McCann said: "When we've got a mass die-off of elephants near human habitation at a time when wildlife disease is very much at the forefront of everyone's minds, it seems extraordinary that the government has not sent the samples to a reputable lab".
The unusual number of elephant carcasses were first recorded at the start of May, McCann said.
The number had more than doubled by the middle of June, with local sources saying that they believed around 70 percent of the deaths to be gathered near waterholes.
Dr. McCann said 169 dead elephants were spotted "in a three-hour flight".
Poachers in Zimbabwe are known to use cyanide to poison elephants, and while it remains a distinct possibility, scavengers aren't dying by the elephants.
This variation has made it hard for officials to identify the toxin that could be causing these deaths. The southern Africa nation is home to nearly a third of the continent's elephants, and numbers have grown to 130,000 from 80,000 in the late 1990s, because of well-managed wildlife reserves. The way the animals appear to be dying - many dropping on their faces - and sightings of other elephants walking in circles points to something potentially attacking their neurological systems, Dr McCann said.
Poisoning or an unknown pathogen are thought to be the most likely causes.
The tusks of deceased elephants have not been removed and conservationists have urged authorities to guard the carcasses so that poachers do not take them.
"We do not suspect poaching since [the] animals were found with tusks", he said.
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.
There are about 15,000 elephants in the delta, 10% of the country's total.
There have been no reports of elephant deaths in any neighbouring countries.
There is insufficient data to see whether something that is killing the elephants could cross into the human population.
"We are still awaiting results on the exact cause of death", regional wildlife coordinator Dimakatso Ntshebe told Reuters.
The deaths of the elephants come amid the COVID-19 pandemic that has killed lakhs of people across the world.
But after failing to send the samples earlier - and then to a laboratory without the required capacity - there are fears the samples are now old and "of dubious origin".