Lassa virus UK—is the rat-borne disease something we need to worry about?

A potentially fatal disease has been identified in the UK, for the first time in over 10 years. 

Two patients have been diagnosed with Lassa fever in England, the UK Health Security Agency has confirmed. 

It's understood that the dangerous virus—which is typically transmitted from rats to humans—was imported into the country by travelers arriving from Africa, where it has been endemic for decades. 

The worrying news marks the first time Lassa has been reported in the UK in over a decade, with the last case dating back to 2009. 

What is Lassa fever? 

Lassa fever is an acute illness caused by the Lassa virus, which belongs to the arenavirus family of viruses. The disease, which was first identified in 1969, exists across the world but is most commonly found in populations in West Africa. 

How is Lassa fever spread? 

Lassa virus is typically spread to humans by rodents, with rats being the most notable culprit of transmission. The African rat, a.k.a. the Mastomys natalensis, is most commonly associated with Lassa. This species of rodent tends to produce large quantities of babies, which can colonize human settlements and increase the risk of animal-to-human contact. 

Rats generally spread the Lassa virus to people through their urine, faeces, or nesting materials. Humans can unknowingly expose themselves by cleaning areas where the infected rats are present, or by eating food contaminated by the rodents. Breathing in air that contains the virus or touching one's face with unwashed hands can also lead to transmission. 

Can humans spread Lassa to other humans? 

Yes, humans can also spread the Lassa virus to one another. Person-to-person infection is most associated with clinical or healthcare settings that are not adequately equipped with infection prevention supplies and protocol. It can also be transmitted through infected bodily fluids. 

How long does it take for Lassa symptoms to show? 

The incubation period for Lassa fever is anywhere between 2 and 21 days, according to the WHO (World Health Organization). 

What are the symptoms of Lassa fever? 

It's estimated that 80% of people infected with the Lassa virus do not show 'observable' symptoms. Unfortunately, the remaining 20% will develop Lassa fever—a multisystem disease characterized by the following: 

• Fever
• Headache
• Sore throat
• Muscle pain
• Chest pain
• Nausea
• Vomiting
• Diarrhoea 
• Cough
• Abdominal pain.

The WHO has warned that some Lassa patients will experience more severe problems, such as 'facial swelling, fluid in the lung cavity, bleeding from the mouth, nose, vagina or gastrointestinal tract and low blood pressure.' Deafness has also been known to occur in 25% of recovered patients. 

Symptoms of Lassa usually surface gradually, with fever and malaise mostly commonly reported as the first red flags. 

Fatality rates for those hospitalized with the fever is around 15%, while the overall fatality rate stands at 1%. Early diagnosis and immediate treatment is crucial to reducing the risk of death. 

What is the treatment for Lassa fever? 

Unfortunately, the treatment options for Lassa fever are extremely limited. 

There is currently no vaccine available to protect humans against the Lassa virus, nor are there any approved medications to treat infected patients. The antiviral drug ribavirin has shown to be helpful in alleviating symptoms if taken in the early stages of the disease, but it has not been proven as an effective medication overall. 

Health authorities have urged the public to practice prevention methods instead, by prioritizing good "community hygiene." The WHO has advised populations where the Mastomys natalensis is endemic to store food in rodent-proof containers, dispose rubbish far from the home and maintain clean households. The organization also recommended keeping cats, if possible, who will can help keep the rodent population down. 

Should you be worried about getting Lassa fever in the UK? 

The risk of contracting Lassa fever if you're in the UK remains 'very low', according to Dr Susan Hopkins, chief medical advisor at UKHSA. 

“Cases of Lassa fever are rare in the UK and it does not spread easily between people," she said.

It's understood that, of the two patients who have been diagnosed with Lassa in the UK this week, one has made a full recovery. The other is currently receiving specialist care at the Royal Free London NHS Foundation Trust.