Hi everyone, thanks for the support messages it's being quite tough...
Answering some of your questions, I did ask the breeder in France, and she never heard of anything like this before.
Poisoning has been quite ruled out as I have another basenji boy (2 years old), and he could have been exposed too. Since the beginning of symptoms, the dogs have been 24h with me as I work home based, and I've been walking them on leash so the probability he would eat something repetedly witout me noticing is almost non-existent...
Stil waiting for some additional tests that take longer to come back from the lab, let's see what the final theory from the vets will be.
Thanks again everyone for the kind words and the interest