Many dogs will snack on "kitty crunchies" when they find an open litter box. Not only is it plenty disgusting to you, your dog can pick up diseases and parasites from eating the cat's feces.
Your cat can host internal parasites that your dog can acquire by eating the cat's feces. Cats can carry roundworms, hookworms, whipworms and tapeworms, and their eggs may wind up in the cats' feces. When your dog eats your cat's poop, he ingests the worm's eggs and becomes infected. Roundworms produce up to 200,000 eggs in a day. They are so hardy that they can remain viable in dirt and feces for years. Other worms, such as tapeworms, lay eggs in packets and may be ingested by your dog when he eats cats feces. Worms can make your dog incredibly ill, or can kill him if not treated. If you suspect your dog has contracted worms, bring a stool sample to your veterinarian for analysis.
Toxoplasmosis is a disease that cats can contract from eating infected mice. When a cat contracts toxoplasmosis for the first time, her feces will become infected with toxoplasmosis gondii oocysts or eggs. Your dog can contract toxoplasmosis when he eats the infected poop. Symptoms of toxoplasmosis include fever, lethargy, diarrhea, refusal to eat and weight loss.
Giardia is protozoan that can infect your dog, cat and you. Symptoms include diarrhea, vomiting, chronic diarrhea, weight loss, dehydration and bloody diarrhea. It can make your dog very sick and can be life-threatening. Giardia infects through microscopic cysts excreted by an infected host animal. Your dog can get infected by eating contaminated fecal matter, drinking contaminated water or even sniffing a contaminated area. Your dog may sniff the contaminated area and then lick his nose, thus ingesting the cysts.
Salmonella can live in your cat's intestinal tract. Your cat can contract salmonella from eating rodents or birds infected with this disease. This is the same bacterium that causes people to get sick from food poisoning. Your dog can contract salmonella from your cat by eating your cats' feces. Your dog may not show any symptoms or may display vomiting, diarrhea, fever, nausea or other symptoms associated with salmonella.