A parasite is a living organism that depends on what is known as a host—another living organism—for nourishment. It can live on this host or inside it.
External Parasites
Ectoparasites are also known as external parasites. Many are hematophagous—meaning they feed on the host’s blood by biting—, such as:
- Lice and fleas
- : only biting lice are hematophagous, while chewing lice feed on skin debris
- ticks
- sandflies and mosquitoes;
Others feed on skin flakes or hair (keratin, more precisely), like Demodex or Sarcoptes scabiei, a mite responsible for scabies.
Some parasites specifically inhabit the ears of our dogs and cats. Examples include Notoedres cati, responsible for the well-known feline mange, and Otodectes cynotis.
Ectoparasites live on their host either permanently (as is the case with lice and scabies mites) or only during one phase of their life cycle. Mosquitoes, for example, only land on their host long enough to feed.
Internal Parasites
When discussing internal parasites, the first thought is usually those colonizing the digestive tract. They are indeed numerous and classified as roundworms (nematodes) and flatworms (cestodes). What is less commonly known is that parasites can also reside in the respiratory tract, such as *Aelurostrongylus abstrusus* in cats, or even in the bloodstream. This is notably the case with *Babesia canis canis*, which specifically inhabits the red blood cells of dogs and causes piroplasmosis, or *Leishmania infantum*, which spends part of its life cycle in the dog’s macrophages—a type of white blood cell—and is responsible for leishmaniasis.
Zoonoses: Diseases Transmitted Between Animals and Humans
Many parasitic infections in our four-legged companions are zoonoses. This means these parasites can also infest you.
You’ve likely already heard of toxoplasmosis (Toxoplasma gondii), a condition particularly feared by non-immune pregnant women, which can be transmitted through undercooked meat, raw vegetables, or even cat feces.
Toxocara canis is likely the most common cause of intestinal infestation in young children. Parasite eggs are ingested after contact with soil contaminated by the feces of infected dogs or cats. Sandboxes pose a major exposure risk.
Echinococcosis, familiar to forest walkers, frequently affects foxes but can also infect dogs and cats—and be transmitted to humans. Echinococcus is the culprit, with two forms found in our regions (E. granulosus and E. multilocularis).
Other examples: ticks, which may occasionally (meaning if they have nothing else to feed on) bite humans. As for lice, they are quite specific: dog lice only feed on dogs, cat lice only feed on cats, and human lice only feed on humans!
Don’t panic, though—good hygiene (washing your hands after cleaning your cat’s litter box or picking up after your dog, and before sitting down to eat) along with deworming and external parasite control protocols tailored to your four-legged friend’s lifestyle can help minimize the risk of contamination. Talk to your vet about it.
When Parasites Become Disease Carriers
Some parasites (referred to as intermediate hosts, IH) can transmit other disease agents by infesting or biting the definitive host (DH). For instance, fleas (IH) can transmit Dipylidium caninum, a digestive worm, to our dogs and cats (DH). Ticks can also play this role by transmitting Borrelia burgdorferi, the bacterium responsible for Lyme disease, and Babesia canis. Sandflies may carry Leishmania infantum or Dirofilaria immitis (heartworm)...
