Diagnosing internal parasites in cats

Most of us are aware that numbers of cats have surpassed dogs as pets in United States households. The same is true in many European countries. Even in countries where cats are less common pets than dogs, the numbers of cats appear to be increasing. As stated by Bowman and co-authors (Feline Clinical Parasitology, Iowa State University Press, 2002), "regardless of national boundaries, with the ever-increasing limitations of time, money and space, the cat is rapidly becoming the cosmopolitan pet of the 21st century". Interestingly, even with these evident trends, often little attention or effort is given to determining or defining prevalences, disease problems or control strategies for internal parasites of cats (See "Possble reasons why ...," p. 21). Many are unaware that cats and dogs often harbor different parasites that are sometimes acquired in different ways. More importantly, feline internal parasites may vary in their susceptibilities to available canine parasite control products, often requiring different dosages or regimens. This article addresses prevalences of feline internal parasites, perceptions of their importance (including zoonotic potential), and diagnostic techniques and control strategies. I also will suggest methods to increase accuracy and sensitivity of fecal diagnostic techniques. Perhaps an increased awareness of feline parasites, improved fecal examination procedures and a knowledge of feline parasite control products will enhance our capability to control major feline intestinal parasites.

Table 1: Overview of selected internal parasites of cats

Analyzing feline parasiteprevalence data

Unlike dogs, few recent comprehensive internal parasite surveys have been conducted in cats. However, a review of available survey results indicates that the roundworm, Toxocara cati, and the hookworm Ancylostoma tubaeforme are the more common worm parasites. The feline roundworm usually ranks as the most common parasite recovered, regardless of the geographic region in which the surveys were conducted. This can be explained by the variety of routes by which T. cati can be acquired, and the capability of the embryonated egg to survive under a variety of adverse environmental conditions (Table 1, p. 17). The hookworm, A. tubaeforme, occurs with greater frequency and often predominates in areas where climatic conditions are mild - a reflection of the susceptibility of free-living larval stages to extremes of temperature and humidity.

Results of recent surveys provide interesting insights into prevalences of parasites in cats of different ages and habits (shelter animals vs. pet animals). A survey published by researchers at Cornell University reported prevalences of T. cati in shelter and pet cats less than 1 year of age. The overall prevalence of T. cati was 33 percent. As expected, the prevalence in shelter kittens (37 percent) exceeded that of pet kittens (27 percent). However, the latter rate of 27 percent in pet kittens reminds us that the feline roundworm remains a prevalent and potentially dangerous zoonotic parasite. Pet owners and some veterinarians are not aware that the feline roundworm can also be a significant cause of larva migrans, an ocular, visceral or neural disease of humans resulting from migration of ascarid larvae. We at Auburn University recently initiated a fecal survey of internal parasites of shelter cats in East Central Alabama. Results of our survey again support the predominance of T. cati and A. tubaeforme in the survey population. After completion of 52 fecal examinations from cats of different ages, T. cati eggs were observed in 23 percent of the samples. Ancylostoma tubaeforme eggs were recovered from 27 percent. We concluded that hookworm eggs were those of A. tubaeforme, because A. braziliense is restricted to the coastal areas of the southeast. As mentioned above, these prevalences reflect the capability of hookworm larvae to survive in the milder climates that prevail in the deep South. What is perhaps most surprising about our results thus far is that 8 of 12 cats that harbored T. cati and 11 of 14 cats that harbored A. tubaeforme were 2 years old or older. No differences in prevalence of these parasites were observed between spayed or neutered, and intact cats. Certainly, available prevalence data, limited as it is, should encourage veterinarians to be more aggressive in conducting fecal examinations on cats, and not to presume that these parasites occur too infrequently to deserve our attention.

Table 2: Selected feline internal parasiticides

The importance of fecal examination procedures

Skill in the conduct and interpretation of fecal examinations is important if feline internal parasites are to be diagnosed accurately and effectively treated. The performance of reliable and accurate fecal examinations requires a knowledge of the procedures, a thorough familiarity with the important parasites of cats (see Table 1), and how to use this information in a reliable strategy. In this section, I will review in-clinic fecal examination procedures and make specific recommendations about improvements in the conduct and sensitivity of feline fecal examination procedures. In most cases, a few simple improvements will result in marked increase in sensitivity of tests and the diagnostic proficiency of clinic staff.

Select the appropriate procedure

A wide variety of feline fecal examination procedures are available. These include direct examination of feces (direct smear), simple fecal flotation, centrifugal fecal flotation, sedimentation and commercially-available immunologic procedures. Direct smears are most suited to recovery of live protozoal stages (i.e. Giardia, see Table 1) and larvae that are not easily concentrated by flotation. Direct smear techniques require the use of a small amount of feces, which markedly decreases test sensitivity. The simple flotation technique and the centrifugal flotation technique are similar except that the latter uses the force of centrifugation to float parasite stages more effectively. Centrifugation increases sensitivity and is particularly helpful when parasite stages are present in low numbers. The technique requires either a fixed-angle or swinging-bucket centrifuge and only a slight increase in conduct time. Most reference laboratories (ours included) perform the centrifugal procedure because of the desired increase in sensitivity. I would encourage all veterinarians to perform the centrifugal procedure if possible (See "Recommendations," p. 22). We have generated data in our laboratories that indicate that when egg or cyst numbers in feces are low, a situation that might be encountered in pet cats, the centrifugal procedure significantly increases the likelihood of recovery of fecal stages.

Possible reasons why cats are believed to harbor fewer internal parasites than dogs

The sedimentation procedure is preferred when stages of target parasites are too large or dense to float in standard flotation solutions. In this procedure, parasite stages are sampled from the bottom (sediment) of the container where they are concentrated. It is most effective when combined with centrifugation, although simple sedimentation techniques can be performed. Sedimentation procedures are used more commonly for diagnosis of large animal parasites than for canine or feline parasites. Fecal immunologic tests for cats are generally used only for detection of feline enteric protozoa such as Giardia or Cryptosporidium. These tests are quite sensitive, but often require numerous procedural steps (Elisa format) or expensive microscopes for viewing prepared specimens (immunofluorescent antibody format). The Elisa format, short for enzyme-linked immunosorbent assay, is similar to that used for detecting heartworm antigen in blood. This technique is the more likely of the two to be accepted and used by veterinarians.

Fecal flotation solutions

A number of fecal flotation solutions are available for use in flotation procedures. Among them are sodium nitrate, sodium chloride, magnesium sulfate, zinc sulfate and sucrose. Sodium nitrate is the most common solution used by veterinarians. All of the solutions can effectively recover fecal stages of feline internal parasites if prepared and maintained properly. One possible exception is the use of zinc sulfate for recovery of cysts of Giardia. It seems that the combinations of parasite density, specific gravity of zinc sulfate (1.18-1.20) and the viscosity of zinc sulfate are matched quite well. Zinc sulfate floats cysts of Giardia without imposing significant specimen distortion. Given that zinc sulfate also will recover other major feline parasites such as T. cati and A. tubaeforme equally well, I would encourage veterinarians to consider the use of zinc sulfate as their flotation solution of choice.

Recommendations for fecal flotation procedures in cats

Regardless of the solution used, it is wise to examine preparations immediately to minimize changes in parasite structure, and to prevent formation of salt crystals beneath the coverslip. Crystals can interfere with the ability to visualize parasite stages in the preparation. We use sucrose in our centrifugal procedure in the diagnostic laboratory because specimen integrity is preserved, crystallization of media generally does not occur and specimens can be read later in the day if laboratory workload prohibits immediate examination. Sucrose generally requires conduct of the centrifugal procedure because the viscosity of the solution necessitates that the preparation remains undisturbed for at least 30 minutes when conducting the passive procedure.

Specimen attributes

Use of fresh feces is a must when conducting most fecal examinations. Older specimens may contain parasites that have developed to a stage beyond the stage that is the normally recognized in feces. This is particularly important for eggs that hatch to first stage larvae (i.e. hookworm eggs). These larvae must be differentiated from larvae of other parasites and also from free-living larvae. If clients collect samples but cannot deliver them immediately, they (if they are willing) can be refrigerated (not frozen) until they can be delivered. Also remember that when the fluid content of feces is high (e.g. diarrhea) the numbers of recoverable parasite stages may decrease, due to a dilution effect. This results in a smaller of amount of solid material in the sample. This problem can be remedied in some cases by increasing the amount of feces that is examined in proportion to the degree of diarrhea.

Although it is unlikely for cats, if specimens are collected from the ground, advise the client to sample fresh feces and to sample several areas not in contact with soil. This will prevent the contamination of the sample with free-living organisms. Also, remember that the sensitivity of the assay is dependent upon the amount of feces that is used. This will likely to be more of a problem for cats than for dogs and other hosts. Contact with the client to encourage them to bring a litter-box sample can be helpful. We generally recommend that at least a gram (specimen is approximately one-half inch cube) of feces is used. Sometimes it is necessary to use the amount obtainable with a fecal loop, but veterinarians and veterinary technicians should be aware that a negative result does not reliably indicate that parasites are not present. One last point to emphasize is the incubation time for simple (non-centrifugal) flotation procedures. We recommend that preparations should stand undisturbed for at least 15 minutes before microscopic examination. Examination of the specimen after shorter periods of incubation can result in false negative results, particularly when the numbers of eggs in the fecal sample is low.

Improving accuracy of results

I believe that is it a good idea to assign one support person to examine of fecal specimens. This allows a single individual to become quite skilled at recognizing even the most challenging of fecal stages. That individual can share observations and acquired skills with other personnel when time allows, but should remain the "go to" person for fecal analysis. I encourage veterinarians and technicians not to under emphasize the knowledge and skills required to practice good feline clinical parasitology. The personal satisfaction and improved patient care gained by developing this knowledge and these skills are rewarding.

Future trends

Given the increasing popularity of cats as pets, it is important to establish effective monitoring strategies for feline internal parasites. Convincing pet owners of the importance and zoonotic potential of certain feline parasites, and encouraging them to collect adequate samples for fecal examination will be a challenge. With some success, regular fecal examinations using proper techniques, combined with use of available broad spectrum products (See Table 2, p. 20), can serve as a foundation for effective feline internal parasite control.

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