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
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
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: