Just how common are canine and feline intestinal parasites?

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.

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.

Table 1. Numbers of fecal specimens received from the different survey regions for national parasite-prevalence study

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.)

Eggs of the canine whipworm T. vulpis were observed with greatest frequency in dogs over 6 months of age. Increased prevalence of T. canis in young dogs (less than 6 months) and lower prevalences in dogs greater than 6 months was expected due to transplacental infection of T. canis and some acquired immunity to T. canis develops as dogs age.

We observed that hookworms remained prevalent in dogs of all ages, despite reports of age resistance to the canine hookworm, or reduced prevalences in older dogs. As mentioned previously, we could not enumerate parasites in our survey population. It is probable that older dogs harbor fewer hookworms (or other parasites) than younger dogs. The decreased prevalence of T. vulpis in dogs less than 6 months old compared to older dogs is easily explained by this parasite's three-month developmental cycle.

Intact female and male dogs were infected with T. canis, A. caninum and T. vulpis or combinations of these parasites with greater frequencies than either spayed female or neutered male dogs. Reduced prevalence of parasites in spayed or neutered animals has been reported in several surveys. Many believe that elective spay/neuter procedures reflect a higher degree of owner stewardship or responsibility for pets, and that these pets are more likely to be examined for parasites or to have received parasiticidal medications.

Results of more recent surveys continue to support the results of our national parasite survey. All surveys reinforce the need to support the year-round parasite control guidelines recommended by the Companion Animal Parasite Council, and emphasize the need to examine dogs regularly for intestinal parasites. It is important to remember that shedding of parasites in feces can create heavily contaminated environments, increasing the probability of reinfection of existing pets or infection of newly acquired ones. Of equal or greater importance is the growing concern over human infections with canine parasites, particularly tapeworms, Ancylostoma spp. and T. canis.

Less information on cats

Compared to dogs, relatively few internal parasite prevalence surveys have been conducted in cats in North America. This can be attributed to several factors. First, except for those practices that are exclusively feline, fewer cats than dogs usually are seen by veterinarians on a regular basis. Secondly, cats comprise a smaller proportion of animal-control or shelter facilities than dogs, making it more difficult to acquire the numbers of cats that we must acquire to obtain data comparable to what has been reported for dogs.

These factors, together with the greater difficulty in obtaining fecal specimens from cats for fecal surveys, have resulted in fewer feline parasite prevalence surveys. However, some information is available.

The feline roundworm (Toxocara cati) appears to be the most commonly observed internal nematode parasite in North American surveys. As reported for T. canis in our canine survey, T. cati appears to parasitize cats in most of the regions in which surveys were conducted.

Also similar to T. canis, this can be attributed to the many ways in which this parasite can infect the cat. The second most common gastrointestinal parasite of cats is the hookworm Ancylostoma tubaeforme. Both of these parasites can cause clinical parasitism in cats, particularly kittens. The feline roundworm also is a potentially important zoonotic agent.

Results of research on its ability to cause disease (known as larva migrans) in humans indicate that T. cati can induce both visceral and ocular disease. Given the availability of effective broad-spectrum parasite control agents in cats, the Companion Animal Parasite Council also recommends a year-round strategy of internal parasite control for cats.

Dr. Blagburn is a Distinguished University Professor at Auburn University's College of Veterinary Medicine.

Acknowledgment

This article is based on Dr. Blagburn's prior publication titled, A Prevalence of Canine Parasites Bases on Fecal Flotation, which appeared in The Compendium on Continuing Education for the Practicing Veterinarian, Vol. 18, No. 5, May, 1996. The author acknowledges the following co-authors of the published article: D.S. Lindsay, J.L. Vaughan, N.S. Rippey, J.C. Wright, R.C. Lynn, William J. Kelch, G.C. Ritchie and D.I. Hepler.