Parasites are critters that use a host animal for part of their life cycle. 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. 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. Canine viruses include distemper, infectious hepatitis, parvovirus, rabies, Bordatella, adenovirus, and parainfluenza.