Given the number and kinds of effective de-wormers that are available, parasitologists often are asked why parasites continue
to be prevalent and important disease agents of companion animals.
The presumption is that, as we introduce newer, more effective products, we should reduce the prevalence or maybe even eradicate
common parasites such as roundworms, hookworms and whipworms.
Interestingly, when we compare surveys of prevalences of canine gastrointestinal parasites, in most cases we do not notice
a dramatic decline in the rates for common canine intestinal worms. Let me give you an example:
Not long ago we attempted to obtain a comprehensive view of canine parasite prevalences by conducting a national survey. We
used a centrifugal fecal flotation method to sample shelter dogs from all geographic regions of the United States. The survey
was the first of its kind and provided helpful information on national and regional prevalences of canine gastrointestinal
parasites. It also allowed us to compare results of our study to the results obtained in several previous surveys. 
Table 1. Numbers of fecal specimens received from the different survey regions for national parasite-prevalence study
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To get a comprehensive assessment of which parasites are found where, we collected 6,458 fecal specimens from dogs housed
in animal shelters. Table 1 depicts the numbers of specimens obtained from the four different geographic regions where the
samples were obtained. Shelter personnel who assisted us were instructed to sample only those dogs that had not been treated
with deworming medications.
We also requested the following additional information about each dog from which a specimen was obtained: approximate age,
breed and sex, including reproductive status (i.e., male, female, intact, neutered or unknown). Fecal specimens were examined
using a centrifugal sucrose flotation procedure, a very sensitive method for detecting fecal parasite stages in feces.
The canine hookworm, Ancylostoma caninum, was observed most often (19.19 percent) in all dogs. The common roundworm, Toxocara canis (14.54 percent) and the whipworm, Trichuris vulpis (14.3 percent) were observed with similar frequencies. Surprisingly, almost 36 percent of all dogs, and 52 percent of dogs
sampled from southeastern states, harbored at least one major intestinal worm parasite. The numbers of dogs harboring combinations
of parasites also surprised us. Those that were concomitantly infected with A. caninum and T. vulpis (5.7 percent) were most common, followed by T. canis and A. caninum (4.55 percent) and T. canis and T. vulpis (3.21 percent). It is noteworthy that 1.39 percent of the dogs harbored all three major intestinal nematode species.
Prevalences of nematodes in individual regions generally mirrored trends observed nationally. Exceptions were the Southeast,
where prevalences of nematode parasites were considerably greater than those observed nationally, and in the West, where prevalences
were somewhat lower. These results are consistent with previous surveys. Differences can be attributed to environmental factors
such as soil type, ambient temperature, relative humidity and the nature of the pet population (i.e., predominantly urban
or rural; nature of care, i.e., well-cared-for dog or poorly-cared-for dog).
Prevalence based on age, gender
Eggs of T. canis were recovered more frequently from dogs less than 6 months of age. Thereafter, prevalences diminished as dogs aged, but
were still present even in dogs over 7 years of age. Infections with A. caninum occurred with similar frequencies in dogs of all ages, although the numbers of worms in older dogs may have been less. (We
could not determine the numbers of parasites in each dog.)
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