Lassa fever is an animal-borne, or zoonotic, acute viral disease. It is endemic in some parts of West Africa including Sierra Leone, Liberia, Guinea and Nigeria. Neighboring countries are also at risk, as the animal vector for Lassa virus, the “multimammate rat” (Mastomys natalensis) is distributed all through the region.
The disease was discovered in 1969 and is named after the town in Nigeria where the first cases took place.
An estimated 100,000 to 300,000 cases of Lassa fever occur annually, with roughly 5,000 deaths. Surveillance for Lassa fever is not consistent; therefore, these estimates are crude. In some areas of Sierra Leone and Liberia, approximately 10-16% of patients admitted to hospitals annually have Lassa fever, demonstrating the serious imapct the disease has on the geographical region.
The Lassa virus is transmitted to humans by infected multi-mammate rats, the mastomys natalensis species complex which is the reservoir host. People become infected from direct contact with the urine and feces of the rat which has the virus, through touching soiled objects, eating contaminated food, or exposure to open cuts or sores. Transmission from person to person can take place following exposure to the virus in the form of blood, tissue, urine, faces or other bodily secretions of an infected person. Hospital-acquired (nosocomial) transmission from person to person are very common, and importantly can happen if appropriate Personal Protective Equipment (PPE) is not worn when managing suspected cases.
Lassa fever appears with symptoms and signs similar to those of many febrile diseases, thus making it difficult to diagnose clinically. The incubation time is between 6-21 days. It leads to a syndrome characterized by fever, muscle aches, sore throat, nausea, vomiting, chest and abdominal pain. Although fever is not a steady symptom, Lassa fever should be suspected in patients with fever (≥380C) not responding adequately to normal anti-malarial and antibiotics. It should also be suspected in any outbreak setting with patients showing a compatible syndrome. It also likely has a vast spectrum of disease presentation, from relatively mild illness to severe haemorrhagic manifestations. Case definitions can be used as guide for diagnosis.
Lassa fever is most often examined by using enzyme-linked immunosorbent serologic assays (ELISA), which identifies IgM and IgG antibodies as well as Lassa antigen. Reverse transcription-polymerase chain reaction (RT-PCR) can be used in the early phases of the disease. The virus itself can be cultured in 7 to 10 days, but this procedure must only be done in a high containment laboratory with excellent laboratory practices. Immunohistochemistry, carried out on formalin-fixed tissue specimens, can be used to make a post-mortem diagnosis.
Ribavirin an antiviral medicine, has been used with success in Lassa fever patients. It has been shown to be most effective when given early in the course of the disease. Patients should also get supportive care consisting of maintenance of appropriate fluid and electrolyte balance, oxygenation and blood pressure, as well as medication for any other complicating infections.
Primary transmission of the Lassa virus from its host to man can be prevented by avoiding contact with Mastomys rodents, especially in the geographic regions where outbreaks do occur. Putting food away in rodent-proof containers and keeping the home clean help to put off rodents from entering homes. Using these rodents as a food source should be discouraged. Trapping in and around homes can help limit rodent populations; however, the wide distribution of Mastomys in Africa makes total control of this rodent reservoir impractical.
When giving care to patients with Lassa fever, further transmission of the disease through person-to-person contact or nosocomial routes can be prevented by taking preventive measures against contact with patient secretions. Such measures may include wearing protective clothings, masks, gloves, gowns and goggles etc., using infection control measures, like sterilization of equipments; and isolating infected people from contact with unprotected persons until the infection has run its course.
Further, educating people in high-risk areas about ways to reduce rodent populations in their homes will help in the control and prevention of Lassa fever. Others include developing more rapid diagnostic test kits and increasing the availability of the drug treatment like ribavirin. Research is currently under way to develop a vaccine for Lassa fever.
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