Humans may be infected either by eating cysts in meat or by ingestion of sporulated oocysts from contaminated soil. The relative risk of infection in the USA, Canada and Europe is considered to be higher from the ingestion of undercooked meat, but in societies with little meat in the diet, oocysts are more important (Frenkel et al., 1995). Studies of vegetarians show a lower incidence of toxoplasmosis in this population in industrialized countries (Roghmann et al., 1999) and the tropics (Rawal, 1959). Birds and rodents are important in picking up oocysts from soil and scavenging bradyzoite cysts from infected animals (Frenkel, 1973, 1997). Grazing food animals, e.g. sheep, are probably infected by soil oocysts, but swine are omnivores and may also ingest infected rodents (Dubey, 1998; Dubey and Beattie, 1998). The incidence of Toxoplasma in swine is quite variable. Bovine and fowl Toxoplasma levels are low (Dubey, 1992).
The distribution of T. gondii is worldwide; all genotypes are found on all continents except Antarctica. Islands without T. gondii have been found in the Pacific, and along the coast of Central America (Etheredge and Frenkel, 1995; Wallace, 1969). Hot, dry climates have a lower incidence of toxoplasmosis than temperate, moist climates, and rates decrease with increase in altitude (Etheredge and Frenkel, 1995; Walton et al, 1966). The role of the cat in the transmission of toxoplasmosis is established, but the epidemiology of transmission also includes the possible role of dogs as carriers of infectious oocysts. Dogs are associated in epidemiological surveys with increased rates of toxoplasmosis. Their habit of rolling in cat feces or eating cat feces suggests a possible mechanism for transfer of infectious oocysts (Frenkel, 1997). Freshwater contaminated with oocysts was implicated as the source of an outbreak in troops training in a jungle in Panama (Benenson et al., 1982). Toxoplasma infection is also acquired by trans-placental transmission (Desmonts and Couvreur, 1974a) and, less commonly, through organ transplantation (Ruskin and Remington, 1976) and laboratory accidents (Jacobs, 1974). Although Toxoplasma DNA can be detected by PCR in blood from chronically infected individuals (Dupon et al., 1995), transmission of toxoplasmosis by transfusion of banked blood has not been established as a public health problem (Kimball et al., 1965). Man is a deadend host for T. gondii, which is of importance in the understanding of the epidemiology of drug resistance. Strains can become resistant to particular chemotherapeutic agents, but cannot be passed from person to person.
Serological surveys demonstrate wide variation in prevalence of infection in various geographic locations (Zuber and Jacquier, 1995). In Paris, France, where rates of infection reach 90% by the fifth decade (Desmonts and Couvreur, 1974b; Remington et al., 1995; Thulliez, 1992) transmission appears to be related to preferences of ingesting poorly cooked or raw meat, especially lamb. In contrast, antibody positivity in the UK and Finland is approximately 20% in the total population (Joynson, 1992; Koskiniemi et al., 1992). In moist tropical areas of Latin America and sub-Saharan Africa, where cats are abundant and the climate favors survival of oocysts, the prevalence may approach 90% (Etheredge and Frenkel, 1995; Frenkel et al., 1995; Onadeko et al., 1992; Sousa et al., 1988). In comparison, rates in hot, dry regions, such as North Africa, usually do not exceed 20% (Hamadto et al., 1997). Rates in the USA also vary, with a recent survey in military recruits showing rates from 3% in the mountain states to 13% in the mid-Atlantic and east-south-central states (Smith et al., 1996). These results are about one-third lower than earlier surveys, and may indicate a decrease in the load of infectious organisms in the meat supply, since the number of cats has increased in recent years.
Recent statistics on the incidence of toxo-plasmosis presenting as an opportunistic infection of the brain in AIDS have been published by the Centers for Disease Control and Prevention in Atlanta, GA (Prevention, 1999). Between 1992 and 1997, the incidence of toxoplasmic encephalitis declined from 20.7/1000 to 7.0/1000 person-years in HIV-infected persons. The percentage of males dying of AIDS in this period who had toxoplasmic encephalitis at any time in the course of their illness was 7.1% overall, but 13.0% in those who acquired HIV through heterosexual contact. These figures are lower than those quoted at the outset of the AIDS epidemic.
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