Background: Lassa fever is caused by a viral haemorrhagic arenavirus that affects two to three million people in West Africa, causing a mortality of between 5,000 and 10,000 each year. The natural reservoir of Lassa virus is the multi-mammate rat Mastomys natalensis, which lives in houses and surrounding fields. With the aim of gaining more information to control this disease, we here carry out a spatial analysis of Lassa fever data from human cases and infected rodent hosts covering the period 1965–2007. Information on contemporary environmental conditions (temperature, rainfall, vegetation) was derived from NASA Terra MODIS satellite sensor data and other sources and for elevation from the GTOPO30 surface for the region from Senegal to the Congo. All multi-temporal data were analysed using temporal Fourier techniques to generate images of means, amplitudes and phases which were used as the predictor variables in the models. In addition, meteorological rainfall data collected between 1951 and 1989 were used to generate a synoptic rainfall surface for the same region. Methodology/Principal Findings: Three different analyses (models) are presented, one superimposing Lassa fever outbreaks on the mean rainfall surface (Model 1) and the other two using non-linear discriminant analytical techniques. Model 2 selected variables in a step-wise inclusive fashion, and Model 3 used an information-theoretic approach in which many different random combinations of 10 variables were fitted to the Lassa fever data. Three combinations of absence:presence clusters were used in each of Models 2 and 3, the 2 absence:1 presence cluster combination giving what appeared to be the best result. Model 1 showed that the recorded outbreaks of Lassa fever in human populations occurred in zones receiving between 1,500 and 3,000 mm rainfall annually. Rainfall, and to a much lesser extent temperature variables, were most strongly selected in both Models 2 and 3, and neither vegetation nor altitude seemed particularly important. Both Models 2 and 3 produced mean kappa values in excess of 0.91 (Model 2) or 0.86 (Model 3), making them ‘Excellent’. Conclusion/Significance: The Lassa fever areas predicted by the models cover approximately 80% of each of Sierra Leone and Liberia, 50% of Guinea, 40% of Nigeria, 30% of each of Coˆ te d’Ivoire, Togo and Benin, and 10% of Ghana.

/pdf/risk-maps-of-lassa-fever-in-west-africa-4wt9t1t706.pdf

Risk maps of Lassa fever in West Africa.

Lassa fever is a severe febrile illness of man, first recognized in West Africa in 1969 During an epidemic in Sierra Leone, Lassa virus was isolated for the first time from wild rodents of Mastomys natalensis A high prevalence of infected Mastomys was found in houses occupied by patients with Lassa fever The data presented provide the first demonstration of an extra-human cycle of Lassa virus transmission and suggest that rodent control may be an effective method of limiting the disease

http://science.sciencemag.org/content/sci/185/4147/263.full.pdf

Lassa Virus Isolation from Mastomys natalensis Rodents during an Epidemic in Sierra Leone

We measured levels of virus in sequential specimens from 137 patients with Lassa fever. The probability of fatal disease increased significantly with the level of viremia measured either on admission or during the course of illness. The odds ratio of death in patients with viremia greater than 10 TCID50/ml was 3.7 (90% confidence interval, 1.9-7.2). The same ratio in patients with viremia greater than 10 TCID50/ml and with levels of aspartate aminotransferase greater than or equal to 150 IU/liter was 21.5 (95% confidence interval, 5.2-99.0). Virus was found in throat cultures from 39% of viremic patients, compared with 14% of nonviremic patients (P less than .002); however, the level of virus was usually less than or equal to TCID50/ml. Fewer than 3% of patients were viruric during acute illness, and virus was isolated from three of three samples of cerebrospinal fluid. On admission, 53% of patients had IgG antibodies, and 67% had IgM antibodies. Recovery was not associated with the presence of either IgG or IgM. Virus was isolated from greater than 100 serum specimens that also contained high titers of IgG. Clinical Lassa fever was shown to be a disseminated systemic, primary viral infection, with an outcome highly associated with viremia but not with development of antibody.

Clinical Virology of Lassa Fever in Hospitalized Patients

A restricted, hospital-centred outbreak of Lassa fever involving 28 persons occurred early in 1970 in Jos, Nigeria 24 patients were in hospital Every patient, save the index, could be traced to some prior known human source of infection Complement-fixing antibody to Lassa virus was detected in the sera of only 4 of 172 family and hospital contacts bled These 4 persons had all been ill Animal studies have so far failed to reveal a non-human host

Lassa fever. Epidemiological aspects of the 1970 epidemic, Jos, Nigeria.

A large outbreak of haemorrhagic fever (subsequently named Ebola Fever) occurred in the Southern Sudan between June and November 1976. There was a total of 284 cases; 67 in the source town of Nzara, 213 in Maridi, three in Tembura, and one in Juba. The outbreak in Nzara appears to have originated in the workers of a cotton factory. The disease in Maridi was amplified by transmission in a large, active hospital in that town. The overall mortality rate was 53". Transmission of the disease required close contact with an acute case and was usually associated with the act of nursing a patient. The incubation period was between 7 and 14 days. Although the link was not well established, it appears that Nzara could have been the source of infection for a similar outbreak in the Bumba zone of Zaire. In June and July 1976, a severe viral haemorrhagic fever erupted in Nzara in Western Equatoria Province of Southern Sudan. From Nzara, the disease spread to Tembura, Maridi, Juba, and Khartoum in the Sudan and also involved the Bumba zone of northern Zaire where a large outbreak followed.

Ebola virus haemorrhagic fever

Lassa virus was isolated from specimens collected from 12 of the 13 patients involved in an epidemic of Lassa Fever which occurred in Jos during 1970. Sero-epidemiological studies indicate that Lassa virus infection occurs over a wide area of western Africa extending at least from Guinea to Nigeria.

Lassa fever virological and serological studies

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Lassa fever virological and serological studies