Parasites are organisms that are detrimental to the health of their host. They draw their nourishment from their host and provide no benefit in return. Parasites come in many shapes and sizes, ranging from microscopic, single-celled protozoa to large organisms such as tapeworms which can be feet in length.
Parasites can be split into two large groups: external parasites and internal parasites.

External Parasites
Organisms found on or in the skin of the host.

Fleas and Flea Allergy Dermatitis

Fleas
Fleas are wingless, brown or black, bloodsucking insects with flattened bodies and three pairs of legs.

Flea Allergy Dermatitis
Although not a parasite, it is the most common skin disease of animals in the world. It is caused by a hypersensitivity to certain components of flea saliva.

Ticks

Mites

Demodicosis

Canine Scabies

Cheyletiellosis

Ear Mites

Lice

Spiders


Internal Parasites
These parasites live within the host, mainly in the intestinal tract, liver and lungs. Some parasites are the direct cause of disease, or pathogenic, but many others are simply a factor in the development of disease. A crucial determining factor in the onset of disease is the number of ineffective-stage parasites to which the host is exposed. Almost all parasites are pathogenic if present in large numbers. Another is the competency of the host's immune defenses. If the host's immune system is unable to respond or responds ineffectively the potential for disease is increased. Malnutrition, stress, other disease, and immunosuppresent drugs all lower the body's ability to keep parasitic numbers in check.

Internal parasites effect the body in the following ways, according to the UC Davis Book of Dogs:

• competition for and absorption of nutrients intended for the host.
• interference with the absorption of nutrients.
• loss of bodily fluids (blood, lymph or tissue fluid.
• anemia through the destruction of red blood cells.
• obstruction of hollow organs, ducts and vessels.
• tissue necrosis through the exertion of pressure on surrounding tissue.
• physical and chemical damage to tissue during parasite migration through the body.
• chemical injury resulting from the action of toxins or other secreted substances.
• tissue injury resulting from irritation or chemical effects.
• infliction of wounds or disruption of body processes, predisposing the host to secondary infections.
• transmission of other infectious disease agents.
• stimulation of abnormal immune or allergic reactions.
• debilitation and/or interference with normal growth and development of the host.
• inflammatory responses induced in the host by death and decomposition of the parasite.

Nematodes (roundworms)
There are probably more nematodes in the world than any other type of animal, except insects. Nematodes are generally long, cylindrical, unsegmented, cream-colored worms.

Ascarids
Ascarids are among the most common and important gastrointestinal parasites of dogs, Toxocara canis and Toxoscaris leonina being the most prevalent.

Hookworms
Hookworms are found in the small intestine of the dog and other domestic species.

Whipworms
Adult whipworms are usually found in the large intestine, specifically the colon. There the female lays eggs that pass out in the feces, infecting other animals through ingestion.

Threadworms
Threadworms can be found in two different life cycles, a parasitic form and a free-living form. The parasitic form is found in the tissue of the small intestine, larvae are passed out of the body. When the larvae enter the soil, they can either develop into a larva that will infect a new host, or a larva that will live in the soil, producing other larvae that will develop into one of the two forms.

Lungworms
The term "lungworm" is used to describe nematodes whose adult form is found in the respiratory tract, pulmonary tissue and/or in pulmonary blood vessels.

Heartworm
Heartworm disease is a serious problem caused by internal parasites but circulated through the population by mosquitoes. Heartworm disease is caused by the parasite Dirofilaria immitis, and is found in all 50 states. (2)

Transmitted by mosquitoes from one infected animal to another, in can occur in both dogs and cats. Mosquitoes deposit immature larvae into the dog's skin. Two months later the juvenile worms migrate throughout the bloodstream to the right side of the heart. Growing into adult worms (some up to 12 inches long), the female then produces offspring, called microfilaria. Producing up to 1,000 offspring, they feed off of the blood of the host. By the age of 6 months, the offspring are up to 3/4 of an inch long. They lodge themselves in the heart and the large blood vessels going from the heart to the lungs. Adult worms can live up to 7 years in the dog's body, and if left untreated, dogs usually die of heart failure. Even dogs that only go out to use the bathroom are at risk, as a lone mosquito can find its way into the house. (2)

Signs of heartworm disease:

The first outward signs may not be apparent for the first year of infection. It then may start as a soft cough, and as the disease progresses, they will find it more difficult to breathe. Decreased appetite, and weight loss, as well as reluctance to exercise are also symptoms

Other Nematodes Affecting Dogs

• Spirocerca Lupi
• Physaloptera
• Giant Kidney Worm

Cestodes (tapeworms)
Cestodes are highly adaptive internal parasites found in all aquatic and terrestrial vertebrates. Typically they are long, flattened, ribbon like and segmented.

Dipylidium Caninum
By far the most common tapeworm in dogs in the United States, transmitted by fleas.

Taeniids

Pseudophyllideans

Trematodes (flukes)
Flukes are flattened, oval-shaped, worm-like parasites. Some are external, most that infect dogs are internal.

Alaria

Nanophyetes Salmincola

Paragonimus Kellicotti

Protozoan Parasites
Protozoa are among the simplest single-celled organisms in the animal kingdom.

Coccidia

Toxoplasma

Neospora Caninum

Giardia

Leishmania

Trypanosoma Cruzi

Babesia



References:

1- UC Davis Book of Dogs: A Complete Medical Reference Guide for Dogs and Puppies, by the Faculty and Staff, School of Veterinary Medicine, University of California at Davis. Harper Collins, 1995.

2 - Drs. Foster & Smith Jan 2008 catalog, pgs 6 & 7.