1. Toxoplasma gondii cosmopolitan distribution
seropositive prevalence rates vary
generally 20-75%
generally causes very benign disease in immunocompetent adults
tissue cyst forming coccidia
predator-prey life cycle
felines are definitive host
infects wide range of birds and mammals (intermediate hosts)
Definitive Host
adult forms
sexual reproduction
Intermediate Host
immature forms
asexual reproduction

2. Typical Isospora Life Cycle in Felines

3. Typical Isospora Life Cycle in Felines
fertilization within infected host cells
immature oocysts in feces
sporulation in environment (1-4 d)

4. Tachyzoite Stage sporozoites  merogony rapid replication
dissemination via macrophages
reticuloendothelial cells
acute stage infection

5. Endodyogony

6. Tachyzoite Stage sporozoites  merogony rapid replication
dissemination via macrophages
reticuloendothelial cells
acute stage infection

7. Tachyzoite Stage sporozoites  merogony rapid replication
dissemination via macrophages
reticuloendothelial cells
acute stage infection

8. Bradyzoite Stage dormant, slowly replicating
due to host immune response
chronic or latent infection
tissue cysts in brain and muscle

9. Bradyzoite Stage dormant, slowly replicating
due to host immune response
chronic or latent infection
tissue cysts in brain and muscle

11. “Fatal Attraction in Rats Infected with Toxoplasma gondii”
non-infected rats (n=32)
infected rats (n=23)
visits to scented areas
own
neutral
rabbit
cat
From Berdoy et al (2000) Proc. R. Soc. (Biol.) 267:1591

13. Human Transmission
ingestion of sporulated oocysts (cat feces + incubation)
ingestion of zoites (undercooked meat)
congenital infection (only during acute stage)
organ transplants
chronic infection in donor
immunosuppression
blood transfusions (only during acute stage)

14. Acquired Postnatal Toxoplasmosis
1-2 week incubation period
acute parasitemia persists for several weeks until development of tissue cysts
often asymptomatic (>80%)
a common symptom is lymphadenopathy without fever
occasionally mononucleosis-like (fever, headache, fatigue, myalgia)
likely persists for life of patient
immunosuppression can lead to reactivation (eg., organ transplants)

15. Toxoplasmic Encephalitis
common complication associated with AIDS during the 1980's
recrudescence of latent infection
multifocal disease associated with immunosuppression
lesions detectable with CT or MRI
little spread to other organs
symptoms include: lethargy, apathy, incoordination, dementia
progressive disease  convulsions

16. Congenital Toxoplasmosis
1o infection must occur during pregnancy
can only occur once
1/3 will pass infection to fetus
incidence ~1 per 1000 births
severity varies with age of fetus
move severe early in pregnancy
more frequent later in pregnancy
infection can result in: spontaneous abortion, still birth, premature birth, or full-term ± overt disease

17. typical disease manifestations include: retinochoroiditis, psychomotor disturbances, intracerebral calcification, hydrocephaly, microcephaly

18. Ocular Toxoplasmosis
retinochoroiditis: likely due to both active parasite proliferation and immune hypersensitivity
generally a recrudescence--rarely from primary infection
congenital infection
20% exhibit ocular symptoms at birth
82% by adolescence
most lesions are focal and self-limiting
rapidly destructive in AIDS patients

19. Ocular Toxoplasmosis
retinochoroiditis: likely due to both active parasite proliferation and immune hypersensitivity
generally a recrudescence--rarely from primary infection
congenital infection
20% exhibit ocular symptoms at birth
82% by adolescence
most lesions are focal and self-limiting
rapidly destructive in AIDS patients

20. Diagnosis seldom by direct parasite demonstration
biopsy
inoculation into mice or cell culture (only acute stage)
various serological tests
active (acute) vs chronic infection
compare samples at 2 week intervals
IgM > IgG;  Ab titers

22. Prevention

23. Frenkel et al (1995)
AJTMH 53:458
But dog contact is highly correlated with Toxoplasma transmission.
Several studies show no correlation between cat contact and Toxoplasma.
An Enigma