Lassa fever: Meaning (information, definition, explanation, facts)

Lassa fever is an acute viral hemorrhagic fever first described in 1969 in the Nigerian town of Lassa in the Yedseram River valley. Clinical cases of the disease had been known for over a decade ealier but not connected with this viral pathogen.
The infection is endemic in West African countries, causing many deaths. Outbreaks of the disease have been observed in the following countries:

• Nigeria
• Liberia
• Sierra Leone
• Guinea
• Central African Republic

but it is believed that human infections exist also in:

• Democratic Republic of the Congo
• Mali
• Senegal

It is also the most common hemorrhagic fever that is exported beyond its endemic area to countries like the United States, United Kingdom, The Netherlands, Japan and Israel.

The virus and epidemiology

The virus belongs to Arenaviridae family; it is a enveloped, single-stranded, bisegmented RNA virus. It has been determined that the virus is zoonotic (transmitted from animals), and that it spreads to man from rodents, multimammate rats (Mastomys natalensis). This is probably the most common rodent in equatorial Africa, ubiquitous in human housholds and as a delicacy eaten by up to 90% of people in some areas. In these rats infection is in a persistent asymptomatic state that is well tolerated by the animals. The virus is shed in their excreta.

In fatal cases Lassa fever is characterized by impaired or delayed cellular immunity leading to fulminant viraemia.

The dissemination of the infection can be assessed by prevalence of antibodies to the virus in populations of:

• Sierra Leone 8-52%
• Guinea 4-55%
• Nigeria approx. 21%

Unlike other hemorrhagic fevers Lassa fever can be transmitted human-to-human. It can be contracted by an airborne route or with direct contact with infected human material: blood, urine, or semen.

The route of infection to humans is by respiratory tract or by gastrointestinal tract when someone comes in contact with excrement of an infected rodent or eats some food contaminated with animal excrement. Inhalation of tiny particles of infective material (aerosol) is believed to be the most significant. It is also possible to acquire the infection through broken skin or mucous membranes that are directly exposed to infective material. Another route of transmission is man-to-man transmission when someone comes in close contact with an disease-stricken person. The virus is excreted in semen for three months after infection, however it has not been established how frequently it may be transmitted through sexual contact.
Similarly healthcare personnel can contract the disease while caring for Lassa fever patients.

In 80% of cases the disease is inapparent but in the remaining 20% it takes a complicated course. It is estimated (no definitive data exist) that the virus is responsible for about 5000 deaths annually.

After an incubation period of 6-21 days, an acute illness with multiorgan involvement develops. Nonspecific symptoms include fever, facial swelling, and muscle fatigue as well as conjunctivitis and mucosal bleeding. The other symptoms arising from the affected organs are:

• Gastrointestinal tract
  • nausea
  • vomiting (bloody)
  • diarrhea (bloody)
  • stomach-ache
  • constipation
  • dysphagia (difficulty swallowing)
  • hepatitis
• Cardiovascular system
  • pericarditis
  • hypertension
  • hypotension
  • tachycardia (heart rate increase)
• Respiratory tract
  • cough
  • chest pain
  • dyspnoea
  • pharyngitis
  • pleuritis
• Nervous system
  • encephalitis
  • meningitis
  • unilateral or bilateral hearing deficit
  • seizures

Clinically Lassa fever infections are hard to distinguish from febrile illnesses such as malaria and other viral hemorrhagic fevers such as Ebola.

The virus is excreted in urine for three to nine weeks and in semen for three months.

There is a range of laboratory investigations that are performed to diagnose the disease and assess its course and complications. ELISA test for antigen and IgM antibodies gives 88% sensitivity and 90% specificity for the presence of the infection. Other laboratory findings in Lassa fever:

• lymphopenia
• thrombocytopenia
• raised aspartate aminotransferase

Control of the Mastomys rodent population is impractical, so measures are limited to keeping rodents out of homes and general food and personal hygiene. Gloves, masks, white coats, and goggles are advised while in contact with or caring for a sick person. No vaccine is available yet, although it is being worked on. The Mozambique virus closely resembles Lassa fever, while lacking its deadly effects. This is being considered for possible use as a vaccine.

All persons suspected of Lassa fever infection should be admitted to isolation facilities and their body fluids and excreta properly disposed of.

Early and aggressive treatment using ribavirin was pioneered by Joe McCormick, starting in 1979. After extensive testing, it was determined that early administration is critical to success. Additionaly, it is almost twice as effective when given intravenously as when taken by mouth. The drug interferes with the virus metabolism, decreasing its replication. The drug is relatively inexpensive, but the cost of the drug is still very high for many of those in poverty-stricken West African states. Fluid replacement, blood transfusion and fighting hypotension is usually required.

About 15%-20% of hospitalized patients with the illness will die. It is guessed that the overall mortality is 1%, however during epidemics mortality can be as high as 50%. Thanks to new treatment with ribavirin there are great prospects for improvement. Needless to say that the vaccine would be the best option.

The terrorist attack on 11 September 2001 and threat of biological warfare attack alerted governmental agencies and scientists. Lassa fever virus is also regarded as a possible biological weapon.