The filarioidea are a superfamily within the order Spirurida. All the species parasitic in the cat are representatives of the Onchocercidae which have microfilariae that occur dispersed in various tissues of the host, typically the blood or subcutaneous tissues. Cats are hosts to quite a few of these pathogens, but very few of those appear to be strictly parasitic in the domestic cat. In fact, most of the species found in the cat are considered to be parasites of other hosts that can infect the cat. Unfortunately, some of the nematodes can cause serious disease in the cat, and the disease in the cat caused by some of the forms that occur primarily in tropical developing countries have not really been examined for their effects on the health of the cat as cats themselves. Experimentally infected cats have been used to model some of the pathology observed in humans, but there has been very little done to examine the effects on any naturally infected cats living where the infections are common.
Cats are hosts to species of Brugia and species of Dirofilaria. Brugia represents relatively small worms with adults that live in the lymphatic tissue of their final hosts. The microfilariae that are produced are found in the blood and transmission is by mosquitoes. The members of this genus are mainly found in Asia, although there are species in the racoon in the United States that might be capable of infecting cats. Dirofilaria is best known in North America for the species, Dirofilariaimmitis, which causes canine heartworm disease. This worm will also cause disease in cats. In Europe and Asia, cats also become infected on rare occasion with Dirofilariarepens which is a subcutaneous dwelling member of this genus. This long worms, up to 17 cm in length, occur in the subcutaneous tissues, and microfilariae are found in the blood. In North America, bobcats are infected with Dirofilariastriata, a similar subcutaneous species, but we know very little about how often this parasite actually infects domestic cats.
The basic filarioid nematode is a hairlike structure with relatively simple features. The females produce microfilariae which circulate in the peripheral blood. The microfilariae are infective to a blood -feeding arthropod, and in the case of those parasitic in cats, all the arthropod vectors are mosquitoes. In the mosquito, the larva undergoes some essential development before it is ready to infect its next host when the mosquito takes another blood meal. Once inoculated into the cat, the larvae mature over a period of several months to the adult stage, and then the cycle is repeated.