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Basics ⬇

Basics

Definition

Toxoplasma gondii-an obligate intracellular coccidian protozoan parasite that infects nearly all mammals; Felids are the definitive hosts; all other warm-blooded animals are intermediate hosts.

Pathophysiology

  • Severity and manifestation-depend on location and degree of tissue injury caused by tissue cysts.
  • Infection-acquired by ingestion of tissue cysts in intermediate hosts or oocysts shed by felids; organisms spread to extra-intestinal organs via blood or lymph; results in focal necrosis to many organs (heart, eyes, CNS).
  • Acute disseminated infection rarely fatal.
  • Chronic disease-tissue cysts form; low-grade disease; usually not clinically apparent unless immunosuppression or concomitant illness allows organism to proliferate, causing an acute inflammatory response.
  • Clinical disease-often associated with other infections that cause severe immunosuppression (e.g., canine distemper, FIP, and FeLV).

Systems Affected

  • Multisystemic-usually the same in cats and dogs.
  • Ophthalmic-approximately 80% of affected cats have evidence of intraocular inflammation, most commonly uveitis.

Incidence/Prevalence

  • Approximately 30% of cats and up to 50% of people serologically positive for T. gondii.
  • Most animals asymptomatic.

Geographic Distribution

Worldwide

Signalment

Species

Cats more commonly symptomatic than dogs

Mean Age and Range

In one study, mean age 4 years; range 2 weeks–16 years

Predominant Sex

Male cats-more common

Signs

General Comments

  • Determined mainly by site and extent of organ damage.
  • Acute-at the time of initial infection.
  • Chronic-reactivation of encysted infection; caused by immunosuppression.

Historical Findings

  • Non-specific signs of lethargy, depression, and anorexia.
  • Weight loss.
  • Fever.
  • Ocular discharge, photophobia, miotic pupils (cats).
  • Respiratory distress.
  • Neurologic-ataxia; seizures; tremors; paresis/paralysis; cranial nerve deficits.
  • Digestive tract-vomiting; diarrhea; abdominal pain; jaundice.
  • Stillborn kittens.

Physical Examination Findings

Cats

  • Most severe in transplacentally infected kittens, which may be stillborn or die before weaning.
  • Surviving kittens-anorexia; lethargy; high fever unresponsive to antibiotics; reflect necrosis/inflammation of lungs (dyspnea, increased respiratory noises), liver (icterus, abdominal enlargement from ascites), and CNS (encephalopathic).
  • Respiratory and gastrointestinal (postnatal)-most common; anorexia; lethargy; high fever unresponsive to antibiotics; dyspnea; weight loss; icterus; vomiting; diarrhea; abdominal effusion.
  • Neurologic (postnatal)-seen in <10% of patients; blindness; stupor; incoordination; circling; torticollis; anisocoria; seizures.
  • Ocular signs (postnatal)-common; uveitis (aqueous flare, hyphema, mydriasis); iritis; detached retina; iridocyclitis; keratic precipitates.
  • Rapid course-acutely affected patients with CNS and/or respiratory involvement.
  • Slow course-patients with reactivation of chronic infection.

Dogs

  • Young-usually generalized infection; fever; weight loss; anorexia; tonsillitis; dyspnea; diarrhea; vomiting.
  • Old-tend to localized infections; mainly associated with neural and muscular systems.
  • Neurologic-quite variable; usually reflect diffuse neurologic inflammation; seizures; tremors; ataxia; paresis; paralysis; muscle weakness; tetraparesis.
  • Ocular-rare; similar to those found in cats.
  • Cardiac involvement-occurs; usually not clinically apparent.

Cause

T. gondii

Risk Factors

Immunosuppression-may predispose to infection or reactivation: FeLV, FIV, FIP, hemotrophic mycoplasma, canine distemper, and glucocorticoid or antitumor chemotherapy or post-renal transplant.

Diagnosis ⬆ ⬇

Diagnosis

Differential Diagnosis

Cats

  • Intraocular disease (anterior uveitis)-FIP; FeLV; FIV; immune-mediated; trauma; lens-induced; corneal ulceration with reflex uveitis.
  • Dyspnea (respiratory signs)-asthma; cardiogenic; pneumonia (bacterial, fungal, parasitic); neoplasia; heartworm disease; pleural disease (effusions); diaphragmatic hernia; chest wall injury.
  • Neurologic (causes of meningoencephalitis)-viral (FIP, rabies, pseudorabies); fungal (cryptococcosis, blastomycosis, histoplasmosis); parasitic (cuterebriasis, coenurosis, aberrant heartworm migration); bacterial; idiopathic disease (feline polioencephalomyelitis).

Dogs

  • Often associated with other immunosuppressive diseases-e.g., signs of distemper may be seen.
  • Differentiate from Neospora or Sarcocystis which are similar morphologically.
  • Consider other conditions causing multifocal signs-infectious or inflammatory toxicity; metabolic disease.

CBC/Biochemistry/Urinalysis

CBC (Cats)

  • Most show mild normocytic normochromic anemia.
  • Leukopenia-approximately 50% of patients with severe disease; mainly owing to lymphopenia.
  • Neutropenia-alone or in addition to lymphopenia and a degenerative left shift.
  • Leukocytosis-may occur during recovery.

Biochemistry

  • ALT and AST-marked increase in most patients.
  • Hypoalbuminemia.
  • Cats-icterus seen in approximately 25% of patients; mildly low serum calcium concentrations often seen with pancreatitis; amylase levels unreliable.

Urinalysis (Cats)

  • Mild proteinuria-small proportion of patients.
  • Bilirubinuria-especially with icterus.

Other Laboratory Tests

Serology

  • IgM, IgG, and antigen serum titers-most definitive information from one sample; determine type of infection (active, recent, chronic) with a follow-up sample taken 3 weeks later.
  • IgM-single serologic titer of choice for diagnosis of active infection; elevated 2 weeks post-infection (usually coincides with onset of clinical signs); persists for a maximum of 3 months; then falls; prolonged titer: reactivation or delay in antibody class shift to IgG (result of immunosuppression from FeLV or FIV infection or steroid therapy).
  • IgG-titers rise 2–4 weeks post-infection; persist >1 year; single high titer not diagnostic for active infection; four-fold increase over a 3-week period suggests active infection.
  • Antigen-positive 1–4 weeks post-infection; because it remains positive during active or chronically persistent infections, does not add much to antibody titer results.
  • A tentative antemortem diagnosis of clinical disease can be based on clinical and serologic parameters: (a) serologic evidence of recent or active infection-high IgM titers, four-fold change in IgG titers, (b) exclusion of other causes of the clinical syndrome, and (c) beneficial response to anti-Toxoplasma drugs.
  • PCR-used to verify presence of T. gondii in biologic specimens; available from several laboratories.

Imaging

Radiographs-may see mixed pattern of patchy alveolar and interstitial pulmonary infiltrates, pleural and abdominal effusions, and hepatomegaly.

Diagnostic Procedures

  • PCR may be the most prudent choice in suspect cases since many protozoa are morphologically similar and are difficult to distinguish in tissues.
  • CSF-high leukocyte count (both mononuclear cells and neutrophils) and protein in encephalopathic patients.
  • Cytology-organism rarely detected in body fluids during acute infection (CSF, pleural or peritoneal effusions); bronchoalveolar lavage effective in identifying organisms in affected cats with signs of pulmonary involvement.
  • Fecal-evaluation with Sheather's sugar solution may be diagnostic; fecal oocyst shedding rarely occurs during clinical disease; oocysts may be detected on routine examination in asymptomatic cats but are morphologically indistinguishable from Hammondia spp. and Besnoitia; distinguish organisms via mouse inoculation.

Pathologic Findings

  • Necrotic foci-up to 1 cm; most often in liver, pancreas, mesenteric lymph nodes, and lungs; necrosis of brain (1 cm areas of discoloration).
  • Ulcers and granulomas-may be seen in stomach and small intestine.
  • Potentially no gross lesions.

Treatment ⬆ ⬇

Treatment

Appropriate Health Care

  • Usually outpatient.
  • Inpatient-severe disease; patient cannot maintain adequate nutrition or hydration.
  • Confine-patients with neurologic signs.

Nursing Care

Dehydration-intravenous fluids

Client Education

  • Cats-prognosis guarded in patients needing therapy; response to therapy inconsistent.
  • Neonates and severely immunocompromised animals-prognosis worse.

Medications ⬆ ⬇

Medications

Drug(s) Of Choice

  • Clindamycin-25–50 mg/kg PO or IM daily, divided into two doses, for at least 2 weeks after clinical signs clear.
  • 1% prednisone drops-every 8 hours for 2 weeks for uveitis; use concurrently.

Precautions

Clindamycin-anorexia, vomiting, and diarrhea (dose-dependent).

Alternative Drug(s)

  • Sulfadiazine (30 mg/kg PO q12h) in combination with pyrimethamine (0.5 mg/kg PO q12h) for 2 weeks; can cause depression, anemia, leukopenia, and thrombocytopenia, especially in cats.
  • Folinic acid (5 mg/day) or brewer's yeast (100 mg/kg/day)-correct bone marrow suppression caused by sulfadiazine/pyrimethamine therapy.

Follow-Up ⬆ ⬇

Follow-Up

Patient Monitoring

Clindamycin

  • Examine 2 days after initiation treatment-clinical signs (fever, hyperesthesia, anorexia, uveitis) should begin to resolve; uveitis should resolve completely within 1 week.
  • Examine 2 weeks after initiation of treatment-assess neuromuscular deficits; should partially resolve (some deficits permanent owing to CNS or peripheral neuromuscular damage).
  • Examine 2 weeks after owner-reported resolution of signs-assess discontinuing treatment; some neuromuscular deficits permanent.

Prevention/Avoidance

Cats

  • Diet-prevent ingestion of raw meat, bones, viscera, or unpasteurized milk (especially goat's milk), or mechanical vectors (flies, cockroaches); feed only well-cooked meat.
  • Behavior-prevent free-roaming to hunt prey (birds, rodents) or to enter buildings where food-producing animals are housed.

Expected Course and Prognosis

  • Prognosis-guarded; varied response to drug treatment.
  • Acute-prompt and aggressive therapy often successful.
  • Residual deficits (especially neurologic) cannot be predicted until after a course of therapy.
  • Ocular disease-usually responds to appropriate therapy.
  • Severe muscular or neurologic disease-usually chronic debility.

Miscellaneous ⬆

Miscellaneous

Associated Conditions

  • Young dogs-distemper.
  • Cats-FeLV, FIP, and FIV; FIV infection does not affect clinical outcome or the ability of the animal to mount a protective immune response to subsequent reinfection; renal transplant.

Age-Related Factors

Disease worse in neonates

Zoonotic Potential

  • Biggest danger is infection of pregnant women or immunocompromised individuals.
  • Young cats are most likely to be shedding oocysts.
  • Important: oocysts need to be sporulated to be infectious. Unsporulated oocysts are shed in the feces and need at least 24 hours to sporulate.
  • Cats-healthy animals with a positive antibody titer pose little danger to humans; animal with no antibody titer at more risk of becoming infected, shedding oocysts in the feces, and constituting a risk to humans.
  • Avoid contact with oocysts or tissue cysts-do not feed raw meat; wash hands and surfaces (cutting boards) after preparing raw meat; boil drinking water if source is unreliable; keep sandboxes covered to prevent cats from defecating in them; wear gloves when gardening; wash hands and vegetables before eating to avoid contact with oocyst soil contamination; empty cat litter boxes daily; disinfect litter boxes with boiling water; control stray cat population to avoid oocyst contamination of environment.
  • Pregnant women-avoid all contact with a cat that is excreting oocysts in feces; avoid contact with soil and cat litter; do not handle or eat raw meat (to kill organism, cook to 66°C; 150°F).
  • T. gondii causes abortion in sheep; prevent cats from ingesting placenta or aborted fetuses and keep cats from defecating in sheep feed to break the life cycle and decrease the zoonotic potential.

Pregnancy/Fertility/Breeding

  • Parasitemia during pregnancy-spread of organism to fetus; probably does not happen unless first-time infection of dam occurs during pregnancy (as with humans).
  • Placental transmission rare.

Abbreviations

  • ALT = alanine aminotransferase
  • AST = aspartate aminotransferase
  • CNS = central nervous system
  • CSF = cerebrospinal fluid
  • FeLV = feline leukemia virus
  • FIP = feline infectious peritonitis
  • FIV = feline immunodeficiency virus
  • PCR = polymerase chain reaction

Suggested Reading

Dubey JP, Lappin MR. Toxoplasmosis and neosporosis. In: Greene CE, ed., Infectious Diseases of the Dog and Cat, 3rd ed. St. Louis, MO: Saunders Elsevier, 2006, pp. 754–775.

Schatzberg SJ, Haley NJ, Barr SC, et al. Use of a multiplex polymerase chain reaction assay in the antemortem diagnosis of toxoplasmosis and neosporosis in the central nervous system of cats and dogs. Am J Vet Res 2003, 64:1507–1513.

Author Matt Brewer

Consulting Editor Stephen C. Barr

Client Education Handout Available Online