History and exam
Key diagnostic factors
common
presence of risk factors
Key risk factors include: living/working in, or arrival from, an endemic area (Nigeria, Sierra Leone, Liberia, and Guinea; cases have also been reported in Ghana, Benin, Togo, Burkina Faso, and the Democratic Republic of the Congo) in the previous 21 days; contact with infected body fluids; occupational exposure; or butchering/consumption of meat from infected (or potentially infected) animals.
fever ≥37.5°C
Fever is present in virtually all patients with Lassa fever.[4] However, it may not be continuous; therefore, being afebrile on presentation does not rule out Lassa fever.
The combination of known exposure and having a fever >38°C for less than 3 weeks with absence of local inflammation (to rule out more common bacterial infections) is required for admission to the Lassa ward in the endemic region of Sierra Leone.
elevated respiratory rate
On admission to hospital, the mean respiratory rate in patients with Lassa fever was 24 breaths per minute amongst those who had this measured.[4]
low systolic blood pressure
On admission to hospital, the mean systolic blood pressure in patients with Lassa fever was 104 mmHg amongst those who had this measured.[4]
malaise
Experienced by 82% of hospitalised patients.[4]
nausea/vomiting
Experienced by 75% of hospitalised patients.[4] This generalised symptom makes differentiation difficult.
headache
Presenting complaint in 55% of cases, but experienced by 75% of patients during hospital stay.[4]
sore throat/pharyngitis
May be reported by patients. Inflamed and swollen posterior pharynx/tonsils is seen on examination in around 70% of cases.[4] In one large study, the combination of sore throat and bleeding was identified as the most predictive feature of Lassa fever.[4]
White or yellowish exudate and small vesicles or shallow ulcers on the tonsils and pharynx may also be seen.
A sore throat may help to differentiate Lassa fever from other causes of fever, such as malaria, leptospirosis, and yellow fever.
conjunctivitis
Seen in around 33% of hospitalised patients. Independently associated with Lassa fever compared with controls and reported to have a specificity of 82%.[4] It should be noted that conjunctivitis is also seen in Ebola, which was not reported in Sierra Leone when the data for Lassa fever were analysed.
chest pain/cough
Over 70% of 441 patients admitted to a hospital in Sierra Leone experienced retrosternal chest pain and/or cough.[4]
A dry non-productive cough may help to differentiate Lassa fever from other causes of fever.
uncommon
deafness (sensorineural)
Developing late in the course of disease or early in convalescence, unilateral or bilateral hearing loss was noted in 29% of 49 acutely febrile patients with confirmed Lassa fever in a hospital in Sierra Leone, who were prospectively studied.[6]
May be permanent in some cases.
facial oedema
Facial oedema may occur in the later stages of disease. It has been reported in approximately 20% of cases.[4]
bleeding
Bleeding is seen in around 17% of hospitalised patients with Lassa fever, although in one large study Lassa fever was identified in 74% of patients who were admitted to a hospital in Sierra Leone with bleeding.[4] In this study, the combination of bleeding and sore throat was identified as the most predictive feature of Lassa fever.
Bleeding is usually from mucosal sites (e.g., gums, eyes, nose, and rectum) and is not usually associated with thrombocytopenia.[4] In women, particularly pregnant women, severe vaginal bleeding can occur.
Other diagnostic factors
common
uncommon
confusion and altered Glasgow Coma Scale or seizures
Encephalopathy with Lassa virus RNA detected in cerebrospinal fluid (CSF) has been reported infrequently.[7][30]
Encephalopathy is quite common among symptomatic patients who present after more than 6 days of symptoms. However, detection of Lassa virus RNA in CSF is rarely reported as lumbar puncture is rarely performed in these patients.
effusions
Pleural or pericardial effusions are rare complications (2% to 3%), often seen late in the illness and linked to proteinuria.[4] Because diagnosis is difficult due to infection risk, effusions may be underdiagnosed.
Risk factors
strong
occupational exposure
Healthcare workers in contact with infected patients are at high risk, and most epidemics have resulted in numerous infections in healthcare professionals. Risk of infection can be significantly reduced if protective measures and proper sterilisation methods are used by healthcare professionals.
living/working in, or arrival from, endemic area
Individuals at greatest risk of Lassa fever are those who live/work in, or travel from, endemic regions (Nigeria, Sierra Leone, Liberia, and Guinea). Cases have also been reported in Ghana, Benin, Togo, Burkina Faso, and the Democratic Republic of the Congo. The risk of exposure may exist in other West African countries where Mastomys rodents commonly reside.
Residing in, or having resided in, a house with evidence of rodent infestation or where individuals have been infected previously with Lassa fever is a risk factor.
contact with infected body fluids
Direct exposure to blood or body fluids from infected rats or infected humans is the primary method of human transmission. Infected Mastomys rodents secrete the virus in urine and faeces;[21] therefore, transmission may occur via ingestion of excreta from an infected rodent, viral exposure to open cuts or sores, or inhaling air contaminated with infected rodent excretions.
Human-to-human transmission occurs via contact with body fluids (e.g., sweat, blood, urine, faeces, vomit, saliva, genital secretions [including semen], and breast milk) from infected patients. A patient can excrete the virus in urine for between 3 and 9 weeks after the onset of illness. Sexual transmission has been reported. Patients may be able to transmit the virus via semen for up to 3 months after the onset of illness; therefore, sexual transmission may be possible after the infection has resolved.[8]
Contacts of infected patients (including healthcare workers and household contacts) are at risk of infection if the person was exposed to body fluids of the infected patient without appropriate protective equipment. Household contacts of infected patients are at a higher risk of infection if there is active diarrhoea, vomiting, or bleeding.
Casual contact (including skin-to-skin contact without exchange of body fluids) does not spread Lassa virus. Furthermore, there is no evidence supporting airborne spread between humans.
butchering and/or eating rodent meat
The multimammate rat (Mastomys natalensis) is the natural host for the Lassa virus. This rodent is considered a cheap source of protein by subsistence farmers who routinely encounter it when they burn their fields ahead of the planting season.[7] Consumption of the rodent is associated with serological evidence of infection.[22]
weak
bioterrorism
In the US, Lassa virus is a National Institute of Allergy and Infectious Disease (NIAID) category A priority pathogen. Category A pathogens are organisms/biological agents that pose the highest risk to national security and public health.
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