Diseases transmitted by insects and ticks
Insects (mosquitoes, lice, fleas, bed bugs) and ticks are able to transmit a number of diseases caused by infectious agents: viruses (chikungunya virus, yellow fever, dengue fever, etc.), bacteria (Lyme disease, plague, etc.), parasites (malaria, sleeping sickness, leishmaniasis, filariasis, etc.).
These diseases thrive
mainly in tropical environments. Malaria is the dominant problem.
The
vectors
Mosquitoes
There
are different species of mosquitoes (Anopheles, Aedes, Culex) and these have
completely different preferred habitats, times when they are active and types
of bite. The lavae that produce the adults develop in areas of stagnant water
(receptacles, reservoirs, ponds, lakes, etc.). They transmit certain specific
diseases according to the specific species, climate and habitat: malaria
(Anopheles), chikungunya virus, dengue fever, yellow fever, Japanese
encephalitis, lymphatic filariasis.
Their
flight can be more or less noisy; their bite is not always painful. Half of all
mosquito bites occur through clothing.
Flies, midges
The
Chrysops (horse flies) are the vector for loa loa filariasis. Simuliidae (black
flies) transmit onchocerciasis. Certain members of the glossina genus (tsetse
flies) transmit African trypanosomiasis (sleeping sickness). Certain of the
phlebotominae (sand flies) transmit leishmaniasis. Their bites are not always
felt.
Besides
their direct role in the transmission of microorganisms, flies can be the cause
of conditions linked to the development and to the migration of their lavae
beneath the skin (the Cayor worm is linked to the passage through the skin of
the lavae of Condylobia anthropophaga which lays its eggs on the ground but
also on linen). The contamination occurs either by lying down on ground
contaminated by the eggs laid by the fly, or via clothes on which the flies
have laid their eggs while drying. Prevention is achieved by not lying down in
the ground and by avoiding drying one's clothes in the open air. It is also
recommended that one irons one's clothes after they are washed in order to
destroy, with the heat from the iron, any lavae present.
Bed bugs
On
the South American continent, the triatomines or reduviidae (bed bugs) transmit
Chagas disease (American trypanosomiasis). They live in the walls of damp
houses and are only active at night.
Lice, fleas
Lice
transmit different bacterial infections: bartonellosis (Trench fever),
borrelliosis (relapsing fever), and certain types of rickettsiosis (typhus).
Rat fleas are a vector for the plague.
Ticks
There
are numerous species of ticks, each one having a relatively specific habitat.
They are the vector for numerous diseases: Lyme disease, tick-borne
meningoencephalitis, Crimean–Congo hemorrhagic fever, tick-borne relapsing
fever, Q fever, the tick-borne spotted fevers, babesiosis, ehrlichiosis,
tularemia.
Tick
bites result from contact with grassland. This risk is generally restricted to
certain well-defined regions. Once the ticks are on the skin, they migrate
towards the major skin folds (groin, armpits) where they implant themselves.
They do not begin to feed until 12 to 24 hours have passed and so the risk of
infection is low if they are quickly removed.
Besides
the transmission of various diseases, a tick bite can, in its own right, be the
cause of a local inflammation reaction that can sometimes be highly pronounced.
The diseases
The
frequency of the diseases mentioned before is continuously changing. Some of
these have become rare as a result of progress in hygiene and vaccinations
measures (yellow fever, Japanese encephalitis, typhus, plague). Others, after
having virtually disappeared, are re-emerging (trypanosomiasis). Finally,
others are spreading, notably due to changes in climate and the changes that
this brings about in the ecology of the vectors (dengue fever, chikungunya
virus, Lyme disease, etc.). We will briefly present the most common diseases.
Malaria
Malaria
represents the primary health risk for travellers due to being very widespread
in the tropical and subtropical regions around the globe and due to its
potentially fatal outcome. The disease is caused by a unicellular
blood-dwelling parasite of the genus Plasmodium, transmitted by a mosquito of
the genus Anopheles. Different species exist. The study of this disease is the
subject of a specific sheet.
The chikungunya virus
This
is an emerging viral disease caused by an Alphavirus, the main reservoir of
which consists of monkeys and other vertebrates including infected humans. The
transmission to humans occurs as a result of the bite of the Aedes genus
mosquito. This is essentially a diurnal vector. This disease affects Africa,
Southeast Asia and the entire Indian subcontinent. The disease progresses in
the form of epidemic outbreaks.
The
disease can go unnoticed. In its usual form, it leads to a feverish state with
intense joint and muscular pains. It is sometimes accompanied by mild
haemorrhaging. The disease progression is normally favourable but complications
and death can occur. There is a long recovery period with long-lasting residual
asthenia. The signs are not very specific, in such a way that without a
serology test, the diagnosis can be confused with other diseases causing fever
and pain, such as dengue fever, malaria, etc.
No
specific anti-viral treatment exists. The treatment is purely symptomatic
(paracetamol-type non-salicylate analgesics). The disease confers long-lasting
immunity. There is no vaccine; prevention consists of avoiding mosquito bites.
Dengue fever
Dengue
Fever is a viral disease caused by a Flavivirus. It is undergoing a strong
resurgence. It is transmitted by the bite of a mosquito of the genus Aedes,
which reproduces in stagnant water locations around habitations. The disease
occurs in Southeast Asia, Australia, Oceania, the Indian Ocean, the Caribbean,
America (from Southeastern USA to northern Argentina), and in sub-Saharan
Africa. It is endemic in all the overseas French regional départements and
territories. It follows an endemic-epidemic transmission pattern. There are 4
distinct serotypes of the disease.
After
an incubation period of 7 days, the clinical picture is one of a fever and
aches with a rash. After a remission with a drop in temperature, the
established stage occurs with resumption of the symptomatology. The disease
lasts for a week, the recovery period is long, and is marked by a long-lasting
asthenia. There are subclinical forms and, on the contrary, severe forms
resulting in death. The clinical picture is not very specific; it is shared by
other arboviral infections that present Dengue fever-like syndromes. The
definitive diagnosis of isolated cases depends on serology testing.
The
treatment is purely symptomatic (paracetamol-type non-salicylate analgesics). A
vaccine is under study. The disease resulting from a virus of a given group
does not confer immunity with regard to the viruses of the other groups. On the
contrary, the fact that a patient has already suffered dengue fever exposes
them to a new, more severe illness in the event of a new infection by a virus
from a different group.
Yellow fever
Yellow
fever is a viral haemorrhagic fever caused by a Flavivirus: the yellow fever
virus. It is transmitted by the bite of a mosquito of the Aedes genus. The
natural host of the virus is a particular species of monkey living in forest
regions. The virus can be transmitted, accidentally, to human communities. The
disease follows an endemic-sporadic transmission pattern and gives rise to
epidemics. It affects the tropical and subtropical regions of South America and
Africa. The disease is absent from Asia, the Pacific and the Indian Ocean. It
is currently highly present in Africa where small epidemics are regularly
observed (Ivory Coast, Cameroon, Senegal).
The
disease typically begins with a highly feverish state with headaches and lower
back pain. It then progresses, in the typical forms, in 2 stages. A red stage
with fever, congested appearance to the face, headaches. There is a remission
in the 3rd-4th day, then a yellow stage with recurrence of the fever,
deterioration of the general condition, jaundice, black vomit (vomitus
containing blood), haemorrhaging, reduction in urine volume. The progression
can be fatal. There are many milder or subclinical forms.
The
treatment is purely symptomatic. There is a vaccination that is very effective.
This vaccination is compulsory for travellers visiting countries where the
disease is likely to exist. The vaccination is administered at the approved
vaccination centres. It is subject to inclusion on the international
vaccination record.
West Nile disease
It
is an infection caused by a Flavivirus: the West Nile virus. The vector is a
mosquito of the Culex genus. The reservoir of the virus consists of birds. The
disease initially affected Africa, part of central and southern Europe, the
Middle East, and India. A gradual expansion is being observed with a spread
across the American continent from East to West, and to Eastern Europe and
Russia. In France, the disease is present in Camargue where it affects horses,
but also humans.
The
disease is frequently asymptomatic but it can cause feverish states accompanied
sometimes by neurological signs presenting a clinical picture of encephalitis
or flaccid paralysis. There is a risk of death or serious sequelae.
The
treatment is symptomatic. Vaccine trials are underway.
Japanese encephalitis
Japanese
encephalitis (JE) is caused by a Flavivirus. It is transmitted to humans via
the bite of a mosquito of the Culex genus. The reservoir of the virus consists
of wild aquatic animals and pigs, from which the disease can spread to humans
in rural areas. JE affects the far south-east of Russia, the whole of Asia,
India and the far north of Australia. It is endemic in rural areas with rice
fields and irrigation throughout the year. It is epidemic in rural and urban
areas during the monsoon. There are 30,000 to 50,000 new cases per year, mainly
among children, with 25,000 deaths. The risk of travellers contracting the
disease is roughly 1/1,000,000.
There
is one visible form (one case) for every 250 contaminations. The clinical
picture formed by the visible forms is not specific. It can consist of an
isolated feverish state, or a meningitis or meningoencephalitis clinical
picture. It is estimated that the mortality rate is 30% and that 30% of
patients will recover with sequelae.
The
treatment is symptomatic. There is a preventive vaccination. There are two
types of vaccines on the market: a cell culture vaccine in Western countries
and a suckling mouse brain vaccine. This vaccination is only available at
approved vaccination centres.
Vaccination
is recommended for :
- Adults
who are expatriates or who need to reside for more than 30 days in Asia;
·
Adults who are visiting these
regions, undertaking significant outdoor activity, in particular in rice fields
or marsh areas, during the virus transmission period, in particular during the
rainy season, regardless of the duration of the stay. The following activities
are considered to present a risk: sleeping outdoors without a mosquito net,
camping, working outdoors, cycling, hiking, etc.; especially in areas where
flood irrigation is practised.
The vaccine marketed in France is currently reserved for persons
aged 18 and over.
Filariasis
Filariasis
is a group of tropical diseases linked by the development, within the body, of
adult worms (filariae) and their lavae or microfilariae. These diseases
constitute a real public health problem for the local populations, but
infection is rare among travellers. They are transmitted by specific insects
acting as vectors, which explains their presence in certain specific areas. A
distinction can be made between lymphatic filariasis and cutaneous filariasis
depending on whether the adult worms live in the lymphatic system or beneath
the skin.
Lymphatic
filariasis :
- There
are three species: Wuchereria bancrofti (cosmopolitan), Brugia malayi and
Brugia timori (Southeast Asia). Each species is transmitted by a specific
mosquito (Culex, Anopheles, Aedes, Mansonia) and has a well-defined
geographical distribution. The disease is endemic in tropical regions
where 120 million people are infected.
- The
symptoms are linked with the disruption of the lymphatic drainage caused
by the adult worms, giving rise to oedema of the limbs, lymphangitis and
superinfections. They are also allergenic in nature due to the
microfilariae. Without treatment, the oedema progress to a chronic state,
causing a thickening of the tissues and resulting in a clinical
presentation of elephantiasis.
- Treatment
uses anti-parasitic drugs. The WHO launched a mass treatment program in
2000 aiming to stop the transmission of the disease.
Loa
loa filariasis :
- This
is a cutaneous filariasis found in the forest regions of central Africa.
It is caused by the Loa-loa. It is transmitted by the bite of a horse fly:
the Chrysops. The microfilariae live in the blood circulation.
- The
symptomatology is characterised by itching, urticarial episodes, migrating
oedema of the limbs. The movement of the adult worm beneath the skin is
the cause of the tortuous cords; they move at a speed of roughly 1 cm per
minute. The passage of the worm beneath the conjunctiva of the eye causes
tearing, red eye and foreign body sensation. This occurrence is dramatic
but relatively benign. The disease progresses over a long period of time.
Later stage complications can be observed, particularly affecting the
heart.
- Treatment
uses anti-parasitic drugs. They must be used carefully.
Other
types of filariasis :
There
are other types of filariasis transmitted by specific vectors: Onchocerciasis,
filarial pleural effusion. They are rarer.
Sleeping sickness
Human
African typanosomiasis or sleeping sickness is a parasite disease caused by the
Trypanosoma protozoa (unicellular blood-dwelling parasites), several different
species of which exist. This disease, which virtually disappeared during the
1960s as a result of the actions of the major disease mobile teams founded by
Jamot, is in full resurgence due to troubles disrupting the various health
systems. It is transmitted by the bite of a fly: Glossina or tsetse fly. It is
found only in Africa. It is found in homes in the tropical and subtropical
regions. The disease affects rural populations; the risk to travellers is
limited.
The
disease progresses in two stages: a lymphatic/blood stage during which the
parasite is found in the lymphatic system, then a stage called the
"neurological phase" where the central nervous system is affected.
The progression occurs over a long period of time. Without treatment, the
disease is invariably fatal.
The
disease is treated with anti-parasite drugs; the treatment is very well
codified but relatively hard to undergo. It is very effective.
The leishmaniases
The
leishmaniases are parasite diseases caused by Leishmania (unicellular parasites
that infect the reticulohistiocytic system) transmitted by a species of sand
fly (subfamily Phlebotominae). A distinction can be made, according to the type
of parasite, between the cutaneous, mucocutaneous and visceral forms.
The
cutaneous and mucocutaneous forms have a different aspects on the American
continent compared with Asia. The various forms of the disease have as a common
feature, the creation of chronic ulcers, more or less hollow, without a
tendency to heal.
The
visceral forms or Kala-azar are observed in the Mediterranean basin. They are
characterised by a severe deterioration of the general condition of the patient
with an increase in the size of the lymph nodes and of the spleen, and a drop
in blood cell numbers.
These
diseases have specific treatments, which need to be followed over long periods
of time in order to achieve a cure.
Lyme disease
Lyme
disease is an infection caused by a a species of bacteria of the Borrelia
genus: Borrelia burgdorferi transmitted by the bite of a tick of the Ixodes
genus. 10 to 15% of them carry the Borrelia bacteria. The risk of contamination
by tick bite varies between 1 and 6% depending on the region. The disease is
found in the northern hemisphere, in America and in Eurasia during the active phase
of the ticks, between May and October. It is not uncommon in France.
The
disease begins with a red skin lesion centred around the tick bite that
gradually spreads out and ends up disappearing within a few days or a few
weeks. A few weeks or a few months later, other signs may appear, involving the
joints, the nervous system (paralysis and pain), heart rhythm abnormalities
and, more rarely, other symptoms. The expression of the disease is usually more
severe in Northern America than in Europe.
The
treatment relies on antibiotics. Admission to hospital can be necessary.
Tick-borne meningoencephalitis
Tick-borne
meningoencephalitis is a disease of the central nervous system caused by a
Flavivirus (Tick Born Encephalitis Virus). It is transmitted by the bite of a
tick of genus Ixode, in spring and summer. In the past, a distinction was made
between the European forms (Central Europe and Eastern Europe) and the Asian
forms (from Russia to Japan), but in fact it consists of the same disease
transmitted by different tick species. In the areas affected, the disease is
restricted to limited territories. It has been expanding continuously over the
last few years. The disease is present in Alsace.
After
an incubation period of 7 days, the symptomatology progresses in two stages. A
first stage that lasts 2 to 4 days is characterised by a flu-like state. The
second stage does not always occur, and if it does, appears 10 days later. It
is characterised by neurological signs of varying severity: meningitis,
meningoencephalitis, meningo-encephalo-radiculitis. In general the disease
progresses towards recovery. Sequelae can be observed. The mortality rate is
1%.
The
treatment is purely symptomatic; there is no medicine that acts on the virus
itself. A preventive vaccine is available. The vaccination against tick-borne
encephalitis is recommended for travellers staying in endemic rural and forest
regions in central, eastern and northern Europe, between spring and autumn.
The tick-borne spotted fevers
The
tick-borne spotted fevers are diseases caused by bacteria of the Rickettsia
genus. They are transmitted by tick bites. More than 20 species of the
Rickettsia genus are pathogenic for humans. Besides the various spotted fevers,
they are responsible for typhus (epidemic typhus, endemic typhus, scrub
typhus).
The
spotted fevers are found in Europe, on the North American continent and in
sub-Saharan Africa (especially southern). Various types of the disease have
been described: Mediterranean spotted fever, Israeli tick typhus, Indian tick
typhus, Rocky Mountain spotted fever, African tick bite fever.
These
various diseases present a common symptomatology. After an incubation period of
5 days, the following signs appear: fever, an eschar at the point of the tick
bite with satellite lymphadenopathy (lymph glands) and, not systematically,
skin rash with blisters. Progression can be severe for Mediterranean spotted
fever and especially Rocky Mountain spotted fever. For the other types of the
disease, progression is benign.
The
severe forms are treated with antibiotics.
Geographical
distribution
The
table below presents the distribution, according to the major geographical
regions, of the main diseases transmitted by insects and ticks.
Geographical regions
|
Diseases
|
North America
|
The leishmaniases
|
Lyme disease
|
|
West Nile disease
|
|
Central and South America
|
Malaria
|
Dengue fever
|
|
Yellow fever
|
|
The leishmaniases
|
|
Chagas disease
|
|
West Nile disease
|
|
North Africa
|
The leishmaniases
|
Lyme disease
|
|
West Nile disease
|
|
Sub-Saharan Africa
|
Malaria
|
Chikungunya virus
|
|
Dengue fever
|
|
Yellow fever
|
|
Lymphatic filariasis, loa
loa filariasis, onchocerciasis
|
|
The leishmaniases
|
|
Human African
trypanosomiasis (sleeping sickness)
|
|
West Nile disease
|
|
Asia
|
Malaria
|
Chikungunya virus
|
|
Dengue fever
|
|
Japanese encephalitis
|
|
Lymphatic filariasis
|
|
The leishmaniases
|
|
Europe
|
The leishmaniases
|
Lyme disease
|
|
Tick-borne
meningoencephalitis virus
|
|
Oceania
|
Malaria
|
Chikungunya virus
|
|
Dengue fever
|
|
Japanese encephalitis
|
|
Lymphatic filariasis
|
|
The leishmaniases
|
.
.
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