Many flea species exist, but most have strong preferences for specific animals. In a pinch, though, hungry fleas will hop onto any nearby warm-blooded creature, sink their mouthparts into the skin and settle in for a blood meal. That's how fleas infect dogs with diseases they carry, some of which can also infect people. When fleas make themselves at home in your home, their eggs get scattered around, thereby starting a cycle of infestation that can be difficult to break. Flea prevention is always the best medicine.
Allergic Reactions Cause Skin Lesions
Dogs who are are allergic to flea saliva suffer terribly when bitten. First, the skin around the bite site becomes inflamed. The dog, trying to relieve the discomfort, scratches, bites and licks at it. The fur gets scraped away. If the skin breaks, infection can set in. A vet might not see any direct evidence of live fleas or flea feces on a dog because he's licked it all away, which can create another potential health problem: tapeworm. Treatment with shampoos, anti-inflammatory medications and antibiotics can help relieve symptoms but can't cure the allergy.
One Parasite Gives Birth to Another
When fleas consume tapeworm eggs and a dog swallows the fleas, the fleas' bodies are digested but the eggs aren't. They hatch, and the larvae attach to the lining of the dog's intestine. As they mature, they shed segments through the dog's anus. The dog will try to relieve the irritation by licking or scooting. The expelled segments contain eggs that flea larvae eat, perpetuating the cycle. If tapeworms get into the stomach, they cause digestive distress -- so you might find live worms in your dog's vomit. A tiny nematode, A. reconditum, can also be transmitted by fleas but lives just under the dog's skin and is considered harmless.
Flea-Borne Typhus Can Pass to Humans
Dogs can be carriers of flea-borne, or murine, typhus but don't get sick. But infected fleas on a dog can infect people, who will. Typhus-causing Rickettsia bacteria mainly live inside fleas infesting wildlife but can also jump aboard our furry friends. If that happens, you don't even have to be bitten to be infected. The bacteria survive in flea feces, which you can inhale or transfer from your hands to your mouth after touching your dog. Symptoms, which are often mistaken for flu, include severe head and body aches, high, fever, weakness, confusion, light sensitivity and, in some cases, skin rashes.
Canine Bartonellosis: Trench Fever
Bartonella bacteria are nasty customers indeed. Infected fleas can transmit bartonellosis to cats, dogs and humans; infected cats can transmit the disease directly to people by scratching them; but it's unclear whether an infected dog could do the same. Trench fever, the human version of the disease, was so named because it was transmitted by infected body lice among men fighting in close proximity to each other in trenches during World War 1. In dogs and people, symptoms include high fever, heart problems including lesions on the aortic valve, and inflamed growths in the lymph nodes and nasal passages. Antibiotics can clear up symptoms, but relapses are possible.
Bubonic Plague: Unlikely but Possible
Statistically, you have a better chance of winning the lottery than contracting bubonic plague from any source, let alone a dog flea, in the United States. However, in 2010, two cases of bubonic plague in Oregon were linked to infected fleas that were believed to have jumped from a rodent to a dog and from there to the affected humans. An average of seven cases a year are reported to the CDC, most from Arizona, California, Colorado and New Mexico. To protect yourself, the CDC suggests not letting your dog sleep in your bed if you live in those regions.
Tularemia: Rare but Often Fatal
Tularemia is an extremely rare and often fatal plaguelike disease that can affect dogs as well as humans. All cases are supposed to be reported to the CDC, which classifies it as a tick-borne disease. The Center for Food Security and Public Health at Iowa State University says, "Fleas are thought to be of little importance as vectors," but the Companion Animal Parasite Council mentions that they might have the capability. According to Healio Infectious Disease News, tularemia has "occasionally spread from cats to humans."
- Bayer Animal Health: Tick- and Flea-Borne Infectious Diseases in Dogs and Cats (Backgrounder)
- Virginia–Maryland Regional College of Veterinary Medicine: Flea Allergy Dermatitis
- Cornell University College of Veterinary Medicine: Baker Institute for Animal Health: An Overview of Canine Tapeworm Infections
- Orange County Vector Control District: Flea-Borne Typhus
- Centers for Disease Control and Prevention: Bartonella Infection
- Canine Vector-Borne Diseases: Canine Bartonellosis
- SkinVet Clinic: Flea Allergy Dermatitis
- Centers for Disease Control and Prevention: Travelers' Health: Rickettsial (Spotted & Typhus Fevers) & Related Infections (Anaplasmosis & Ehrlichiosis)
- Encyclopedia Britannica: Trench Fever
- Companion Animal Parasite Council: Ectoparasites - Fleas
- Centers for Disease Control and Prevention: Protect Yourself from Plague
- The Centers for Disease Control and Prevention: Plague: Ecology and Transmission
- The Center for Food Security and Public Health: Iowa State University
- Healio Infectious Disease News: Tularemia: A Threat to Animals and Humans
- American Heartworm Society: Pet Owner Glossary: Acanthocheilonema Reconditum (A. Reconditum)
- Centers for Disease Control and Prevention: Notes from the Field: Two Cases of Human Plague --- Oregon, 2010
- Centers for Disease Control and Prevention: Plague: Maps and Statistics
- Thinkstock Images/Stockbyte/Getty Images