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More than 100 viruses can cause encephalitis or haemorrhagic
fever. Almost all are zoonoses, where the human is an accidental
host that has come into contact with the natural life cycle. They
are transmitted by direct contact with blood and body fluids or by
the bite of arthropods, such as mosquitoes, ticks and sandflies.
Some infections are associated with a high mortality.
Rabies is a rhabdovirus infection that, once symptoms develop,
causes a fatal encephalomyelitis.
- It is a bullet-shaped, negative-sense RNA enveloped virus.
- It infects warm-blooded animals worldwide.
- The virus is found in saliva and is transmitted to humans through
the bite of an infected animal.
- Two epidemiological patterns exist: urban rabies, which is transmitted
by feral and domestic dogs; and sylvatic rabies, which is
endemic in small carnivores in the countryside. Dog-bites are
responsible for most infections.
- Bats, raccoons and skunks are an important reservoir and vector
of infection in the Americas; the red fox is the reservoir of infection
- The virus enters via the motor endplates, spreading up the axons
to enter the brain. Sites with short neural connections to the
central nervous system have the shortest incubation period (7
days), whereas a bite on the foot may have an incubation period
of 100 days.
- Bite depth and viral inoculum also influence the incubation
A prodromal fever, nausea and vomiting precede disease, which
takes one of two forms: furious rabies (hyperexcitability, hyperreactivity,
hydrophobia) or dumb rabies (an ascending paralysis).
Disease is progressive and inevitably fatal. Diagnosis is based on
the clinical and epidemiological features, confirmed by specific
fluorescence in corneal scrapings, by brain biopsy, or by the finding
specific rabies antibody.
The disease may be prevented by pre-exposure vaccination, wound
care, local antiserum, systemic hyperimmunoglobulin and a postexposure
vaccination course with the human diploid cell vaccine. Preexposure
vaccination is reserved for those in a high-risk group (e.g.
vets and travellers to remote regions of endemic countries).
Yellow fever virus is a flavivirus, an enveloped positive-sense RNA
virus, transmitted by Aedes aegypti. Yellow fever is a zoonosis in
which humans are an accidental host (sylvatic disease), but an
urban cycle results in periodic human epidemics.
- Infection may be asymptomatic or may cause acute hepatitis and
death from hepatic necrosis.
- A short incubation period is followed by fever, nausea and vomiting,
and later by jaundice.
- Haemorrhagic manifestations may develop and vomitus may be
black because of digested blood (vomito negro).
- The mortality rate is high, but patients who recover do so
Diagnosis is clinical, supported by nucleic acid amplification test
(NAAT), culture and serology. Disease prevention is by mosquito
control and vaccination with the live attenuated vaccine.
Japanese B encephalitis
- Dengue is a flavivirus related to yellow fever virus with four
- It is transmitted by Aedes mosquitos.
- The incubation period is 2-15 days.
- It is found throughout the tropics and the Middle East.
- Epidemics occur when a new serotype enters the community or
a large number of susceptible individuals move into an endemic
area. Urban epidemics can be explosive and severe.
- Common features include a sudden onset of fever and chills, and
headache with pains in the bones and joints. The fever may be
biphasic and a mild rash may also be present.
- Dengue haemorrhagic syndrome causes severe shock and bleeding
with mortality of 5-10%.
- Diagnosis is by NAAT, serology or culture.
- Prevention is by mosquito control.
- Treatment is symptomatic.
West Nile virus
- This is a mosquito-borne flavivirus infection that causes encephalitis
with a high mortality.
- The natural reservoir is in pigs.
- It causes abrupt-onset fever and severe headache, nausea and
vomiting. Convulsions can occur.
- There may be permanent cranial nerve or pyramidal tract
- Prevention is by vaccination.
This is a flavivirus infection from Africa, which has been found in
North America since 1999 and has spread across the continent into
Canada, Latin America and the Caribbean, that causes an
Ebola and Marburg virus
- Lassa fever is a severe haemorrhagic fever caused by an
- It is endemic in West Africa.
- It is transmitted from house rats to humans and from person to
person by contact.
- Patients present with fever, mouth ulcers, myalgia and haemorrhagic
- Diagnosis is clinical and depends on exposure history.
- Confirmation is by NAAT or serology.
- Ribavirin improves outcome if given early and may be given as
postexposure prophylaxis to contacts.
- Special isolation is required in hospital.
- These viruses are found in Africa and are transmitted to humans
from primates or from a rodent reservoir.
- They cause haemorrhagic disease with high fever and
- They may be transmitted in the hospital environment.
- Treatment is supportive and with hyperimmune serum.
- Control is not possible as the reservoir is not confirmed.
- Special isolation is required in hospital.
- A vaccine using vesicular stomatitis virus encoding Ebola and
Marburg antigens is in development.
This bunyavirus infection is transmitted to humans from rodents
and causes either a haemorrhagic fever with renal failure or hantavirus
pulmonary syndrome. The disease occurs widely throughout
the world. Person-to-person spread does not appear to take place.
The incubation period is 2-3 weeks, followed by fever, headache,
backache and injected conjunctiva and palate. Hypotension, shock
and oliguric renal failure follow. The mortality rate is about 5%.
Diagnosis is based on NAAT, serology and culture.
Nipah and Hendra virus
Nipah virus, a paramyxovirus, causes severe disease in humans
and animals. It is found in South Asia and causes febrile encephalitis
with a high mortality rate. The reservoir is probably fruit bats,
with human infection from contact with bats or an intermediate
animal host such as pigs. Person-to-person spread occurs. The
related, rarer Hendra virus is also acquired from bats and causes
an influenza-like syndrome or encephalitis.