"Months after the initial carcasses were discovered, there is still no answer as to why that many elephants are dead", Kat wrote in a blog post, criticising Botswana's government for being slow to protect the animals that are vitally important to the country's tourism - its second largest industry.
The tusks of the elephants have not been removed by poachers.
Dr McCann, of the UK-based charity National Park Rescue, told the BBC local conservationists first alerted the government in early May, after they undertook a flight over the delta.
Either way, without knowing the source, it is impossible to rule out the possibility of a disease crossing into the human population - especially if the cause is in either the water sources or the soil.More news: Launch of NASA Mars rover delayed again, 2 weeks left to fly
Poison has not been ruled out, according to the NPR, though it could also be disease or a naturally occurring toxin.
More than 350 elephants have been found dead in the north of Bostwana in a mysterious mass death.
More than 350 elephants have mysteriously died in the Okavango Delta since May.
Dr McCann added: 'If you look at the carcasses, some of them have fallen straight on their face, indicating they died very quickly.
Scientists are describing the event as a "conservation disaster", with the cause of death still unknown.More news: Bubba Wallace: Nascar president said it was right to fear hate crime
"Unless the government starts to cooperate with NGOs with the resources and capacity to deal with the emergency, elephants will continue to die". The deteriorating health of the living elephants in the region suggests the number of fatalities will increase in the coming weeks. This variation has made it hard for officials to identify the toxin that could be causing these deaths.
Dr Cyril Taolo, acting director for Botswana's department of wildlife and national parks, told the Guardian they had so far confirmed at least 280 elephants had died, and were in the process of confirming the rest. Without proper testing, the cause cannot be identified, and Covid-19 restrictions could mean results will take a couple of weeks.
Between 2007 and 2015 the total population of savannah elephants decreased by 30 per cent.
The Okavango Delta, where the carcasses were found, is home to around 10% of the country's elephants, McCann said.More news: Italy seizes 14 tons of amphetamines, said to be ISIS made