Macadamia nuts are dangerous for dogs to eat. Your healthy adult dog is not likely to die from a few of these nuts since they are only mild to moderately toxic, but if he has other health problems, is very young or old or chows down on a whole lot of them, it could be fatal. Something in macadamia nuts just doesn't agree with dogs, although researchers aren't sure exactly what. Lack of muscle coordination, high fever, tremors and an increased heart rate are the most common symptoms of ingestion. Adding to the danger of macadamia nuts is the fact that they are often found, and eaten by dogs, in combination with chocolate. The combined effects of a reaction to the nuts and the chocolate can be fatal for even the healthiest dog.
It's not so much that walnuts are toxic for dogs, although they might cause a stomach ache, it's that they can cause intestinal blockages. Dogs aren't so good at chewing up walnuts. They tend to get in a hurry and swallow large chunks. These chunks don't break down very easily in the stomach and can move into the intestine and stop up the entire digestive system. If it goes untreated, the blockage could be fatal. Walnut shells can have the same effect.
Like walnuts, pistachios are not really toxic to dogs, and an occasional pistachio isn't likely to have much effect on a healthy adult dog. A large amount of pistachios, however, can cause pancreatitis, or a swelling of the pancreas. Enzymes released by the pancreas can cause damage in other parts of the body, causing death. Many cases of pancreatitis can be successfully treated, but if too much damage is done there is not much the veterinarian can do to save your dog. Even though they aren't necessarily poisonous, it's best to keep pistachios away from your pooch.
Any Moldy Nuts
Most dogs will eat just about anything they think they're not supposed to, including moldy nuts. Moldy walnuts and pecans are often eaten by dogs, but any moldy nut can cause serious problems. These rotten nuts can cause seizures and loss of motor skills. In large amounts they can be deadly.
General Nut Risks
Dogs are not really meant to eat any type of nut, even though many appear to enjoy the taste of them (though the dogs are probably enjoying the salty seasoning more than the nut itself -- which presents its own dangers). While some nuts, like almonds, aren't toxic to dogs, they are very high in fat. In large amounts, just about any type of nut could be fatal, either by causing pancreatitis (due to high consumption of fat) or salt toxicity. Many dogs, just like people, are also allergic to certain nuts. A severe reaction could cause the throat and nose to swell rapidly, choking your dog to death. Dogs don't need nuts as part of their diet, so it's better to be safe and avoid them altogether.
So what about peanuts, since so many dog treats come in peanut butter flavor? Though not technically a "nut" but a legume, they're generally considered as nuts -- so we should definitely mention them here. Obviously, as they're a popular pooch treat, peanuts and peanut butter isn't toxic to dogs, but while small amounts of unsalted, unsweetened varieties are ok to use in dog cookies and other treats, they are high in fat which dogs have a hard time digesting. Again, small amounts and moderation is key!