Nigeria remains on high alert for a further spike in the number of cases of Lassa fever, despite signs that the largest outbreak the country has ever seen is slowing down.
For the last three months Nigeria has been battling an unexpectedly large outbreak, with more than 1,300 people suffering from the disease.
Of these, 365 cases have been confirmed in the lab and there have been around 90 deaths. However, the real numbers will be much higher, say the Nigerian authorities.
The viral haemorrhagic fever – which is spread by rats, causes a high temperature, bleeding from the mouth and nose and, in some cases, can be fatal.
But while the numbers are slowing – there have been 18 new cases in the week up to March 25, compared to 54 cases in the week up to February 25 – the number of cases is still about three to four times higher than in Nigeria’s normal Lassa fever season, which usually runs from January to March.
There were more cases in the first two months of this year than during the whole of 2017.
Key facts | Lassa fever
- Lassa fever is an acute viral haemorrhagic illness of 2-21 days duration
- It is transmitted to humans via contact with food or household items contaminated with rodent urine or faeces
- Person-to-person infections and laboratory transmission can also occur
- The disease is endemic in Benin, Ghana, Guinea, Liberia, Mali, Sierra Leone, and Nigeria, but probably exists in other West African countries as well
- Symptoms can include muscle pain, nausea, vomiting, facial swelling and bleeding from the mouth, nose, vagina or gastrointestinal tract.
Wondimagegnehu Alemu, WHO representative to Nigeria, said that the decline in the number of cases should be regarded with caution.
“The Lassa fever season is not yet over. We need to maintain vigilance and response operations, and ensure continued engagement with communities to help curb the further spread of Lassa fever,” he said.
The disease does not spread as rapidly as its close cousin, Ebola, but is hard to contain because it is mainly spread by rats, rather than humans. Health care workers and close family members are most likely to pick up the disease.
For the first time during an outbreak doctors in Nigeria, in collaboration with WHO and researchers in Germany, have conducted whole genomic sequencing of the virus to determine its origin, said Chikwe Ihekweazu, chief executive of the NCDC.
“This has shown that there hasn’t been any significant change in the virus so we have to look for other causes for the increase in numbers,” he said.
At the moment experts in Nigeria and from outside do not know why the outbreak is better this year but are working on a number of theories.
One is that Nigeria has improved its diagnostic and surveillance capacity with the number of laboratories increasing from two to four in the last year, with plans to increase to six.
“We’re definitely picking up more cases but whether that is enough to account for the size of the outbreak, that’s analysis that will happen soon,” said Dr Ihekweazu.
Population growth could be behind the increase in numbers or the rat that carries the disease may be living closer to humans, said Dr Ihekweazu.
He added: “Most of the transmission happens from food being contaminated by rat excreta. People leave their food out and rats come into the home. This is what happens when rats can’t find food in their natural habitat.”
Hilary Bower, an epidemiologist from the London School of Hygiene and Tropical Medicine, has just returned from Nigeria as a member of the school’s rapid reaction team deployed to assist in disease outbreaks.
She said that Lassa fever was a complicated disease because of the interaction between humans and rats. Rats are part of the diet so asking people to stop eating them was not an option, she said.
“But you can tell people that if they prepare meat they must wash their hands afterwards. That’s a simple and not too big behavioural change,” she said.
She said that experts were also looking at which sectors of the population were more at risk from contracting the disease and what steps they should take to minimise their risk.
The treatment for Lassa, ribavirin, is expensive and Dr Ihekweazu said the price needed to come down if it was to become more widely available.
He added that it worked best when it was administered early on but because the symptoms of Lassa, such as high fever and aching joints, are similar to other viral diseases a diagnosis is made quite late on in the disease course.
“Often by the time patients come they have been treated for malaria or other diseases. We need better diagnostics so we can make an earlier diagnosis,” he said.
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