Fish absorb crap from the water, like mercury. In Africa I'd guess dioxin or chlorinated pesticides.
Industry, there is all bottom line driven, to an extreme. Manufacturing, agriculture, mining, all with very little concern of the environmental repercussions. I'm sure several world health groups are trying to zero in on the epidemiology.
Worst thing, most Africans rely on fish for a big chunk of their protein. So on top of the strain from covid, this is something they need to unravel quickly. Bill Gates.......write them a check.