Chapter 12 Drugs for the Treatment of Protozoal Infections
Protozoal infections can provide diagnostic as well as therapeutic challenges for the small animal clinician. The situations can range from the simple treatment of a young kitten with coccidiosis to the more robust challenges of chronic giardiasis in a breeding kennel. Although several of the protozoa are well characterized and relatively easy to treat, others are poorly understood and have no specific agents available for therapy. Three types of infections caused by major pathogens are presented here: common enteric coccidia, toxoplasmosis, and giardia. The pathogens are known to affect dogs and cats, and two are significant zoonotic agents.
This chapter does not include the protozoa that appear only sporadically in the veterinary literature (Balantidium, Pentatrichomonas, Entamoeba, Hammondia, Besnoitia, and Sarcocystis spp.). These organisms are not adequately documented as pathogens of dogs and cats and thus require no therapy. Also excluded are selected protozoal pathogens that are partially characterized but have no effective treatment available. Textbooks of parasitology or infectious disease should be consulted for a complete discussion of these sporadic, spurious, or untreatable pathogens.1,2
Therapy of protozoal infections, which are often zoonotic, must include use of therapeutic agents along with supportive therapy and proper hygiene and husbandry to clean up the environment and prevent spread to other animals and people. No therapeutic agent, no matter how safe or effective, can be expected to treat these diseases without supportive therapy and hygiene. Table 12-1 lists the therapeutic agents discussed in the text. These drugs are discussed below as well as in specific chapters.
Common Enteric Coccidiosis
Biology
The most common protozoa in small animal veterinary medicine are the coccidians, which cause a condition termed coccidiosis. Coccidia are very host specific. Dogs and cats are infected with several species in the genus Isospora. Diagnosis is readily made by conventional fecal floatation techniques using concentrated sugar or salt solutions. Careful identification of coccidia oocysts may reveal the presence of spurious coccidia from other genera, especially Eimeria spp., which commonly parasitize food animals, thus indicating coprophagy. Nevertheless, coccidia are ubiquitous in young dogs and cats and commonly cause disease, especially in those with suboptimal nutrition, immune status, or stress.
Coccidia are obligate intracellular parasites that depend on dispersion of fecal oocysts for transmission. This fact alone illustrates the importance of hygiene. There are four species that infect dogs (Isospora canis, Isospora ohioensis, Isospora burrowsi, and Isospora neorivolta) and two that infect cats (Isospora felis and Isospora rivolta). Although direct ingestion of the oocyst is the primary means of infection, rodents can serve as paratenic hosts if they ingest the oocyst and then are eaten by the definitive host.
Coccidia have life cycles that are more complex than other infectious agents (Figure 12-1). Each life cycle includes both sexual and asexual phases. This is important to remember because the therapeutic agents used to treat and control coccidia are primarily effective against the asexual stage of the life cycle.
Figure 12-1 Life cycle of Isospora felis, which is typical of the Isospora spp. The mode of transmission may be direct, via ingestion of sporulated oocysts from the environment, or indirect, via ingestion of cysts in prey animals. Sexual and asexual reproduction of the parasite occurs in the intestines of the definitive host (in this case, a cat), and unsporulated oocysts are shed in the feces of definitive hosts.
(From Dubey JP, Greene CE: Enteric coccidiosis. In Greene CE, editor: Infectious diseases of the dog and cat, ed 2, Philadelphia, 1998, Saunders, p. 511.)
The oocyst is passed in the feces, and, after suitable exposure to air, heat, and moisture, the oocyst sporulates. This process may take only a few hours or a few days, depending on the species of coccidia and on the environmental conditions. During sporulation each oocyst develops into two sporocysts that contain four sporozoites each; thus each oocyst contains a total of eight infective sporozoites.
After ingestion the sporozoites are liberated from the oocyst and invade the enterocytes that line the small intestine. Once inside the enterocytes, the sporozoites turn into trophozoites, which undergo asexual fission (properly termed schizogony or merogony) to produce many daughter schizonts. After 4 days the enterocyte ruptures and releases the multiple schizonts (or meronts). This schizont stage is the place in the life cycle where therapeutic agents have a chance to break the life cycle. Because the schizonts are released from the cell only every 4 days, the therapeutic agent should be present in the gut for several multiples of this period, usually 14 to 21 days. The daughter schizonts are capable of infecting new enterocytes and repeating the cycle of fission into many daughters and rupture of the subsequent enterocytes. The number of asexual cycles has been determined for each species of coccidia; the small animal pathogens typically have two or three asexual cycles before entering the sexual stage of the life cycle.
The schizont, produced by the last cycle of asexual fission, enters another enterocyte and develops into either a male or a female gametocyte. The female gametocyte enlarges and forms a singular large cellular structure within the enterocyte. The male gametocyte undergoes fission to produce many small biflagellate male sex cells. The enterocytes rupture, and the motile male sex cells fertilize the female gametocytes, which mature to a zygote and then pass out in the feces in the form of an oocyst. The fresh oocysts are exposed to the external environment, where they sporulate and infect new hosts.
The repeated intracellular invasion of enterocytes and subsequent rupture can produce substantial pathology to the gut, especially if the infected host is young, weak, malnourished, or stressed. Normal animals, in otherwise good health, usually experience coccidial infection followed by an effective immune response that limits and eliminates the infection without therapeutic intervention. Most clinicians prefer to intervene when coccidia are identified in a fecal floatation. Therapy is usually successful in eliminating the coccidial oocysts, although it is not known how many of these animals would have spontaneously cleared the infection without intervention.
Treatment
Sulfas and Potentiated Sulfas
Use of sulfonamides is the treatment of choice for small animal coccidia as well as a number of other protozoal organisms. Unfortunately there is a paucity of research information to support their efficacy. Two pivotal studies on sulfamethoxine and sulfaguanidine against coccidia support their utility; however, these two agents are no longer available in the United States.3,4 Clinicians have empirically substituted more readily available sulfonamides and enjoyed apparent clinical success.5 Currently there is one simple sulfa and three potentiated sulfas that are commonly used in the United States: sulfadimethoxine (Albon), sulfadimethoxine with ormetoprim (Primor), sulfadiazine with trimethoprim (Di-Trim, Tribrissen), and sulfamethoxazole with trimethoprim (Bactrim, Septra).
Sulfonamide Chemistry and Mechanism of Action
The sulfonamides are discussed in more depth in Chapter 7. Each is a structural analog of para-aminobenzoic acid that competitively inhibits the dihydropterate synthetase step in the synthesis of folic acid, which is required for synthesis of RNA and DNA. Inhibition by sulfas impairs protein synthesis, metabolism, and growth of the pathogen. A vast array of sulfa agents have been created and described; all but a few have been lost in the sands of time. The important differences among these agents involve their solubility, duration of action, and activity against key pathogens. Fortunately, the three sulfas included in this discussion demonstrate acceptable performance in all three categories; solubility is adequate, they are given once or twice daily, and they have a reasonably broad spectrum of action. The sulfa drugs are primarily effective against the schizont stages of coccidian; thus prolonged treatment may be required for the drug to effectively block the life cycle.
Potentiator Chemistry and Mechanism of Action
The diaminopyrimidine potentiators (trimethoprim and ormetoprim) act in concert with sulfonamides by blocking the next step (dihydrofolate reductase) in folic acid synthesis. Chemically the diaminopyrimidines are related to pyrimethamine, which has antimalarial properties. The agents are highly selective inhibitors of dihydrofolate reductase. This sequential blockade produces significant potentiation of activity. It is a classic case of drug potentiation.
Drug Disposition
The sulfonamides are weak acids that are well absorbed from the gastrointestinal tract and are widely distributed in the body. Sulfadimethoxine and sulfamethoxazole have high serum protein binding, which provides decreased body clearance and long half-lives. They undergo metabolic alteration in the liver and subsequent renal clearance. Trimethoprim and ormetoprim are also well absorbed from the gut, widely distributed, and then hydroxylated and excreted through the urinary tract.
Toxicity and Adverse Effects
The long history of sulfa use in veterinary medicine has resulted in a wide array of toxic and idiosyncratic reactions in animals. Historically, the most common and most avoidable reactions result from crystallization in the urinary tract with secondary crystalluria, hematuria, and urinary obstruction. Recent reviews in human medicine indicate that the improved solubility of the modern preparations has decreased the risk of crystalluria; nevertheless, it is still prudent to ensure adequate water intake and proper hydration during sulfa therapy.6 The human-medicine literature also suggests that the sulfonamides may be directly nephrotoxic.6 Hematopoietic disorders (thrombocytopenia and leukopenia) have also been reported as a result of sulfa therapy. Sulfaquinoxaline especially has been associated with hypothrombinemia, hemorrhage, and death in puppies receiving therapy for coccidia.7
Idiosyncratic reactions in animals and people often include immune-mediated phenomena such as hypersensitivity reactions, drug fever, urticaria, nonseptic polyarthritis, focal retinitis, and hepatitis. Fortunately, these reactions occur at very low rates when the drugs are used at recommended dose rates and for less than 2 weeks.
Preparations
There are four sulfa products that are currently available for use in small animal medicine: sulfadimethoxine, sulfadimethoxine with ormetoprim, sulfadiazine with trimethoprim, and sulfamethoxazole with trimethoprim. Each is available in a variety of formulations.
Sulfadimethoxine
Sulfadimethoxine is a rapidly absorbed, long-acting sulfonamide. It is not acetylated in the dog and is excreted unchanged in the urine. It is approved for treatment of coccidiosis in dogs and cats. It has a wide margin of safety; dogs given multiple oral doses of 160 mg/kg by mouth daily for 13 weeks showed no signs of toxicity.8
It is important that all treated animals receive adequate water intake to prevent dehydration and crystalluria, as well as proper nutrition, during therapy for coccidiosis. Therapy is available as a 40% injection (Albon); in 125-, 250-, and 500-mg tablets (Albon); as a pleasant-tasting 5% suspension (Albon); and as a 12.5% oral solution (Albon, Di–Methox). The approved therapy is an initial dose of 55 mg/kg, orally or by subcutaneous or intravenous injection, for the first day and subsequent doses of 27.5 mg/kg orally once daily for 12 to 21 days. It seems reasonable that, because coccidia are enteric pathogens, the oral route would be most effective.
Sulfadimethoxine with Ormetoprim
Sulfadimethoxine with ormetoprim is the most recently approved potentiated sulfonamide. It constitutes a rational combination that potentiates the action of both drugs by blocking two sequential steps in the synthesis of folic acid. Ormetoprim is a diaminopyrimidine potentiator with very low mammalian toxicity. The available tablets contain 100/20, 200/40, 500/100, or 1000/200 mg sulfadimethoxine/mg ormetoprim, respectively (Primor). The tablets are designated by the total weight of active ingredient in each tablet; thus Primor 120 contains 100 mg of sulfadimethoxine and 20 mg of ormetoprim. The approved starting dose is 55 mg/kg orally on the first day of treatment and then 27.5 mg/kg orally once per day for 14 to 21 days. Treatment should not extend beyond 21 days.8
It is interesting to note that the only recent controlled study of coccidiosis therapy for dogs was conducted with this drug combination. In that study, 32.5 mg/kg or 66 mg/kg was given continuously in the food for 23 days, subsequent to experimental oocyst infection. The higher dose of 66 mg/kg provided better results and did not produce any adverse reactions.9
Sulfadiazine or Sulfamethoxazole with Trimethoprim
Sulfadiazine with trimethoprim is the potentiated sulfa with the most years of actual use in veterinary medicine. For many years it was the only potentiated sulfa approved for use in animals. Trimethoprim is a diaminopyrimidine potentiator with very low mammalian toxicity. The available tablets contain 25/5, 100/20, 400/80, or 800/160 mg sulfadiazine/mg trimethoprim, respectively (Tribrissen, Di-Trim). The tablets are designated by the total weight of active ingredient in each tablet; thus Tribrissen 30 contains 25 mg sulfadiazine and 5 mg trimethoprim. The approved dose is 30 mg/kg orally or 26.4 mg/kg by subcutaneous injection daily for up to 14 days. The preferred dose for bacterial infections in dogs and cats is 30 mg/kg once or twice daily and may be indicated for severe coccidial infections. The manufacturer recommends that animals with marked hepatic parenchymal damage, blood dyscrasias, or previous sulfonamide sensitivity should not be given this product.8,10
Sulfamethoxazole with trimethoprim is a readily available product approved for use in people (Bactrim, Septra); it is not currently approved for use in animals. Because of its similarity to veterinary potentiated sulfonamides and because low-cost generics are available, it is widely used in veterinary medicine. There is some controversy regarding the appropriate dosing regimen for this human-labeled product in animals, but many clinicians gain acceptable clinical results using the same dose as sulfadiazine.
Sulfamethoxazole with trimethoprim is available in a fixed combination of 5:1 sulfamethoxazole to trimethoprim as tablets and pediatric suspension. The available single-strength tablets contain 400/80 mg and double-strength tablets contain 800/160 mg trimethoprim, respectively (Bactrim, Septra). The pediatric oral suspension contains 40 mg sulfamethoxazole and 8 mg trimethoprim per milliliter. The dose for bacterial infections and coccidiosis in dogs and cats is 30 mg/kg once daily for 10 days10 and may be indicated in severe coccidial infections.
Amprolium
Amprolium (Amprol, Corid) is an antiprotozoal drug that is a structural analog of thiamine. It is freely soluble in water, methanol, and ethanol. The close structural similarity between amprolium and thiamine allows amprolium to compete with thiamine for absorption into the parasite. It is most effective against the first-generation schizont stage and thus is more effective for prevention than treatment.
At very high doses, amprolium may produce thiamine deficiency in the host. Thiamine deficiency can be treated by adding thiamine to the diet, although excessive thiamine supplementation may decrease the efficacy against the pathogen. In dogs adverse reactions are apparently rare and may consist of neurologic abnormalities, depression, anorexia, and diarrhea.10
Amprolium is approved for use in the drinking water or feed of poultry and cattle for the prevention and treatment of coccidia. Treatment for dogs and cats requires adapting the approved formulations to small animal use. The target dose for treatment of dogs is 100 to 200 mg/kg by mouth daily in food or water.10 Dogs may be treated by mixing 30 mL (2 Tbs) of 9.6% amprolium solution to 1 U.S. gallon (3.8 L) of drinking water and offering it as the sole source of drinking water.11 Alternatively, 1.25 g of 20% amprolium powder can be mixed with daily ration sufficient for four puppies.12 Amprolium should be provided in either the food or the water but not in both for a period of 7 days. It may be given as a treatment for coccidia or as a preventive measure for 7 days before puppies are shipped or to bitches just before whelping.
Cats may be treated at a dose of 60 to 100 mg/kg by mouth once daily for 7 days, which may be accomplished by direct oral administration.13 Placement of medication in food or water may be more unreliable for cats than for dogs owing to the finicky eating habits of many cats.
Furazolidone
Although the nitrofurans (nitrofurazone and furazolidone) have been reported in the literature as being effective in the treatment of coccidiosis and were once widely available for oral treatment of food animals, they have been systematically eliminated from the veterinary marketplace in the United States because of concerns regarding carcinogenicity. Furazolidone apparently inhibits numerous microbial enzyme systems, especially those related to carbohydrate metabolism, but the actual mechanism of action remains to be determined.14 Furazolidone is still available in a dosage form that is approved for human use (Furoxone). Potential toxicity includes gastrointestinal disturbance, peripheral neuritis, decreased spermatogenesis, and weight gain.15 Dogs and cats can be treated with 8 to 20 mg/kg orally, 1 to 2 times daily, for 5 days.10,13 The product is available in 2 formulations approved for use in people (Furoxone): 100-mg tablets and an oral liquid containing 3.34 mg/mL.
Toxoplasmosis
Biology
Toxoplasmosis is caused by Toxoplasma gondii, an obligate intracellular coccidian parasite. The parasite and the disease occur worldwide and have serious zoonotic impact. The domestic cat and other cats serve as the definitive hosts of this parasite. All other warm-blooded animals serve as intermediate hosts. In the United States, infection rates range from 0% to 100% in cats, 30% in dogs, and 30% to 60% in people. Although infection and seroconversion are common, clinical disease and diagnosis are rare.16
Enteroepithelial Life Cycle
The enteroepithelial life cycle of T. gondii in cats is similar to the life cycle of the common enteric coccidia (Figure 12-2). Toxoplasma oocysts are ingested from the environment; alternatively, tissue cysts may be ingested by carnivorism. Once ingested, bradyzooites are released that penetrate the epithelial cells and begin a cycle of asexual reproduction. The sexual stage of the cycle proceeds when the zooites differentiate into microgametes and macrogametes. The macrogametes are fertilized by the microgamete, and the resulting union produces an oocyst that is shed in the feces to begin the cycle again. It is believed that the enteroepithelial life cycle and the resulting oocysts occur only in cats; therefore only cats shed infective oocysts.
Extraintestinal Life Cycle
The extraintestinal life cycle occurs in all warm-blooded animals, including cats. This cycle begins when oocysts or infected tissues are ingested. The bradyzoites or sporozoites penetrate intestinal cells and undergo asexual reproduction and then break out of the gastrointestinal tract to infect virtually all other tissues, including the brain, striated muscle, and liver. After entering these extraintestinal tissues, they penetrate the cell and multiply until the cell is destroyed. The tachyzoites are released to infect other cells, and the cycle repeats. Eventually, the tachyzoites form tissue cysts that remain viable and infective for the life of the animal. These tissue cysts are infective to all warm-blooded animals and infect all animals who ingest the infected tissues. It is the ubiquitous tissue migration and replication across innumerable species that makes the pathogen so insidious and dangerous.
Congenital Transmission
If the host is infected during pregnancy, then the tachyzoites move across the placenta to infect the developing fetus. Infection during the first half of the pregnancy leads to more severe disease in the fetus. Women infected during pregnancy risk congenital malformation, mental retardation, and death of the unborn fetus. Women should be cautioned to avoid exposure to cat feces and to refrain from consuming undercooked meat during pregnancy.
Clinical Signs
Clinical signs in cats are most severe in prenatally infected kittens. Such kittens may be stillborn or die before weaning. Clinical signs relate to pathology in the liver, lungs, and central nervous system. Adult cats typically demonstrate anorexia, lethargy, dyspnea, weight loss, icterus, vomiting, diarrhea, stiff gait, shifting leg lameness, or neurologic deficits. Ocular lesions may include uveitis in the anterior and posterior chambers, iritis, iridocyclitis, and detached retina. In severe cases respiratory or central nervous system involvement may cause death.17
Infected dogs may show clinical signs related to respiratory, neuromuscular, or gastrointestinal pathology. Generalized toxoplasmosis is characterized by fever, tonsillitis, dyspnea, diarrhea, and vomiting.17 More devastating clinical signs may be seen in dogs with neurologic or muscular involvement. Seizures, neurologic deficits, tremors, ataxia, paresis, or paralysis may be seen in these animals. Ocular lesions in dogs are infrequently reported.17
Diagnosis
Antemortem diagnosis of toxoplasmosis is a significant diagnostic challenge primarily because of the usual lack of clinical signs in infected animals. Fecal floatation for infected cats may reveal small oocysts that are indistinguishable from other coccidia. It is also important to realize that infected cats shed oocysts for only 1 to 2 weeks after their first exposure, thereafter forming a protective immunity that prevents further shedding of oocysts. Many other tools have been applied to the diagnosis of toxoplasmosis, including clinical chemistry, which may reveal elevated liver enzymes; cytology, which may detect tachyzoites; radiology, which could suggest inflammation of target organs; serology, which would reveal a past infection; and parasite isolation. Unfortunately, no simple, specific, and timely diagnostic tool is available to detect an active case of toxoplasmosis.
Treatment
Treatment of toxoplasmosis may have several goals: to prevent shedding of oocysts from infected cats, to prevent transmission of toxoplasmosis by ingestion of infected tissues, to prevent tachyzoite replication in nonfeline host tissues, and to prevent prenatal infections. In some cases the goal may be to alleviate clinical signs of an active infection.
Clindamycin
Clindamycin is currently considered the drug of choice for treating toxoplasmosis. Structurally, clindamycin is a congener of lincomycin. Clindamycin is well absorbed (90%) after oral administration and is widely distributed in most tissues, except the central nervous system. It readily crosses the placenta and is extensively bound to plasma proteins. The drug is metabolized in the liver and excreted primarily in the urine and bile.18 Gastrointestinal upset is sometimes reported in animals receiving clindamycin. Severe, even fatal, pseudomembranous enterocolitis has been reported in people, caused by overgrowth of Clostridium difficile.
Treatment of systemic Toxoplasma infection in dogs can be accomplished with oral or intramuscular clindamycin at 10 to 20 mg/kg twice daily for 2 weeks.17,19 Cats can be treated for systemic infections with oral or parenteral clindamycin at 10 to 12.5 mg/kg twice daily for 2 to 4 weeks; this antimicrobial is also useful to control shedding of oocysts.20 The drug should be given with caution to cats with pulmonic toxoplasmosis; parenteral administration to experimentally infected cats resulted in several deaths.10
Clindamycin is available in two veterinary formulations (Antirobe): capsules containing 25, 75, or 150 mg and an oral solution containing 25 mg/mL. Similar clindamycin formulations are available for use in humans (Cleocin): 75- and 150-mg oral capsules, an oral pediatric suspension (15 mg/mL), and an injectable solution containing 150 mg/mL.
Sulfa Plus Pyrimethamine
The more time-tested therapeutic regimen for toxoplasmosis is a combination of sulfonamide and pyrimethamine. The sulfonamides were discussed previously. Pyrimethamine is structurally and pharmacologically similar to the folic acid antagonist trimethoprim. Pyrimethamine is primarily used in veterinary medicine to treat toxoplasmosis and equine protozoal myelitis, or “equine toxoplasmosis.” Little pharmacokinetic data are available for pyrimethamine use in dogs and cats, but in humans it is well absorbed after oral administration. It is well distributed to the kidneys, liver, spleen, and lungs. The metabolic pathway is unclear, but pyrimethamine metabolites may be found in the urine.
Pyrimethamine can cause anorexia, malaise, vomiting, depression, and myelosuppression. Concomitant oral administration of folinic acid or brewer’s yeast may help alleviate some of these clinical signs. Because toxicity may develop rapidly in cats, they should have frequent hematologic monitoring. It is a teratogen in rats but is sometimes used by pregnant women.10
Dogs and cats are treated for systemic Toxoplasma infections at a dose of 30 mg/kg sulfa and 0.25 to 0.5 mg/kg pyrimethamine orally twice daily for 2 weeks. Cats may be treated to control shedding of oocysts at a dose of 100 mg/kg sulfa and 2 mg/kg pyrimethamine orally once daily for 1 to 2 weeks.17
Pyrimethamine alone is available in 25-mg tablets (Daraprim) and in combination tablets containing 25 mg pyrimethamine and 500 mg sulfadoxine (Fansidar). These dosage forms are likely to be difficult for most cat owners to administer.
Monensin
Monensin is an ionophore coccidiostat that is fed to poultry and cattle to enhance feed efficiency. It forms ionic complexes that move across biological membranes. The net effect is disturbance of mitochondrial function, which inhibits growth of the pathogen. It is not well absorbed from the gastrointestinal tract, and thus oral administration provides effective concentrations only in the gastrointestinal tract. It can be toxic if high doses are given. Shedding of Toxoplasma oocysts may be controlled in cats by mixing monensin in the feed at 0.2% on a dry matter basis and feeding for 1 to 2 weeks.17 Monensin is available in several feed-additive formulations designed for incorporation into a finished feed. Formulating such feeds for cats is beyond the capabilities of most cat owners.
Toxoplasmosis is the most common infection of the central nervous system in human patients with acquired immune-deficiency syndrome, resulting in a number of studies in human patients regarding efficacy. Standard regimens include combinations of pyrimethamine with either sulfadiazine or clindamycin, although side effects limit use. A potentially less toxic combination includes atovaquone suspension (approximately 20 mg/kg every 12 hours) and either pyrimethamine (1 mg/kg once daily after a loading dose of approximately 3 mg/kg) or sulfadiazine (20 mg/kg every 6 hours). In one study up to 82% of patients receiving atovaquone as part of combination therapy responded therapeutically, with 28% discontinuing therapy as a result of gastrointestinal adverse events (including taste of atovaquone). The combination of atovaquone and pyrimethamine was recommended as the first-choice combination, with sulfadiazine substituting for pyrimethamine in patients unable to tolerate the former.21
Giardia
Biology
Giardia (Giardia duodenalis = Giardia lamblia) are protozoan parasites that are motile by means of flagella. They exist extracellularly in the lumen of the gastrointestinal tract. They are ubiquitous pathogens that can inhabit and cause disease in most mammals and are well-known pathogens in dogs, cats, and people. Recent surveys show that 36% of puppies in the United States are infected with Giardia.22 Despite this prevalence, the condition in pet animals remains underdiagnosed on account of inappropriate fecal examination techniques. It is a pathogen with zoonotic potential, insofar as Giardia frequently causes disease in people, but there is some uncertainty as to whether the same species and strains infect people and pets. More extensive information about this issue can be found elsewhere.1,23–26
The life cycle of Giardia is direct and simple (Figure 12-3). The cyst of the Giardia is passed in the feces. It is nonmotile and protected by a distinct wall. Although very susceptible to drying, the cysts can survive for weeks or months in cool water. Infections are often traced back to contaminated drinking water. Once ingested, the cysts break open and trophozoites are released into the small intestine. The trophozoites are flattened on one side with a ventral sucking disk that attaches to the brush border surface of the villous epithelium. The trophozoites obtain their nutrients by way of the host cell membrane. The flagella provide locomotion from one attachment site to another. The trophozoites reproduce by binary fission. After several divisions the trophozoites encyst and are shed in the feces; they are immediately infective.
Figure 12-3 Schematic representation of Giardia life cycle and structural features of Giardia cysts and trophozoites visible by light microscopy. Infection begins when a cyst (1) is ingested by the host. Excystation (2) in the upper small intestine results in the release of an incompletely divided pair of trophozoites. Trophozoites attach to villous epithelial surfaces (3) or swim freely (4) in the lumen of the small intestine, where asexual division of trophozoites (5) also takes place. Encystment (6) of trophozoites, probably in the lower ileum or in the colon, results in the passage of infective cysts in the feces. Trophozoites (7) may also be passed into the environment, but they die quickly. Key to organelles: a, cyst wall; b, nuclei; c, axonemes; d, adhesive-disk fragments; e, median bodies; and f, flagella.
(From Kirkpatrick CE: Giardiasis. Vet Clin North Am Small Anim Pract 17: 1377, 1987.)
Conventional fecal floatation techniques make identification of the parasite difficult because concentrated salt floatation media distort the trophozoites. Preferred methods of diagnosis include microscopic examination of saline smears, which readily show the motile trophozoites. Alternatively, a zinc sulfate centrifugal method may be used to concentrate the giardial cysts and improve sensitivity.1 Improved techniques for evaluation of duodenal aspirates and fecal enzyme-linked immunosorbent assay (ELISA) testing have recently become available.
Treatment
Albendazole
Albendazole is a broad-spectrum benzimidazole commonly used for treatment of nematode and trematode infections in large animals. Early evidence suggested that albendazole is 100% effective in treating giardiasis in dogs.27 The dose given in that study was 25 mg/kg orally twice a day for four doses. Albendazole is available in an oral suspension (Valbazen) containing 113.6 mg/mL.
Albendazole, like other benzimidazoles, is well absorbed (about 50% bioavailable) and converted in the liver to its active metabolites, albendazole sulfoxide and albendazole sulfone. These active metabolites are thought to bind to tubulin molecules, which prohibits the formation of microtubules and disrupts cell division. There is also evidence that benzimidazoles can inhibit fumarate reductase, which blocks mitochondrial function, thus depriving the parasite of energy and resulting in death. The parent drug and its metabolites are excreted primarily in the urine.
Albendazole has been shown to be teratogenic, thus limiting its use in pregnant animals. Dogs treated with 50 mg/kg twice daily may develop anorexia, and cats treated with 100 mg/kg per day for 14 to 21 days showed weight loss, neutropenia, and mental dullness.10 More recently, the drug was shown to be toxic to dogs and cats in clinical use.28,29 Reported toxicities include myelosuppression (anemia, leukopenia, and thrombocytopenia), abortion, teratogenicity, anorexia, depression, ataxia, vomiting, and diarrhea. Veterinarians are advised to use due caution with this product in dogs.
Fenbendazole
Fenbendazole is currently approved by the U.S. Food and Drug Administration (FDA) for removal of gastrointestinal helminthes in dogs. Recently, it has shown excellent activity against Giardia.28,30,31 The approved dose and the effective dose against Giardia is 50 mg/kg orally once daily for 3 days. Treatment of giardiasis is not an approved use for this product.
The drug is well tolerated and has a good safety profile. The only reported adverse effects are vomiting and diarrhea. This time-tested anthelmintic should enjoy more widespread use in the treatment of Giardia in dogs. Fenbendazole (50 mg/kg orally once daily for 5 days) was variably effective in treating cats concurrently infected with Giardia and Cryptosporidium parvum.32 Although the number of oocysts decreased the first week after treatment (compared with controls), no difference was detected the second week (the power of the study was not addressed). Only 50% (4 of 8) of cats treated with fenbendazole were negative 3 weeks after treatment.
Furazolidone
There is a report of successful treatment of giardiasis in cats with furazolidone.33 As noted previously, furazolidone was once widely available for oral treatment of food animals. Presently, the nitrofurans have been systematically eliminated from the veterinary marketplace in the United States because of concerns regarding carcinogenicity. Furazolidone is still available in a dosage form that is approved for human use (Furoxone). Toxicity includes gastrointestinal disturbance, peripheral neuritis, decreased spermatogenesis, and weight gain.15 Cats can be treated with a dose of 4 mg/kg twice daily for 7 to 10 days.26,34 The product is available in two formulations approved for use by people (Furoxone): 100-mg tablets and an oral liquid containing 3.33 mg/mL.
Metronidazole
The nitroimidazoles represent a very useful class of drugs that have broad-spectrum activity against trichomonads, amebas, Giardia, as well as anaerobic cocci and Bacillus spp. The prototypical nitroimidazole is metronidazole, which has become the drug of choice for treatment of Giardia. Other drugs in the class (ipronidazole, tinidazole, nimorazole, ornidazole, and benznidazole) have been used to control Giardia, although none of these is currently available in the United States. None of the nitroimidazole drugs is approved for use in animals. The FDA strongly warns against their use in food-producing animals because this class of drug has been shown to produce tumors in laboratory rodents.
Metronidazole (Flagyl) is well absorbed from the gastrointestinal tract. It has very low protein binding and is well distributed in the body. After entering the target cell, it interacts with the protozoal DNA, where it causes a loss of helical structure and strand breakage.12 The liver extensively metabolizes the drug, and in humans hepatic transformation is responsible for 50% of the elimination. Patients receiving cimetidine or phenobarbital may require adjustment in the dosage because of drug interaction. Metronidazole toxicity may be seen with high doses. Neurologic toxicity includes ataxia, nystagmus, seizures, tremors, and weakness.12,35 Numerous studies have demonstrated that metronidazole is an effective treatment for giardiasis,36-40 although efficacy is rarely 100%. Dogs may be treated orally with 12.5 to 32.5 mg/kg twice daily; therapy should be continued for 8 days. Cats may be treated orally with 17.4 mg/kg once daily for 8 days.12 The commercially available product (Flagyl) is formulated in 250- and 500-mg tablets. Parenteral formulations are also available, but their usefulness is questionable insofar as the giardial trophozoites remain in the lumen of the gastrointestinal tract. Efficacy of metronidazole benzoate (25 mg/kg orally twice daily) for treatment of giardia was studied prospectively in experimentally infecting cats that had been vaccinated for giardiasis (n = 16) and cats that had not been vaccinated (n = 16). All cats had been shedding giardia for 3 months and were confirmed positive 1 week before treatment. All cats were negative by the 15-day posttreatment period.
Quinacrine
Quinacrine has also been shown to be useful in treating giardiasis in dogs and cats.40 Unfortunately, commercial production of the product (Atabrine) was discontinued in 1993.
Miscellaneous Drugs and Protozoal Infections
Drugs
Folate Antagonists
Folate antagonists used to treat protozoal disease are categorized as type 1 or type 2. Type 1 antagonists include the sulfonamides and sulfones, which mimic para-aminobenzoic acid, thus targeting dihydropteroate synthase. Type 2 antifolates include the diaminopyrimidines, such as pyrimethamine, biguanidines, the triazine metabolites (discussed later), and quinazolines. These drugs target dihydrofolate reductase, preventing the formation of tetrahydrofolate, a cofactor necessary for the biosynthesis of thymidylate, purine nucleotides, and selected amino acids.41 Type 1 folate antagonists are discussed in greater depth in Chapter 7. Pyrimethamine is characterized by an 80- to 90-hour half-life in humans. Although not a first-line antimalarial drug, it, along with a sulfonamide (sulfadiazine), is the first choice for treatment of toxoplasmosis. Other drugs with which it is given include clindamycin and the macrolides; the drug also has been administered with dapsone, but this may increase the risk of agranulocytosis. The drug is relatively well tolerated in humans.
Dapsone also is a folate antagonist, (similar to para-aminobenzoic acid, it should be considered a type 1 folate antagonist) shown to be useful against Pneumocystis carinii.42 Synergy has been demonstrated toward some organisms when dapsone is combined with pyrimethamine and trimethoprim; efficacy was greater compared with sulfonamide combinations toward selected organisms.
Atovaquone
Atovaquone is a hydroxymapthoquinone used for the treatment and prevention of selected protozoal disease in humans, including malaria. It is available as either the sole drug, or in combination with proguanil. Proguanil is a biguanide prodrug that must be converted to the triazine metabolite, cycloguanil. The combined drug minimizes the risk of resistance. Synergy also has been demonstrated with the combination toward some organisms. The mechanism of action of atovaquone includes, but probably is not limited to, impaired mitochondrial function in the protozoal organism, probably at the level of electron transport. Dihydroorotate dehydrogenase has been the suggested mitochondrial enzyme targeted by atovaquone.41 Resistance to atovaquone, reflecting single point mutations, occurs rapidly when atovaqone is used alone.41 The pharmacokinetics have been cited as a contributing factor to resistance: slow uptake and high lipophilicity may result in prolonged exposure of the protozoa to subtheraeputic concentrations.
Triazines
Toltrazuril is a triazine coccidiostat used in poultry; ponazuril is the major metabolite. Toltrazuril targets the mitochondrial respiratory chain of the susceptible protozoal organism; at higher concentrations, it also blocks pyrimidine formation.43 Ponazuril has been shown to be active against Sarcocystis neurona both in vitro and in vivo and has been approved for use in horses to treat protozoal encephalitis caused by Sarocystis. Efficacy also has been demonstrated in vivo against Neospora caninum.44 In vitro studies with toxoplasmosis indicates that ponazuril interferes with tachyzoite division. Disease regressed in mice treated with 100 mg/kg of toltrazuril and ponazuril, but recrudescence occurred with the ponazuril-treated mice but not those treated with toltrazuril.43 At 10 or 20 mg/kg, ponazuril treatment 1 day before or 3 days after infection followed by 10 days of therapy completely protected mice against acute toxoplasmosis. Treatment with 20 mg/kg but not 10 mg/kg once before and daily for 6 days after infection, followed by 10 days of therapy, also protected against fatal toxoplasmosis.45
Using a murine model of neosporosis, either toltrazuril or ponazuril administered at 20 mg/kg in drinking water completely prevented the formation of cerebral lesions and decreased polymerase chain reaction (PCR) detection by 90%.46 Ponazuril is being used at 20 mg/kg once weekly for 2 weeks for coccidia in dogs (see other doses).
Other Nitroimidazoles
Tinidazole and ronidazole are newer nitromidazoles that have been used with variable success for treatment of protozoal disease. Of the two, the kinetics have been described for tinadazole in dogs and cats after single intravenous (15 mg/kg) and oral doses (15 mg/kg or 30 mg/kg). Oral bioavailability was described as completed, with peak plasma concentrations after oral administration of 15 mg/kg being 17.8 and 22.5 μg/mL in dogs and cats, respectively, and after 30 mg/kg, 37.9 μg/mL in dogs, and 33.6 μg/mL in cats. The apparent volume of distribution in dogs and cats was 0.67 and 0.54 L/kg, respectively. The elimination half-life in dogs was 4.4 hours, compared with 8.4 hours in cats, suggesting 8- to 12-hour dosing intervals in both species. However, plasma drug concentrations were above the minimum inhibitory concentration of tinidazole-susceptible bacteria for 24 hours in cats and 12 hours in dogs after a single oral dose of 15 mg/kg. No adverse events were reported.
Other Nitrofurans
Numerous nitrofurans have been studied for efficacy against protozoal organisms. Among them, nifurtimox demonstrated significant efficacy against Chagas disease. Although nifurtimox is available in the United States, access can be gained only through the Centers for Disease Control and Prevention (CDC), presumably to allow tracking of human cases of Chagas infections. Trypanocidal effects reflect partial reduction and formation of reactive oxygen radicals; the organisms have low concentrations of glutathione and thus have a limited capacity to scavenge the oxygen radicals. However, like the organisms, host toxicity also reflects oxygen radical formation. The ability of N-acetylcysteine to protect the host apparently has not been studied. Although absorbed well after oral administration, metabolism is sufficiently rapid that concentrations of the parent compound remain low; efficacy of the metabolites is not known. Side effects include gastric upset and weight loss.
Quinolones
The quinolone-containing drugs are classified as two types: Type 1 drugs include the 4-aminoquinolones such as chlorquinolone, whereas the type 2 drugs include the aryl–amino alcohols such as quinine and quinidine. Their use has been principally for the treatment of malaria in humans, but they occasionally are used to treat other parasites. Quinine is the primary alkaloid of cinchona, the bark of a South American tree, and has been used medicinally since the early 1600s. The drug is still derived primarily from natural sources. Quinidine differs from quinine only by the orientation of an alcohol group, resulting in greater potency and toxicity. Chloroquine is characterized by less toxicity and greater efficacy compared with quinine.47 However, widespread resistance to chloroquinine has markedly reduced its use. The mechanism of action of these drugs is not well known, with a number being proposed. These include inhibition of protein sythnthesis, inhibition of FV lipase or aspartic proteinases and inhibition of DNA or RNA synthesis.41 Quinine and similar drugs increase the refractory period of muscle, antagonizing the action of physostigmine on skeletal muscle.47 Respiratory distress and dysphagia occur in humans with myasthenia gravis. Quinine is well absorbed after oral administration. The drug is extensively metabolized and is characterized by an elimination half-life of about 11 hours, but this can increase (owing to decreased clearance) to close to 20 hours with repetitive dosing.47 Side effects also include hypoglycelmia (which can be life-threatening), hypotension, and central nervous system side effects.
Diaminidines
The diaminidines include diminazene aceturate and pentamidine isethionate (approved in the United States to treat Pneumocystis pneumonia), and a carbanilide member, imidocarb diproprionate. The latter is approved for use in the United States to treat canine babesiosis. The mechanism of action of these drugs is not clear and may vary for the individual parasite. Proposed mechanisms include cationic interactions with DNA or nucleotides or interference with polyamine uptake or function. Efficacy within a genus of protozoa may vary with the species. Efficacy of pentamide is particularly good against Pneumocystis, hence its importance in human medicine. As a class, the diamidines are fungicidal toward some organisms, although their usefulness was largely replaced by amphotericin B.47
In humans pentamidine has a half-life of about 6 hours but is very slowly eliminated in the urine. The drug is extensively accumulated in tissues, which may explain its apparently prophylactic efficacy against for some organisms. Side effects can be life threatening and may reflect anticholinergic- or histaminergic-like responses (perhaps reflecting mast cell degranulation and histamine release). Clinical signs include tachycardia, dyspnea, vomiting, and (in humans) fainting or dizziness. Sterile abscesses have been reported in humans after intramuscular injection. Pancreatitis and either hyperglycemia or hypoglycemia (the latter potentially life-threatening) also have been reported.47
Sodium Stibogluconate
Sodium stibolugonate (SSG) is a pentavalent antimonial compound used for the treatment of leishmaniasis. As with several other antiprotoazoal drugs, SSG is available only through the CDC. Its mechanism of action is not known, but bioenergetics, including glycolysis and fatty acid metabolism and generation of ATP and GTP of amastigotes is impaired. The drugs may be prodrugs with generation of Sb3+ being the toxic compound. The preservative chlorocresol may contribute to the activity of the compound. The drug is eliminated in humans in two phases: a short 2-hour phase followed by a longer 30- to 75-hour phase. Accumulation of the drug in macrophages may facilitate efficacy. Resistance is limiting efficacy of the drugs for treatment of leishmaniasis; increasingly higher doses are required. Currently, when given (20 mg in humans) daily for 10 days, the drug yields an 85% to 90% cure rate.47 The drug is relatively well tolerated, with pain at the injection site, chemical pancreatitis (high incidence), hepatitis, bone marrow suppression, myalgia, and malaise being reported side effects in humans.47
Miscellaneous Protozoal Infections
Leishmaniasis
Leishmaniasis is caused by protozoa of the Trypanosomatidae family. In mammals the organisms reside in macrophages as amastigotes. Two major forms occur. The cutaneous form is caused by a number of species, including Leishmania major, Leishmania tropica, and Leishmania mexicana. The visceral form is caused by Leishmania of the donovani complex. The dog is the major reservoir for human visceral leishmaniasis in Mediterranean countries.48 Whereas the cutaneous form has been reported in the United States and other countries, the visceral form has largely been limited to Asia and the Middle East, with afflicted dogs generally having traveled to endemic countries.49 Recently, the visceral form has been described in a kennel of Foxhounds in Oklahoma; additional multiple dog outbreaks have since been reported.
Two reviews have addressed treatment of leishmaniasis.50,51 The latter is a systemic review of the literature. The goal of treatment ranges from resolution of clinical signs to eradication of organisms. Because experimental infections are difficult to establish, clinical trials tend to focus on spontaneous disease. Therapeutic response is based on decreased Leishmania-specific antibody concentrations and return of parasite-specific cell mediated immunity.50 Improvement persists unless organisms have not been cleared.
A number of studies have focused on the role of altered immune response in refractory infections caused by intracellular protozoal infections.48 Resistance to leishmaniasis may reflect Th-1–mediated immunity. Among the cytokines important for an effective Th-1 response, IL-1 is among the most potent inducers. Depending on the immune response, infection in dogs can cause manifestations that range from asymptomatic subclinical disease to complete manifestation.52 In one study dogs experimentally infected with Leishmania infantum failed to express IL-4 compared with control dogs. Further, in infected but asymptomatic dogs, although both Th-1 and Th-2 cytokines were produced, cell-mediated immunity reflected preferential expression of Th-1 cytokines.52 IL-12 augmented interferon-gamma production by peripheral blood mononuclear cells in dogs either experimentally or naturally infected with canine visceral leishmaniasis, suggesting that IL-12 may be a feasible cytokine therapy in infected dogs refractory to therapy.48
Treatment with antiprotozoal drugs traditionally has included pentavalent antimony compounds, including SSG (obtainable through the CDC) and meglumine antimoniate. Baneth and Shaw50 reviewed the efficacy of antimony for treatment and cure of leishmaniasis in dogs. Cure rate is considered low, although clinical signs often improve. Use has been limited more recently by the emergence of L. infantum, which is resistant to therapy.
KEY POINT 12-5
Successful therapy of leishmaniasis ultimately may require appropriate immunomodulation.
The use of a liposomal antimony preparation for treatment of leishmaniasis was studied in experimentally infected Beagles (n = 6). Dogs first received an intravenous injection of a commercial product (Glucantime; Rhône–Mérieux, France; 9.9 μg/kg antimony), and 1 week later the same dose of antimony was given as a liposomal preparation intravenously for 2 days, then subcutaneously for 8 days. The cycle was repeated 10 days later. The disposition of antimony was characterized for both antimony preparations. Regarding the disposition, for the liposomal and nonliposomal preparations, the results were, respectively: Cmax at steady state (μg/mL) 51, 114; volume of distribution at steady state (L/kg), 0.4 ± 0.16 and 0.2 ± 0.06; clearance (l∗hr/kg) 0.009 ± 0.005 and 0.17 ± 0.05; area under the curve (μg/hr/mL) 1494 ± 1038 and 61.2 ± 14; and elimination half-life∗hr) 33.1 ± 13.8 and 6.1 ± 0.7. Response to therapy was addressed only in terms of total protein and gamma globulin concentrations treatment and at long-term follow-up. These increased 3 months after treatment with the free form of antimony but not the liposomal form.53
Allopurinol is metabolized by Leishmania to an inactive analog of inosine, which is then incorporated into the RNA of the parasite, causing faulty protein translation. It can be used alone or in combination with other drugs. Although clinical signs are likely to improve, clinical cure is unlikely. Survival (78% for more than 4 years in one study) is more likely if renal insufficiency is not present when therapy is begun; the proportion of survivors is similar to that in dogs treated with antimony. Among the clinical sequelae of leishmaniasis is progressive renal disease. The efficacy of allopurinol (10 mg/kg twice daily for 6 months) in slowing the progression of renal disease associated with proteinuria was prospectively studied in dogs (n = 12; 5 no treatment controls) without proteinuria or renal insufficiency, asymptomatic dogs with proteinuria (n = 10; 5 no treatment controls), and symptomatic (azotemic) dogs (n = 8) with proteinuria and renal insufficiency.54 Compared with untreated controls, proteinuria was decreased in dogs that already had proteinuria and the progression slowed of tubular but not glomerular disease in all asymptomatic dogs. Finally, azotemia resolved in the symptomatic dogs. Cure of visceral leishmaniasis with the combined use of fluconazole and allopurinol for 4 months has also been reported.55
A number of other drugs have been studied for their potential efficacy. Among the drugs used to treat leishmaniasis is the antifungal agent, amphotericin B (see Chapter 9). Baneth and Shaw50 reviewed use of amphotericin B for treatment of leishmaniasis. In general, efficacy is limited by emerging renal disease after total cumulative doses ranging from 8 to 26 mg/kg. This study reported that 27 of 28 naturally infected dogs that responded to amphotericin B (out of a total of 30 infected dogs) remained in clinical remission for 12 months after treatment of 0.5 to 0.8 mg/kg intravenously 2 to 3 times a week for a total cumulative dose of 15 mg/kg (as reviewed by Baneth and Shaw50). In another study 100% of 16 dogs diagnosed with leishmaniaisis were clinically cured (no organisms in the bone marrow and 87% negative on PCR) at the end of 4 to 5 weeks of amphotericin B prepared as the desoxycholate salt in soybean oil (0.8 to 2.5 mg/kg). However, 6 dogs (including 2 dogs that never became negative) subsequently became PCR positive for infection at some point in the posttreatment follow-up. Of these 6 dogs, 3 became clinically ill again. The authors concluded that a single negative PCR result should not be interpreted as clinical cure.56 Oliva and coworkers57 reported the failure of liposomal amphotericin B (3 to 3.3 mg/kg for 3 to 5 treatments) to cure dogs (n = 13) with cutaneous leishmaniasis. Although animals responded clinically very rapidly, recrudescence of clinical signs returned 4 months later and lymph node aspirates remained positive. Of 17 dogs treated with amphotericin B prepared in a lipid, 14 were treated successfully, as indicated by negative bone marrow PCR results at 1 to 3 months after therapy; however, the timing of recheck may have been inappropriately short.
Pentamidine (4 mg/kg intramuscularly every 3 days for 8 treatments) has also been used successfully in resolving clinical signs in a small number of dogs naturally (n = 3) or experimentally (n = 5) infected with L. infantum. Two of the experimental animals were cured. Other drugs that have been used with variable efficacy include metronidazole, itraconazole, ketoconazole, and terbinafine.
Noli and Auxilia51 systematically reviewed the literature for evidence of effective therapy for treatment of L. infantum. Clinical trials that addressed prevention or treatment (n = 47) were reviewed from 1980 to 2004. Good evidence was found to support the use of meglumine antimony (at least 100 mg/kg for 3 to 4 weeks). Fair evidence existed for pentamidine (4 mg/kg twice weekly) and aminosidine (5 mg/kg twice daily) for 3 to 4 weeks. Aminosidine is an aminoglycoside antimicrobial whose efficacy is similar to that of antimony, but its use is limited by emergent nephrotoxicity. Insufficient evidence existed for allopurinol alone, amphotericin B, ketoconazole, enrofloxaxcin, metronidazole, or combinations thereof.
Trypanosomosis
Trypanosomiasis is caused by protozoa of the genus Trypanosoma spp. Two major diseases occur: that associated with T. brucei in subsahara Africa and that associated with T. cruzi, occurring largely in Latin America. Infection with T. bruceia is transmitted by the tsetse fly and occurs either in small vessels or connective tissue. The disease is of major regional economic importance, impacting food producing and companion animals as well as humans. Resistance has emerged toward the three primary chemotherapeutic agents, with underdosing and delay in treatment being major mitigating factors.51a Treatment of experimentally induced T. brucei brucei in dogs has been studied. The use of diaminazene aceturate (7 mg/kg) as a single dose, or pentamidine esethionate (4 mg/kg, IM, at either every other day for 6 treatments [Group 2] or at 3, 2, 14,16 days [Group 3]), was studied in dogs (n = 4 per group and 3 untreated controls) 14 days after experimental infection with T. brucei brucei. All dogs were parasitemic by day 7 of infection. By day 7 posttreatment, parasitemia had cleared in all dogs but not the control animals (these died by day 28 postinfection). However, one dog in the diaminazene group relapsed at day 42 (dog died at day 70); the remaining dogs remained free of parasites through the 77 day study period. Two dogs treated with pentamidine (group 3) also died, without evidence of parasitemia. Serum liver enzymes increased more significantly in the diaminazine group. The authors concluded that pentamidine at 4 mg/kg every other day for 2 weeks was a reasonable choice for treatment of trypanosomosis in dogs. Chagas disease is caused by T. cruzi which is transmitted by the kissing (reduviid) bug. Infection occurs after rubbing feces from the bug into the bitewound made by the bug; infection also can be acquired by eating the bug. The acute stage occurs within weeks after initial infection and is followed by a latent and then chronic stage. The chronic stage in humans impacts the nervous (neuritis and other neuropathies), digestive (e.g., megaesophagus or megacolon; reflecting in part neurologic damage) and cardiovascular systems (cardiomyopathy). Cardiovascular disease in dogs is generally associated with right-sided cardiac dysfunction, and its sequelae, including arrhythmias, pleural effusion, ascites, and hepatomegaly. Treatment focuses on parasiticides and supportive care. Intracellular amastigotes destroy intramural neurons; inflammation, fibrosis, and cell death result in organ damage and clinical signs. Antiparasiticides include azoles (benznidazole) or nitros (nifurtimox). The latter may be associated with gastrointestinal signs, peripheral neuropathy, and hemolytica anemia.
Cryptosporidiosis
Cryptosporidiosis is a ubiquitous protozoal disease spread by the fecal–oral route and associated with immunosuppressive diseases in dogs or cats. C. parvum is the strain that most commonly infects mammals. Clinical manifestations reflect malabsorption or secretory diarrhea through unknown mechanisms. Signs are similar to those associated with giardiasis, although mucus is more common with infections by the latter. The syndrome may be self-limiting. Like giardia, cryptosporidiosis is a zoonotic concern. In humans nutritional approaches (including probiotics, low-fat diets, high-fiber diets, simple carbohydrates) to treatment are generally effective, with antimicrobial therapy reserved for nonresponders. Antiprotozoal therapy has largely been ineffective. Tylosin (11 mg/kg by mouth twice daily for 28 days) may help reduce diarrhea. Clindamycin was ineffective in single cases for treatment of cryptosporidiosis in cats. The aminoglycoside paramycin may be effective following oral (not parenteral) administration. When paramycin is used in cats (150 mg/kg every 12 to 24 hours orally for 5 days), fecal oocysts were cleared from infected cats,58 but when it was used to treat feline trichomoniasis, 25% of the cats developed acute renal failure. Azithromycin has been used with some success in the treatment of cattle and might be considered as an alternative therapy in dogs or cats. Nitazoxanide, approved for use in humans to treat Giardia, is a derivative of nitrothiazole found to be effective against a wide range of parasites and bacteria. The use of this drug in dogs and cats has not yet been reported.
Fenbendazole (50 mg/kg orally once daily for 5 days) was variably effective in treating cats concurrently infected with Giardia and C. parvum.32 Although the number of oocysts decreased the first week after treatment compared with controls, no difference was detected the second week (the power of the study was not addressed). Only 50% (four of eight) of cats treated with fenbendazole were negative 3 weeks after treatment.
Trichomoniasis
Tritrichomonas foetus, the cause of trichomoniasis in cattle, is an emerging protozoal enteric pathogen of domestic cats. Infection occurs in the lumen of the colon, where inflammatory colitis and explosive, chronic, foul-smelling diarrhea result.59 The disease may be prevalent in high-density cat populations such as catteries or show environments and therefore may be more prevalent in purebred cats. A number of treatments have been tried without consistent therapeutic success, including metronidazole, fenbendazole, albendazole, pyrantel pamoate, sulfadimethoxine, trimethoprim–sulfadiazine, furazolidone, tylosin, enrofloxacin, amoxicillin, clindamycin, paromomycin, and erythromycin.59
Trichomonads generate pyruvate through glycolysis and reductive fermentation. The latter pathway is targeted by 5-nitroimidazole antibiotics such as metronidazole. Reduction of the nitroimidazoles in hydrogenosomes generates anion radicals, which accumulate to toxic concentrations in the protozoal cell. Metronidazole as sole therapy generally has been ineffective against T. foetus infection in cats: Transient improvement is generally followed by recrudescence of clinical signs.60 Gookin and coworkers60 found that, based on in vitro comparison of metronidazole, ronidazole, and tinidazole, ronidazole was the most potent toward T. foetus isolated from a cat with spontaneous disease. Ronidazole was then prospectively studied for its efficacy in a cat naturally infected, and 10 specific pathogen-free (SPF) kittens experimentally infected with T. foetus. Ronidazole at 30 to 50 mg/kg every 12 hours for 14 days resolved clinical signs and cured (based on PCR results) all 11 cats. Relapses occurred at 10 mg/kg. Kather and coworkers61 reported in vitro susceptibility for T. foetus collected from four Bengal cats with spontaneous disease. Omeprazole and paromomycin were ineffective, and metronidazole, ronidazole, and furazolidone were equally effective at 0.625 to 2.5 μg/mL. However, ronidazole was characterized by greater lethality.
In SPF kittens experimentally infected with T. foetus, ronidazole (10 mg/kg every 12 hours for 2 weeks) initially was effective but relapse occurred in 5 of 5 cats 2 to 20 weeks after treatment. However, at 30 to 50 mg/kg, 10 of 10 cats remained negative for 21 to 30 weeks after treatment. Likewise, treatment with 10 mg/kg ronidazole for 2 weeks resolved clinical signs of infection in a spontaneously diseased cat, with clinical signs returning at 85 days.60
Cytauxzoonosis
Cytauxzoonosis is a generally fatal tick-borne protozoal disease that afflicts cats. The bobcat appears to be the reservoir host. Cytauxzoon has an erythrocytic phase, which morphologically cannot be distinguished from babesiosis, as well as a tissue phase. Schizonts develop in monocytes, which then marginate into the vascular endothelium, often occluding the vessel. Schizonts then develop into merozoites, which, on release into the blood and tissue, invade other cells, including erythrocytes. Clinical signs rapidly progress, with the course of the disease generally being less than 1 week. Clinical signs generally include pale mucous membranes, dehydration, dyspnea, icterus, anorexia, and severe lethargy. Treatment with either diminazene aceturate (2 mg/kg subcutaneously, repeated in 2 to 4 weeks) or imidocarb dipropionate (5 mg/kg, subcutaneously, repeated in 2 weeks) should be considered.62 In one report, 6 of 7 cats diagnosed with cytauxzoonosis responded to 2 intramuscular injections (2 to 4 weeks apart) of either diminazene aceturate or imidocarb dipropionate (2 mg/kg). However, the seventh cat died after the first injection of diminazine.63 Drugs generally shown to be ineffective include paravaquone (20 or 30 mg/kg intramuscularly daily), buparaquone (5 or 10 mg/kg intramuscularly daily), sodium thiacetarsamide, and tetracyclines.62,64
Babesiosis
Babesiosis is a tick-borne hematozoan disease afflicting dogs and cats. Dogs are known to host Babesia canis (large and more important form) and Babesia gibsoni (small form), both of which cause hemolytic anemia, whereas cats are infected with Babesia felis, among others. However, other body systems may also be involved. In dogs the disease can present as hyperacute, acute, chronic, and subclinical. Greyhounds appear to have a higher prevalence compared with other breeds of dogs. Treatment with imidocarb dipropionate (approved by the FDA for this use) is generally successful. A number of protocols for treatment have been described. Imidocarb at 7.5 mg/kg can be followed with diminazene (3.5 mg/kg) or given in two doses (5 to 6.6 mg/kg 14 days apart). Imidocarb is the preferred drug with concurrent infection with Erhlichia. A single dose of imidocarb should be protective. Phenamidine (15 mg/kg once a day, for two doses), pentamidine (16.5 mg/kg, two doses 24 hours apart), or trypan blue (10 mg/kg of a 1% solution intravenously, once) also have been suggested for severe infections when the anticholinergic properties of the diamidine derivatives are a concern. Clindamycin at 25 to 50 mg/kg daily for 7 to 10 days also may be useful. For cats the antimalarial drug primaquine, (0.5 mg/kg orally or intramuscularly) has been recommended. However, because the lethal dose is 1 mg/kg, extreme caution should accompany use of this drug in cats. Another drug that may be useful is atovaquone.
The mechanism of action of atovaquone against other protozoa is believed to involve the inhibition of cytochrome b and electron transport. The antiprotozoal drug atovaquone has proved effective for treatment of at least two Babesia species, Babesia microti and Babesia divergens, when combined with azithromycin.65 The combined use of atovaquone (13 mg/kg orally every 8 hours) and azithromycin (10 mg/kg orally once daily) was successful in the treatment of babesiosis in dogs.65 Of 11 dogs remaining infected with B. gibsoni (Asian genotype) despite therapy with either imidocarb. diproprionate alone (dose: 6 to 6.6 mg/kg intramuscularly) for two doses 1 to 2 weeks apart, diminazene aceturate (3.5 mg/kg intramuscularly) for two doses 2 weeks apart, or a combination of the two, 8 became PCR negative compared to 11 of 11 controls that remained positive. 65
Evidence (PCR) of infection was absent in 80% of treated dogs, whereas it was present in all dogs in a placebo group of similar size. No adverse events were reported in treated animals. Treatment was limited to animals that did not require hospitalization, indicating a need to study acutely ill animals. However, the lack of any other known effective therapy warrants consideration of this combination for treatment in acutely ill animals as well. Of the two preparations (atovaquone or atovaquone combined with proguanil) currently available in the United States, the single product is probably less likely to cause gastrointesintal side effects.65
Hepatozoonosis
Hepatozoonosis is a tick-transmitted hemosporazoon disease infecting dogs. The organism is similar to Babesia. The clinical syndrome is characterized by a stiff gait and myalgia, inactivity, and weight loss and profound leukocytosis (mature neutrophilia: 70,000 to 200,000 cells/μL). Periosteal proliferation and increased alkaline phosphatase may be present. Severe hyperesthesia may be manifested as stiffness. Analgesic therapy, probably with nonsteroidal antiinflammatory drugs, is indicated. Tepoxalin (Zubrin) might be considered for its ability to target leukotrienes (prevalent in white blood cells) as well as prostaglandins. Definitive diagnosis appears to have been facilitated using an ELISA-based test. Treatment with triple combination therapy (TCP) may resolve clinical signs and has dramatically improved the prognosis in dogs. Therapy includes trimethoprime sulfadiazine (15 mg/kg orally, twice daily), clindamycin (10 mg/kg orally, three times daily), and pyrimethamine (0.25 mg/kg orally, once daily), each administered for at least 14 days. Efficacy with imidocarb dipropionate (5 mg/kg subcutaneously, given once) alone or combined (at 6 mg/kg subcutaneously, every 14 days) with tetracycline (22 mg/kg orally, thrice daily) for 14 days) is less effective. Treatment with the coccidiostat toltrazuril (5 to 10 mg/kg subcutaneously or orally, once daily for 3 to 5 days, or 5 mg/kg orally, twice daily for 4 days) may resolve clinical signs, but the risk of relapse may be great. Decoquinate (10 to 20 mg/kg orally, twice daily; continuous therapy) may be indicated to prevent relapses after TCP.
Neospora Caninum
N. caninum is a Toxoplasma gondii-like protozoan for which canids are the only known definitive host. The life cycle involves cysts containing bradyzoites in the intermediate host and rapidly dividing tachyzoites in multiple tissues of the definitive host. The intestinal phase in the definitive host is similar to that of coccidiosis and leads to passage of nonsporulated oocysts in feces. Clinical disease, which is most severe in puppies, presents as hind limb paresis that rapidly progresses to rear limb paralysis and occasionally rigid hyperextension of limbs. Other signs include dysphagia, jaw muscle paralysis, muscle atrophy, and heart failure. Neosporosis in adult dogs generally includes neurologic disease but also may manifest as encephalomyelitis, polymyositis, myocarditis, or dermatitis. Puppies can be infected transplacentally. A canine kidney cell model, recombinant canine interferon alpha (IFN-alpha), beta (IFN-beta), and gamma (IFN–gamma) inhibited the growth of N. caninum tachyzoites. However, the effect was associated with the suppression of the host cell viability.66 Treatment is similar to that for toxoplasmosis.
Protothecosis
Prototheca are achlorophyllous algae, related to green algae and ubiquitous in the environment. Their importance as a cause of infection in immunocompromised infections is increasing in human medicine.67 Infections are either subcutaneous to cutaneous, synovial or systemic. Infection is generally preceded by skin or wound infections. Therapy with amphotericin B requires high concentrations, which may include local as well as systemic therapy. Duration of treatment in humans ranges from several days to months. Tetracyclines (doxycycline) may act synergistically. Imidazoles and flucytosine have been used systemically successfully, and a number of local therapies have been successful (gentian violet, polymyxin B, clotrimazole, neomycin, hydrogen peroxide, and potassium iodide). Protothecosis in cats usually is cutaneous, whereas dogs may present with a variety of clinical manifestations. The most common presenting sign in dogs is protracted hemorrhagic diarrhea, with the colon the most common site.68 Treatment in dogs has included amphotericin B rectally in an enema form (3% cream), itraconazole, and ketoconazole.
Summary
Therapy of protozoal infections in small animals may range from simple to complex therapeutic dilemmas. The best treatment of each case must be determined by considering the life cycle of the pathogen, the general physical condition of each animal, and the animal’s environment. Therapy must include adequate attention to supportive therapy to control clinical signs and support normal body function. Therapy also should include adequate hygiene to limit reinfection and disease transmission. The selection and administration of the specific antiprotozoal agent are only parts of the overall therapeutic picture.
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