Heartworm (Dirofilaria immitis) is a type of nematode. Heartworm causes a potentially fatal disease called, unsurprisingly, heartworm disease. Many types of mammals can get heartworm, including dogs, cats, wolves, foxes, ferrets, sea lions, and humans.

Let's follow "Bob" the heartworm through his entire life cycle.

Bob's mother "Phyllis" lived in the right ventricle of the heart of a Yorkshire Terrier. Phyllis was lucky to end up living in a dog. Dogs allow heartworms to progress unimpeded through their complete life cycle. If Phyllis had ended up in a cat, for example, her chances of successfully giving birth to Bob would have been much smaller.

Phyllis gave birth to Bob about six months after she arrived in her host dog. Bob was born alive (viviparous) as a microfilarium, his first stage of development. As a microfilarium, Bob looked like a headless snake, though far smaller. Stretched out, Bob was about as long as thirty canine blood cells laid end-to-end.

Bob circulated through the terrier's bloodstream as a microfilarium for a while. Then, by chance, a female mosquito landed on Bob's host dog, penetrated the dog's skin with her proboscis, and ingested some of the dog's blood... along with our hero Bob. (This mosquito couldn't have been a male mosquito, by the way, since male mosquitos do not have mouth parts suitable for biting animals. Male mosquitos feed exclusively on the nectar of flowers.)

Bob was well equipped for this turn of events. In fact, he was counting on it. Mosquitos are the transmission vector for heartworms. That is, mosquitos are the means by which heartworms are transmitted from host animal to host animal.

Once in the mosquito, Bob migrated to the mosquito's Malpighian tubules. Malpighian tubules clean an insect's blood and deposit waste into its hindgut for excretion. (Bob also could have been perfectly happy in the mosquito's salivary glands.)

Heartworm development in the mosquito is temperature sensitive, requiring two weeks of temperatures at or above 80º F (27º C). A fortnight of warm weather later, Bob had transformed himself from a microfilarium into a larva. As a larva, Bob was shaped more like a grain of rice than a snake, though he still was tiny. He was now in his infective stage, ready to be transmitted to a new host.

The mosquito, feeling peckish once more, landed on the back of a yet-uninfected German Shepherd. In the process of drawing the German Shepherd's blood, the mosquito spat Bob into the small wound she created in the German Shepherd's back.

Bob flowed through the German Shepherd's bloodstream until he reached its pulmonary artery, which is the major vessel that supplies the lungs with blood. Bob attached himself there and began to grow. Within two months after Bob's arrival in his German Shepherd host, Bob had transformed himself into his adult form, which resembled nothing so much as cooked angel hair pasta. As a male heartworm, Bob was a little smaller than the average female heartworm his age. He also had the coiled tail that is characteristic of male heartworms. (Female tails are straight.)

As time went by, twenty other heartworms also infected the German Shepherd and set up their own homesteads in its heart and pulmonary artery. As a result, the German Shepherd became lethargic and acquired a chronic cough. It became anemic, its breathing became labored, its abdomen swelled, and it drank an excessive amount of water.

The German Shepherd's owners were concerned and took it to the veterinarian. The vet gave the correct diagnosis: heartworm disease. To combat its infestation, the German Shepherd was given the arsenical compound thiacetarsamide sodium intravenously, which killed Bob and all his fellow adult heartworms within a month or so. After the adult heartworms were dead, the German Shepherd was given another type of drug that killed the microfilaria in its bloodstream, leaving it completely free of heartworm infestation.

The German Shepherd was lucky to survive such an extensive heartworm infestation. Dead heartworms often clog a dog's veins and arteries, leading to stroke or sudden death.

At the time of his death, Bob was about as large as a heartworm can grow: ten inches long (twenty-five centimeters).

Hawcroft, Tim. The Howell Book of Cat Care. New York: Macmillan, 1991.

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