Anne, I had a similar experience in Puerto Rico almost 15 years ago. I didn't expect it to be like the mainland United States, although it is a territory of my country, but I was shocked that it seemed so much like a Third-World country. Trash was everywhere - blowing down highways, scattered over beaches, overflowing from public trash cans that obviously did not get emptied often enough. Numerous times I saw people throw trash from moving vehicles.
Mangy, skinny stray dogs and cats were everywhere. I was shocked to find out that, at the time, there was no official animal control on the island. My travel mate, Mary Jo, like to get up early and walk on the beach where she was always followed by three or more hungry dogs. Fortunately, they only followed her and didn't bother her. Still, it was scary.
I was also very shocked at how people treated their pets - really not much better than they treated strays. Domestic animals were throw-aways. Fed scraps, rarely if ever seen by a veterinarian, given very little attention. Most only had homes so they could provide some level of security. The "pets" looked nearly as bad as the strays.
Like you wrote, you can't unsee these things and they definitely lessened my ability to enjoy my time on the island. In fact, it sickened me, especially since my daughter and grandchildren were living there at the time. Fortunately, they returned to Florida the following year. I couldn't bear the thought of my grandchildren growing up to be so uncaring to animals and the environment.