A petechia -- a small, red or purplish dot on the dog's skin or mucous membrane -- is caused by minor hemorrhaging, or bleeding under the skin. Usually present as a group of dots instead of just one, petechiae might be nothing more serious than small bruises due to minor injury. Other times, however, these dots indicate graver medical problems such as a bleeding or clotting disorder, infectious disease or cancer. Some cancer treatments and medications may also cause petechiae.
Cloud-shaped blood cells called platelets normally bind together, or clot, to seal off leaking blood vessels. Such repairs are made successfully all the time on a microscopic level, as long as blood contains healthy platelets in sufficient numbers. When platelet numbers are too low -- a condition called thrombocytopenia -- the vessels are not sealed off but are allowed to hemorrhage, producing petechiae and sometimes larger bruises. Thrombocytopenia can result from one of four conditions: Bone marrow produces too few platelets; more platelets than normal are already being used; the immune system destroys platelets or platelets are removed from circulation.
Low Platelet Production
Some bone marrow diseases such as leukemia and aplastic anemia cause bone marrow to produce fewer platelets than normal, which often results in thrombocytopenia and petechiae. Other possible causes of low platelet production include chloramphenical and estrogen medications; estrus, or "heat"; ehrlichiosis, a disease transmitted by ticks and chemotherapy or radiation therapy.
Increased Platelet Use
If the demand for platelets suddenly increases, platelets may be used faster than they can be produced. Platelet supply and clotting ability decline, which can result in hemorrhaging in the form of petechiae. Some conditions that can substantially increase platelet demand include endotoxic shock, a reaction to a bacterial infection; a dangerously exaggerated clotting condition called disseminated intravascular coagulation; vasculitis, or inflammation of the blood vessels, sometimes caused by an infection like Rocky Mountain Spotted Fever or a cancer called hemangiosarcoma.
If a dog's immune system does not recognize its own platelets, it will attack them as it would any other foreign body. This reduces the number of platelets available for clotting and sometimes results in petechiae. This attack on platelets can result from autoimmune diseases such as lupus; infections like ehrlichiosis, heartworm and babesiosis; some vaccines and sulfa medications and some toxins.
Sometimes the platelets are not allowed to circulate in the blood and perform as they should because they are trapped in the spleen, which filters blood as an aid to the immune system. Platelets can become sequestered if a dog's spleen is twisted, which occasionally happens in larger dogs, or if the spleen becomes enlarged.
In the case of thrombocytopathy, a sufficient quantity of platelets circulates in the blood supply, but the platelets themselves malfunction so clotting fails and petechiae and other hemorrhaging may result. Very often, a thrombocytopathic dog also will bleed excessively or for no apparent reason from mucous membranes such as the nose, ears, mouth or anus. These disorders may be hereditary or acquired in response to painkillers, anesthetics, antibiotics or nonsteroidal anti-inflammatory drugs. Thrombocytopathies may also occur as secondary conditions in animals suffering from kidney disease, liver disease, inflammation of the pancreas, parasitic disease or cancer. Breeds prone to certain hereditary thrombocytopathies include Basset hounds, Great Pyrenees, gray collies and some American cocker spaniels.