Lassa fever or lassa is an acute haemorrhagic viral disease caused by lassa virus (LASV), an Arenavirus in the viral family Arenaviridae. The natural host of LASV is the multimammate rat known as Mastomys natalensis. M. natalensis is the rodent species that naturally harbour LASV; and the rat is found throughout West Africa – thus making the disease to be endemic in the region. Lassa fever is geographical distributed in West Africa (including Nigeria, Liberia, Sierra Leone and Guinea); and the causative agent of the disease is responsible for several outbreak of Lassa fever in the region. The disease can be mild to severe in occurrence, and in some cases it can be fatal and thus cause the death of infected victims. Lassa fever is characterized by severe systemic febrile illness with changes in vascular permeability and vasoregulation; and the disease is also associated with bleeding as is obtainable with Ebola virus disease. The case fatality rate (CFR) of Lassa fever is 15-20 % but it can reach 50 % depending on the severity of the outbreak.
Lassa virus was first discovered in 1969 in a town known as Lassa in the Northern part of Nigeria. The first case of Lassa fever originated in this town, and thus the disease was named after the town. Lassa fever killed two missionary nurses stationed in the region (Lassa town) where the disease was first discovered. Lassa virus (LASV) like the Ebola virus is zoonotic in nature (i.e. it is animal-borne and can be transmitted from infected animals to humans). Lassa fever has caused quite a few epidemic outbreaks in several West African countries. According to the World Health Organization (WHO) and the Center for Disease Prevention and Control (CDC), about 100,000 to 300,000 people are supposed to be infected annually by LASV in the West African sub region. Also, over 5000 deaths due to LASV infection occur annually in West African countries.
PATHOGENESIS OF LASSA FEVER
The symptoms of Lassa fever include malaise, back pain, sore throat, diarrhea, vomiting, unexplained fever, conjunctivitis, severe prostration, swelling of the face and mucosal bleeding. Human infection with LASV occurs when human have direct contact with the body fluids of the rodent reservoir of the virus such as urine and animal droppings. The rodent rat urinates and defecates as it moves around, and its fecal droppings and urine can contaminate raw food meant for human consumption especially when the foods are left uncovered. Though close body contacts are possibly required for the case-to-case transmission of LASV amongst human population; casual body contact with lassa virus infected individuals is not a usual route of disease transmission. However, blood contacts with infected individuals can aid in the transmission of LASV. Another source of transmission of LASV is via the inhalation of contaminated air especially in rat infested homes. Nosocomial transmission of LASV amongst healthcare workers and patients is possible through the use of hospital equipments and reused invasive devices such as needles contaminated by body fluids of infected patients. The incubation period of the infection is usually 7 to 21 days. LASV has a broad tissue/cell tropism i.e. it affects several tissues/cells of the host body including the liver, kidney and the nervous system.
LABORATORY DIAGNOSIS OF LASSA FEVER
The prompt and accurate diagnosis of LASV infection is critical in administering the proper type of therapy to affected individuals since the pathogenesis of the disease is usually sporadic and unpredictable in occurrence. The laboratory diagnosis of Lassa fever is usually done using enzyme linked immunosorbent assay (ELISA). ELISA is used to detect immunoglobulins (particularly IgM and IgG antibodies) produced by the host’s immune system against the virus. It can also be used to detect lassa virus antigens. Tissue/cell culture techniques can also be employed in the laboratory diagnoses of LASV infection. And LASV can also be directly detected from clinical samples such as blood using reverse transcription-polymerase chain reaction (RT-PCR). Nevertheless, RT-PCR is mainly used in reference laboratories for research purposes aimed at studying and understanding the pathogenesis and/or pathology of LASV infection in humans.
TREATMENT OF LASSA FEVER
LASV infection is usually treated with the antiviral drug ribavirin. Ribavirin (which is chemically known as: 1-β-D-ribofuranosyl-1, 2, 4-triazole-3-carboxamide) is a synthetic nucleoside analog of guanosine. Its mode or mechanism of action is based on the ability of the agent to interfere with the biosynthesis of guanylic acid nucleotides in the pathogenic virus; and thus interfere with the synthesis of RNA in the organism. Ribavirin is generally an inhibitor of RNA synthesis; and the drug is also used to treat other viral infections including those caused by parainfluenza viruses, Influenza viruses, Bunyaviruses, Picornaviruses and other Arenaviruses. The trade name for ribavirin in the market is virazole. Ribavirin also increases the rate of mutation in the RNA of the infecting virus; and the drug is most active when it is given at the early stage of LASV infection. No vaccine currently exists for the prevention of LASV infection in humans, but vaccine development for the effective control and prevention of the disease is still underway.
Human convalescent serum has also been found to be effective in the treatment of LASV infection in humans. As is applicable with Ebola virus infection, LASV infection in humans also require additional supportive therapy aside the administration of the antiviral drug ribavirin to the infected patient. Since LASV infected patients undergo severe gastrointestinal disorder leading to the loss of excess fluids form the body, it is important to ensure proper fluid and electrolyte replacement to avoid hypotension and shock that may cause the death of the affected individuals. Any other secondary or complicating infections should be treated alongside the Lassa fever disease; and patients infected with LASV should be placed on proper oxygenation and their blood pressure should be regularly checked as part of the supportive therapy administered.
PREVENTION AND CONTROL OF LASSA FEVER INFECTION
The people that are most at risk at getting the LASV infection are those who live in endemic regions where the disease is widespread especially in regions with high population of the natural host of the virus (i.e. M. natalensis multimammate rat). Hospital personnel’s who observed the requisite preventive measures when handling LASV infected individuals are less likely to become infected. The main preventive measure is avoiding contact with the multimammate rat that naturally harbours the virus. Barrier nursing techniques and proper isolation methods as well as using the correct personal protective equipments (including eye goggles, hand gloves, masks and gowns) both in the hospitals and in the field in the event of LASV disease outbreak is paramount to prevent outbreak or disease spread amongst healthcare workers. Food meant for human consumption should be put away or stored in rodent-proof containers so that the rat does not defecate or urinate on them.
Though the complete eradication of the multimammate rat in West Africa may not be feasible due to its high population in the region; the use of traps around the house can help to reduce their population in a given area. And homes especially in the rural areas should always be kept clean so that these rats do not find a haven to stay and transmit the LASV. In addition, the proper enlightenment and public awareness of people in LASV endemic regions in West Africa will go a long way in preventing and controlling the disease especially in the area of educating the people about sustainable measures that can be employed in keeping the natural reservoir of LASV (i.e. M. natalensis multimammate rats) at bay. Such measures aimed at reducing the population of the rats around residential areas will help to reduce the spread of the virus in LASV endemic regions.
Acheson N.H (2011). Fundamentals of Molecular Virology. Second edition. John Wiley and Sons Limited, West Sussex, United Kingdom.
Alan J. Cann (2005). Principles of Molecular Virology. 4th edition. Elsevier Academic Press, Burlington, MA, USA.
Alberts B, Bray D, Johnson A, Lewis J, Raff M, Roberts K and Walter P (1998). Essential Cell Biology: An Introduction to the Molecular Biology of the Cell. Third edition. Garland Publishing Inc., New York.
Balows A, Hausler W, Herrmann K.L, Isenberg H.D and Shadomy H.J (1991). Manual of clinical microbiology. 5th ed. American Society of Microbiology Press, USA.
Barrett J.T (1998). Microbiology and Immunology Concepts. Philadelphia, PA: Lippincott-Raven Publishers. USA.