Not much to know. Hits dogs suddenly, mostly smaller breeds but has impacted larger breeds as well. Vets are uncertain as to its cause.
Whenever I hear about sudden onset of diseases like these I automatically think about diet and the by-products used in so many dog foods. Food whch is loaded with grains and corn which contain pesticides, insecticides,herbicides, Meat by roducts from animals which have been ill treated. Small Breeds are more suseptible to toxics becuase they are closer to the ground, where cleaning products and chemicals put on plants and lawns are more easily inhaled or ingested. I hope they find a cause and cure soon.

We see a lot of HGE during the holiday season, and the article above is pretty good at explaining it, how it is diagnosed and treated. I don't have much to add to it, other than very severe cases sometimes do not survive, but well over 95% do well with hospitalization, fluids, antibiotics, antiemtics and close monitoring.