Lassa fever

Among six arenaviruses so far known to cause human illness, only one, Lassa virus, is from Africa, where its reservoir is the ubiquitous rodent Mastomys natalensis. Among the arenaviruses, the Lassa fever virus, found throughout West Africa, affects by far the largest number of humans. After an incubation period of 7-18 days, Lassa fever begins insidiously with fever, weakness, malaise, severe headache, usually frontal, and a very painful sore throat, often with retrosternal chest pain, nausea, vomiting or diarrhea. Severe pulmonary edema and adult respiratory distress syndrome are common in fatal cases with gross head and neck edema, Marked lymphopenia and later relative or absolute neutrophilia occur which may reach as high as 30 x 10(9)/L in severely ill patients. Platelet function is markedly depressed or even absent. Increasing viremia is associated with increasing case fatality. Elevated viremia (> 10(3)) and AST (greater than or equal to 150 IU) together carry a risk of death of nearly 80%. Case fatality of hospitalized patients is about 16%. However, the death rate for all Lassa virus infections (including non-hospitalized) may be as low as 2-3%. The case-fatality rate may be as high as 30% in the third trimester of pregnancy. Nearly 30% of patients with Lassa virus infection suffer an acute loss of hearing in one or both ears. Fetal loss in infected pregnant women is as high as 87%, and does not seem to vary by trimester. Lassa fever is common in children, but may be difficult to diagnose because manifestations are general. In one hospital study in Sierra Leone, 21% of pediatric admissions had Lassa fever, with 12% fatalities. As many as 13% of all medical admissions to two study hospitals were acute Lassa virus infections, and 27% of medical deaths in the same two hospitals were attributable to laboratory-confirmed Lassa virus infections. The transmission of Lassa virus to humans is the result of contact between the rodent, M. natalensis, which prefers to occupy human habitats, and humans living in villages in many areas of West Africa. Rodents are infected in utero, and remain infected throughout life, excreting between 1000 and 10,000 infectious viral particles/mL of urine. Experimental data show that virtually all rodents excrete virus once infected. The earliest cases of Lassa fever were associated with hospital transmission and it continued to be associated with nosocomial transmission. Contact in households with persons ill: or recently ill with Lassa fever is also an important risk factor. In Sierra Leone, human antibody prevalence ranges from around 5% in coastal villages to 40% in secondary forest and savannah areas. In some highly endemic villages, 5-20% of susceptible (antidoby-negative) persons per year are infected, and seroconversion to Lassa virus antigen has been observed in 5-13% of all febrile illnesses in these villages. In Nigeria, the prevalence of antibody was from 5 to 25%, and the average prevalence was 21%. The key to prevention and control is either to interrupt the contact between infectious sources and susceptible persons or to avoid disease in the event of infection. A vaccine against Lassa virus has been made and tested in primates with substantial success. Patients treated with ribavirin at any time during their illness had significantly lower case-fatality than untreated patients with the same risk factors. Survival after convalescent plasma therapy was no higher than in those untreated. Ribavirin can prevent death in Lassa fever when given at any point during the illness, but is more effective when given early, and perhaps more effective given intravenously.