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Mechanisms of human infection with Toxoplasma
gondii |
1) Ingestion of sporulated oocysts shed in cat feces:
- It's critically important to realize that once oocysts are
shed in the feces of a cat, they require a period of at least
1-5 days for sporulation before they are infectious (even longer
at colder temperatures).
- Hence, contact with fresh feces is of much less risk than
contact with feces in soil or feces left sitting in a litter
box for several days.
- However, sporulated oocysts are very resistant in the environment
and may remain infectious for over 18 months under favorable
conditions.
- It must also be stressed that cats only shed the oocysts
for 10-14 days following initial infection. Thereafter, the organism
is found only as tissue cysts in the body.
- In an experiment by Dubey, 4/9 cats experimentally rechallenged
6 years after primary infection went through a second phase of
shedding, so the potential for shedding upon subsequent exposure
may exist.
- However, the risk of reactivation and shedding following
intercurrent disease/stress in cats appears to be quite low.
Daily oral prednisone at doses <10mg/kg does not appear to
cause renewed shedding. Likewise, shedding patterns do not appear
to be altered by secondary infections with FeLV or FIV.
- Latently infected cats may shed T. gondii oocysts
for a short period of time after secondary infection with Isospora
felis.
- At any one time, <1% of cats in the U.S. are shedding
Toxoplasma oocysts.
2. Ingestion of the bradyzoite form of the organism encysted
in meat:
- Despite the role of cats as the definitive host for T.
gondii, multiple studies have shown that cat ownership is
not a specific risk factor for human infection with Toxoplasma.
INGESTION OF UNDERCOOKED MEAT IS THE MAJOR ROUTE OF INFECTION
FOR HUMANS, NOT CONTACT WITH CAT FECES!! This fact has been
substantiated by studies in the United States and a large, multi-center
study of pregnant woman in Europe with acute T. gondii infections.
In the later study (Cook et al., 2000), the risk factor
most strongly predictive of infection was eating undercooked
meats. 30-63% of infections were attributed to the consumption
of undercooked meats.
- Up to 25% of lamb and pork meat samples in the U.S. contain
the organism, and a survey in Iowa and Illinois found that 20-22%
of pigs were seropositive for T. gondii.
- The numbers in sheep are less clear, but small scale studies
indicate that seropositivity in lambs is even higher than in
pigs.
- Poultry is of minimal risk because poultry meat is often
frozen during processing, which kills the organism. (And the
public is already aware [we hope!] of the need to thoroughly
cook poultry in order to kill Campylobacter and Salmonella
organisms.)
- Infection can also occur from ingestion of undercooked game
meat such as venison, bear (very high rates of infection) and
even kangaroos!
- In December 1994, 12 people in Australia were infected by
eating kangaroo meat, which is apparently often prepared quite
rare.
- Infection has also been documented following consumption
of unpasteurized goat's milk and municipal drinking water.
- In 1995, 100 people were infected in an unusual multi-person
outbreak in Victoria, British Columbia. The only apparent link
among the cases was residence in a water district supplying unfiltered
but chlorinated water from a single reservoir
- Cases occurred following two periods of heavy rain, with
run-off into the reservoir and increases in water turbidity.
(Very reminiscent of the Cryptosporidium outbreak in Milwaukee,
WI in 1993.) A similar water-borne outbreak of toxoplasmosis
associated with a single water reservoir occurred in Brazil in
2001-2002.
3. Transplacental infection following active replication
in the placenta and tachyzoite invasion of the fetus:
(See Congenital Toxoplasmosis page)
(Click
here to see a graphic depicting the sources of infection with
Toxoplasma gondii.)
