Endemic toxoplasmosis in pigs on a farm in Maryland: isolation and genetic characterization of Toxoplasma gondii

Toxoplasma gondii Infection in Swine: Implications for Public Health

Toxoplasmosis, due to Toxoplasma gondii, is a parasitic disease with global importance. Among livestock, chronic T. gondii infection has been reported in higher rates in pigs and small ruminants, but with subclinical infections in case commonly encountered in pigs. Seroprevalence in the global pig population ranges according to the age or species of pigs, geographical distribution, production programs, and systems. Generally, T. gondii infections are noticed in low prevalence rates in conventional pig farms with high hygiene standards. In contrast, higher prevalence is common on free-ranging farms, outdoor or backyard small pig fams, as well as in farmed or hunted wild boars. The T. gondii average worldwide seroprevalence in pigs is reported to be 13% in Europe, 21% in Africa, 25% in North America, 21% in Asia, and 23% in South America. Human toxoplasmosis outbreaks have been correlated with the consumption of raw or undercooked meat, especially from infected pigs or wild boars, as well as of contaminated drinking water. The risk of infection in processed pork products is lower compared with fresh pork, as meat processing can reduce or inactivate T. gondii tissue cysts. Hence, the prevalence of T. gondii in the pig population may be a useful indicator of the risk of human toxoplasmosis associated with the consumption of pork products. The lack of obligatory screening methods at farm level for the detection of antibodies in farmed animals or the viable T. gondii in carcasses at slaughterhouse level increases the risk of contaminated pork or meat products. For this reason, the application of biosecurity and surveillance programs at farm level is very important to prevent a T. gondii infection.

All about Toxoplasma gondii infections in pigs: 2009-2020

Toxoplasma gondii infections are common in humans and animals worldwide. Toxoplasma gondii infection in pigs continues to be of public health concern. Pigs are important for the economy of many countries, particularly, USA, China, and European countries. Among the many food animals, pigs are considered the most important for T. gondii transmission in USA and China because viable parasites have rarely been isolated from beef or indoor raised chickens. Besides public health issues, T. gondii causes outbreaks of clinical toxoplasmosis in pigs in China, associated with a unique genotype of T. gondii (ToxoDB genotype #9 or Chinese 1), rarely found in other countries. The safety of ready to eat pork products with respect to T. gondii infection is a matter of recent debate. Here, we review in detail seroprevalence, prevalence of viable and nonviable T. gondii, epidemiology, risk assessment, diagnosis, and curing of pork products containing T. gondii for the past decade. This review will be of interest to biologists, parasitologists, veterinarians, and public health workers.

A partition of Toxoplasma gondii genotypes across spatial gradients and among host species, and decreased parasite diversity towards areas of human settlement in North America

Toxoplasma gondii counts among the most consequential food-borne parasites, and although the parasite occurs in a wide range of wild and domesticated animals, farms may constitute a specific and important locus of transmission. If so, parasites in animals that inhabit agricultural habitats might be suspected of harbouring genetically distinct parasite types. To better understand habitat effects pertinent to this parasite's transmission, we compiled and analysed existing genotypic data of 623 samples from animals across a proximity gradient from areas of human settlement to the wilderness in North America. To facilitate such analysis, T. gondii isolates were divided into three groups: (i) from farm-bound animals (with the most limited home ranges on farms); (ii) from free-roaming animals (with wider home ranges on or near farms); and (iii) from wildlife. In addition, parasite genotype distribution in different animal species was analysed. We observed no absolute limitation of any of five major genotypes to any one habitat; however, the frequency of four genotypes decreased across the gradient from the farm-bound group, to the free-roaming group, then the wildlife, whereas a fifth genotype increased along that gradient. Genetic diversity was greater in free-roaming than in farm-bound animals. The genotypic composition of parasites in wildlife differed from those in farm-bound and free-roaming animals. Furthermore, parasite genotypes differed among host species. We conclude that T. gondii genotype distributions are influenced by the spatial habitat and host species composition, and parasite diversity decreases towards areas of human settlement, elucidating facts which may influence transmission dynamics and zoonotic potential in this ubiquitous but regionally variable parasite.

Toxoplasma gondii as a Parasite in Food: Analysis and Control

Foodborne infections are a significant cause of morbidity and mortality worldwide, and foodborne parasitic diseases, though not as widespread as bacterial and viral infections, are common on all continents and in most ecosystems, including arctic, temperate, and tropical regions. Outbreaks of disease resulting from foodstuffs contaminated by parasitic protozoa have become increasingly recognized as a problem in the United States and globally. Increased international trade in food products has made movement of these organisms across national boundaries more frequent, and the risks associated with infections have become apparent in nations with well-developed food safety apparatus in place.

A Systematic Meta-Analysis of Toxoplasma gondii Prevalence in Food Animals in the United States

Toxoplasma gondii is a widely distributed protozoan parasite. The Centers for Disease Control and Prevention reported that T. gondii is one of three pathogens (along with Salmonella and Listeria), that together account for >70% of all deaths due to foodborne illness in the United States. Food animals are reservoirs for T. gondii and act as one of the sources for parasite transmission to humans. Based on limited population-based data, the Food and Agriculture Organization/World Health Organization estimated that approximately 22% of human T. gondii infections are meatborne. The objective of the current study was to conduct a systematic meta-analysis to provide a precise estimation of T. gondii infection prevalence in food animals produced in the United States. Four databases were searched to collect eligible studies. Prevalence was estimated in six animal categories (confinement-raised market pigs, confinement-raised sows, non-confinement-raised pigs, lamb, goats, and non-confinement-raised chickens) by a quality-effects model. A wide variation in prevalence was observed in each animal category. Animals raised outdoors or that have outdoor access had a higher prevalence as compared with animals raised indoors. T. gondii prevalence in non-confinement-raised pigs ranked the highest (31.0%) followed by goats (30.7%), non-confinement-raised chickens (24.1%), lambs (22.0%), confinement-raised sows (16.7%), and confinement-raised market pigs (5.6%). These results indicate that T. gondii-infected animals are a food safety concern. The computed prevalence can be used as an important input in quantitative microbial risk assessment models to further predict public health burden.

Toxoplasma gondii in livestock in St. Kitts and Nevis, West Indies

BACKGROUND

Toxoplasma gondii is a ubiquitous protozoan parasite capable of infecting all warm-blooded animals including livestock. In these animals, the parasite forms cysts in the tissues which may pose a risk to public health if infected meat is consumed undercooked or raw. The aim of this study was to determine the exposure of livestock to T. gondii in St. Kitts and Nevis.

METHODS

Sera and/or heart tissue and meat juice were collected from pigs (n = 124), sheep (n = 116) and goats (n = 66) at the St. Kitts Abattoir. Sera and meat juice were screened for reactive antibodies to T. gondii using an in-house ELISA. Heart tissue was screened for T. gondii DNA using quantitative PCR and positive samples were genotyped using RFLP.

RESULTS

Antibodies to T. gondii were detected in sera from 48% of pigs, 26% of sheep and 34% of goats tested. Antibodies were also detected in the meat juice from 55% of pig hearts, 22% of sheep hearts and 31% of goat hearts tested. There was a significant positive correlation between serology and meat juice results. T. gondii DNA was detected in heart tissue of 21% of pigs, 16% of sheep and 23% of goats tested. Preliminary PCR-RFLP analysis identified a predominance of the Type III genotype of T. gondii.

CONCLUSIONS

These results suggest widespread environmental contamination with T. gondii oocysts and that livestock could be a potentially important source of T. gondii infection if their infected meat is consumed (or handled) undercooked.

Prevalence and risk factors for Toxoplasma gondii infection in meat animals and meat products destined for human consumption

Toxoplasma gondii is a protozoan parasite that is responsible for approximately 24% of all estimated deaths attributed to foodborne pathogens in the United States. Human infection results from accidental ingestion of oocysts from the environment, in water, or on insufficiently washed produce or from consumption of raw or undercooked meat products that contain T. gondii tissue cysts. This review focused on studies of T. gondii in meat because many human T. gondii infections are acquired through consumption of raw or undercooked meat. Prevalence of T. gondii is higher in conventionally reared pigs, sheep, and poultry than in cattle and is greater in meat products from organic than from conventionally reared meat animals because of outdoor access, which poses substantially greater opportunities for exposure to infected rodents, wildlife, and oocyst-contaminated feed, water, or environmental surfaces. Risk factors related to T. gondii exposure for livestock include farm type, feed source, presence of cats, methods of rodent and bird control, methods of carcass handling, and water quality. This review serves as a useful resource and information repository for informing quantitative risk assessment studies for T. gondii infection in humans through meat consumption.

Avirulent strains of Toxoplasma gondii infect macrophages by active invasion from the phagosome

Unlike most intracellular pathogens that gain access into host cells through endocytic pathways, Toxoplasma gondii initiates infection at the cell surface by active penetration through a moving junction and subsequent formation of a parasitophorous vacuole. Here, we describe a noncanonical pathway for T. gondii infection of macrophages, in which parasites are initially internalized through phagocytosis, and then actively invade from within a phagosomal compartment to form a parasitophorous vacuole. This phagosome to vacuole invasion (PTVI) pathway may represent an intermediary link between the endocytic and the penetrative routes for host cell entry by intracellular pathogens. The PTVI pathway is preferentially used by avirulent strains of T. gondii and confers an infectious advantage over virulent strains for macrophage tropism.

When should a trophically and vertically transmitted parasite manipulate its intermediate host? The case of Toxoplasma gondii

Parasites with complex life cycles are expected to manipulate the behaviour of their intermediate hosts (IHs), which increase their predation rate and facilitate the transmission to definitive hosts (DHs). This ability, however, is a double-edged sword when the parasite can also be transmitted vertically in the IH. In this situation, as the manipulation of the IH behaviour increases the IH death rate, it conflicts with vertical transmission, which requires healthy and reproducing IHs. The protozoan Toxoplasma gondii, a widespread pathogen, combines both trophic and vertical transmission strategies. Is parasite manipulation of host behaviour still adaptive in this situation? We model the evolution of the IH manipulation by T. gondii to study the conflict between these two routes of transmission under different epidemiological situations. Model outputs show that manipulation is particularly advantageous for virulent strains and in epidemic situations, and that different levels of manipulation may evolve depending on the sex of the IH and the transmission routes considered. These results may help to understand the variability of strain characteristics encountered for T. gondii and may extend to other trophically transmitted parasites.

Toxoplasma gondii prevalence in farm animals in the United States

Toxoplasmosis, caused by Toxoplasma gondii, is one of the most common parasitic infections of humans and other warmblooded animals. It has been found worldwide and nearly one-third of humans have been exposed to the parasite. Congenital infection occurs when a woman becomes infected during pregnancy and transmits the parasite to the foetus. Besides congenital infection, humans become infected by ingesting food or water contaminated with sporulated oocysts from infected cat faeces or through ingestion of tissue cysts in undercooked or uncooked meat. Food animals (pigs, chickens, lambs and goats) become infected by the same routes, resulting in meat products containing tissue cysts, which can then infect consumers. Toxoplasma infection is common in food animals in the United States. Implementation of management factors such as biosecure confinement housing are important in reducing the levels of infection in animals destined for human consumption.

Foodborne toxoplasmosis

Toxoplasmosis can be due to congenital infection or acquired infection after birth and is one of the leading illnesses associated with foodborne hospitalizations and deaths. Undercooked meat, especially pork, lamb, and wild game meat, and soil contaminated with cat feces on raw fruits and vegetables are the major sources of foodborne transmission for humans. The new trend in the production of free-range organically raised meat could increase the risk of Toxoplasma gondii contamination of meat. Foodborne transmission can be prevented by production practices that reduce T. gondii in meat, adequate cooking of meat, washing of raw fruits and vegetables, prevention of cross contamination in the kitchen, and measures that decrease spread of viable oocysts into the environment.

Loop-mediated isothermal amplification (LAMP): early detection of Toxoplasma gondii infection in mice

Background

Toxoplasmosis is a widespread zoonotic parasitic disease that occurs in both animals and humans. Traditional molecular assays are often difficult to perform, especially for the early diagnosis of Toxoplasma gondii infections. Here, we established a novel loop-mediated isothermal amplification targeting the 529 bp repeat element (529 bp-LAMP) to detect T. gondii DNA in blood samples of experimental mice infected with tachyzoites of the RH strain.

Findings

The assay was performed with Bst DNA polymerase at 65°C for 1 h. The detection limit of the 529 bp-LAMP assay was as low as 0.6 fg of T. gondii DNA. The sensitivity of this assay was 100 and 1000 fold higher than that of the LAMP targeting B1 gene (B1-LAMP) and nested PCR targeting 529 bp repeat element (529 bp-nested PCR), respectively. The specificity of the 529 bp-LAMP assay was determined using the DNA samples of Trypanosoma evansi, Plasmodium falciparum, Paragonimus westermani, Schistosoma japonicum, Fasciola hepatica and Angiostrongylus cantonensis. No cross-reactivity with the DNA of any parasites was found. The assay was able to detect T. gondii DNA in all mouse blood samples at one day post infection (dpi).

Conclusions

We report the following findings: (i) The detection limit of the 529 bp-LAMP assay is 0.6 fg of T. gondii DNA; (ii) The assay does not involve any cross-reactivity with the DNA of other parasites; (iii) This is the first report on the application of the LAMP assay for early diagnosis of toxoplasmosis in blood samples from experimentally infected mice. Due to its simplicity, sensitivity and cost-effectiveness for common use, we suggest that this assay should be used as an early diagnostic tool for health control of toxoplasmosis.

Genetic characterisation of Toxoplasma gondii in wildlife from North America revealed widespread and high prevalence of the fourth clonal type

Little is known of the genetic diversity of Toxoplasma gondii circulating in wildlife. In the present study wild animals, from the USA were examined for T. gondii infection. Tissues of naturally exposed animals were bioassayed in mice for isolation of viable parasites. Viable T. gondii was isolated from 31 animals including, to our knowledge for the first time, from a bald eagle (Haliaeetus leucocephalus), five gray wolves (Canis lupus), a woodrat (Neotoma micropus), and five Arctic foxes (Alopex lagopus). Additionally, 66 T. gondii isolates obtained previously, but not genetically characterised, were revived in mice. Toxoplasma gondii DNA isolated from these 97 samples (31+66) was characterised using 11 PCR-restriction fragment length polymorphism (RFLP) markers (SAG1, 5'- and 3'-SAG2, alt.SAG2, SAG3, BTUB, GRA6, c22-8, c29-2, L358, PK1 and Apico). A total of 95 isolates were successfully genotyped. In addition to clonal Types II, and III, 12 different genotypes were found. These genotype data were combined with 74 T. gondii isolates previously characterised from wildlife from North America and a composite data set of 169 isolates comprised 22 genotypes, including clonal Types II, III and 20 atypical genotypes. Phylogenetic network analysis showed limited diversity with dominance of a recently designated fourth clonal type (Type 12) in North America, followed by the Type II and III lineages. These three major lineages together accounted for 85% of strains in North America. The Type 12 lineage includes previously identified Type A and X strains from sea otters. This study revealed that the Type 12 lineage accounts for 46.7% (79/169) of isolates and is dominant in wildlife of North America. No clonal Type I strain was identified among these wildlife isolates. These results suggest that T. gondii strains in wildlife from North America have limited diversity, with the occurrence of only a few major clonal types.

Genetic analyses of atypical Toxoplasma gondii strains reveal a fourth clonal lineage in North America

Toxoplasma gondii is a widespread parasite of animals that causes zoonotic infections in humans. Previous studies have revealed a strongly clonal population structure in North America and Europe, while strains from South America are genetically separate and more diverse. However, the composition within North America has been questioned by recent descriptions of genetically more variable strains from this region. Here, we examined an expanded set of isolates using sequenced-based phylogenetic and population analyses to re-evaluate the population structure of T. gondii in North America. Our findings reveal that isolates previously defined by atypical restriction fragment length polymorphism patterns fall into two discrete groups. In one case, these new isolates represent variants of an existing lineage, from which they differ only by minor mutational drift. However, in the second case, it is evident that these isolates define a completely new lineage that is common in North America. Support for this new lineage was based on phylogeny, principle components analysis, STRUCTURE analyses, and statistical analysis of gene flow between groups. This new group, referred to as haplogroup 12, contains divergent genotypes previously referred to as A and X, isolated from sea otters. Consistent with this, group 12 was found primarily in wild animals, as well as occasionally in humans. This new lineage also has a highly clonal population structure. Analysis of the inheritance of multilocus genotypes revealed that different strains within group 12 are the products of a single recombination event between type 2 and a unique parental lineage. Collectively, the archetypal type 2 has been associated with clonal expansion of a small number of lineages in the North, as a consequence of separate but infrequent genetic crosses with several different parental lines.

Seroprevalence of Toxoplasma gondii infection in domestic pigs in Durango State, Mexico

Little is known concerning the prevalence of Toxoplasma gondii infection in pigs in Mexico. Accordingly, antibodies to T. gondii were determined in 1,074 domestic pigs in Durango, Mexico using the modified agglutination test. Two groups (A, B) of pigs were sampled: Group A pigs (n  =  555) were raised in 3 geographical regions in Durango State and Group B pigs (n  =  519) were from Sonora State but slaughtered in Durango City. Overall, antibodies to T. gondii were found in 136 (12.7%) of 1,074 pigs with titers of 1∶25 in 29, 1∶50 in 23, 1∶100 in 18, 1∶200 in 22, 1∶400 in 12, 1∶800 in 8, 1∶1,600 in 2, and 1∶3,200 or higher in 22. Of the pigs raised in Durango State, seroprevalence varied with age, management, and the geographic region; pigs raised in backyards in the mountainous region had a significantly higher seroprevalence (32.1%) than those raised in the valley (13.0%) and the semi-desert regions (14.0%). In Group A pigs from Durango, seroprevalence of T. gondii infection was significantly higher in pigs older than 8 mo (19.5%) than in younger pigs (10.9%). In the whole pig population (Groups A and B together), seroprevalence was higher in pigs raised in Durango (16.0%) than in those raised in Sonora (9.1%) and higher in mixed-breed pigs (15.7%) than in pure-bred pigs (10.3%). This is the first, in-depth study on the seroprevalence of T. gondii infection of pigs in Mexico and the first report on pigs from Durango State, Mexico. Results indicate that infected pork is likely an important source of T. gondii infection for humans in Durango State.

Toxoplasma gondii infection in slaughter pigs in Serbia: seroprevalence and demonstration of parasites in blood

A seroepizootiological study of Toxoplasma gondii infection involving a total of 488 slaughter pigs (468 market-weight pigs and 20 sows) in the Belgrade area, also included examination of the presence of T. gondii in the blood. Blood sampled at the slaughter line was examined for specific antibodies by modified direct agglutination, and blood clots of those seropositive at titres of 1:50-1:12800 were bioassayed in mice. The overall seroprevalence was 9.2%, significantly higher (p = 0.0063) in sows (30.0%) than in market-weight pigs (8.3%). Amongst the 22 bioassays performed, a total of 16 (72.7%) were positive, by observation of T. gondii cysts (12), seropositivity (7, including 3 in which cysts were not detected), and/or detection of T. gondii DNA by real-time PCR (12, including one otherwise negative). The positive bioassays originated from the blood of 12 market-weight pigs and 4 sows. Despite a general increase in the rate of demonstration of T. gondii with the increase in the specific antibody level, the association was not significant (p = 0.101). The risk of infection was 41-fold increased in sows vs market-weight pigs, and 15-fold in pigs from smallholders' finishing type farms vs those from large farrow-to-finish farms. The presence of viable T. gondii in a proportion of the samples indicates that some of the pigs had an active parasitaemia at the time of slaughter, which, along with the seroprevalence established, points to a potential source of human infection in Serbia. This is the first report on parasitaemia in naturally infected swine.

Genetic diversity of Toxoplasma gondii in animals and humans

Toxoplasma gondii is one of the most widespread parasites of domestic, wild, and companion animals, and it also commonly infects humans. Toxoplasma gondii has a complex life cycle. Sexual development occurs only in the cat gut, while asexual replication occurs in many vertebrate hosts. These features combine to create an unusual population structure. The vast majority of strains in North America and Europe fall into three recently derived, clonal lineages known as types I, II and III. Recent studies have revealed that South American strains are more genetically diverse and comprise distinct genotypes. These differences have been shaped by infrequent sexual recombination, population sweeps and biogeography. The majority of human infections that have been studied in North America and Europe are caused by type II strains, which are also common in agricultural animals from these regions. In contrast, several diverse genotypes of T. gondii are associated with severe infections in humans in South America. Defining the population structure of T. gondii from new regions has important implications for transmission, immunogenicity and pathogenesis.

Moving towards an integrated approach to molecular detection and identification ofToxoplasma gondii

The development of simple, sensitive and rapid methods for the detection and identification ofToxoplasma gondiiis important for the diagnosis and epidemiological studies of the zoonotic disease toxoplasmosis. In the past 2 decades, molecular methods based on a variety of genetic markers have been developed, each with its advantages and limitations. The application of these methods has generated invaluable information to enhance our understanding of the epidemiology, population genetics and phylogeny ofT. gondii. However, since most studies focused solely on the detection but not genetic characterization ofT. gondii, the information obtained was limited. In this review, we discuss some widely used molecular methods and propose an integrated approach for the detection and identification ofT. gondii, in order to generate maximum information for epidemiological, population and phylogenetic studies of this key pathogen.

Sexual recombination punctuated by outbreaks and clonal expansions predicts Toxoplasma gondii population genetics

The cosmopolitan parasitic pathogen Toxoplasma gondii is capable of infecting essentially any warm-blooded vertebrate worldwide, including most birds and mammals, and establishes chronic infections in one-third of the globe's human population. The success of this highly prevalent zoonosis is largely the result of its ability to propagate both sexually and clonally. Frequent genetic exchanges via sexual recombination among extant parasite lineages that mix in the definitive felid host produces new lines that emerge to expand the parasite's host range and cause outbreaks. Highly successful lines spread clonally via carnivorism and in some cases sweep to pandemic levels. The extent to which sexual reproduction versus clonal expansion shapes Toxoplasma's current, global population genetic structure is the central question this review will attempt to answer.

Toxoplasma-safe meat: close to reality?

In 2008, the centennial of the discovery of Toxoplasma gondii was celebrated. However, toxoplasmosis is still seen as a neglected and underreported disease, despite having a disease burden similar to that of salmonellosis and campylobacteriosis. Human vaccines are not available and current antiparasitic treatment is disappointing. This has led to an urge to focus more on prevention. Food, soil or water contaminated with oocysts from cat faeces and undercooked meat from infected intermediate hosts are important routes of infection. Oocyst contamination is difficult to control, whereas in Western countries, the control of T. gondii in meat should be feasible. Here, we discuss strategies aimed at developing a Toxoplasma-safe meat chain.