Lassa virus was first described in Lassa, a village of northern Nigeria, in the central region of Africa. Small outbreaks have occurred in Zorzor, Liberia, and various other parts of Nigeria. Serological evidence of human infection have been documented in Guinea, Senegal, Mali, Sierra Leone and Zaire. The virus is largely confined to West Africa, but there have been incidences where the virus has been introduced outside of Africa mainly through health care workers.

The natural reservoir of Lassa is Mastomys natalensis, a rodent prevalent throughout Africa south of the Sahara. In this rodent, Lassa causes a chronic, inapparent infection with persistent viremia. Lassa is excreted from the rat through urine and saliva. While nosocomial spread (aerosols) are rare for most arenaviridae. Lassa can be spread through aerosols or contact. Reservoir hosts are congenitally transmitted, and horizontal transmission to uninfected rats are through the mentioned secretions. Humans are primarily infected by eating an infected rat, or by eating food contaminated with rat excretions. Infection is also possible through the inhalation of dried infecte urine. Low levels of sanitation and the high prevalence of the natural reservoir favor promote the spread of Lassa virus. The most efficient mechanism of infection is by a rodent bite.

Reported cases do not favor a particular sex, and both children and adults can be infected, with a lower incidence in children. Interhuman transmission of arenaviridae is rare except in the case of Lassa. Person to person transmission is mainly via direct contact, and is possible through contamination of skin breaks with infected blood, and aerosol spreads. Hospitals and dispensaries are ideal environments for the spread of Lassa, and the worst epidemics documented have occurred in these places. As the virus is present in bodily secretions such as urine, salive, and semen, it can be spread through sexual contact.

• The number of cases of Lassa fever annually is estimated at 200000 - 400000 cases with several thousand fatal. Overall mortality among hospital cases is about 15%. In untreated cases, mortalility rises to 60%. Mortality is 2-3x higher in pregnant/post portum women, and pregnancies invariably end in abortion/neonatal death. In endemic areas such as Sierra Leone, antibodies to the virus in the population may reach 40%, and overall mortality is <2%, and hospitalized cases at 12%. Clearly, hospitals select the most severe cases, and caregivers are at high risk of infection. Patients should be treated using gloves, mask, gown, and respiratory protection against small aerosols. Patients may excrete the virus weeks after recovery, thus bodily fluids should be monitored for infectivity before hospital discharge.

INTERNATIONAL scientists are investigating the worst recorded outbreak of the highly contagious Lassa fever virus, which has killed at least 23 people and infected more than 150 others in eastern Sierra Leone since the beginning of March.

The statistics are based on confirmed cases admitted to hospital in the eastern provincial capital Kenema, and physicians believe the number infected may be much higher.

Lassa fever, initially spread by rats and then from person to person, causes fever, haemorrhaging, vomiting, chest pains and low blood pressure. It can kill within seven days: in 1996 about 25% of those infected died.

Although Lassa fever is endemic in eastern Sierra Leone, scientists are baffled by the recent huge increase in cases. About 35 are confirmed each year. There were only 10 reported cases in March last year.

One of the visiting scientists from the Centre for Disease Control in Atlanta, Georgia, said: "Since the rat population had been left undisturbed for several years in villages abandoned because of the war, there has been an explosion in their numbers. People now returning to their homes often don't have the resources to protect their food or kill rats, while overcrowding increases the risk of human transmission. But the truth is we don't exactly know the reasons why."

The disease takes its name from Lassa village in northern Nigeria, where it was first recorded in 1969. Although cases of Lassa fever appeared throughout West Africa, it was in Sierra Leone that scientists from the disease control centre first isolated the virus in rats in 1971.

"Everyone was focusing on the new killer disease," he said. "At that time there was considerable interest by both the United States and Russian armies. Although since the end of the cold war military interest in biological warfare has decreased. Since more is now known about Lassa, it would not really be useful as a biological weapon anyway."

The only known cure for Lassa fever, Ribavirin, was until recently produced by the US company Viratech, which holds the patent rights. The cost of Ribavirin shot up to R4 445 a dose when it was falsely believed to be useful in the treatment of the HIV virus and the fatal Hunter's virus.

The centre, which donates Ribavirin for the treatment of Lassa fever, says its stocks are depleted but for a small emergency supply. The other main supplier to the region, the British Medical Emergency Relief International, says its stocks are exhausted.

Two unlicensed sources of the drug have been identified in China - the Beijing and Shanghai Medical companies. The Chinese Red Cross has offered to donate a consignment of Ribavirin to the Sierra Leone government. The unlicensed form of the drug costs about R270 a dose.

Producing a Lassa vaccine is not a priority for the centre, as outbreaks of the virus have been restricted to a relatively small area. Social and political unrest in the region is considered to make investment in research commercially unviable.

The centre admits that interest in "exotic" diseases such as Lassa fever stems from fear that they could spread to America - even though there has been no transmission of secondary cases outside Africa.

Lassa fever is a recently discovered viral infection found in the tropical regions of the world, especially West Africa. Epidemics have been recognized in Nigeria, Sierra Leone and Zaire. This fever is a major public health concern because it is highly contagious and can cause a severe or fatal illness. The rapid spread of the infection has been clearly identified in the case of hospital outbreaks. The lassa virus has been found in one rodent species but the exact mode of transmission to humans is uncertain. The virus is present in all body fluids of an infected person and persists in the urine for several weeks after recovery.

The incubation period of lassa fever is 7 to 21 days. The infection begins with increasingly high fever, vomiting, cough and general weakness lasting for several days. If a person has traveled to West Africa and has a severe fever within 21 days of return, the illness should be reported to a physician who would test for lassa fever. Due to the highly contagious nature of this illness and because the mode of transmission is not known, strict isolation precautions are to be followed.

At this time, the treatment of lassa fever is symptomatic (treating the symptoms of high fever, etc.), supportive (making the infected person comfortable) and preventive (following strict isolation procedures).