Intestinal parasites are common in dogs in the United States, and many species are zoonotic.1 Common intestinal parasites include roundworms, hookworms, whipworms, Giardia spp, and Cystoisospora spp.2 A large national fecal survey of pet dogs conducted in 2009 revealed evidence of infection with at least 1 of these 5 parasites in 12.5% of all tested dogs (n = 1,199,293).2 In the western United States, including Arizona, the overall prevalence of intestinal parasitism was 14.0%, with 6.3% of dogs infected with Giardia spp, 5.2% with Cystoisospora spp, 2.8% with roundworms, 1.4% with hookworms, and 0.5% with whipworms. In a national study3 of intestinal parasitism in dogs in animal shelters in the United States, < 10% of dogs in the western region (including Arizona) were infected with at least 1 of these 5 parasites.
Dogs are typically infected with intestinal parasites early in life, particularly by transplacental and transmammary transfer. The first indication of infection is lack of growth and loss of body condition. Migrating roundworm and hookworm larvae also may cause an eosinophilic pneumonia, manifested by a persistent cough. Diarrhea may also be evident. Severe infections can result in severe pneumonia, anemia, diarrhea, anorexia, ascites, fatty liver, and mucoid enteritis.4
Surveys of incidence or prevalence of intestinal parasitism in the US human population are rare. One estimation is that 4 million Americans are infected with roundworms, with prevalence greatest in Appalachia and the southern United States.5 The zoonotic potential is greatest when pets, particularly dogs and cats, are infected and can pass them to their owners, either directly6 or through environmental fecal contamination.7 Epidemiological studies8,9 have shown that having pet dogs, particularly puppies, and pica (dirt eating) are principal risk factors for human infection.
A nationwide survey of animal shelters revealed that nearly 36% of dogs harbored parasites capable of causing human disease.3 As an example of the transmissibility, a single female roundworm in a puppy can produce > 100,000 eggs/d, resulting in millions of infective eggs in the environment.10 The primary objective of the study reported here was to determine and compare the prevalence of intestinal parasites in fecal specimens from pet dogs and AFR shelter dogs in the metropolitan area of Phoenix.
Materials and Methods
Dogs and specimens
Convenience samples of canine fecal specimens were collected from November 2014 through March 2015 within the boundaries of the Phoenix metropolitan area. Fecal specimens (n = 175) from pet dogs were collected from 3 widely spaced public dog parks, recently owner-surrendered dogs at 2 animal shelters, and dog owners within the general community and Midwestern University students and faculty. Fecal specimens (n = 188) from AFR shelter dogs were obtained from the Arizona Humane Society shelter and a Maricopa County Animal Care and Control facility, both of which take in stray animals from the entire Phoenix Valley. Dogs chosen for this portion of the study included only those apprehended within the previous 72 hours by Humane Society or Animal Control officers and not owner-surrendered dogs, with the intention of representing the free-roaming dog population in the area. Although an identification microchip was not identified in any of these dogs, we could not know for certain whether they were ownerless or merely stray. When known, dog age was recorded for the associated fecal specimen.
Specimen collection
Fecal specimens were handled as similarly as possible from collection to laboratory submission. Fecal specimens from the dog parks included freshly voided specimens obtained with owner permission and specimens collected from closed-lid feces-disposal receptacles. Care was taken to ensure specimens collected from disposal receptacles were only from intact, tied bags to minimize cross-contamination. Only bags that were resting on the top layer were opened to collect specimens, and any specimen from that top layer suspected of being more than a few hours old, subjectively assessed for dryness by the same researcher throughout collection, was not included in the study. To help minimize collection of multiple specimens from the same dog, only 1 specimen was obtained per similar bag type. At the animal shelters, fecal specimens were obtained opportunistically from the nonporous kennel floors of singly housed dogs within 72 hours after facility admission. Collection was performed immediately after shelter personnel had performed their morning kennel cleaning rounds to ensure specimen freshness.
Every fecal specimen was placed into a 30-mL fecal collection tube, and the tube was then subsequently placed inside its own zip-closure bag. After daily completion of specimen collection, a laboratory courier was immediately notified and specimens were housed at room temperature (varied) inside the diagnostic laboratory shipment container while awaiting pickup. Regardless of source, fecal specimens were analyzed within 36 hours after initial collection to ensure consistent results. No approval by an institutional animal care and use committee was required for this study because the dogs were not handled.
Fecal parasite detection
To ensure consistent analyses and results, all fecal specimens were processed at a contract laboratorya for detection of roundworms, hookworms, whipworms, and Cystoisospora organisms by use of a standard centrifugal flotation procedure. Briefly, a 2- to 5-g portion of each fecal specimen was mixed with 10 mL of zinc sulfate solution (specific gravity, 1.18) and passed through a strainer to remove large particles. The mixture was then transferred to a 15-mL centrifuge tube and centrifuged at 1,000 × g for 5 minutes at room temperature (approx 22°C). The meniscus of the resulting column was raised to the top of the tube by injecting fresh zinc sulfate solution below the surface. A coverslip was placed on the meniscus for 10 minutes and then removed and examined microscopically at 100× magnification and subsequently at 400× for identification of oocysts.b
For Giardia detection, a previously validated ELISA was used. This test had a sensitivity of 95% and specificity of 96%.b
Statistical analysis
A statistical software programc was used before the study began to determine the required sample size, revealing that a total of approximately 370 fecal specimens from different dogs would be required to detect prevalence differences between groups. Our estimated prevalence difference was 4.0%, and for this 2-tailed calculation, α was set at 0.05 and power at 60%. After determining the prevalence of various internal parasites in each dog group (pet dogs and AFR shelter dogs), the χ2 test was applied to compare prevalence between groups. Additionally, to reduce the bias of the prevalence estimation, the PRG was calculated, which provides an estimate of true prevalence in the presence of an imperfect diagnostic test.11
The association between dog age and intestinal parasitism (ie, positive results for ≥ 1 type of parasite) and Giardia detection specifically was evaluated for dogs with a known age in both groups and in each group separately. For this analysis, dogs < 1 year of age were considered young and those ≥ 1 year of age were considered adult. The Fisher exact test was used when the number of dogs in any 1 cell for the calculations was < 20 (ie, for the young dog group). Values of P < 0.05 were considered significant for all comparisons.
Results
A total of 363 fecal specimens were collected: 175 (48.2%) from pet dogs and 188 (51.8%) from AFR shelter dogs. Results of fecal parasite testing were summarized (Table 1). Eighty-five (45.2%) fecal specimens from AFR shelter dogs had positive results for at least 1 of the 5 evaluated parasites; 24 (13.7%) fecal specimens from pet dogs had similar positive results, for an overall prevalence of 30.0%.
Number (%) of fecal specimens from AFR shelter (n = 188) and pet (175) dogs in the Phoenix metropolitan area with positive results of intestinal parasite testing.
| Parasite | Shelter | Pet |
|---|---|---|
| Giardia spp alone | 67 (35.6) | 19 (10.9) |
| Cystoisospora spp alone | 6 (3.2) | 0 (0) |
| Roundworms alone | 0 (0) | 1 (0.6) |
| Hookworms alone | 3 (1.6) | 2 (1.1) |
| Whipworms alone | 0 (0) | 1 (0.6) |
| Giardia spp and Cystoisospora spp | 8 (4.3) | 1 (0.6) |
| Giardia and hookworms | 1 (0.5) | 0 (0) |
In general, Giardia spp were most common, distantly followed by Cystoisospora spp, hookworms, roundworms, and whipworms. Of the 109 dogs for which parasites were detected, 99 (90.8%) were infected with a single parasite and 10 (9.2%) were infected with multiple parasites. Of the 96 dogs for which Giardia spp were detected, all (100%) were positive by ELISA testing and 47 (49.0%) were positive by centrifugal flotation testing.
The prevalence of intestinal parasitism with the evaluated organisms was significantly (P < 0.001) greater in AFR shelter dogs than in pet dogs. Given that the prevalence of Giardia spp detection was remarkably high in both groups and the sensitivity and specificity of the ELISA were known, separate statistical comparisons were made for this organism specifically. The prevalence of Giardia spp in AFR shelter dogs was 40.4% (PRG = 0.400 [40.0%]), and that in pet dogs was 11.4% (PRG = 0.082 [8.2%]; P < 0.001).
Age was known for 83 pet dogs (47.4%; 6 young and 77 adult) and was estimated on the basis of dentition for 187 AFR shelter dogs (99.5%; 48 young and 139 adult). In all dogs combined, intestinal parasitism with the evaluated organisms (P = 0.004) and with Giardia spp specifically (P = 0.006) was significantly more prevalent in young dogs than in adult dogs. Similar differences between age groups were identified for pet dogs in regard to intestinal parasitism with the evaluated organisms (P = 0.01) and with Giardia spp specifically (P = 0.046). For AFR shelter dogs, however, neither the prevalence of intestinal parasitism with the evaluated organisms (P = 0.23) nor with Giardia spp specifically (P = 0.20) differed significantly between age groups.
Discussion
Intestinal parasites are common in dogs and may result in clinical signs. Some of these parasites also have zoonotic potential.1 Knowledge of the epidemiology of these parasitic infections may guide development of intervention strategies for their prevention and management.
National data are available from 2 reports of results for > 2 million pet dogs evaluated for parasites by either a diagnostic service12 or a network of companion animal hospitals.13 In these national reports, the data were further stratified down to specific states13 and counties.12 However, the data obtained in those studies pertained to pet dogs only, which are generally cared for differently than AFR shelter dogs. Indeed, compared with findings in those reports,12,13 shelter dogs in Florida14 and Canada15 have a considerably higher prevalence of intestinal parasitism.
Maricopa County, which encompasses the Phoenix metropolitan area, has a high volume of animals entering its shelters each year, with 52,673 animals admitted in 2015, of which 42,904 were placed up for adoption as pets.16 Therefore, we considered it important to determine the prevalence of intestinal parasites in shelter dogs in Phoenix as well in pet dogs.
The overall prevalence of Cystoisospora spp, roundworms, hookworms, and whipworms in the present study was within (or slightly below) the expected range, compared with findings of previous studies2,17 involving dogs in the United States. The prevalence of Giardia detection in fecal specimens, however, was notably higher in the Phoenix metropolitan area than previously reported.2,18,19 The reason for this difference is not immediately apparent, but we speculate that a few factors could have been involved.
First, the previous studies2,19 involved use of microscopy as the sole method of detection for Giardia organisms in fecal specimens, whereas in the present study, data included results that were positive on ELISA but negative on microscopy. If fecal specimens were considered positive for Giardia detection only when results of centrifugal flotation testing (and microscopy) were positive (47 specimens), the overall prevalence of Giardia detection in the present study would have been only 12.9% (47/363). That overall prevalence calculation compares favorably with previously reported findings18 for dogs in general but is still considerably higher than that in the US pet dog population. Second, the arid, hot climate of Arizona is different from environmental conditions in many other US regions. This type of climate may compel dogs to drink more and use certain water sources vulnerable to Giardia contamination (eg, man-made lakes or decoration fountains) more often than dogs in other regions. As a result, they may be more likely to ingest larger doses of infectious cysts. Third, because natural water sources in Arizona are rarer than in other US regions, greater numbers of animals may frequent those few available sources, thereby concentrating the burden of infectious cysts and increasing the likelihood of ingestion.
The zoonotic potential of Giardia organisms is dependent on their genetic assemblages, and the prevalence of various genetic assemblages varies geographically. For example, in Germany and Japan, dogs carry mostly dog-specific Giardia assemblages of low zoonotic potential.20,21 However, 1 study22 showed that zoonotic Giardia assemblages are common in dogs living in urban environments. The zoonotic potential of the Giardia spp detected in the present study remains to be determined. Of the other parasitic organisms detected, only hookworms and roundworms are known to have clinically important zoonotic potential.23
The present study had several limitations. With respect to the representative findings for the Phoenix metropolitan region, certain zip codes were overrepresented. In particular, 45 fecal specimens (25.7% of pet dog specimens) pertained to the zip code 85308, which represents the Arizona campus of Midwestern University. This bias was introduced through the convenience sampling strategy used. Along with this geographic bias, dog owners represented by this zip code were associated with a health sciences university and therefore likely had more knowledge about intestinal parasitism and its prevention than other dog owners.
The results pertaining to AFR shelter dogs (which could have included lost, owned dogs), as opposed to stray dogs, in the present study may be difficult to interpret. Although some dogs may have acquired intestinal parasites before or during their shelter stay, fecal specimens were collected within the first 72 hours after arrival at the shelter, suggesting that most positive results were attributable to a prior infection because the prepatent period for the 5 evaluated parasites typically exceeds 72 hours. Specifically, the prepatent period is 21 to 56 days for roundworms,24,25 13 to 28 days for hookworms,25,26 70 to 90 days for whipworms,25 4 to 21 days for Giardia spp,27 and 6 to 11 days for Cystoisospora spp.28 In addition, compared with a healthy dog, a sick dog would be more likely to be captured by animal control officers, so the findings for AFR shelter dogs in the present study may not pertain to the general population of free-roaming dogs. An additional study limitation is that all fecal specimens were collected in the late fall and winter, and the findings might be expected to differ had the specimens been collected at a different time of year.
Intestinal parasitism is more common in young than adult dogs, mainly because young dogs lack acquired immunity to the parasites29 and have more potential sources of exposure (eg, transplacental and transmammary).30 In the present study, intestinal parasitism was significantly more common in pet dogs < 1 year of age than in older pet dogs, which is similar to data in other reports.2,30–34 However, AFR shelter dogs in both age groups had equally high prevalences of intestinal parasitism. This might have reflected a higher likelihood of contact with infected animals, greater exposure to a high parasite burden in the environment, and less preventative health care provided than for pet dogs. Previous studies involving shelter dogs, stray dogs, or both in similar age groupings have yielded conflicting results. In some studies, no significant difference was identified between age groups in the overall prevalence of intestinal parasitism35 or infection with Giardia spp,36 whereas in other studies, significant differences were identified.15,36,37
The present study showed that the prevalence of intestinal parasitism with the evaluated organisms in dogs in the Phoenix metropolitan area was similar to previously reported prevalence data from national or regional surveys, with the exception of the prevalence of Giardia spp, which was notably higher. Additionally, the prevalence of all evaluated organisms was significantly higher in AFR shelter dogs than in pet dogs. Giardia sp was the most prevalent organism in this select sample. Findings may be useful for the development of intervention strategies for the prevention and management of intestinal parasitism among dogs living in the Phoenix metropolitan area.
Acknowledgments
Supported by a Midwestern University College of Veterinary Medicine Intramural Research Grant and Student Research Fellowship.
The authors thank Drs. Kathleen Croteau and Kimbal Cooper for technical assistance.
ABBREVIATIONS
| AFR | Apprehended free-roaming |
| PRG | Rogen-Gladen prevalence estimate |
Footnotes
Antech Diagnostics, Phoenix, Ariz.
Klodja T, Antech Diagnostics, Irvine, Calif: Personal communication, 2014.
G*Power: statistical power analyses for Windows and Mac, Heinrich-Heine-Universität Düsseldorf, Düsseldorf, Germany. Available at: www.gpower.hhu.de/en.html. Accessed Apr 23, 2015.
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