Lassa fever is an animal-borne (zoonotic) virus that is transmitted via contact with contaminated rat urine or feces. Rural regions of the West African countries Nigeria, Sierra Leone, Guinea, and Liberia known as the “Lassa belt” experience intermittent ongoing outbreaks of Lassa fever. Following infection with Lassa virus, 80% of persons remain symptom free, or develop mild symptoms, while the remaining 20% develop a more severe illness. Symptoms increase in severity as infection progresses, with neurological problems and sometimes death occurring in the later stages. Lassa fever is treated via antiviral drugs in addition to symptom management. As no vaccine is available, prevention methods focus on avoiding contaminated material, avoiding rats, and taking precautions while in close contact with infected people.
Following peace agreements within endemic countries previously upset by civil unrest, progress has been made in the treatment and prevention of Lassa fever. Furthermore, work is underway to improve diagnostic testing for Lassa fever, as well as to discover a vaccine for the virus.
Lassa fever was first described in the 1950s, although the virus responsible for the infection was not identified until 1969, when missionary nurses in Nigeria, West Africa, died from an infection caused by a virus identified as the Lassa virus. Lassa virus is a member of the Arenaviridae family and is transmitted to humans via contact with infected urine or droppings of certain species of rats.
Rats from the genus Mastomys are the reservoirs of Lassa virus. They are efficient hosts due to their high frequency of breeding and large number of offspring. Furthermore, they tend to colonize human habitats, increasing the chances of human exposure. Mastomys rats become infected with Lassa virus, but do not become ill from it. Humans become infected following exposure to infected rat excreta, either directly or indirectly. The virus is transmitted when humans touch objects or eat food that is contaminated with rat excreta, or when excreta comes in contact with cuts and sores. In addition, inhaling small particles of excreta in the air transmits the virus, as does consuming infected rats as food. Lassa fever is also transmitted between humans. This occurs following contact with infected body fluids such as blood, excretions, secretions, and tissues from infected humans.
Lassa fever is asymptomatic or mild in 80% of infected people. However, the remaining 20% experience severe disease in which many organs within the body are affected. Symptoms include fever, aches, vomiting, diarrhea, conjunctivitis (inflammation, redness of the conjunctiva of the eye), facial swelling, protein in the urine, and mucosal (mucous membranes such as in the nose and mouth) bleeding. Symptoms increase in severity as the disease progresses, leading to neurological problems such as hearing loss, tremors, and coma in the later stages. Symptoms usually take one to three weeks to appear, and generally last for one to four weeks. Mortality rates have been estimated to be 1% in total, and up to 15% in hospitalized patients. In fatal cases, death usually occurs within two weeks following the arise of symptoms.
Lassa fever occurs predominantly in West Africa. While it is endemic (occurs naturally) in certain regions, such as Guinea, Liberia, Sierra Leone, and Nigeria, the disease may exist in adjoining regions due to the wide distribution of the host rodent species. Imported cases of Lassa fever have been reported from the United States where, in both cases, the patients were travelers who had returned from endemic regions of West Africa.
The number of annual infections within West Africa is estimated to be between 100,000 and 300,000, and annual deaths from the disease number about 5,000. As disease surveillance for Lassa fever is not uniformly undertaken, these estimates are rudimentary and subject to error. According to the Centers for Disease Control and Prevention (CDC), Lassa fever tends to be restricted to the rural regions of West Africa, particularly in areas where humans live in close proximity to the rats that are the main reservoir of the virus. Infections have also occurred as a consequence of laboratory exposure elsewhere in the world.
The people who tend to be most at risk of infection with Lassa virus are those who reside in areas with high densities of Mastomys rats, or those who come in contact with infected humans. Therefore, populations living in rural areas in which rat populations are high, as well as hospital staff in these areas, are at the greatest risk. However, hospital staff greatly reduce their risk by taking preventative measures including standard and isolation precautions in order to avoid contact with the virus.
WORDS TO KNOW
ENDEMIC: Present in a particular area or among a particular group of people.
RESERVOIR: The animal or organism in which the virus or parasite normally resides.
SPECIAL PATHOGENS BRANCH: A group within the U.S. Centers for Disease Control and Prevention (CDC) whose goal is to study highly infectious viruses that produce diseases within humans.
ZOONOSES: Zoonoses are diseases of microbiological origin that can be transmitted from animals to people. The causes of the diseases can be bacteria, viruses, parasites, and fungi.
Lassa fever is treated using the anti-viral drug ribavirin. This drug has been shown to be effective against early stages of Lassa fever, but does not appear to be as effective if given during the later stages of the illness. In addition to drug treatment, patients should also receive supportive care. This includes caring for the fever symptoms, and maintaining fluid and electrolyte balance, along with blood pressure and oxygenation levels.
There is no vaccine for Lassa fever, and thus, prevention consists mainly of avoiding contact with potentially contaminated materials. Contamination from rat excreta can be avoided by discouraging rats from human living quarters through removing garbage from the home, keeping cats, and maintaining clean living quarters. Furthermore, keeping food stored in rodent-proof containers prevents food becoming contaminated with infected rat excreta. People in close contact with infected persons, such as family members and health care workers can prevent contact with blood and body fluids by taking precautions such as wearing gloves, gowns, face shields, and masks while in contact with the person.
Complete eradication of Mastomys is unlikely to occur due to their high prevalence in endemic areas. Therefore, avoidance rather than eradication appears to be the most effective way of preventing infection via rat excreta. In order to achieve avoidance, good hygiene practices are being promoted within infected communities.
The symptoms of Lassa fever are common to a variety of viral fevers, and thus diagnosis is difficult and often requires diagnostic testing that can be both expensive and time consuming. Therefore, improved testing procedures would lead to increased accuracy of diagnosis and a more accurate idea of infection prevalence. Research is also being completed to develop a vaccine for Lassa fever.
Changing the cultural attitudes among traditional peoples of Sierra Leone about the nature of Lassa fever presents a continuing challenge to workers in the London-based charity Merlin's Lassa fever group. Many traditional people in Sierra Leone still consider Lassa fever an inevitable fact of life, and are hesitant to invest in efforts to reduce exposure to rats when other diseases such as malaria remain ever-present, and are often not distinguished from Lassa fever when a person becomesill. Workers with Merlin and other agencies continue to travel from village to village, conducting education campaigns that explain the particular connection between rats and Lassa fever, and discouraging trapping, eating, and sharing living areas with rats.
Exportation of Lassa fever, as well as many other diseases such as malaria and typhoid occurs when travelers pass through endemic areas and become infected. This creates the threat that these diseases will become introduced to areas previously unaffected by these diseases.
The Lassa virus is also considered a potential candidate for use as an agent of bioterrorism.
IN CONTEXT: ERADICATION PROGRAM EFFECTIVENESS
Control of Lassa fever has been set back by civil unrest within endemic countries such as Guinea, Liberia, and Sierra Leone. However, peace initiatives have led to steps being taken by these three countries to develop prevention and coping strategies for Lassa virus. These developments have been led by the formation of the Mano River Union Lassa Fever Network, which has begun enhancing diagnostic testing, improving clinical management, and performing environmental control. In addition, better care facilities are being constructed for patients suffering from Lassa fever.
SOURCE: World Health Organization (WHO)
See AlsoAirborne Precautions; Animal Importation; Antiviral Drugs; Malaria; Rapid Diagnostic Tests for Infectious Diseases; Travel and Infectious Disease; Typhoid Fever; Vaccines and Vaccine Development; War and Infectious Disease.
Arguin, P.M., P.E. Kozarsky, and A.W. Navin. Health Information for International Travel 2005–2006. U.S. Department of Health and Human Services, 2005.
Centers for Disease Control and Prevention. “Lassa Fever.” December 3, 2004 <http://www.cdc.gov/ncidod/dvrd/spb/mnpages/dispages/lassaf.htm> (accessed February 22, 2007).
tanford University. “Lassa Fever Virus.” 2005 <http://www.stanford.edu/group/virus/arena/2005/LassaFeverVirus.htm> (accessed February 22, 2007).
World Health Organization. “Lassa Fever.” April 2005 <http://www.who.int/mediacentre/factsheets/fs179/en/> (accessed February 22, 2007).
Lassa fever is a highly infectious and sometimes fatal viral disease that occurs in western Africa.
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Lassa fever is an infectious illness caused by a virus. It is named after the town in Nigeria where it was discovered. Most people infected with the virus have only mild symptoms. But one out of five people with Lassa fever becomes very ill. Lassa virus affects approximately 100,000 to 300,000 people in western Africa each year.
Lassa virus is spread to humans by the Mastomys rodent, which is found in the grasslands and forests of tropical Africa, as well as in human homes. A person can catch the virus by touching objects that have been contaminated with the urine and droppings of the rodents. It is also possible to catch Lassa virus by breathing air near rat droppings, or by eating the rats for food. In addition, person-to-person transmission is common in village settings and in hospitals.
The U.S. and the World
Lassa fever was first identified in 1969.
- Since 1969, Lassa fever has been killing about 5,000 people a year and infecting as many as 300,000 in West Africa, the only region where it is found. Those numbers may underestimate the extent of the disease, because of poor reporting in some countries.
- About 15 to 20 percent of people who are hospitalized with Lassa fever die. In some areas with high rates of Lassa fever, like Sierra Leone and Liberia, approximately 15 percent of all hospital admissions involve people with Lassa fever.
Symptoms of Lassa fever may include fever, pain in the chest, sore throat, cough, vomiting, and diarrhea. The virus is so infectious that medical personnel diagnosing the disease must take special precautions. One-third of people with Lassa fever will develop deafness that is sometimes permanent. One percent of people infected with the virus will die from it.
Lassa fever can often be successfully treated with an antiviral drug called ribavirin when it is given within the first six days of illness. Because Mastomys rodents are found all over western Africa, however, it is unlikely that the virus can be prevented by getting rid of the rats. More promising methods of prevention include educating people about how to keep their homes free of rodents and developing a vaccine for Lassa fever.
Garrett, Laurie. The Coming Plague: Newly Emerging Diseases in a World Out of Balance. New York: Farrar, Straus, and Giroux, 1994.
The World Health Organization’s Communicable Disease Surveillance and Response division posts a fact sheet about Lassa Fever at its website. http://www.who.int/inf-fs/en/factl79.html
The U.S. National Institute of Allergy and Infectious Diseases posts a fact sheet about emerging infectious diseases at its website. http://www.niaid.nih.gov/factsheets/eid.htm