Toxoplasma gondii, an apicomplexan zoonotic protozoa, is one of the most successful parasites widely prevalent in humans and animals worldwide. Felids are its definitive host, and almost all other warm blooded animals, including humans, are its intermediate hosts. Toxoplasmosis is acquired by the infection of T. gondii through ingestion of tissue cysts from undercooked meat, or consuming food or water contaminated with oocysts. Another route of infection is from infected mother to her fetus via placental transmission, a process known as congenital toxoplasmosis [1–3]. Congenital toxoplasmosis may cause stillbirth or abortion in addition to serious damages to the fetus, such as severe neurological disorders after delivery [4,5]. Most toxoplasmosis cases show only mild symptoms or are asymptomatic as only a small percentage of exposed adult humans develop clinical symptoms following exposure. The infections can transform into a chronic status, especially in the central nervous system of the individuals affected. The infection is sometimes reactivated in immune compromised patients to cause toxoplasmic lymphadenitis, meningoencephalitis, or ocular toxoplasmosis [6–9].

Many surveys on the seroprevalence of toxoplasmosis among various patient groups have been performed in Korea using various test methods [10–15]. The seroprevalence resulted in the range from 1.9 to 5.8% with indirect latex agglutination (ILA) test [10] and up to 7.7% with the more sensitive ELISA [11]. On the other hand, all of these were still significantly lower than those of the other countries with seroprevalences of 30–70% [3]. However, recent surveys have demonstrated higher seroprevalences ranging from 12.9 to 17.0% in certain parts of Korea [10,12–14]. Although ELISA has been used widely for diagnosis of toxoplasmosis, it is a time-consuming procedure and requires a skilled technician. Thus, it is being replaced gradually by a rapid diagnostic test (RDT), the current universal trend for diagnosis.

Korea has an almost 100% full health-coverage of its population, yet its seroprevalence of toxoplasmosis in certain areas is high or surging recently, which suggests some changes in the toxoplasmic environment [14]. The current study presents 4-year-results of a survey on the seroprevalence of toxoplasmosis performed by RDT among the residents in Seokmo-do (Island) in Ganghwa-gun, Incheon City, localized in the Yellow Sea. We tried to postulate its significance and also the cause of high prevalence of toxoplasmosis in this area.

The positive seroprevalence of the residents in Seokmo-do was high from the beginning of the survey in 2010 at 29.2% (91/312). The seroprevalence was maintained high in subsequent years, and simultaneously showed an increasing trend, to 35.3% (121/343), 38.7% (151/390), and 45.3% (164/362) in 2011, 2012, and 2013, respectively (Table 1; Fig. 2).

According to gender, the positive rate of males was always higher than that of females, maintaining roughly a 2-fold higher ratio throughout the survey period. On the other hand, the positive rate of females showed a steeper increase, while males showed a more gradual increase. In regard to age, positive rates increased gradually in all age groups, regardless of gender. However, a sharp drop was observed in the < 49 years-old male group in 2012. Additionally, during the early stages of the survey (2010 and 2011), the positive rates were fairly low in the < 49 years-old female group.

When comparing the age groups according to gender, the positive rate of men was always higher than that of women in the same age group, as noted above. Nevertheless, positive seroprevalence showed a steady increasing trend in both genders (Fig. 2). RDT reactivity changes were also observed in seropositive cases during the survey period. The ratio of the mid-reactivity group was relatively constant. However, the strong-reactivity group showed a gradual increase in its ratio, while the weak-reactivity group showed a gradual decrease (Fig. 3).

Since Soh et al. [19] first reported the seropositive rate of 5.6% in 373 Koreans by skin test using toxoplasmin in 1960, several serological methods such as indirect fluorescent antibody test (IFAT), ILA, and ELISA have been developed for the detection of T. gondii infection. However, most of them are expensive and time-consuming, and recently RDT using recombinant proteins as antigens has been developed to address these problems [18].

The seroprevalence of the residents in Seokmo-do was high from the beginning of the survey in 2010 at 29.2% (91/312). The seroprevalence was maintained high in subsequent years, and simultaneously showed an increasing trend. This high seroprevalence may have been caused in part by peculiar changes in the toxoplasmic environment of the island as it is a relatively isolated area preserving its natural habitat while also being connected by a bridge to the mainland.

Meat consumption is increasing in Korea, but domestic meat supply is limited, so a large amount must be imported. As the world becomes more global day by day, the meat source also becomes more varied. Some of them may have been imported from high endemic nations. Indeed, pork has been imported without any inspection on T. gondii infection in Korea [15].

Ganghwa-gun is known as an outbreak site of toxoplasmic lymphadenitis by the ingestion of tissue-cyst from pork which was fed in a backyard pigsty. Regardless of pig farm or house pigsty, pigs have been bred under conditions where cats can access easily. They are served as popular foods in domestic festivals, wedding receptions and funeral receptions, and local markets. This may presumably explain the high positive rates observed on the area, if pork had been served undercooked. These factors, combined with its isolated location may have provided a high parasite burden to the inhabitants of the island.

Like most other advanced countries, Korea has seen an alarming increase in the elderly population recently. People in rural areas have traditionally kept many pets, but the increase in single households and life expectancy has caused an explosive increase in these pets. This again has resulted in an increase in abandoning them, as most of them cannot be cared when their owners pass away. Most of them become feral and may contribute to the increase in toxoplasmosis prevalence. Interaction among pets and these feral counterparts are also common in the countryside. Especially troublesome is an increase in the number of feral cats, they became ubiquitous in Korea, and have occupied the top of the food chain in the Korean fauna, with increasing oocyst-shedding rate. Additionally, as mentioned above, Seokmo-do is connected to the mainland by a bridge, so access for feral cats is far easier than the past.

Korea is becoming economically ever more affluent, and health consciousness is increasing, along with environmental awareness. Hence, there is an increase in organic agriculture with far less use of pesticides throughout the country [20,21]. On the other hand, the improvement in the environment has caused an increase in the local fauna, and thus the potential for intermediate toxoplasma hosts have also increased [22]. Sometimes they are captured by a snare to be consumed among families and relatives.

Another significant source of infection, albeit minor, could be migratory birds [23,24] as Ganghwa-gun has been renowned for its extensive tideland marshes, an important shelter for these birds. During the recent avian influenza crisis, a high number of migratory birds have been positively identified with the virus, so these birds may also transmit toxoplasmosis from high endemic regions [25].

The positive seroprevalence of men was always higher than that of women, maintaining roughly a 2-fold higher ratio throughout the survey period. Some Koreans, mostly men, still have the misbelief that the raw viscera of animals, both domestic and wild, have special nutrients for their stamina, so they eat them raw. This may in some part explain the higher Toxoplasma seroprevalence in men observed in our study, besides being more socioeconomically active. On the other hand, the positive rate of women showed a steeper increase, while males showed a more gradual increase. Recent increase in consumption of imported pork, and more intimate relationship between elderly women and their pets may have been partially responsible. Further study is deemed necessary to address this problem.

In regard to age, positive rates increased gradually in all age groups, regardless of gender, but a sharp drop was observed in <49 years-old male group (2012), and they were absent or fairly low in <49 years-old female group. These may reflect the demographic changes in rural areas where young generations leave for cities for better opportunities.

A peculiarity of RDT needs to be addressed in regard to our study. Our previous study has determined the overall specificity and sensitivity of RDT to be 100% and 97.1%, respectively [18]. RDT uses rSAG1A-GRA2 antigen. A chronic infection history implies abundant antibodies against various antigens, such as SAGs, GRAs, and MICs, which could be detected by ELISA, whereas RDT would be limited in this respect, as only 2 antigens are detected. RDT reactivity changes were also observed in seropositive cases during the survey period. The ratio of the mid-reactivity group was relatively constant, but the strong-reactivity group showed a gradual increase in its ratio, while the weak-reactivity group showed a gradual decrease. Taking these factors together, it may be implied that the high positive seroprevalence indicates an increasing incidence of new cases, which would be better detected by RDT. These cases may represent an early or acute toxoplasmosis.

Toxoplasmosis is prevalent throughout the world, in both humans and our environment, but clinical manifestations are somewhat uncommon, so it is being mainly neglected in major health policies. However, our study reveals a different story, and that a worrying one. A major surge in seroprevalence, coupled with a change in disease characteristics at any point, may cause a major health threat.

In conclusion, it is necessary to find both symptomatic and asymptomatic toxoplasmic patients and confirm the risk factors for further infection in the area. Repeated surveys at a certain time interval are recommended to monitor the infection status, including new infections. Indeed, the problems may not be limited to the island, so the postulated causes in this study may be applied to a full-scale national survey to screen the national toxoplasmic seroprevalence.