978 
ANIMAL DISEASES 
cutaneous tissues. The first week involves ad- 
justment of the parasite to a completely new 
and different environment, the vertebrae host. 
The end of this period of adjustment is marked 
by a molt occurring seven to ten days after the 
larvae have penetrated into the subcutaneous 
tissues of the dog. The larvae during this time 
migrate quite freely in the subcutaneous tissues 
insinuating themselves between cells and, char- 
acteristically, not evoking a cellular response. 
Migration to the heart is first seen about 70 
days after infection and by 90 days worms can 
no longer be recovered from the subcutaneous 
tissues. Although the migration is to the heart, 
it should be noted that many of these juvenile 
worms are recovered from the distal portions of 
the pulmonary arterial tree and the pathological 
changes noted in pulmonary arteries, to be dis- 
cussed later, suggest that they spend most of 
their time in this location. The reproductive 
system in these worms becomes functional early 
in the fourth month of the infection and fully- 
developed MF are present in the uteri and va- 
gina of the worms at six months. Thus, 25 to 
30 weeks after infection MF can be detected in 
the peripheral circulation.^ 
Pathological changes attributed to Dirofilaria 
immitis can be considered under two headings : 
changes attributable to developing larvae or 
adult worms and changes associated with the 
MF. The most obvious problem associated with 
adult worms is mechanical. The presence of 
these large worms in the right ventricle and pul- 
monary arteries blocks and embarasses circula- 
tion and may result in hypertrophy of the 
heart, fibrosis of valves and chronic passive 
congestion of vital organs. In general the sever- 
ity of these changes is directly related to the 
number of worms present. Secondly, but equally 
important is the fibrosis of the endothelium of 
the pulmonary arteries which can be quite ex- 
tensive. Until recently it was believed that this 
fibrosis was elicited by mechanical insult to 
the endothelium, but studies at Brooks Air 
Force Base, Texas, suggest strongly that this 
may be an allergic reaction to products of the 
worms.2 
Although documentation of antigenic differ- 
ences between adult worms and MF has been 
available for a number of years ^^'^'^ and dis- 
ease specifically attributable to MF has been 
extensively documented in human filariases, it ^ 
is only in recent years that disease associated L 
with MF of D. immitis in the dog has been in- 
vestigated. Casey documented extensive dam- j, 
age to glomeruli in dogs infected with D. im- 
mitis? He indicated that this damage could be \ 
associated with the deposition of antigen-anti- \ 
body-complement complexes. ( 
Although pathological changes associated \ 
with developing third- or fourth-stage larvae in | 
the vertebrae have not been considered damag- | 
ing to the host, effects of sensitization resulting | 
from the extensive tissue invasion have not been \ 
studied. 
It is well known, and extensively documented, [ 
that D. immitis is highly endemic throughout \ 
the southeastern portion of the United States. [ 
However, there is a lack of documentation on \ 
the incidence of the parasite in other parts of \ 
the country. Some recent reports, such as that of \ 
Mallack et al. of dogs maintained by three hunt [ 
clubs in Maryland, suggest that the incidence of 
D. immitis is over 50 percent.''^ However, sur- 
veys done in dogs that were not housed in the || 
same relatively restricted quarters suggest that 
in fact the incidence is much lower, probably 
of the order of 5 percent. There are few surveys 
of large populations of dogs in the midwestern 
region of the country, but in these the reported 
incidence of heartworm is relatively low, less 
than 5 percent. In the western states the in- ^ 
cidence of D. immitis is indeed very low. There 
are occasional reports of dogs infected with the { 
parasite, but these are so rare that one would ) 
suspect importation of the dog or close associa- i 
tion with an imported dog. 
A major problem associated with surveys for \ 
D. immitis is the widespread distribution of | 
the related filaria, Dipetalonema reconditum. | 
It was only in 1956 that Newton and Wright \ 
recognized that a second filarial worm com- j 
monly parasitized dogs in the United States.^ ) 
Although MF of this parasite usually are not |! 
abundant, when observed they must be differen- j 
tiated from MF of D. immitis. In some instances j 
it is clear that the investigators involved in \ 
the survey were carefully separating the two 
species,^ but in others it is quite evident that [ 
differentiation between D. immitis and D. re- , 
