Feline Toxoplasmosis and its Zoonotic Implications

Kari Addante, DVM; Holly A. Moore, DVM; Bruce E. LeRoy, DVM, PhD

Class of 2006 (Addante) and Department of Pathology (Moore, LeRoy), College of Veterinary Medicine, University of Georgia, Athens, GA 30602-7388

Introduction

The coccidian Toxoplasma gondii is an obligate intracellular parasite that can infect virtually all warm-blooded animals including humans. Members of the species Felidae, however, are the only known definitive hosts for the sexual stages of T. gondii and, thus, serve as the primary reservoir for the pareasite. Numerous other species are paratenic hosts and serve as intermediate hosts required for completion of the parasite’s life cycle but in which developmental changes do not occur.  In the United States, the percentage of domestic cats seropositive for T. gondii is estimated to be between 16 - 43% based on regional variances.^1,2 The high serological prevalence of toxoplasmosis and its zoonotic potential underscore the importance of veterinarians’ full understanding of both the disease course and the role of domestic cats in the T. gondii life cycle.³

Epidemiology

T. gondii sporulated oocysts, bradyzoites, and tachyzoites may all cause infection.² Cats, as obligate carnivores, primarily contract toxoplasmosis through ingestion of infected host tissue cysts. As definitive hosts, cats alone can complete the enteroepithelial life cycle. Extraintestinal cycling of T. gondii occurs in cats, as well, along with all other susceptible species that serve as intermediate hosts. In cats, the enteroepithelial and extraintestinal developmental cycles may occur simultaneously.⁴

Enteroepithelial Life Cycle

Upon ingestion of tissue from infected hosts, proteolytic digestive enzymes degrade the tissue cysts and release the bradyzoites.² These slowly dividing forms of the coccidian are capable of a sexual reproductive process called merogany that occurs only in cats.⁵ Through merogany, bradyzoites undergo repeated nuclear/cytoplasmic fission to yield microgamonts (‘male-like’) and macrogamonts (‘female-like’).⁵ Microgamonts further divide to form microgametes which are then able to fertilize the macrogamont.⁵ A protective wall encapsulates fertilized macrogamonts to form the zygote-containing oocyst. Unsporulated oocysts are non-infective and are shed into the environment from the feces in most naïve cats, 10 days or less after tissue cyst ingestion.^2,6 Oocyst sporulation to the infective form occurs approximately 1-5 days later, after exposure to ambient air and humidity.² This is the period of most concern for exposure to pregnant women who may be cleaning the cat litter box. Each sporulated oocyst contains eight sporozoites that can survive in the environment for up to 18 months⁷, and insects such as roaches and earthworms may function as transport hosts by dispersing oocysts from their initial shedding point .⁷

There are several reasons why cats who ingest oocysts or tachyzoites develop T. gondii infections less frequently (approximately 20%) and exhibit a prolonged prepatent period when compared to those cats ingesting tissue cysts.⁶ Bradyzoites are able to serve as direct precursors to enteroepithelial cycling (and thus a shorter prepatent period) whereas oocysts or tachyzoites must first develop to the bradyzoite stage.^{2, 5} Moreover, the decreased infection rate may be related to the fact that each tissue cyst contains a large number of bradyzoites whereas only eight sporozoites are within an oocyst.² Finally, tachyzoite ingestion is less likely to result in patency as they are often destroyed by gastrointestinal digestive enzymes.²

Extraintestinal Life Cycle

Ingested sporulated oocysts release sporozoites into the gastrointestinal tract of the host.⁶ Sporozoites enter cells of the intestine and lymph nodes where they undergo endodyogeny to form rapidly dividing tachyzoites that disemminate throughout the body through the blood and lymphatic vasculature.⁶ The tachyzoite tissue phase that develops within a host or is ingested may result in bradyzoite tissue cyst formation within the brain, skeletal muscle, and liver (see Figures 1,2). These tissue cysts may persist for the life of the host,⁷ and the potential for recrudescence exists as tissue cysts rupture and release bradyzoites.² Both ingested bradyzoites and those released through host cyst rupture are capable of asexual development to tachyzoites, thereby perpetuating the cycle.⁵

Figure 1. Raccoon brain. Multiple Toxoplasma gondii tissue cysts (arrow) within white matter. H&E stain, 40X.	Figure 2. Dog muscle. Myonecrosis and myositis with intralesional T. gondii tissue cysts (arrows) containing developing bradyzoites. H&E stain, 40X.

Vertical and Iatrogenic Infections

Transplacental and transmammary transmission occurs in carnivores, omnivores, and herbivores as a result of naïve host infection during pregnancy.^2,8 In congenital transmission, T. gondii replicates and infects placental tissue whereby it gains access to the fetus. Outcome in these cases is most severe (e.g. abortion, stillbirth) when infection occurs during the first half of the pregnancy.² Alternatively, when tachyzoites are shed in milk, transmammary transmission may occur and result in clinical illness of varying degrees in newborns.² Additionally, although less commonly observed, infection is also possible in recipients of donor fluids or tissue thereby underscoring the importance of thoroughly screening donors as T. gondii can survive at 4°C for up to 50 days in stored blood products.⁹

Clinical Disease

Although there is a high seroprevalence of T. gondii infection among Felidae, significant clinical disease in domestic cats is relatively infrequent. When disease does occur, it may be attributed to a primary infection in which an inadequate immune response failed to arrest invasive tachyzoites.^1,2,4 Alternatively, disease may result from reactivation of subclinical infection in an immunocompromised individual with encysted bradyzoites which may then form rapidly multiplying tachyzoites.² Such incidents are thought to relate directly to variables such as the age and sex of the host, presence of immunodeficient states (such as feline leukemia virus or feline immunodeficiency virus infection), concomitant stress or illness, and organism load.²

Clinical illness appears to be most frequent among cats less than 2 years of age which may be due in part to an insufficient immune response by young cats.^1,2,4 In one study involving 25 kittens experimentally infected with neonatal T. gondii infection, 3 kittens were stillborn.¹⁰ Among the 22 live kittens, approximately 95% exhibited proliferative interstitial pneumonia, necrotizing hepatitis, myocarditis, and skeletal myositis to the extent that euthanasia was necessary.¹⁰ Infected neonates also often develop central nervous system infections.²

Organism ingestion followed by initial enteroepithelial replication may lead to a self-limiting small bowel diarrhea that occurs due to an IgA response elicited by T. gondii resulting in increased intestinal secretions.² Clinical illness may progress when naïve hosts are infected or when prior infections are reactivated. Extraintestinal spread of the parasite may occur as rapid multiplication of tachyzoites within host cells results in cell rupture, promoting organism dissemination and, ultimately, tissue necrosis.^1,2,4 Disease onset may be sudden or insidious, and clinical signs are often multiple and varied as most types host cells are vulnerable to infection.¹¹ Respiratory, CNS, hepatic, pancreatic, cardiac, and ocular tissues were among the most commonly affected tissues in a group of 100 adult cats with confirmed toxoplasmosis.⁷ Associated clinical signs included dyspnea, tachypnea, intermittent fever, icterus, vomiting, weight loss, hyperesthesia, shifting lame lameness, ataxia, dermatitis, and death.²

Clinicopathologic Findings

Laboratory testing may detect several abnormal parameters in animals with acute systemic toxoplasmosis. A nonregenerative anemia may be detected with a concurrent neutrophilic leukocytosis, lymphocytosis, monocytosis, and eosinophilia.² The biochemical profile of cats with chronic toxoplasmosis may demonstrate hyperglobulinemia due to chronic antigenic stimulation and immune response. Hepatocellular disease may result in elevated alanine aminotransferase (ALT) enzyme activity, and muscle damage may cause an increase in aspartate aminotransferase (AST) and creatine kinase (CK) activity.² Hyperbilirubinemia may occur in cats with cholangiohepatitis or hepatic lipidosis secondary to liver dysfunction.²

Histologically, lesions associated with toxoplasmosis are a result of cell death secondary to intracellular replication of T. gondii. The associated inflammatory reaction is primarily composed of macrophages in adult cats and neutrophils and macrophages (pyogranulomatous), with or without a lymphoplasmacytic component, in neonates.^10,11 Tissue cysts often persist in the absence of host reaction.

In the CNS, encephalitis may result from tachyzoites primarily infecting astrocytes.¹¹ A resultant diffuse necrotizing and nonsuppurative lymphocytic infiltrate may develop in the brain and extend to the meninges.¹¹ Necrotizing hepatitis with focal areas of coagulative lobular necrosis may be observed with the presence of few organisms.¹¹ Other gross lesions include pulmonary edema and congestion with failure of lung collapse as well as multifocal areas of firm white, yellow, or gray discoloration in the pulmonary parenchyma.¹¹ Toxoplasma organisms invade type 1 and 2 pneumocytes as well as pulmonary alveolar macrophages, fibroblasts, endothelial cells, and smooth muscle cells.¹¹ The subsequent proliferative reaction in alveolar walls may resemble adenomatosis. Severe lymphadenopathy may occur.¹¹ Pericardial effusion is occasionally reported and likely due to organisms invading the myocardium.² Intestinal lymphatic tissue invasion may result in small bowel ulcerative disease.¹¹ If the muscularis is involved, occasionally a chronic necrotizing process leads to large granulomatous nodules that can impede the transport of luminal contents and possible lymphangiectasia.^2,11 Ocular lesions are frequent and may cause inflammation of the retina or anterior segment (anterior uveitis) with granulomatous inflammation being the prominent cytologic feature.² Placental lesions can include focal necrosis with or without foci of mineralization.⁷

Diagnosis

Diagnosis of clinical toxoplasmosis in cats is challenging due to the high seroprevalence of the infection among non-affected cats as well as the many potential antibody responses that may occur in disease as well as in health. Since seroconversion also occurs in the absence of clinical disease, titers should be interpreted in conjuction with clinical signs consistent with toxoplasmosis. Accordingly, the exclusion of other causes of clinical disease paired with serologic evidence (such as a four-fold increase in titer) and a positive response to appropriate therapy may be simultaneously employed in order to reach a tentative diagnosis.²

Serology

Near 80% of cats are thought to develop IgM antibodies within 1-2 weeks after exposure. These antibodies may remain detectable for months to years.² As a result, the presence of IgM antibody cannot reliably be used to predict recent infection and oocyst shedding. Similarly, IgG antibodies may not develop for 4-6 weeks and may peak in 2-3 weeks with some cats remaining high for several years.^1,2,4

T. gondii titers may be by indirect fluorescent antibody testing, modified agglutination, enzyme-linked immunosorbent antibody assays, and Sabin-Feldman serologic testing.⁷ Because many cats have T. gondii antibodies from previous exposure, it is necessary to demonstrate either a four-fold rise in IgG titers over a 2-3 week period or a single high IgM titer (> 1:64).^1,2,4,7 Clinical signs may develop before seroconversion occurs in 1-2 weeks or not until after peak titers have developed.^1,2,7 For these reasons, antibody titers may be difficult to interpret and single antibody titers are often insufficient for definitive diagnosis.²

Cytology

Definitive diagnosis for T. gondii is possible through identification of the organisms in body tissue or fluids (see Figures 3,4). In acute illness, tachyzoites may be present in large numbers in body fluids such as abdominal and pleural effusions.² Peripheral blood smears, cerebrospinal fluid (CSF), fine needle aspirates of tissues, and airway washings are less likely to provide direct organism detection.² Such biologic samples can be used for bioassays in mice, tissue cultures, or PCR to demonstrate the presence of T. gondii organisms.^2,7

Figure 3. Transtracheal washing from a cat. A large binucleate phagocytic cell contains intracytoplasmic Toxoplasma tachyzoites (arrow). Wrights stain, 50X.	Figure 4. Fine needle aspirate from cat lung. A ruptured large mononuclear cell with multiple Toxoplasma tachyzoites. Wrights stain, 100X.

Fecal samples may also be examined for the presence of T. gondii oocysts but are relatively insensitive and nonspecific. Clinical signs usually do not develop until after oocyst shedding has ceased, and oocyst shedding may occur in the absence of clinical toxoplasmosis.^1,2,7 Furthermore, the microscopic appearance of T. gondii oocysts is indistinguishable from the oocysts of other coccidians such as Hammondia and Besnoitia that may also infect cats.^2,7

CNS and Aqueous Humor Analysis

Diagnosis of toxoplasmosis-induced uveitis or encephalitis is also possible through antibody testing and subsequent calculation of Goldman-Witmer coefficients.^{2, 7} Antibody measurements for T. gondii and another agent-specific-antibody (ASA) in aqueous humor or CSF must be compared to the same measurements obtained for serum.^2,7 The ASA should be from an agent that is expected to have a high serum titer but does not cause CNS disease. Calicivirus antibody has been used for this purpose in cats.² This procedure allows for higher antibody levels occurring simply due to increased vascular permeability that may result from inflammation of any cause.² To calculate the coefficient, the ratio of the T. gondii antibody level in the aqueous or CSF over the T. gondii antibody level in the serum is multiplied by the ratio of the ASA level in body fluids (i.e. in the aqueous or CSF) over the serum ASA level.^2,7 A coefficient greater than 1 is significant, and a result greater than 8 is considered compatible with T. gondii infection.^2,7

Treatment

Clindamycin at 10-12mg/kg every 8-12 hours for 4 weeks, by oral or parenteral route, is often used for treatment of cats with clinical toxoplasmosis.^2,7 Resolution of clinical signs may begin within 24 to 48 hours of instituting therapy.² Clindamycin doses necessary to treat toxoplasmosis in cats may lead to such adverse reactions as anorexia, vomiting, and diarrhea.¹² If clindamycin cannot be tolerated at the recommended dose, pyrimethamine or trimethoprim may be combined with a sulfonamide for treatment. However, the risk of anti-folate drug related bone marrow suppression necessitates frequent monitoring for hematologic abnormalities.^2,7 Dietary supplementation with folic acid (5mg/day) or yeast (100 mg/kg) may aide in correcting hematopoietic disruption.^2,7 The prognosis for cats with toxoplasmosis is guarded, as available drugs are unable to completely eliminate the parasite and the risk for relapse of clinical disease is possible.² Recurrence is particularly common in immunocompromised cats.^2,7

Zoonotic Potential

An estimated 30-50% of the human population is currently infected with Toxoplasma in the asymptomatic cyst form.³ In immunocompromised patients, the cyst form may potentially lead to serious disease.^3,13 Because of the potential for disastrous outcomes in infections that occur during pregnancy, women planning to become pregnant may opt for T. gondii antibody testing.¹³ Positive antibody detection indicates previous infection and suggests that the likelihood of congenital transmission upon re-exposure to the parasite during pregnancy is limited.¹³ Conversely, antibody-negative women are at far greater risk of transmitting Toxoplasma to the fetus if they should become infected during pregnancy.¹³ This risk becomes magnified as 70 million domestic cats are kept as pets in American households, making them the most numerous in the nation among domestic companion species.¹⁴ Clearly, it is vital to clarify the role of cats in the transmission of Toxoplasma to humans.

The primary means by which most humans are infected with T. gondii are through oocyst-contaminated soil and eating undercooked infected meat such as lamb and pork.³ People who own cats are not at a significantly higher risk for T. gondii infection than those who do not.⁷ Since oocysts are highly resistant to environmental conditions, exposure can be minimized by wearing rubber gloves during contact and washing hands thoroughly after possible exposure to potentially contaminated soil.^1,3,13 Areas such as sandboxes should be kept covered to avoid oocyst contamination. ^1,3,13 Meat should routinely be cooked to an internal temperature of 70°C (158°F) for at least 15 to 30 minutes in order to destroy tissue cysts.¹³

Restricting the access of pet cats to hunting and disallowing the ingestion of uncooked meat may prevent cats exposure to T. gondii. When they are exposed. most healthy cats will shed oocysts only during acute infection.^1,2,7,13 As the infected cat develops an immune response, oocyst shedding is halted, and the development of tachyzoites is arrested with the resultant formation of bradyzoites (slowly replicating forms of the organism) conyained within tissue cysts.² Cats previously unexposed to T. gondii usually begin shedding oocysts between 3 and 10 days after ingestion of infected tissue and continue shedding for 10-14 days, during which time many millions of oocysts may be produced.^1,2,7,13 However, once a cat has developed an immune response, further shedding of oocysts is extremely rare.^1,2,7,13 In the few cats that do re-excrete oocysts after another exposure to Toxoplasma, the number of oocysts shed is lower and may even be insufficient to transmit the parasite effectively.^2,7 An antibody-negative cat, particularly a kitten, is most susceptible to infection and will shed oocysts for one to two weeks post-exposure to T. gondii.² In a healthy cat, positive antibody titers are suggestive that the cat is immune, not excreting oocysts, and an unlikely source of infection.² In any case, daily cleaning of feces from litter boxes, paired with regular disinfection of the boxes, will prevent any oocysts that are shed from sporulating to the infective form.^1,2,7,13 It is still recommended, however, that women who are or may become pregnant avoid cleaning the litter box.

References

1. Lappin MR. General Concepts in Zoonotic Disease Control. Vet Clin North Am Small Anim Pract 2005;35:1-20.

2. Dubey JP, Lappin MR. Toxoplasmosis and Neosporosis. In: Greene C, ed. Infectious Diseases of the Dog and Cat, 3^rd ed. St. Louis, MO: Elsevier, 2006. pp 754-768.

3. Toxoplasmosis. United States Center for Disease Control, 22 Nov. 2004. <http://www.dpd.cdc.gov/dpdx/HTML/Toxoplasmosis.htm>

4. Lindsay DS, Blagburn, BL. Coccidial Parasites of Dogs and Cats. Compend Contin Educ Pract Vet 1991;13:759-765.

5. Bowman, DD. Georgi’s Parasitology for Veterinarians. St. Louis, MO: Saunders, 2003. pp 100-102.

6. Dubey, JP, Speer CA, et al. Oocyst-induced murine toxoplasmosis: life cycle, pathogenicity, and stage conversion in mice fed Toxoplasma gondii oocysts. J Parasitol 1997;83:870-872.

7. Lindsay, DS, Blagburn, BL: Feline toxoplasmosis and the importance of the Toxoplasma gondii oocyst. Compend Contin Educ Pract Vet 1997;19:448-461.

8. Bonametti AM, Passos JN, Da Silva EMK, and Macedo ZS. Probable transmission of acute toxoplasmosis through breast feeding. J Trop Ped 1997;43:116.

9. Freij BJ, Sever JL. Toxoplasmosis. Paed in Rev 1991;12:227-36.

10. Dubey JP, Mattix ME, Lipscomb TP. Lesions of neonatally induced toxoplasmosis in cats. Vet Pathol 1996;33:290-295.

11. Jones TC, Hunt RD, King NW. Veterinary Pathology. Baltimore, MD: Lippincott, 1996. pp 555-561.

12. Plumb DC: Veterinary Drug Handbook. Ames, IA: Iowa State Press, 2002. pp 206-209.

13. Montoya JG, Rosso F: Diagnosis and Management of Toxoplasmosis. Clin Perinatol 2005;32:705-726.

14. Veterinary Market Statistics. AVMA. 2002. <http://www.avma.org/membshp/marketstats/comp_exotic.asp>

Acknowledgement

"Cat Nap" watercolor by Sinclair Stratton (c) 2002 is from the Gallery section of his website and is used with permission.

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