ARBOVIRUSESand viral
HAEMORRHAGIC FEVERS
ARBOVIRUSES
DISTRIBUTION
They are mostly found in the warmer parts of the world (tropics) but some
range into temperate regions. Rapid transport today means every country is
at some risk.
NATURE
The majority are zoonoses maintained silently in nature by their animal
(vertebrate) hosts which form their natural reservoir. In their normal hosts
they have usually adapted well and do not produce overt disease. They produce
high blood titres of virus which favours their chances of being picked up
by a blood-sucking insect vector. Within that vector they can multiply again,
which favours their chances of being transmitted from the insect vector to
a new member of their definitive animal host. Crossing into non-normal hosts
may produce disease.
HUMAN INFECTION
Man usually forms an accidental, non-normal host, with sporadic - end
point’ infections which have no further transmission. However, there
are some notable exceptions to this generalisation, where human infection
and disease can be spread to other humans via human-feeding insects or even
directly person to person. Human infections thus tend to be isolated or sporadic,
unless climatic conditions have favoured large-scale vector proliferation
when human (and animal) infections may become seasonal and epidemic. Some
vaccines but no specific treatments are available.
TRANSMISSION
Climate has a major role in facilitating transmission, eq. mosquito proliferation
after heavy (warm) rains. Viral replication only occurs in the insect vector
in warm conditions (>25’C). Human activity can bring man into contact
with an otherwise silent cycle in nature, eg. clearing of forests, agriculture.
Creating vector promoting activities, such as dams and irrigation, may enhance
human arbovirus disease potential. Within the vector the virus multiplies
without making the insect sick and in some viruses there is evidence there
may be trans-ovarial (vertical) spread, ie. the virus can overwinter from
one season to the next inside the insect eggs. The virus is present in the
insect saliva and the infection is lifelong in the insect.
LABORATORY
Arboviruses are single-stranded enveloped RNA viruses with haemagglutinating
properties. They grow in suckling mice and/or in cell cultures. Mouse inoculation
is hazardous to human workers and must only be done in special containment
facilities (National Institute for Virology in RSA). Diagnosis is made
by isolation of the virus (usually done in a specialist laboratory)
and by detection of IgM and IgG antibodies by haemagglutination-inhibition,
or ELISA.
CONTROL
Human disease control is limited to prevention by
Infection of man may produce a wide spectrum of disease - from subclinical infections to fulminant haemorrhagic disease with death. By and large, there are 4 major clinical patterns of disease. Any one virus characteristically tends to produce mostly one pattern, but overlapping may occur.
Major Clinical Categories of Arboviral Disease
Severe clinical disease in man attracts attention, and many arboviruses were first discovered (and still are today) by investigation of unexplained, possibly infectious, disease cases. As the investigation progressed, it was commonly found that there were milder disease cases in the vicinity of a severe disease case, aa well as subclinical cases of infection. We now know that subclinical cases are probably more common than frank disease in any one outbreak, eg. even in a yellow fever epidemic. New Arboviruses have also been discovered by intensive cultural and serological investigation of possible insect vectors (mosquitos, ticks) and possible definitive hosts (rodents, birds, small mammals, etc.).
After inoculation there is a short incubation period of a few days, and then
an abrupt onset of disease at a time of high viraemia. There is a sudden
onset of fever with chills; headache; muscle, bone and joint pains; nausea
and vomiting, and often a rash and lymphodenopothy. These clinical features
may be mild or very severe and prostrating.
The disease is typically self-limiting and short lived (few days to a week).
Occasional cases may go on to a more fulminant form (- see haemorrhagic fever
section).
Examples
In South Africa.
Wesselsbron, Chikungunya
Other than South Africa:
Almost all arboviruses may infect the CNS but some characteristically produce
an encephalitis in man.
Symptoms start with fever, progressively severe headache, nausea and vomifing,
neck stiffness, paralysis, decreased consciousness, convulsions and death
in severe cases.
Examples (none in South Africa)
Japanese B Encephalitis
A number of arboviruses (Toga viruses and Bunyaviruses) and some similar
viruses (Arenaviruses and Filoviruses) may cause an illness similar to the
rather non-specific flu-like illness described under (2) above, but which
rapidly progresses to a severe disseminated illness with a marked bleeding
tendency and multi-organ failure.
Mild and subclinical cases also do occur, but are less easily recognised.
Examples:
In South Africa:
Crimean Congo Haemorrhagic Fever
(Eastern Europe, Central and Southern Africa)
See "What is
CCHF?"
A rare disease, but the most common viral haemorrhagic fever in South
Africa.
Most frequent in Kimberley area, but antibodies are widespread in cattle
herds all over South Africa.
Transmitted by ticks with striped legs (bont-poot bosluis, Hyalomma
species).
Outside South Africa:
Illustrations and layout by Linda Stannard, updated November 1999 ©