Toxoplasmosis is caused by infection with Toxoplasma (T.) gondii, a coccidian parasite that can infect humans and animals [2,6,15]. Postnatally, animals and humans generally become infected after ingesting food and water contaminated with sporulated oocysts or by consuming raw or undercooked meat containing tissue cysts [2,7]. Although toxoplasmosis is generally asymptomatic, primary infections in pregnant women and animals may cause abortions, fetal abnormalities, or perinatal death [2,6].

Most T. gondii infections in pigs are subclinical [2], and transplacental infections are less common than post-natal infections [2]. Although abortions related to T. gondii are uncommon, they may occur in sows infected during pregnancy [2]. Pigs infected transplacentally may be born premature, dead, or weak, or they may die soon after birth [15].

In Korea, T. gondii infections have been reported in humans [20] and many domestic animals, including cats [5], dogs [9], and pigs [19]. To our knowledge, however, there have been no reports of abortions in animals related to this parasite in Korea. Herein we describe an abortion outbreak due to T. gondii infection in sows.

Clinical signs and pathological changes may be nonspecific, requiring full panels of microbiological and serological tests to confirm the exact etiology. Diagnostic tests were able to rule out major endemic swine infectious diseases in Korea, including CSF, AD, PRRS, and PCV-2. Histopathologic examination of the major parenchymal organs of sows and aborted fetuses revealed many necrotic lesions associated with protozoan tachyzoites. Using T. gondii specific antibody, the presence of T. gondii tachyzoites was demonstrated in the formalin-fixed tissues from sows and fetuses. In addition, serological tests revealed high antibody titers to T. gondii in aborted sows from this herd. Based on these clinical signs, histopathology, immunohistochemistry and microbiology, the herd was diagnosed with toxoplasmosis. As toxoplasmosis is zoonotic, the remaining 68 pregnant sows present in the same room were culled 2 weeks after the outbreak of this disease.

Although clinical toxoplasmosis has been reported in young piglets, little was known of abortions associated with T. gondii and rates of congenital infection in pigs [2]. Epidemiologically, porcine toxoplasmosis has been classified into sporadic neonatal, postnatal, and epizootic infection [2]. Sporadic fatal toxoplasmosis in piglets has been reported in several countries, including the USA, Japan, and Korea. Although many infected piglets were born dead or sick, or became sick within 3 months of birth, others remained clinically normal.

Epizootic outbreaks of toxoplasmosis have been reported in Italy [4], Singapore [13], and Taiwan [18]. In Italy, there were 4 simultaneous outbreaks of toxoplasmosis with high mortality in different pig herds in 2 different provinces over a 1 month period [4]. The morbidity rate was as high as 60%, and the mortality rate ranged from 10% to 42% in fattening pigs weighing 60 to 180 kg. In Taiwan, 51 of 66 pregnant gilts infected with T. gondii on a single farm aborted within 2 months [18]. Most fetuses were stillborn, some were mummified, and few were born alive but died within a few days. In Singapore, a large outbreak of toxoplasmosis was observed in a herd containing 540 pigs [13]. Many pigs in different age groups, ranging from piglets to sows, became sick and died. The overall morbidity rate was 35.7% and the overall mortality rate was 11.8%.

In this study, higher abortion rates, up to 44%, were observed and unusually high sow mortality rates, up to 19%, were primarily associated with toxoplasmosis over a very short period of time. All clinical signs were restricted to pregnant sows, especially those at any stages of gestation. Although the sample size was relatively small, the high seroprevalence in aborted sows indicated an active toxoplasmosis infection in this particular herd. Transplacental infection of T. gondii were also observed. In sum, most clinical aspects and laboratory results in this herd were very similar to epizootic toxoplasmosis reported in other countries.

At the initial outbreak, toxoplasmosis was not suspected. Therefore, a thorough epidemiological survey was not conducted to determine the origin of T. gondii. In addition, the feed could not be investigated due to a disagreement with the farmer. All remaining pregnant sows were destroyed 2 weeks after the disease outbreak. Hence, the precise origin of T. gondii remains unknown. Studies aiming to clarify the sources of pig infection for T. gondii have suggested that ingestion of oocysts in contaminated feed, water, soil, and living animals were the main sources of infection [2,14]. In some countries, eating infected rodents has been regarded as a source of infection [16]. Cannibalism has been shown experimentally to be another possible route of infection [2]. However, most studies have suggested that oocysts shed by cats are the most common source. Cats may excrete millions of oocysts after ingesting only one bradyzoite or one tissue cyst, and many tissue cysts may be present in one infected mouse [3,6]. Although oocysts are shed only for a short period (1-2 weeks) in the life of a cat, the enormous numbers shed assure widespread contamination of the environment [6].

According to the system used to manage this herd, the pigs in individual rooms of the second pen were fed different feed. As they drank the same water from a private well and new animals had not been introduced into the herd in the period leading up to the disease outbreak, a change of feed source was the only variation within this herd. Because the disease outbreak in this herd was restricted to pregnant sows housed in the second room, and there was no recurrence of infection after the other pregnant sows were destroyed, the source of the parasite may have been the feed source or an animal contaminated with oocysts. Recently, the density of stray cats has been gradually increased in Jeju Island. Many wild rodents and stray cats were freely introduced into the old-fashioned small farms in Jeju. Although the feed for the sows were not able to be investigated, it is assumed that the feed, which had an abnormal stink, might have been contaminated in some way with feces contained T. gondii oocysts from wild animals, such as stray cats. According to a previous study, the seropositive rate of T. gondii in the residents of Jeju Island is relatively high compared to that of the past 30 years in Korea [20]. This suggested that incomplete cooked porcine meat and mammals such as pigs and deer may acts as reservoir hosts for T. gondii on Jeju Island. To prevent T. gondii infection in pigs, pig farms should be treated periodically with rodenticides, and no cats and wild rodents should be allowed to enter their living quarters. Moreover, animal feed should be carefully stored to prevent contamination by cats.