|
 Yellow
Fever
Yellow fever is a viral illness which is spread
by the bite of a mosquito. The species of mosquito
that is responsible for spreading yellow fever
is Aedes Aegypti and is a different
species from the anopheles mosquito that transmits
malaria.
Essentially
yellow fever is a disease of monkeys living in
tropical rain forests. The virus which causes
the disease is one of a group of viruses known
as arboviruses. Humans are infected by
being bitten by rain forest mosquitos carrying
the yellow fever virus.
 As
their name implies "arboviruses" are transmitted
to humans by insects (arthropod borne virus).
The mosquito in particular is well suited to
the transport and spread of the infection due
to its wide distribution throughout the tropics.
The
geographical distribution of yellow fever
is confined to Equatorial Africa and Central South
America as shown on the maps as shown.
Strangely, yellow fever is unknown in Asia
despite the presence of mosquitos capable of spreading
the virus.
Arbovirus illnesses usually have two characteristic
phases, the first when the virus is invading the
host cells, and the second a few days later when
the body's immune system is fighting the infection.
The antibodies produced during the second phase
of illness can cause damage to the blood vessels
which explains why arboviruses often cause bleeding.
Many yellow fever infections are mild and
go unrecognised but severe and life threatening
illness is not uncommon. After an incubation period
of about three to six days fever, headache, abdominal
pain and vomiting develop. After a brief recovery
period, shock, bleeding and signs of liver and
kidney failure develop. Liver failure is associated
with jaundice hence the name "yellow fever".
 There
is no drug available to cure yellow fever
hence treatment is aimed at symptomatic relief.
Overall about 5% of patients die. Those who
recover do so completely and are immune thereafter.
Fortunately yellow fever
is one of the few arboviruses for which a vaccination
is available. A single injection of a live, weakened
(and harmless) virus stimulates the body's immune
defences and confers effective immunity for ten
years.
In general, all travellers going to an
endemic area require a yellow fever vaccination
certificate and travellers going to some parts
of Asia from an endemic region will also require
a certificate.
 Dengue
Fever
This is an unusual arbovirus infection since
no other animals except humans and mosquitoes
play a significant part in perpetuating the infection.
It is present in Africa, South East Asia, the
Pacific area and northern South America.
The disease is spread from person to person
by the bite of a mosquito (Aedes aegypti) and
after about five days incubation period there
is a sudden onset of fever, headache and severe
joint and muscle pains. The initial fever resolves
after about three to five days only to recur with
the appearance of a rash consisting of small white
spots which starts on the trunk and spreads to
the limbs and face. Within a few days the fever
subsides and recovery follows.
Although dengue is a very unpleasant illness,
complications are uncommon and recovery is usually
complete.
There is also a more severe and life
threatening haemorrhagic form of the disease
which has appeared with dramatic outbreaks.
Fortunately this form occurs only rarely.
This is thought to be the
result of a second infection where there is some
remaining immunity from a first attack causing
a vigorous immunological response in which severe
blood vessel damage occurs.
Unfortunately,
immunity to infection does not last long and subsequent
attacks are possible. There is no vaccine available.
Prevention is by avoiding mosquito bites.
 Japanese
B Encephalitis
This
is a rare but serious arboviral infection
with a 20% fatality rate. It occurs in most of
the Far East and South East Asia. The endemic
zone extends from India and Nepal across the whole
of South East Asia to Japan and Korea in the Far
East.
The risk of infection is greatest in long
term visitors to rural areas, and the risk to
short term visitors and visitors to major cities
is small. Precautions against mosquito bites are
essential.
Japanese B Encephalitis is transmitted
by rice field breeding mosquitoes (of the Culex
group) that become infected with Japanese encephalitis
virus.
 Mosquitoes
become infected by feeding on domestic pigs
and wild birds infected with the Japanese encephalitis
virus. Infected mosquitoes then transmit the
Japanese encephalitis virus to humans and animals
during the feeding process.
The incubation period is normally between
5 and 15 days. The illness cannot be passed
on from person to person. There is no specific
treatment. Intensive supportive therapy is indicated.
Mild
infections can sometimes occur without apparent
symptoms other than mild fever with headache.
More severe infection is marked by quick onset,
headache, high fever, neck stiffness, stupor,
disorientation, coma, tremors, occasional convulsions
(especially in infants) and spastic paralysis.
There
is an effective vaccine available in the UK
on a named patient basis. It should be considered
by anyone travelling to Asia for more than a month
or visiting rural areas.
 Tick
Borne Encephalitis
Ticks are blood feeding external
parasites of mammals, birds, and reptiles throughout
the world. Ticks are not insects like fleas, but
arachnids like mites, spiders and scorpions.
Tick-borne
encephalitis (TBE), is a viral disease affecting
the central nervous system. The disease is most
often manifest as meningitis or encephalitis or
sometimes, even meningoencephalitis. It is caused
by the tick-borne encephalitis virus (TBEV), a
member of the family Flaviviridae.
TBE is endemic in many European countries, the
former Soviet Union, and Asia. It is found mainly
in temperate regions. The natural hosts of the
virus are small rodents and deer, with humans
being accidental hosts. After attaching itself
to the host, an infected tick transfers the virus
to the host during feeding.
Map showing TBE
endemic areas |
In
disease endemic areas, people with recreational
or occupational exposure to rural or outdoor settings
such as; hunters, hikers, campers, forest workers,
farmers, etc. are potentially at risk of infection
by contact with the infected ticks.
The incubation period of TBE is usually between
7 and 14 days.
A characteristic biphasic febrile illness follows,
with an initial phase that lasts two to four days.
It is non-specific with symptoms that may include
fever, malaise, anorexia, muscle aches, headache,
nausea, and/or vomiting.
 After
about eight days of remission, the second phase
of the disease occurs in twenty to thirty percent
of patients and involves the central nervous
system with symptoms of meningitis (fever, headache,
and a stiff neck) or encephalitis (drowsiness,
confusion, sensory disturbances, and/or motor
abnormalities such as paralysis) or meningoencephalitis.
TBE is more severe in adults than in children.
Mortality is about two percent with deaths occurring
five to seven days after the onset of neurological
signs.
There
is no specific treatment for TBE. Meningitis,
encephalitis, or meningoencephalitis require hospitalisation
and supportive care.
Like other tick-borne infectious diseases, TBEV
infection can be prevented by using insect repellents
and protective clothing to prevent tick bites.
A vaccine is available in some disease endemic
areas but adverse vaccine reactions in children
limit the use of the vaccine. The vaccine involves
two doses one month apart. If required a booster
is given after a year.
| Symptoms
of West Nile Virus |
- symptoms
of a mild infection: fever, headache,
bodyaches, skin rash, and swollen lymph
glands
- symptoms
of a severe infection may be marked by:
high fever, severe headache, neck stiffness,
stupor, disorientation, tremors, convulsions,
muscle weakness, paralysis, and coma
- Contact
your health care provider if you have
concerns about your health. If severe
symptoms develop, see your doctor immediately.
|
West
Nile Virus
West Nile Virus is a flavivirus commonly found
in Africa, West Asia, the Middle East and the
United States. The virus can infect humans, birds,
mosquitoes, horses and some other mammals.
The principal route of human infection with West
Nile virus is through the bite of an infected
mosquito. Mosquitoes become infected when they
feed on infected birds, which circulate the virus
in their blood for a few days. The virus replicates
in the mosquito and spreads to the mosquito's
salivary glands. During subsequent blood meals,
the virus may be injected into humans and animals,
where it can multiply and possibly cause illness.
At least 36 species of mosquito are known to be
carriers of West Nile Virus. The most common carrier
of West Nile in the United States is the Culex
pipiens (Northern house) mosquito.
 West
Nile fever is usually a case of mild disease
characterized by flu-like symptoms. It typically
lasts only a few days and does not appear to
cause any long-term health effects.
More severe disease due to a person being infected
with this virus can be West Nile encephalitis,
West Nile meningitis or West Nile meningoencephalitis.
Encephalitis refers to an inflammation of the
brain, meningitis is an inflammation of the membrane
around the brain and the spinal cord, and meningoencephalitis
refers to inflammation of the brain and the membrane
surrounding it.
The incubation period is usually 3 to 14 days.
Symptoms of mild disease will generally last a
few days. Symptoms of severe disease may last
several weeks, although neurological effects may
be permanent.
Many
people who are infected with the West Nile virus
will not have any type of illness. It is estimated
that 20% of the people who become infected will
develop West Nile fever: mild symptoms, including
fever, headache, and body aches, occasionally
with a skin rash on the trunk of the body and
swollen lymph glands.
 The
symptoms of severe infection (West Nile encephalitis
or meningitis) include headache, high fever,
neck stiffness, stupor, disorientation, coma,
tremors, convulsions, muscle weakness, and paralysis.
It is estimated that 1 in 150 persons infected
with the West Nile virus will develop a more
severe form of disease.
There is no specific treatment for West Nile virus
infection. In more severe cases, intensive supportive
therapy is indicated, often involving hospitalization,
intravenous fluids, airway management, respiratory
support (ventilator), prevention of secondary
infections (pneumonia, urinary tract, etc.), and
good nursing care.
When travelling to areas where the disease is
endemic you can reduce the risk of becoming infected
with the virus by employing preventive measures
such as protecting yourself from mosquito bites:-
- Apply
insect rellent to your skin and clothes
when going outside
- When
possible, wear long-sleeved shirts and
long pants whenever you are outdoors.
- Place
mosquito netting over infant carriers
when you are outdoors with infants.
- Consider
staying indoors at dawn, dusk, and in
the early evening, which are peak mosquito
biting times.
- Make
sure any window and door screens are intact
so that mosquitoes cannot get indoors.
- Use
mosquito nets around beds at night while
asleep.
|
 Many
of the mosquitoes that carry the West Nile virus
are especially likely to bite around dusk and
dawn. If you are outdoors around these times
of the day, it is important to apply repellent.
But there are also mosquitoes that bite during
the day, and these mosquitoes have also been
found to carry the West Nile virus. The safest
decision is to apply repellent whenever you
are outdoors
Apply insect repellent sparingly to exposed skin.
The more DEET a repellent contains the longer
time it can protect you from mosquito bites. A
higher percentage of DEET in a repellent does
not mean that your protection is better—just that
it will last longer. DEET concentrations higher
than 50% do not increase the length of protection.
Choose a repellent that provides protection for
the amount of time that you will be outdoors.
|
