The domestic dog's health is possibly one of the best-studied areas in veterinary medicine, since the dog has had such a long and close relationship with humans.
Diseases and ailments
Some diseases, ailments, and poisons are common to both humans and dogs; others are different.
Most diseases that affect dogs or humans are not transferable between the two species. There are some exceptions of zoonoses:
Rabies, or Hydrophobia, is a usually fatal disease which can be transmitted to dogs or humans by the bite of an infected mammal, possibly a dog's, cat's, raccoon's, or bat's. Although rodents and similar small mammals can be infected with the disease artificially, they are generally not found infected in the wild; the current hypothesis is that they are not likely to survive any attack that would infect them.
Animals with rabies suffer deterioration of the brain and tend to behave bizarrely and often aggressively, increasing the chances that they will bite another animal or a person and transmit the disease. Areas that are rabies-free, (usually islands) such as Britain, Ireland, Australia, and the American state of Hawaii have strict quarantine laws to keep their territories rabies-free. These require long periods of isolation and observation of imported animals, which makes them unattractive places to move with a pet unless the pet is quite young.
Areas that are not rabies-free usually require that dogs (and often cats) be vaccinated against rabies. A person or dog bitten by an unknown or unvaccinated dog (or other animal) should always be treated without waiting for symptoms, given the potentially fatal consequences of a rabid biter: there has been only one case of someone surviving rabies when treatment was not begun until after symptoms appeared. The biter should be apprehended if possible, as only autopsy of the brain can determine if it was rabid.
This should be a great incentive to dog-owners to vaccinate their dogs even if they feel the risk of their dog contracting rabies is low, since vaccination will eliminate the need for their dog to be euthanized and examined in this fashion should it bite anyone or be suspected of biting anyone, as well as the need for it to be treated for rabies if it is suspected of being bitten.
Parasites, particularly intestinal worms such as hookworms, tapeworms and roundworms, can be transmitted in a dog's saliva or feces. Some parasites have fleas as intermediate hosts: the worm egg must be consumed by a flea to hatch, then the infected flea must be ingested (usually by the dog while grooming itself, but occasionally by a human through various means) for the adult worm to establish itself in the intestines. The worm's eggs then pass through the intestines and adhere to the nether regions of the dog, and the cycle begins again.
Fleas and ticks of various species can be acquired and brought home by a dog, where they can multiply and attack humans (and vice versa). This is particularly important, now that tick-borne Lyme Disease has become endemic throughout a large area, in addition to other similar diseases such as Rocky Mountain Spotted Fever.
Although dogs do not seem to be as susceptible to such diseases as humans, similar rickettsial diseases have been spread by dogs to humans through such mechanisms as a dog killing an infected rabbit, then shaking itself off in the house near enough to its owners to fatally infect most of the family.
Leptospirosis is a bacterial disease that affects humans and animals. It is caused by bacteria of the genus Leptospira. Humans and dogs become infected through contact with water, food, or soil containing urine from infected animals. This may happen by swallowing contaminated food or water or through skin contact, especially with mucosal surfaces, such as the eyes or nose, or with broken skin.
Genetic conditions are a problem in some dogs, particularly purebreeds:
Hip dysplasia primarily affects larger breeds.
Luxating patellas can be a problem for smaller breeds.
Genes for blindness or deafness seem to be carried by some breeds.
In some dogs, such as collies, the blue merle or harlequin coloring is actually the heterozygote of a partially recessive gene preventing proper development of the nervous system; therefore, if two such dogs are mated, on the average one quarter of the puppies will have severe genetic defects in their nervous systems and sensory organs ranging from deafness to fatal flaws.