Travel medicine: A closer look at the world map
Are you looking for information on diseases with regional patterns?
In this Chalk Talk episode, we’ll provide you with a general overview and brief introduction
on diseases to consider.
This includes distribution patterns, modes of transmission, and main symptoms.
So, what are we waiting for?
Let’s go explore the map of the world.
Schistosomiasis is a parasitic disease acquired by contact with contaminated freshwater.
It’s caused by schistosoma trematodes, also known as blood flukes.
The parasites usually live in freshwater, where they mature into snails.
The infectious larvae, also termed cercaria, can penetrate the skin when humans come into
contact with contaminated water.
This can cause a pruritic maculopapular rash at the point of entry, commonly known as swimmer's
itch.
The parasites then migrate to the liver.
Apart from a local reaction, most infections are asymptomatic; however, in some individuals,
the eggs or adult worms are recognized by the immune system, resulting in Katayama fever,
an acute form of schistosomiasis.
Symptoms include fever, cough, and angioedema.
Depending on the schistosoma type, adult worms migrate to the intestine or bladder, resulting
in two main forms of chronic schistosomiasis.
These forms occur in response to egg deposition and are the result of chronic inflammation.
Intestinal schistosomiasis presents with abdominal pain and bloody diarrhea, whereas clinical
findings in urogenital schistosomiasis are dysuria and hematuria.
If the eggs enter the bloodstream, they’re transported to other organs such as the liver,
kidney, or central nervous system.
Schistosomiasis is the leading cause of portal hypertension worldwide.
The regional distribution of the different chronic forms depends on the distribution
pattern of the parasite.
Intestinal schistosomiasis is prevalent in South America and Asia, whereas urogenital
schistosomiasis is more common in the Middle East and Northern Africa.
Both forms are frequently observed in other regions of Africa.
The only preventative measure against infection is to avoid contact with freshwater in endemic
areas.
Schistosomiasis can be effectively treated with the anthelmintic praziquantel.
The most historically feared disease by travellers is malaria.
It clinically presents with nonspecific findings such as fever, nausea, and vomiting after
an incubation period of 1 to 6 weeks.
Despite its prevalence in Africa, South America, and Southeast Asia, approximately 90% of more
than 200 million annual cases of malaria occur in Africa.
Several types of malaria exist, which are caused by different Plasmodium species.
Among these, Plasmodium falciparum is the most common, accounting for approximately
three-quarters of cases worldwide.
The female Anopheles mosquito acts as a vector, transferring sporozoites through their bite.
The sporozoites enter the bloodstream of the human host and migrate to the liver.
After maturation, they re-enter the bloodstream and invade erythrocytes, which rupture and
release merozoites that induce febrile inflammatory response.
As a preventative measure, travellers are strongly advised to use exposure prophylaxis,
such as mosquito repellents and mosquito nets.
In addition, chemoprophylaxis can be used.
The drug of choice depends on the destination and duration of stay, and considers the current
status of drug resistance.
However, antimalarial drugs don’t prevent infection.
Instead, they alleviate clinical manifestations by attacking developmental forms of the parasite.
Due to the strong adverse effects of chemoprophylaxis, a detailed look at the local risk of malaria
should be made.
As a general rule of thumb, there’s no risk of malaria infection in locations with temperatures
below 15°C, which are in various high altitude areas in the tropics.
Another mosquito-borne febrile disease is dengue fever.
It’s caused by infection with dengue virus and is primarily transmitted by the Aedes
aegypti mosquito.
Therefore, the risk areas of dengue infection cover most of the distribution areas of Aedes
aegypti.
Dengue is found in tropical and subtropical regions worldwide, especially in Southeast
Asia, but also in central and South America, Africa, Oceania, and Northeast Australia.
Although approximately 90% of infected individuals remain asymptomatic, a severe course with
hemorrhagic fever occurs in 1–2% of cases.
Dengue hemorrhagic fever is characterized by bleeding, shock, and organ dysfunction,
and can be fatal.
Progression to severe forms occur especially in patients with either concurrent infection
with different serotypes of dengue virus or in patients immune to one or more serotypes
as a result of previous infection.
A live-attenuated vaccine is currently available in some endemic countries; however, it isn’t
recommended for travellers.
Dengue prevention is limited to avoiding mosquito exposure in endemic areas.
Aedes mosquitoes are also involved in the transmission of another haemorrhagic fever,
yellow fever.
This disease is currently more localized than dengue fever and malaria.
Most yellow fever infections occur in sub-Saharan Africa between 15 degrees north and 18 degrees
south of the equator.
Other regions of risk for travellers are the tropical regions of northern South America.
This region is also referred to as the “yellow fever belt”.
The disease is caused by yellow fever virus, which can result in viral hemorrhagic fever.
Clinical manifestations include liver dysfunction, renal failure, disseminated intravascular
coagulation, and shock.
The most effective preventative measure against yellow fever is vaccination, with a single-dose
vaccine available that provides lifelong protection.
Some countries require travellers coming from yellow fever endemic areas to provide proof
of vaccination.
The protozoan parasite Trypanosoma brucei is restricted to sub-Saharan Africa, giving
rise to a disease that is best known as “sleeping sickness”, or African trypanosomiasis.
The parasites are transmitted by the tsetse fly.
There are two different forms of the disease that are caused by different pathogens: The
West African form and East African form, which are designated by their geographic distributions.
Patients initially present with painful, red swelling at the site of the bite.
This is followed by a hemolymphatic phase, where the patients develop intermittent fever,
lymph node swelling, anemia, and immune reactions in different organ systems such as the central
nervous system and the cardiac system.
The neurological phase begins a few months to years after infection in the case of West
African trypanosomiasis or within a few weeks in case of East African trypanosomiasis.
Clinical findings include headache, daytime somnolence, which gives rise to its name sleeping
sickness, ataxia, delayed hyperesthesia, cachexia, and eventually coma.
The only preventative measure is to avoid exposure by using insect repellents, wearing
long sleeve clothing, and avoiding areas inhabited by the tsetse fly.
As the protozoan parasite Trypanosoma is not limited to Africa, it should come as no surprise
that an American form of trypanosomiasis exists.
The disease is widely known as Chagas disease.
Chagas disease is endemic in Central and South America and is transmitted by the triatomine
bug, also known as kissing bug.
Infections reported in Northern America and Europe are either imported or infrequently
acquired through blood transfusion, organ transplantation, or vertical transmission
from mother to fetus.
Shortly after infection, patients develop a characteristic, local skin lesion at the
site of the bite, which is also known as a chagoma.
The acute phase of infection manifests with nonspecific symptoms such as fever, malaise,
and generalized lymphadenopathy.
The asymptomatic intermittent phase is followed by a chronic phase with the development of
cardiomyopathy characterized by arrhythmias, megaesophagus, and megacolon.
In contrast to sleeping sickness, the central nervous system is rarely affected in Chagas
disease.
Prophylactic measures for travelers are preventing exposure to the vector by using insect repellents
and bed nets treated with insecticides.
A disease that poses a risk for individuals travelling to the Northern hemisphere is Lyme
disease, also termed borreliosis.
Lyme disease is caused by bacteria of the Borrelia type, which are transmitted by ticks.
The vector is most prevalent in areas endemic for Lyme disease.
Infected ticks can be found in the Northeastern and upper Midwestern US and Europe, as well
as in Russia.
If left untreated, Lyme disease can proceed in three stages, beginning with an early localized
rash, eventually causing a systemic infection involving joints, skin, and central nervous
system.
As there is no vaccine available against Lyme disease, preventative measures comprise of
preventing exposure to ticks, as well as the immediate removal of the tick after biting.
Removal lowers the risk of infection, as transmission usually occurs after an attachment time between
12 and 24 hours.
In addition, close observation of the bite site is advised, because the presence of a
classical erythema migrans may indicate infection.
However, not all cases of Lyme disease are accompanied by erythema.
Lyme disease can be treated with antibiotics, with early treatment resulting in the highest
success rates.
Therefore, any sign of infection should signal the start of further diagnostics and, if indicated,
treatment.
Another viral infectious disease to be on the lookout for in the Northern hemisphere
is tick-borne encephalitis, which is caused by tick-borne encephalitis virus, in short
TBEV.
Apart from it’s regional distribution, transmission is more frequent during summer, with infection
rates highest during this period.
Approximately 90% of infections remain asymptomatic; however, the remaining 10% initially show
flu-like symptoms and fever.
After a fever-free interval, the central nervous system is affected, typically manifesting
as meningoencephalitis.
Treatment of meningoencephalitis is supportive and based on the severity of symptoms, but
usually leads to full recovery.
As a preventive measure, vaccines against TBEV are available; however, they are only
recommended to individuals travelling to endemic regions where they’re more likely to be
exposed to ticks.
Preventative measures to avoid tick bites are generally advised.
Immediate removal of the tick after biting doesn’t further reduce the risk of infection
with TBEV, as the virus is transferred directly after biting.
One well-known disease that occurs on a global scale is rabies.
The disease is often fatal and is caused by lyssaviruses, which include the rabies virus.
In encephalitic rabies, initial symptoms include agitation, confusion, and hypersalivation,
followed by coma and death.
Rabies is usually transmitted through the bite of an infected animal, most likely a
dog or a bat.
The regions high at risk for rabies infection are Africa and South-East Asia; however, transmission
through wild animals can occur in almost all geographical locations, including central
and South America, and Europe.
As there is no drug available against rabies virus infection, prophylaxis is highly recommended.
Pre-exposure rabies vaccination is ideal; however, treatment consisting of post-exposure
vaccination and rabies immunoglobulin after suspected exposure is particularly effective.
So, let’s summarize what we’ve just covered.
Individuals preparing to travel should be informed about potential risks in the countries
that they're travelling to.
This includes determining whether the regions are at risk for diseases that can be prevented
by administering a vaccine prior to departure.
Also, it’s usually quite helpful to be cautious about drinking water sources and food, especially
if it’s not well cooked.
For any outdoor activities, it’s advisable to minimize exposure to insects, which are
often vectors of pathogens.
We do admit that it’s difficult to remember all of this information.
But don’t worry; it's all available in our library.
So, let’s see how much you know about these travel-relevant diseases in our quiz.