LASSA - Guanarito, Junin, Machupo, Sabia Viruses

History:		In 1969 a missionary nurse working in Lassa, Nigeria contracted a severe fever during an epidemic. She died after about 2 weeks, as did a second nurse who cared for her. A third nurse was repatriated to New York, where a novel virus was isolated from her serum. She recovered after about 8 weeks. A research virologist contracted the disease, but recovered. A worker employed in the same building later became infected and died.
Virus:		Arenaviruses are ss-RNA viruses with a granular appearance on E.M., caused by dark inclusions derived from host ribosomes. The type - species is LCM (lymphoctichoriomeningitis). All are pathogens of rodents , in which they cause symptomless infection with urinary excretion.

Virus

Natural Host

Disease

Lassa

Multimammate rat

Lassa Fever

Guanarito

Venezuelan Haemorrhagic Fever

Junin

Harvest Mouse

Argentine Haemorrhagic Fever

Machupo

Harvest Mouse

Bolivian Haemorrhagic Fever

Sabia

Brazilian Haemorrhagic Fever

Clinical Features

The clinical presentation of Lassa fever is non-specific. Apart from fever, myalgia and prostration, there may be cough, sore throat, abdominal discomfort, loose stools and headache. In established disease, non-putting oedema of the neck and lower face, encephalopathy and varying degrees of haemorrhagic features are often seen.

Fatally-infected patients usually die on the seventh to tenth day. Infants may have a fulminating course with widespread oedema. Many cases are milder, and numerous asymptomatic or trivial infections probably occur in endemic areas where seropositivity rates of l5% - 40% are found in local populations.

Laboratory features

The neutrophil count and total leucocyte account is usually low, but both tend to rise rapidly with escalating tissue damage. Tissue transaminases (ALT and AST) are raised : levels about l50 - 200 iu/l indicate severe disease. Classical coagulometry tests (INR, PT, APTT) are minimally disturbed (haemorrhage is due to deficient platelet aggregation and endothelial damage).

Liver morphology shows focal haemorrhagic necrosis with acute inflammatory infiltrate.

Diagnosis

Malaria should be excluded or treated as a priority.

The high viraemia in early illness permits rapid diagnosis by PCR techniques on serum or whole blood.

ELISA tests for IgM are positive at presentation in most cases. (IFA tests are available, but are less specific).

Lassa virus is easily cultured in VERO cells, and is demonstrated by immunofluorescence.

Management

Blood, tissue and body fluids are highly infectious, so patients should be managed in isolation, with stringent precautions, and laboratory specimens should be handled with particular care in containment level 3+ or 4.

Ribavirin (Tribavirin) reduces viraemia, and also reduces mortality in severe cases. It does not abolish viraemia or viriuria in acute or convalescent cases. It is most effective if given intravenously within four days of onset of fever. A typical dose is 24mg/kg daily in three divided doses. Anaemia is a common adverse effect, which may limit the dose.

Oral ribavirin has been used for prophylaxis of Lassa fever, but no studies of its efficacy are available.

Immune plasma has not been shown effective in prophylaxis or treatment.

Public Health

 Family, health-care and laboratory contacts should be kept under surveillance for the maximum incubation period of 21 days. In the UK suspected and known cases should be referred to the HSIDU, and transferred with ambulance category 3 precautions.