The outbreak began earlier this year in the state of Lagos – the region around the country’s most populous city of the same name – and has now spread to many other parts of the country.
The viral disease does not affect humans, but has around a 100 per cent fatality rate for pigs.
An outbreak of the disease in Asian countries last year saw around 6 million animals culled.
The current outbreak has hit Nigeria’s largest pig co-operative, Oke Aro Farm, where over 300,000 pigs have been killed according to a worker who spoke to the BBC, while another worker at the same co-operative later told The Guardian more than 500,000 pigs had now been culled.
But a wider lack of official record keeping means both overall numbers and the spread of the disease are difficult to measure. African swine fever is currently known to have spread to at least a quarter of Nigeria’s 36 states.
One farmer told The Guardian as many as a million pigs could already have been killed.
African swine fever has become increasingly common in recent years, with more than 60 outbreaks across Africa between 2016 and 2019. But the current outbreak is the worst yet.
“We have never experienced anything of this scale in the past. This is the worst and largest outbreak ever,” Ayo Omirin, a farmer for Oke Aro, who lost more than 600 of his 800 pigs, told The Guardian.
Speaking to the BBC, the same farmer, Mr Omirin said: “We have lost four farmers as a result of shock, two of them slumped and died on the farm.”
The last time African swine fever hit Oke Aro farm was 12 years ago, its president Adewale Oluwalana said.
Pig rearing in Nigeria has soared in recent decades as high demand has resulted in better incomes for farmers.
According to Nigeria’s National Veterinary Research Institute the country’s estimated pig population grew from 2 million in 1984 to about 7 million by 2009, and it is thought to have reached around 14 million animals today.
Last month, British magazine Farmers Weekly reported a new vaccine trial had resulted in 100 per cent of pigs surviving infection with the virus.
UK chief veterinary officer Christine Middlemiss said: “This is a very encouraging breakthrough and it means we are one step closer to safeguarding the health of our pigs and the wider industry’s role in global food supply from African swine fever.
“While there has never been an outbreak of African swine fever in the UK, we are not complacent and already have robust measures in place to protect against animal disease outbreaks.”
But the vaccine is not yet available and the successful trial will be little comfort to Nigerian farmers already facing ruin.
The Nigerian government has reportedly distributed bags of seed and has been working to fumigate pig pens known to be infected.