Författare: Cheryl Walter Lecturer in Biomedical Science University of H

A case of Lassa fever has been reported in Paris, France, sparking lurid warnings about the “horrific Ebola-like bug”.

So what is Lassa fever and are comparisons with Ebola fair?

Lassa virus (Lassa mammarenavirus) belongs to the Arenaviridae family and there are eight species of this family known to infect humans. What makes this group of viruses noteworthy are their relatively recent emergence as threats to human health (the first case was in Lassa, Nigeria, in 1969), the severity of disease they cause, and the fact that they are mostly transmitted to humans by rodents. And where you find people, you invariably find rodents.

As the name suggests, Lassa fever causes a high temperature, but also weakness, headache and “malaise” (a general ill feeling). These rather commonplace symptoms mean that cases are often misdiagnosed and are sometimes confused with malaria, which can mean worse outcomes for the patient and also more time for the virus to spread to other people.

While many people infected with Lassa virus experience no symptoms, in about 20% of infected people, more serious symptoms develop. These include encephalitis (swelling of the brain), shock and bleeding from the gums, eyes and nose.

About 1% of people who catch Lassa fever die. Although this figure is significantly higher in women in the last trimester of pregnancy.

Although there is no cure for Lassa fever, if a general (“broad spectrum”) antiviral such as ribavirin is given early enough, it can be quite effective at preventing serious disease. A Lassa fever vaccine is currently in phase 2 clinical trials – the penultimate phase of testing in humans.

Difference to Ebola

Lassa virus is often likened to Ebola because of the similar symptoms, but they are very different.

While many people have heard of Ebola and high-profile imported cases into non-endemic regions, such as Europe and North America, Lassa fever is relatively unheard of. Although both of these viruses are endemic to west Africa and can result in many of the same symptoms in people, an important difference is how they are transmitted.

Ebola virus is primarily transmitted from one person to another by body fluids such as sweat, spit, semen and blood. In other words, you’ve got to come into contact with infected fluids and these have then got to gain entry through a cut, by ingestion or, occasionally, sex.

Lassa virus, on the other hand, is almost always transmitted by rodents, in particular the common African rat (Mastomys natalensis).

It is now understood that Lassa fever is transmitted by these rodents when people accidentally ingest rat excrement or urine. It is also likely that dried rat urine or faeces containing the virus can act as a source of infection when it is inhaled, for example, when sweeping out a grain store.

Even more concerning, it has been found that other rodent species, such as the common rat (Rattus rattus) and the pygmy mouse (Mus baoulei) can also become infected with Lassa virus.

While the main animal reservoir has long since shown to be the common African rat, it is concerning that there is potential for other rodent species, especially those widely found on other continents, such as the common rat, to also act as carriers of this virus.

A Nigerian Security and Civil Defence Corps official sprays a woman’s hand with hand sanitiser during a Lassa fever outbreak.
NurPhoto SRL/Alamy Stock Photo

Another virus that is also endemic to Africa, mpox, has gained traction in spreading from person to person. We know this because the virus has distinctly evolved over the last seven years and spread to large numbers of people around the world, resulting in a “public health emergency of international concern” in July 2022.

Similarly, the number of reported outbreaks of Lassa fever in Africa has also increased, with almost yearly outbreaks reported in Nigeria. Unsurprisingly, we are now also seeing imported cases of Lassa fever elsewhere, such as the UK (2022) and Europe (2019, 2024).

An increase in the number of cases could result in a virus that transmits more easily from person to person as it has more chance to adapt, or a virus that can infect different rodent reservoirs if it establishes itself in other communities.

Add the complications of delayed diagnosis of Lassa virus in the community and climate change driving changes to rainfall patterns potentially altering the distribution of rodent species into a more urban environment, Lassa virus is another pathogen threat that needs to be carefully monitored. Läs mer…

In 2022, as the world was slowly beginning to recover from the worst of the COVID-19 pandemic, an outbreak of mpox – then still called monkeypox – began spreading at an alarming rate in many countries worldwide. Confirmed cases were cropping up in places where the disease wasn’t normally present (endemic) – and it was spreading quickly among certain groups.

Global mpox infections have fallen significantly since the height of this outbreak. But a recent paper, which has not yet been peer reviewed, has renewed concerns about the potential for mpox to cause another global outbreak like that seen in 2022.

The researchers, who analysed mpox samples from patients in the Democratic Republic of the Congo (DRC), discovered a new cluster of mpox variants. This cluster is genetically distinct from the variants that were circulating in 2022 – and alarmingly, this variant has evolved rapidly.

However, this is just one example of the way mpox has continued to evolve since the 2022 outbreak.

Mpox virus, as the name suggests, is a pox virus. It’s related to smallpox, cowpox and other animal pox viruses. Notably, it’s not related to chickenpox, although there are some shared symptoms.

Mpox symptoms include rash, fever, sore throat and swollen lymph nodes. The rash, which is often the first symptom people notice, can develop into liquid-filled blisters. However, some people exhibit no symptoms.

The virus is transmitted between people through close contact, respiratory droplets or direct contact with open blisters. The case fatality rate varies – and can be as high as 10% in some instances.

Mpox was first discovered in laboratory monkeys in the 1950s. The first human case was reported in 1970 in the DRC. Since then, outbreaks of mpox have been small and traceable to a single person – typically someone who had recently returned from a country where the virus is endemic – such as west and central Africa.

The 2022 outbreak was unlike any previously seen outside of Africa. Infections were reported in 117 countries – with 95,226 cases and 185 deaths confirmed since the height of the outbreak.

Mpox’s genetic evolution

Mpox is a large virus. Its particles are often described as being “brick-shaped”. To match its clunky appearance, it also has a very large genome for a virus. The genome contains all the information the virus needs to produce new virus particles. Because of mpox’s large DNA genome, it tends to evolve more slowly than viruses with smaller genomes – such as SARS-CoV-2, which causes COVID.

A virus can be clustered into categories based on differences in their genome sequence – referred to as clades. Clades are like the leafy branches of a tree. The more leaves you have on a branch (the clade), the more genetic variants you have in that particular clustering.

The number of branches and leaves on the tree vary for a number of reasons – such as how quickly a virus mutates and how many people are infected. Sometimes, these clades will branch even further into sub-clades.

Using genetic sequencing, scientists can wind back the molecular clock and work out how old a variant is. This is important for establishing an outbreak timeline, working out the virus’ mutation rate and where a particular variant fits on the tree branch.

Mpox causes liquid-filled blisters.
Associated Press / Alamy Stock Photo

Before 2022, mpox could be clustered into two distinct clades. Clade I was typically found in the Congo basin of central Africa, while clade II isolates were found in west Africa.

Clade I was associated with severe disease and a higher case fatality rate. Clade II tended to be associated with less severe disease. But around 2016, variants from clade II started readily spreading between people in Nigeria. This means the genome changed, potentially allowing it to be more transmissible.

Scientists also used genetic sequencing to work out the timelines of the 2022 global mpox outbreak. Importantly, they were able to analyse what had changed in new samples compared to older virus samples.

A string of tell-tale mutations in these mpox samples confirmed the virus was spreading from person-to-person at a rapid rate. Previously, mpox outbreaks tended to originate almost entirely via animal-to-human transmission. Furthermore, a new clade – clade IIb – had formed.

There’s still many questions we can’t answer about the 2022 outbreak – including about what events led to the sustained transmission that happened between people. Yet, based on the low case fatality rate during the outbreak and subsequent research in mice, clade IIb appears to have reduced virulence compared to other clade I. Less virulence means it’s less fatal – which might contribute to the virus being able to spread more easily between people.

This recent pre-print paper, which used samples from the 2023 mpox outbreak in the DRC, has now identified a new offshoot from clade I – clade Ib. This shows mpox is continuing to adapt and becoming more genetically diverse.

This is indeed concerning, especially considering mpox from clade I was associated with more severe disease and a higher case fatality rate. We still don’t fully understand why mpox is becoming more genetically diverse within such a short period of time.

Mpox can be a dangerous infection for the very young and those that have weakened immune function. We also don’t have a vaccine for mpox yet, so rely on cross-immunity from often limited stocks of the smallpox vaccine. Treatments for mpox infections are also limited.

It’s important scientists and doctors continue to monitor new cases worldwide so we can understand how the virus is evolving – and hopefully get closer to breaking the cycle of human-to-human transmission. Läs mer…