**3. Epidemiology**

#### **3.1 Distribution**

Lassa fever is endemic in West Africa. However the world is now a global village and the previous geographical gap between the tropics and the developed world has been bridged by international travel. The 6 – 21 days incubation period indicates that a person who contacts Lassa fever in an endemic area in West Africa may travel to a developed country within the incubation period and cause an epidemic.

#### **3.2 Prevalence**

The prevalence of Lassa fever can be assesed by determining the prevalence of antibodies to Lassa fever in communities. The prevalence of Lassa fever in Nigeria, Guinea and Sierra Leone can be up to 21%, 55% and 52% respectively.

#### **3.3 Reservoir**

The reservoir of infection is mastomys natalensis. It is a species of rodent in the Muridae family. It is also known as the Natal multimammate rat, the common African rat, or the African soft-furred rat. It is found in Angola, Benin, Botswana, Burkina Faso, Burundi, Cameroon, Central African Republic, Chad, Republic of the Congo, Democratic Republic of the Congo, Ivory Coast, Equatorial Guinea, Ethiopia, Gabon, Ghana, Guinea, Guinea-Bissau, Kenya, Lesotho, Malawi, Mali, Mauritania, Mozambique, Namibia, Niger, Nigeria, Rwanda, Senegal, Sierra Leone, Somalia, South Africa, Sudan, Swaziland, Tanzania, Togo, Uganda, Zambia, and Zimbabwe. Its natural habitats are subtropical or tropical dry forests, subtropical or tropical moist lowland forests, dry savanna, moist savanna, subtropical or tropical dry shrubland, subtropical or tropical moist shrubland, arable land, pastureland, rural gardens, urban areas, irrigated land, and seasonally flooded agricultural land. In 1972, the Natal multimammate Mouse was found to be the natural host of the deadly Lassa fever virus.

#### **3.4 Transmission**

Lassa fever is transmitted to humans when they ingest food contaminated by the feces and urine of mastomys natalensis. Once humans are infected, transmission also occurs from human to human through contact with fluid and aerosol secretions in the form of sneezing, sputum, seminal fluid, stool, urine and blood. Vertical transmission through breast milk has been observed.

#### **3.5 Host factors**

Men are more commonly affected than women. However the case fatality rate is nearly two times higher in women. Men are more likely to buy food from food vendors especially at lunch time while women are more likely to eat personally cooked food. Contamination of food from this source may be responsible for the higher incidence of Lassa fever in men. Although the high case-fatality of Lassa fever is due to delayed cellular immunity, development of partial immunity as a result of frequent exposure to contaminated food may be responsible for the milder forms of the disease and lower case-fatality rate in men. Research is needed to find out whether Lassa fever infection confers partial or full immunity on affected people.

Lassa Fever in the Tropics 113

shock, Electrolyte imbalance, Disseminated intravascular coagulation, Renal failure,

Lassa fever viremia causes endothelial and platelet dysfunction with consequent leaky capillary syndrome. Bleeding occurs in all organs and from all mucosae leading to

Most Lassa fever victims lose fluid through vomiting and diarrhea and therefore develop

Renal tubular damage may also occur on Lassa fever and in conjunction with the

Lassa fever is especially dangerous in pregnant women. Abortion is common in early pregnancy and intrauterine fetal death is common in later pregnancies. Abortion reduces the mortality rate in affected pregnant women. Prognosis is very poor in pregnant women as

This is the commonest complication of Lassa fever. It is not related to the severity of disease as it may occur with the same frequency in both mild and severe forms of the disease.

Lassa fever is most often diagnosed using ELISAs. The virus can also be detected by reverse transcription PCR (RT-PCR) in all patients by the third day of illness, but

Ribavirin, an antiviral drug, is the current treatment of Lassa fever. The drug is to be administered in a volume of 50-100 ml of normal saline to be infused over 30-40 minutes.

Supportive treatment is usually carried out with intravenous fluids, and treatment of

Although Lassa fever can be treated with ribavirin, early diagnosis and treatment is essential in all cases of Lassa fever. Ribavirin is most effective when given within 6 days of illness. Self-diagnosis and treatment is common in the tropics because of ignorance and poverty. It is only when there is no remission of fever that the patient seeks treatment in a health-care facility. However, many health-care providers are unable to make early

 Followed by a dose of 16 mg/kg (max dose 1.28 g) every 6 hours for the first 4 days Followed by a dose of 8 mg/kg (maximum dose 0.64 g) every 8 hours for the

Sensorineural deafness, pregnancy complications.

hypovolemic shock predispose to renal failure

**5.4 Complications of lassa fever in pregnancy** 

immunofluorescence identifies only 52% of the patients.

Loading dose: 33 mg/kg (maximum dose 2.64 g)

complications such as renal failure and infections may be necessary.

**5.1 Hypovolemic shock** 

**5.2 Electrolyte imbalance** 

mortality rate may be up to 80%.

**5.5 Sensorineural deafness** 

**6. Laboratory diagnosis** 

subsequent 6 days

**6.1 Treatment** 

hypovolemic shock.

electrolyte imbalance.

**5.3 Renal failure** 

Fig. 1. Bar Chart showing Case Fatality Rates of Common Diseases of Medical Inpatients in Irrua Specialist Teaching Hospital (ISTH), Irrua, Nigeria in 2007
