G. PACHECO 
979 
conditum was not done. Thus, at this time it is 
j difficult to determine the precise prevalence of 
this parasitic infection in dogs throughout most 
of the U. S. However, in the southeastern region 
of the U. S. the prevalence is high : 20 to 60 per- 
cent of dogs surveyed. Some careful surveys, 
such as the one mentioned earlier in dogs main- 
tained by hunt clubs'' indicate clearly that 
whenever dogs are housed in close proximity 
to one another, there is a real danger that a 
hyperendemic focus will be established. How- 
ever, existence of such a hyperendemic focus 
does not mean that the incidence in the sur- 
rounding region will equal that of the focus. 
While it is well established that D. immitis 
is transmitted by mosquitoes and laboratory 
transmission of the parasite is routinely done 
with several species of either anopheline or 
j culicine mosquitoes, there is no information 
j on the natural vectors of the parasite in any 
part of the country. Ludlam et al. tested a 
number of wild-caught species of mosquitoes to 
determine their ability to transmit D. immitis 
and found that ten species common to Mary- 
land would transmit the infection in the lab- 
oratory.^" There is no information on whether 
these same species can serve as vectors in na- 
ture. Thus, control of the diseases directed at 
the vector must await solid documentation on 
which mosquito (s) really serve as vectors. 
Diagnosis of infections with D. immitis has 
been based on finding MF in blood but there are 
a number of problems associated with this. 
First, if MF are seen, how is one to be certain 
that there are D. immitis and not D. recondi- 
tum? Some of the early studies emphasized the 
presence of a "button hook" tail on MF or D. 
reconditum ^ ^ but it has since been recognized 
as an unreliable feature. Although the length 
and diameter of these MF are substantially 
I different and, if measured, will serve to readily 
distinguish the two species, the histochemical 
differentiation of the two species provided by 
Chalifoux and Hunt has given practitioners 
and people who work with these filariae greater 
confidence in their ability to distinguish be- 
tween the two species.i2 
i The second problem that has troubled many 
! people who must evaluate the health status of 
dogs is that when MF are not present in large 
numbers they may be difficult to detect. Al- 
though somewhat more cumbersome than exami- 
nation of a simple blood smear, the efficiency 
of the concentration technics that have been 
developed is such that only a very small pro- 
portion of dogs with microfilaremia will be mis- 
diagnosed. ^^'^^ While diagnosis of infection 
when there is microfilaremia is relatively sim- 
ple, it is a common experience to obtain a dog 
free of MF only to find that in fact adult or 
juvenile worms are present in the heart. Diag- 
nosis of these pre-patent or non-patent infec- 
tions is at present not feasible. Several reports 
have indicated that antibodies are readily de- 
tectable in sera from dogs during the pre- 
patent period of the infection, however, all of 
these studies were done with experimentally 
infected dogs in which it was possible to study 
antibody levels before and after infection, thus 
a titer differential could be determined. ^^■^^-^'^ 
Secondly, when currently available serological 
tests have been used with sera from nonin- 
fected controlled dogs housed indoors in areas 
of low endemnicity, there has been a high in- 
cidence — over 20 percent — in which significant 
levels of antibody have been detected. Thus, 
currently available serological tests for filariasis 
suffer from excess cross-reactivity. These tests 
are useful to indicate the extent or degree of 
prepatent infection in a geographic region, i.e., 
as epidemiological tools, but they cannot be 
relied on to provide a diagnosis for individual 
dogs. 
Recently, purified extracts of D. immitis have 
been used in intradermal tests in attempts to 
detect prepatent infections." As with serological 
tests the proportion of false positive reactions 
is approximately 30 percent. Thus, again, there 
are no reliable tests at present that can be used 
to indicate presence of D. immitis in a dog be- 
fore MF are detectable in blood. 
Management of heartworm infections has 
changed significantly during the past few years. 
Emphasis in the past was only on treatment 
once the infection was established, but at pres- 
ent emphasis is on chemophophylaxis with di- 
ethylcarbamazine. Studies in Japan as well as 
in the United States clearly demonstrated that 
chemoprophylaxis is entirely feasible in dogs 
maintained in colonies or groups in which the 
