Lassa fever outbreak in Nigeria is its largest ever
Nigeria is experiencing its largest-ever outbreak of Lassa fever.
Lassa fever is one of a group of tropical and sub-tropical infections - the so called, viral haemorrhagic fevers - which can cause dangerous epidemics, but for which no effective vaccine currently exists. Like its sister disease, Ebola virus disease (EVD), Lassa is endemic in West Africa.
The World Health Organization (WHO) is reporting (March 1st, 2018) that between January 1st and February 25th 2018, Nigeria had identified a total of 1081 suspected cases of Lassa fever, resulting in 90 deaths, in 18 of the country’s 36 states. Of these cases, 317 cases have been classified as confirmed and eight as probable, which has resulted (among these groups) in 72 deaths (the case fatality rate [CFR] for confirmed and probable cases in this outbreak is therefore about 22%). A total of 2845 contacts have been identified. A number of healthcare workers, in managing cases, have themselves become infected.
This outbreak appears to be linked to a smaller, concurrent cluster of Lassa cases in neighbouring Benin. WHO reported on 16th February that Benin had detected 21 Lassa cases (five confirmed, two probable and 14 suspected) including eight deaths. Eight of the cases identified in Benin were residents of Nigeria, who subsequently travelled to Benin.
The WHO and the US’s Centers for Disease Control and Prevention (CDC) are providing support in the field, so the likelihood of a case of Lassa (or a contact) presenting to the health system in Ireland is considered to be low. However, as the incubation period (the interval between exposure to the virus and development of clinical features) of the disease is relatively long (between one and three weeks), a precautionary approach to the possible importation of a case of Lassa fever into Ireland is advisable. Accordingly, any person presenting to an Irish healthcare facility, with symptoms suggestive of Lassa fever, in conjunction with a history of travel to or from Nigeria or Benin in the preceding three weeks, should be considered to be at a high risk of having Lassa, and such a person should be isolated without delay, and immediate expert infectious disease, infection prevention and control, and microbiological advice sought.
Lassa fever can affect many organs, and can inflict severe damage (in a similar manner to EVD) upon the body's blood vessels. Treatment is complex and frequently difficult, even in well-staffed and well-equipped tertiary-level healthcare facilities in developed countries. For the majority (approximately 80%) of Lassa cases, symptoms are mild and non-specific (fever, headache and general weakness ) or are undiagnosed. Signs and symptoms of the infection typically appear 1-3 weeks following exposure. In severe cases of Lassa, the disease can present with a syndrome not dissimilar to that of EVD, with haemorrhaging (from gums, eyes, or nose, or from minor skin abrasions), moderate to severe respiratory distress, repeated vomiting, facial swelling, pain in the chest, back, and abdomen, and, eventually, shock. Occasionally, neurological features, such as hearing loss, tremors, and encephalitis can occur.
The CFR for Lassa fever is typically considered to be about 1%, so this outbreak would appear to have an unusually high fatality rate. Certain demographic groups are at particular risk from Lassa; women who contract the disease late in pregnancy (along with their unborn babies) are at particular risk, and can face an 80% chance of losing their child, or of dying themselves.
Lassa virus is endemic in a broad swathe across West Africa, extending across Benin, Ghana, Guinea, Liberia, Mali, Sierra Leone, and Nigeria, but is likely to exist in other West African countries as well. The virus is transmitted via contact with the urine/faeces of infected multimammate (or Common African) rats; as many as 10% of these rodents have serological evidence of infection with Lassa virus.
WHO acknowledges, that although Lassa is endemic in Nigeria, it considered this outbreak to represent a moderate level of risk at regional level. WHO also state the following: “The diagnosis of Lassa fever should be considered in febrile patients returning from West Africa, especially if they have been in rural areas or hospitals in countries where Lassa fever is endemic. Healthcare workers seeing a patient suspected to have Lassa fever should immediately contact local and national experts for guidance and to arrange for laboratory testing.”
The most recent, previous outbreak of Lassa occurred in Liberia in early 2018.
Further information on Lassa fever can be found on the websites of:
• HPSC
• WHO