(military recruits) declined from 14.9% in 1962 to 9.9% in
1989 (46); prevalence rates in France are reported to range
from 42%-84% of the population (10). Using the tools
currently available, the source of infection for humans cannot
be determined between exposure to oocysts in the
environment or ingestion of contaminated and undercooked
meat. However, the assumption that infection in food animals
does play a significant role in the transmission of
toxoplasmosis to humans is reasonable.
Most species of livestock, including sheep, goats and pigs, are
infected with T. gondii. Prevalence rates vary in pigs as in
humans (6), but generally exceed 10%-20% in most
countries. Infection rates are higher in breeding populations
than in market pigs, reflecting that time of exposure is a factor
in acquiring toxoplasmosis. In the USA, the level of
toxoplasmosis was estimated at 23.9% of pigs in 1983-1984
with higher rates in breeders (42%) than in market pigs (23%)
(7, 12). When pigs from these areas were tested in 1992, the
rate was reduced to 20.8% of breeders and 3.1% of finisher
pigs (50). These results suggest that the incidence of
toxoplasmosis is declining in confinement reared pigs due to a
reduction in risk factors.
Transmission of toxoplasmosis to pigs on the farm occurs by
various means. Risk factors for infection by exposure to tissue
cysts are virtually identical to risk factors for exposure to