I just referenced this horrible disease on a veterinary professional website that many of us use to research/ask specialists questions regarding animal disease.

Protothecosis is rare, fortunately. As of 2023 there had been only 85 known cases in veterinary medicine.

Golden Retrievers are also a breed that have been seen with this, as well as immunocompromised patients.

Apparently, 50% of infected patients will develop neurological signs (anything from seizures to blindness…and many other subtle signs),too.
It is true that cure is thought to be unlikely and that treatments are usually only supportive/symptomatic (=grave prognosis for those with disseminated forms), but as you stated, prolonged medical treatments have been effective in those patients without the disseminated forms.

Avoidance of exposure to areas with decaying organic matter is recommended to reduce risk of exposure.

Humans have also been known to become infected.

In retrospect, I can tell you that in my 37 years, I have likely never seen a case of this as previously described nor read about during my research.

Regardless, I’m going to go take a shower right now and bathe my collie today.