Cats are host to a variety of internal and external parasites. Despite the documented prevalence and zoonotic importance of these parasites, many pet owners and some veterinarians aren't convinced that comprehensive feline parasite control strategies are needed. This viewpoint may stem from the previous lack of safe, effective, and convenient broad-spectrum parasiticides and the difficulties in acquiring adequate fecal samples. Fortunately, newer broad-spectrum agents (Table 1), particularly those with label claims against heartworms and fleas, allow veterinarians to eliminate a higher percentage of feline parasites. Let's review some of the key feline parasites and discuss new strategies for controlling them.
Dr. Byron L. Blagburn
Although cats are host to numerous internal parasites, heartworms, roundworms, and hookworms are the most important. Fortunately, all three are easily preventable with available medications (Table
Heartworms. Serologic studies indicate that cats' exposure to heartworm-infected mosquitoes is common in many regions of the United States. Although most infected cats are asymptomatic, it's impossible to predict when and under what conditions asymptomatic cats will develop clinical heartworm disease. Symptomatic cats present with vague signs such as mild dyspnea, intermittent coughing, and occasional vomiting that's not associated with eating. Respiratory signs are similar to those observed with feline asthma. A small percentage of cats present with peracute dirofilariasis or die suddenly. This peracute presentation also mimics signs of feline asthma or cardiomyopathy (dyspnea). Many of these cats are clinically normal before the acute heartworm-induced event. Some affected cats also lose weight or develop diarrhea without respiratory signs.
Problems associated with diagnosing and treating feline heartworm disease emphasize the need to prevent infection, especially in high prevalence areas. Essentially, feline heartworm disease isn't treatable because there isn't a safe, effective adulticide for cats. You can treat and alleviate clinical signs, but you can't eliminate heartworms safely. To make matters worse, very few infected cats test positive using either antigen or microfilaria detection methods. Many infected cats harbor fewer than three worms (Figure
Table 1 Selected Feline Internal and External Parasiticides
Roundworms. Although surveys indicate that roundworms (Figure
2) are the most common internal parasite in cats, the importance of Toxocara
cati infections is often underestimated. Contrary to the beliefs of many veterinarians and pet owners, these surveys also indicate that cats may harbor roundworms throughout their lives, resulting in substantial environmental contamination (Figure
3). The high prevalence of roundworms in cats stems from the multiple ways infection can occur (e.g., embryonated eggs; transmammary transmission; paratenic hosts, such as rats and mice; and the long life of embryonated eggs in the environment). One T. cati female can produce thousands of eggs per day, and eggs can live several years in protected indoor and outdoor environments.
Figure 1 Most feline heartworm infections consist of only a few worms. Often a single worm is present, as this case illustrates.