Hyalomma ticks are known to transmit
the disease to animals and man. There is a brief but high-titre viraemic
phase in mammals, at which stage their blood is highly infectious.
Note
Antibodies to CCHF have been detected in cattle, sheep and small mammals
over a wide area in Southern Africa without evidence of disease, and antibodies
have been found in farm workers with no confirmation of Congo Fever disease.
Several human outbreaks and individual cases have been reported previously
in South Africa.
In November, 1996, a number of cases of Congo Crimean Haemorrhagic Fever
(CCHF) were identified in individuals working in an ostrich abbatoir
in the town of Oudtshoorn, Western Cape Province, South Africa.
It remained to be determined whether ostriches can
be infected (and themselves serve as an infectious source), or whether they
may merely carry infected ticks (they certainly do carry ticks) that
can be passed to humans.
See: "Some Fascinating Facts about the 1996
CCHF Outbreak," by Bob Swanepoel
What is Congo-Crimean Haemorrhagic Fever?
The following is an extract from an article in the SAMJ, Vol. 62, p576-580,
October,1982
by James H. S. Gear et al.
Congo-Crimean haemorrhagic fever
Congo-Crimean haemorrhagic fever was first observed in the Crimea by Russian
scientists in 1944 and 1945. At that time it was established by studies in
human volunteers that the aetiological agent was filtrable and that the disease
in man was associated with the bite of the tick Hyalomma marginatum.
The agent was detected in the larvae and in adult ticks, as well as in the
blood of patients during the fever. This agent, presumably a virus, was not
maintained in the laboratory and was lost.
Congo virus was first isolated in Africa from the blood of a febrile patient
in Zaire in 1956. In 1967 Simpson et al. described 12 cases of a feverish
illness of which 5 were laboratory infections; the virus was isolated by
the inoculation of blood into newborn mice. Simpson showed that these viruses
were similar to the one isolated in 1956. Casals then showed that the viruses
isolated in cases of Crimean haemorrhagic fever and the Congo virus were
serologically indistinguishable and demonstrated that other virus strains
from Central Asia, the USSR and Bulgaria were similar.
The virus has been classified as a Nairovirus in the genus Bunyavirus
in the family Bunyaviridae. It contains RNA and is inactivated by lipid
solvents and detergents.
Laboratory studies have shown that Congo virus is related to Hazara virus
isolated from ticks in Pakistan, and to Nairobi sheep disease virus; together
they form the Nairovirus group.
In Africa the virus has been isolated from a variety of animals, including
cattle, sheep, goats, hares and hedgehogs, and from a number of ticks which
parasitize them, including Hyalomma sp., Amblyomma variegatum,
Boophilus decoloratus and Rhipicephalus sp.
The most important transmitters of the infection to man are species of the genus Hyalomma, the life history of which is shown in the figure below.
The larval and nymphal stages of some species parasitize birds, including
migratory birds, some of which fly from south-eastern Europe to South Africa
and thus may carry the infection over long distances. To verify whether this
actually happens will require further study of the ticks and their hosts
in South Africa and on their way from Europe.
Clinical picture
The infection is usually transmitted to man by the bite of a tick, but an increasing number of cases have occurred among the medical and nursing staff caring for patients in hospital and in laboratory personnel carrying out investigations of these patients. In these cases the infection has apparently been acquired by contagion, particularly by contact with the patient's blood or blood-contaminated specimens. Exposure to the blood of infected animals, especially cattle and sheep, has led to severe and often fatal infections.
The incubation period is 2 - 7 days. The onset of the illness is sudden, with fever, chills, severe muscular pains, headache, vomiting and pain in the epigastric and lumbar regions. A haemorrhagic state develops from the 3rd to the 5th day and manifests as petechial haemorrhages or purpura in the skin, and bleeding from the mucous membranes manifests as epistaxis, haemoptysis, haematemesis, melaena and haematuria. At this stage the conjunctivae are injected, the face is flushed and the tongue is dry, often coated with dry blood. The pulse is slow in the beginning, but with continuing loss of blood becomes fast and feeble; the blood pressure drops and the heart sounds become weak - clear signs of impending shock and vascular collapse. The liver is enlarged and tender and there is tenderness over the epigastrium and splenic region. In patients who recover, the temperature falls between the 10th and the 20th day and bleeding stops, but convalescence is prolonged up to 4 weeks or longer. In fatal cases, death from massive haemorrhage and cardiac arrest occurs, usually 7 - 9 days after the onset of the illness. Massive haemorrhage into the gastro-intestinal tract, with scattered haemorrhages into the viscera, is found at autopsy.
The diagnosis is suggested on clinical grounds when the patient has a history
of a tick bite or of exposure to ticks in the environment, and after an
incubation period of 2 - 7 days develops an illness of sudden onset of muscle
pains, headache fever and a rapidly evolving severe illness with the development
of a haemorrhagic state with bleeding from the mucous membranes and petechiae
in the skin, associated with thrombocytopenia and leucopenia.
The diagnosis may be confirmed in the laboratory by intracerebral inoculation
of baby mice with blood of a patient; the mice sicken about 1 week after
inoculation. The virus is identified by using known specific Congo virus
antiserum in an immunofluorescent test. The development of antibodies in
patients' serum as the illness progresses may be demonstrated in
immunofluorescent tests using chamber slides with tissue culture cells infected
with Congo virus.
The authors are grateful to Professor 0. W. Prozesky, Director of the National Institute for Virology, and to Dr R. Swanepoel, Dr K. Struthers, Mrs E. Rossouw and Miss G. McGillivray, staff members of the high-security laboratory, and to Dr P. Jupp of the Arbovirus Unit of the National Institute for Virology for undertaking, as a matter of urgency, the investigations which led to the incrimination of the Congo virus as the cause of this patient's fatal illness. The authors are grateful to Mrs M. Anderson, who prepared the chart of its life cycle.
REFERENCES