respiratory illnesses include flu and legionellosis. Very few people who catch a disease abroad actually die from it, and a large proportion of those deaths are from falciparum malaria (the worst form of malaria).
Diagnosis
Diagnosis can be very difficult, as most of these illnesses have non-specific and overlapping symptoms (like fever, fatigue, nausea, etc.). Therefore, it is important to obtain a thorough history that includes travel history and measures taken to avoid risk (e.g. anti-mosquito measures, pre-travel vaccinations).
Sometimes, you get lucky and find a symptom that is specific to an infection. For example, an eschar, which has a dark area in the middle surrounded by a ring of inflammation, is associated with Rickettsia infections. Chancres are associated with primary syphilis. A "migrating track" under the skin" is associated with cutaneous larva migrans (animal hookworms under the skin). A similar-shaped lesion in the eye might indicate loa loa microfilarial worms that cause loiasis. Myiasis- infection by human botflies- can be quite distinctive too.
Incubation periods can also be used to help narrow down a diagnosis. For example, the incubation period for yellow fever is only 3-6 days. If a patient visited a yellow fever-endemic country more than 6 days ago, it is unlikely that their current infection is due to yellow fever. Other incubation periods that might be helpful to know are malaria (9 days - several months), typhoid (3-60 days), dengue (3-14 days) and typhus (Rickettsia infection not to be confused with typhoid. Incubation period for this one is 6-21 days).
Malaria
Since a large proportion of deaths from travel illnesses are due to malaria, it should always be considered in a returned traveller with fever. Malaria is transmitted by Anopheles mosquitoes, and comes in five flavours: Plasmodium vivax, P. ovale, P. malariae, P. falciparum and P. knowlesi. P. falciparum is the most severe and life-threatening. P. knowlesi is not very common (originated in monkeys), but it can also be severe.
The life cycle of malaria is quite complex. The infective form, called a sporozoite, lives in the salivary gland of mosquitoes. Sporozoites are transmitted to us when mosquitoes bite. Once inside us, sporozoites move through the blood and lymphatics to the liver, where they multiply into haploid forms called merozoites, which gather in lesions called schizonts. At this stage, P. vivax and P. ovale can also produce hypnozoites, which are latent forms. Hence, P. vivax and P. ovale malaria can recur.
Eventually, liver schizonts rupture, releasing merozoites into the bloodstream. Merozoites infect red blood cells and develop into the trophozoite form. Trophozoites sometimes appear as rings under the microscope due to the presence of a large vacuole in the middle. As the trophozoites develop, a mature erythrocytic schizont that contains thousands of merozoites is formed. These erythrocytic schizonts can eventually rupture, releasing merozoites to infect fresh red cells. In untreated malaria, this cycle can cause regular waxing and waning of fever.
Some trophozoites can exit the cycle and develop into male and female gametocytes. Gametocytes can be taken up by mosquitoes. Once in the mosquito's gut, the gametocytes undergo sexual reproduction, producing new sporozoites that live in the mosquito's salivary glands. And so the process continues!
Malaria can present in many different ways. Fever is usually the main symptom, but there are many other nonspecific symptoms, from headache, to abdominal pain, to seizures. In severe cases, it may proceed to pulmonary oedema, acute renal failure, severe anaemia, and so on. Therefore, diagnosis of malaria cannot be made clinically, and diagnostic tests are necessary. These diagnostic tests include a blood film to look for parasites, PCR and antigen detection.
Dengue fever
Dengue is another common cause of fever in returned travellers. It is transmitted by Aedes mosquitoes. Just like malaria, dengue fever can present with very non-specific symptoms, mainly fever. The main symptoms of "classical" dengue are fever, rash, severe muscle and joint pain, headache, retro-orbital pain, and mild haemorrhagic phenomena. If a patient is infected twice, with a different serotype each time, they can get dengue haemorrhagic fever or dengue shock syndrome, which are very serious conditions. In dengue haemorrhagic fever, platelets are diminished, leading to haemorrhagic skin rashes, leaky blood vessels and hypotension. Diagnostic tests for Dengue fever include antibody and antigen (NS1) detection and PCR.
Prevention
Some preventative measures can be taken to reduce the risk of getting a travel illness. These include getting appropriate vaccinations, prophylactic drugs for malaria, mosquito avoidance, observing food and water safety, safe sex and avoiding animals.
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