DIROFILARIA IMMITIS: A REVIEW OF PRESENT KNOWLEDGE 
G. Pacheco* 
Dirofilaria immitis, the dog heartworm, is highly 
endemic throughout the southeastern portion of the 
United States. Although its distribution in the rest of 
the country is not as well documented, recent reports 
indicate that it is prevalent. In fact, some reports 
document extremely high focalized incidence. Adult D. 
immitis, commonly found in the right ventricle and 
pulmonary arteries, shed their embryos (microfilariae — 
MF) into the blood stream. The MF are ingested by 
mosquitoes, migrate to the Malpighian tubules and 
develop intracellularly for 12 to 16 days before they 
are infective for a second host. In dogs pathological 
changes ascribed to infection with D. immitis are noted 
in the heart and pulmonary arteries. Additionally, 
immunologic damage of glomeruli has been reported in 
kidneys of dogs with large numbers of MF in circulation. 
In man the "coin-type" of lesion seen radiologically in 
lung is commonly associated with dead juvenile D. im- 
mitis. Although treatment of infections with heartworm 
in the dog remains primitive — organic arsenicals to 
destroy the adults and dithiazinine iodide for the MF — 
prophylaxis is now well established. Daily use of diethyl- 
carbamazine in sentry dogs maintained by the Air 
Force, as well as in colonies privately maintained for 
hunting, has resulted in complete protection against this 
parasite. 
INTRODUCTION 
Taxonomically, Dirofilaria immitis is one of 
a very large group of nematodes known as fil- 
ariae. Most terrestrial vertebrates are parasit- 
ized by at least one of these nematodes which 
have in common the requirement of a blood- 
I sucking arthropod as a combination vector — 
I intermediate host. Adult worms of this group 
of nematodes are commonly found in subcutane- 
ous tissues or in the body cavities and, excep- 
tionally as in the case of the dog heartworm, 
directly in the circulatory system. Reproductive 
products — ^microfilariae (MF) — are in the blood- 
stream or in skin and are ingested by the ar- 
thropod when it feeds. 
*U. S. Department of Health, Education, and Welfare; National 
Institutes of Health; National Institute of Allergy and Infectious 
Diseases; Laboratory of Parasitic Diseases, Bethesda, Maryland 20014. 
Adult male and female D. immitis live in the 
right ventricle and pulmonary arteries of dogs. 
The females are approximately 25 cm long and 
about 1 mm in diameter. The males are shorter, 
approximately 16 cm in length, but have the 
same diameter. Although they are commonly 
found in the right ventricle, they may also be in 
the pulmonary arteries, particularly those of 
the diaphragmatic lobes of the lung. The im- 
pression that one has from this localization is 
that the worms move at will from the heart to 
the arteries and back. In this location the adult 
females shed the MF directly into the blood- 
stream. The MF, which are actively moving 
embryos measuring approximately 300 ju. in 
length by 6 to 7 in diameter are picked up 
from the bloodstream by mosquitoes when these 
take a blood meal. The MF migrate out of the 
stomach and penetrate the Malpighian tubules 
of the mosquito. They take an intracellular 
position in the Malpighian tubules and under 
optimal conditions develop for at least two 
weeks. Development in the Malpighian tubules 
of the mosquito involves extensive growth, 
molting and reorganization, arriving finally at 
a third stage larva. This larva is approximately 
1 mm in length by 25 [x in diameter. It breaks 
out of the Malpighian tubules and migrates to 
the head of the mosquito into the sheath sur- 
rounding the proboscis. Thus, when the mos- 
quito takes a blood meal the sheath is bent, 
ruptured and the infective-stage larvae de- 
posited right next to the puncture wound made 
by the stylets of the biting mosquito. When the 
stylets are withdrawn, the larvae enter the 
vertebrae host through this puncture wound 
and begin their development to adult worms. 
The development from third-stage infective 
larvae to adult worms requires approximately 
six months. From two to two and one-half 
months of this time the worms are in sub- 
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