What is a disease?

Canine illness can take many forms


Disease: Any departure from health; illness in general; a particular destructive process in an organ or organism.(1)

Diseases come in several categories: they can be caused by genes, parasites, viruses, bacteria, protozoa, fungi, structural idiosyncrasies, and the natural aging process. The cause of some diseases is difficult to pinpoint, and some diseases seem to have no discernible cause at all.

It can be tough for the dog owner to recognize when his pet has a disease with few symptoms, especially at the onset, because dogs can’t describe pain, complain of a queasy stomach, or mention that they just feel a bit tired or have a scratchy throat.

Veterinary medicine has progressed in leaps and bounds in the past 20 years. Better vaccines, new diagnostic tests and treatments, more money for canine disease research, improved surgical techniques and equipment, more specialists, and more veterinary clinics have contributed to recovery from many diseases and to overall canine longevity. At the same time, researchers have uncovered more diseases that affect man’s best friend and are struggling to find causes and cures.

There’s no doubt that veterinary medicine faces huge challenges that most pet owners cannot begin to comprehend. We often find out about diseases only when Sassy or Sierra gets sick, but veterinarians must know something about diseases of every type – or must know where to get the information when Pepper tests positive for autoimmune hemolytic anemia or Sport contracts blastomycosis.

Genetic diseases

These diseases are caused by the dog’s own genetic makeup. More than 300 canine diseases are categorized as directly inherited or as inherited tendencies, including hip and elbow dysplasia, deafness, hypothyroidism, progressive retinal atrophy, rheumatoid arthritis, atopic dermatitis, hemolytic anemia, glaucoma, narcolepsy, tracheal collapse, and dozens more.

Diseases caused by recessive genes and diseases caused by interaction of many genes can sometimes be exacerbated or modified by environmental factors. There are indications that immune system stress is a factor in triggering diseases such as hemolytic anemia, that hip dysplasia can be avoided or modified by controlling puppy diets, and that careful breeding practices can greatly reduce the incidence of inherited maladies. The impact of other genetic diseases can be eased by careful management, minor surgeries, or inexpensive treatments.

Some genetic diseases or abnormalities are specific to a breed or group of breeds. Dalmatians are susceptible to bladder stones, but careful breeding practices and a low-protein diet minimize the occurrence of this disease. Poodles and Akitas are susceptible to sebaceous adenitis, a skin disease that results in hair loss, toughening of the skin, and greasy coat, but research projects and better diagnostic tests enable careful breeders and owners to breed away from the disease and treat any cases that do arise.

All animals carry genetic defects, and all genetic departures from health are not equal. Genetic faults that can be corrected with minor surgery, controlled by good management or medication, or simply do not interfere with a dog’s quality of life can be tolerated in a breeding program if breeders make an attempt to weed out serious genetic diseases and are honest with buyers about these faults.

The American Kennel Club Canine Health Foundation awards grants to scientists studying genetic diseases and to mapping of the canine genome so that these diseases can be more quickly identified and eliminated from breeding programs. AKC has donated millions to the foundation, and AKC clubs and breeders give heavily tot he effort. Potential puppy buyers can do their part by carefully researching puppy sources and by spaying or neutering any puppy that has a serious genetic abnormality.

Parasitic diseases

Parasites are critters that use a host animal for part of their life cycle. They can be internal (i.e., worms or protozoa ) or external (i.e., fleas and ticks). Some internal parasites are transported to dogs by external parasites. Fleas are intermediate hosts for tapeworms, ticks can carry protozoal disease, and mosquitoes carry heartworm larvae that are injected in the dog when the insect takes a blood meal. Flea bites can also trigger allergic reactions in susceptible dogs, reactions that lead to severe itchiness, hair loss, and infections when bacteria invade skin scratched and bitten raw.

Protozoa are microscopic, single-cell parasites that live in water, soil, and the feces of infected animals and invade canine digestive systems and blood streams. Protozoal diseases affecting dogs include coccidiosis, giardia, leishmaniasis, and babesiosis. Coccidiosis and giardia affect the intestines and cause mild to severe diarrhea. Leishmaniasis invades white blood cells; the most severe form of the disease causes anemia, diarrhea, and liver and spleen damage. Babesiosis infects the circulatory system and causes anemia. Severe or untreated protozoal infections can be fatal.

Viruses and bacteria

Viruses and bacteria are the culprits in the majority of infectious diseases that affect animals.

Made primarily of protein and genetic material, viruses are microscopic non-living disease agents – they don’t eat, breathe, or excrete, but they nonetheless have the capacity to multiply in susceptible hosts. Canine viruses include distemper, infectious hepatitis, parvovirus, rabies, Bordatella, adenovirus, and parainfluenza. Most state or local health codes require vaccination against rabies, a deadly disease that can be transmitted to people. Dogs can be protected against many other viruses through vaccines as well.

Bacteria are single-celled organisms that can be found throughout the environment. Most bacteria perform necessary functions: they help break down food, synthesize vitamins, and aid in fermentation and decomposition of organic matter. Some bacteria cause disease.

Bacterial diseases in dogs include leptospirosis, enteric campylobacteriosis, streptococcal and staphylococcal infections, brucellosis, tetanus, Lyme disease, erlichiosis, Rocky Mountain spotted fever, and infectious thrombocytopenia. Vaccines are available to protect against Lyme disease and leptospirosis.


Most people think of mushrooms, toadstools, and yeasts when they think of fungi, but there are many other members of this plant family, some of which can cause diseases such as ringworm, candiadiasis (yeast infection of the skin) blastomycosis, histoplasmosis, coccidioimycosis, and aspergillius. There are no vaccines to protect against fungal infections, but in some cases, owners can protect their pets by avoiding areas where fungal infestations can occur. For example, the fungus that causes blastomycosis tends to occur along sandy river banks, histoplasomosis is a problem where bird or bat feces have accumulated for years, and coccidioimycosis can be a problem in the Southwestern US, especially after a period of stormy weather.

Structural abnormalities

While not generally thought of as a disease, structural abnormalities nonetheless can contribute to a decline in quality of health and life for a dog. The incorrect meshing of teeth – referred to as “bad bites” by dog breeders – can interfere with a dog’s intake of food; poor shoulder or hip structure can cause painful or stilted gaits; loose knees can lead to rear end breakdown, arthritis, or reconstructive surgery; and weak pasterns can interfere with strenuous activity as the dog ages.

Good breeders do their best to produce dogs that a structurally sound, but environmental factors also play a part in skeletal health. If a puppy grows too fast or is exercised too strenuously before bones, muscles, and connective tissues have matured, even the strongest systems will be hard put to remain healthy in the long run. Those who compete with their dogs bring them along slowly to prevent future skeletal problems.

If a well-bred puppy grows into a couch potato dog that eats too much and doesn’t get enough exercise, its structure can break down under the strain of too much weight and too little conditioning.


Like all mammals, dogs experience various diseases as their bodies age. Organs slow down or become ill, infections and other stresses take a bigger toll, cancers crop up, Cushings disease invades, bloat becomes a possibility, and arthritis begins to form.

Owners can protect their pets from some of these changes by keeping weight under control and providing some exercise, but the majority of old-age changes cannot be avoided.

In conclusion

Disease is a fact of life. Breeders can do their part by making good decisions that increase the likelihood of producing healthy puppies. Pet owners can help by following breeder suggestions about feeding and exercising puppies and adult dogs and by working with veterinarians on an immunization plan, protection from parasites, and management techniques that minimize opportunities that infectious diseases and parasites will be encountered.

If all of these precautions don’t do the trick, veterinarians have many tricks up their collective sleeves for curing or controlling many diseases and for making dogs comfortable when cures are not in the cards.


UC Davis Book of Dogs, edited by Mordecai Siegal
Control of Canine Genetic Diseases by George A. Padgett DVM


1. Webster’s New World Dictionary, Third College Edition, 1994.

Norma Bennett Woolf

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