|Tuesday, 03 April 2012 00:00|
Toxoplasma gondii is an obligate intracellular parasite in the phylum Apicomplexa. It is ubiquitous in nature and can infect most warm blooded animals ( Dubey 2009). There are three major routes of T. gondii exposure: transmission from mother to fetus, consumption of undercooked meat from tainted meat, and ingestion of oocyst contaminated fruits, vegetables, and drinking water ( Villena et al. 2004) ( Dubey 2009). Prevalence of human toxoplasmosis in many parts of Europe and South America can be as high as 50-90%. In the United Kingdom, prevalence is estimated to be between 16-40%, whereas in the United States, it ranges from 10-30% ( Bahia-Oliveira et al. 2003; Dubey 2009; Jones et al. 2001; Sroka et al. 2010). Recent estimates suggest that at least one million people, or approximately 1% of the total population in the United States, are infected every year (Jones and Holland, 2010). Infection with T. gondii in healthy individuals is generally asymptomatic, but does result in a lifelong chronic infection that can reactivate, and can lead to death, if one becomes immunocompromised. Infection during pregnancy can result in congenital toxoplasmosis, which can cause fetal complications and can be fatal to the fetus ( Montoya and Liesenfeld 2004).
Felids, both wild and domestic, are the definitive host of T. gondii and are the only host in which the sexual part of the life cycle occurs, resulting in the formation of oocysts. During the primary infection, a single cat can shed between 20 - 200 million oocysts in their feces (Dabritz and Conrad 2010; Dubey 1995). Once they are shed in the environment, the oocysts can sporulate and become infective under the optimal conditions, typically in a moist and warm environment (4 - 37ºC). These oocysts are remarkably resistant to environmental stressors and can remain infective in the environment for several months. The infective sporulated oocyst contains two sporocysts, each containing 4 sporozoites. Once ingested, the sporozoites quickly invade the host cell to form unique vacuole called the parasitophorous vacuole (PV). Inside, the PV, the sporozoite quickly develops into the rapidly replicating tachyzoites. In all immunocompetent hosts, following the activation of a robust infection-induced immune response, the tachyzoites quickly differentiate into bradyzoites and encyst in the brain and muscle tissues for the remainder of the host’s life (for images of T. gondii tissue cysts in a mouse brain please visit www.ncbi.nlm.nih.gov/pmc/articles/PMC106833/figure/F11/). The life cycle is completed when bradyzoite containing cysts from infected intermediate hosts or the environmental oocyst form are ingested by a cat ( Dubey 2009) (Figure 1).
Figure 1. Toxoplasma gondii life cycle. The definitive hosts for T. gondii are domestic and wild cats. Following infection, unsporulated oocysts are produced in the gut and are shed in the feces. Once in the environment, it takes 1-7 days for the oocysts to develop into the infective sporulated form of the parasite. Intermediate hosts in nature (all warm blooded animals) become infected after ingesting soil, water or plant material contaminated with sporulated oocysts. Once inside the host, they transform into tachyzoites. These tachyzoites are typically found in neural and muscle tissue where they quickly develop into tissue cyst bradyzoites. Cats become infected after consuming intermediate hosts harboring tissue cysts. Cats may also become infected directly by ingestion of sporulated oocysts. Animals bred for human consumption and wild game may also become infected with tissue cysts after ingestion of sporulated oocysts in the environment. For humans there are several common routes of transmission: 1) eating undercooked meat of animals harboring tissue cysts. 2) consuming water, fruits and vegetables contaminated with cat feces or by contaminated environmental samples (such as fecal-contaminated soil or changing the litter box of a pet cat) , 3) blood transfusion, organ transplantation, or accidental laboratory infections , or 4) transplacentally from mother to fetus. In the human host, the parasites form tissue cysts, most commonly in skeletal muscle, myocardium, brain, and eyes; these cysts remain throughout the life of the host. Modified from http://www.dpd.cdc.gov/dpdx/html/Toxoplasmosis.htm.
Waterborne outbreaks of toxoplasmosis have been reported worldwide ( Baldursson and Karanis 2011; Benenson et al. 1982; Bowie et al. 1997; de Moura et al. 2006; Khan et al. 2006; Palanisamy et al. 2006), and in most cases, the environmental sources of oocyst contamination were not definitively identified. In three reports, oocyst contaminated drinking water was attributed to the outbreaks. One outbreak occurred in a ground water system in Poland. In this case, water collected from shallow wells from farms in poor hygienic state tested positive for T. gondii DNA more frequently than waters from deep wells or from other farms that were in better hygienic condition ( Sroka et al. 2006). Another incident occurred in Santa Isabel do Ivai, Brazil. This outbreak was linked to contamination of the drinking water system (de Moura et al. 2006). Cats living above the tank reservoir were suspected to have contaminated the drinking water distribution system. Leaks and cracks in the reservoir left it unprotected from rainwater runoff that most likely washed oocyst containing cat feces into the water tank. Therefore, failing water infrastructure combined with the lack of appropriate plant treatment processes were likely major factors that led to the outbreak. The third and perhaps most relevant drinking water associated toxoplasmosis outbreak occurred in the Greater Victoria region of British Columbia, Canada ( Bowie et al. 1997). This outbreak was estimated to have infected between 2,894 – 7,718 individuals. The cause was thought to be contamination of the water reservoir with oocysts from cougars found living within the watershed during heavy rainfall events prior to the outbreak (Aramini et al., 1998). Although the treatment plant chloroaminated their finished drinking water, they did not filter it, and since oocysts are highly resistant to chemical disinfectants, the oocyst remained infective. Typically, drinking water treatment plants have a filtration barrier as part of their treatment train. This process is used to remove large sized particulates and contaminants and can considerably increases the probability of successfully removing T. gondii oocysts from the water. Without filtration, it leaves the treatment process vulnerable to T. gondii oocysts breakthroughs, as was the case for the Canada outbreak.
There are currently no practical and sensitive methods available for routine water monitoring efforts to determine the presence of oocysts in water. Studies on the levels of seropositive cats in various countries and estimations on total oocysts shed by infected cats are currently the only means to estimate the oocyst burden in the environment. In Europe, it was found that 26 of the 24,106 cats (0.11 %) tested were actively shedding oocysts ( Schares et al. 2008). Similarly, studies by Conrad and colleagues in Morro Bay, California also found that 3 out of 326 (0.9 %) cats were actively shedding oocysts, with each cat estimated to shed approximately 244-253 million oocysts (Dabritz and Conrad 2010). Taking the seroprevalence and oocyst burden together with total cat fecal deposits in the environment in the US (approximately 1.2-1.3 million tonnes/year), it was estimated that 94 – 4,671 oocysts/m2 are excreted in the environment, suggesting a significant risk of environmental contamination with T. gondii oocysts ( Dabritz et al. 2007).
There are three infective stages of Toxoplasma gondii, tachyzoites (the rapidly replicating, intracellular form of the parasite), bradyzoites (tissue cysts), and oocysts (the environmentally resistant form of the parasite). While tachyzoites and tissue cysts can be transmitted to humans and cause disease, these stages are typically associated with either accidental laboratory infections or ingestion of contaminated meat, respectively. In contrast, sporozoite-containing oocysts are the predominant form found in the environment and are responsible for transmission through contaminated water, soil, fruits, and vegetables. T. gondii is highly infectious and studies have shown that a single oocyst can lead to chronic infection in exposed pigs ( Dubey et al. 1996). Because of its low infectious dose and previous reports of waterborne outbreaks, there is considerable interest in understanding the resistance of oocysts to various disinfectants since chemical disinfection, at levels used by drinking and waste water industries, have little to no effects on the oocysts ( Dubey 2009; Villegas et al. 2010). However, recent studies have shown ultraviolet irradiation to be effective at inactivating oocysts ( Dumetre et al. 2008; Wainwright et al. 2007a; Ware et al. 2010).
Many chemicals typically used to sanitize laboratory and clinical supplies are not particularly effective for T. gondii oocysts. Sodium hypochlorite (10% bleach), 2 % sulfuric acid, 1 % organic iodine (Wescodyne), and alcohols (e.g. 95 % ethanol) have minimal effects on T. gondii oocysts ( Dubey 2009; Villegas et al. 2010). Reports by Wainwright and Dumetre further demonstrated that T. gondii oocysts are highly resistant to ozone inactivation. Ozone concentrations of 6 mg·ml/L or 9.4 mg·ml/L were ineffective at completely inactivating T. gondii oocysts ( Dumetre et al. 2008; Wainwright et al. 2007b). These reports showed that treating T. gondii oocysts with ozone levels used to effectively inactivate Cryptosporidium oocysts, as described in the USEPA Long Term 2 Enhanced Surface Treatment Rule (LT2), are not effective on T. gondii oocysts ( USEPA 2006). Studies to date indicate that chemical-based disinfection practices (e.g. chlorine or ozone) used by the water industry may also not be sufficient at inactivating T. gondii oocysts, although it remains to be determined if combining multiple chemicals and physical inactivation treatment strategies may have a synergistic effect (e.g. chlorine plus ozone or chlorine plus UV).
UV irradiation has also been recently shown to be effective at inactivating T. gondii oocysts ( Dumetre et al. 2008; Wainwright et al. 2007a; Ware et al. 2010). Initial studies by Wainwright and colleagues reported that a broad range of doses between 10-500 mJ/cm2 of low pressure UV was not effective at completely inactivating T. gondii oocysts; however, they did not report log inactivation levels ( Wainwright et al. 2007a). Studies by Dumetre and colleagues observed a 4-log10 inactivation at 40 mJ/cm2 ( Dumetre et al. 2008). More recently, a dose response study on UV inactivation of T. gondii oocysts revealed that a 1-, 3-, and 4-log10 inactivation were achieved at 4, 10, and >15 mJ/cm2, respectively ( Ware et al. 2010) . Overall, these levels suggest that T. gondii oocysts are susceptible to UV irradiation, at levels described in the LT2 rule for Cryptosporidium oocysts ( Bukhari and LeChevallier 2003; Keegan et al. 2003).
Assessing natural attrition rates of oocysts in environmental waters is difficult due to the lack of robust methods to collect them from the environment. However, based on controlled laboratory experiments, oocysts can survive in various water matrices for several years ( Jones and Dubey 2010).
Wastewater: Data assessing survival of oocysts in wastewater matrices is lacking. However, previous studies have reported that oocysts can survive for at least 300 days in fecal suspensions, which contain many of the components found in raw wastewater ( Yilmaz and Hopkins 1972). The fact that oocysts are also resistant to disinfectants typically used in the water industry (e.g. chlorine) underscores the significance of this pathogen in drinking and recreational water system impacted by publicly owned treatment works (POTW) and surface run-offs.
Fresh and marine waters: Data on survival and persistence of T. gondii oocysts in fresh and marine aquatic environment are limited. Several studies have detected T. gondii DNA in surface or drinking water, but the presence of infectious oocysts was not found ( Kourenti and Karanis 2006; Sroka et al. 2006; Villena et al. 2004). T. gondii oocysts were detected in indigenous shellfish in the United States, Italy, and Brazil suggesting the potential for oocysts to survive in marine environments ( Esmerini et al. 2010; Miller et al. 2008; Putignani et al. 2011). In laboratory experiments, partially purified oocysts resuspended in water were stored at 4°C remained infective to mice for at least 54 months ( Dubey 1998). In artificial seawater (15 ppt NaCl), oocysts remained infective to mice for up to 18 months at room temperature, and up to two years at 4°C ( Lindsay and Dubey 2009). Based on these studies, T. gondii oocysts have the potential to easily survive in water for a significantly long period of time.
Data on the infectious dose in humans are not available, but it is suspected to be extremely low. In other natural hosts, as little as a single oocyst can be infective. In one study, one hundred percent of the inbred and out bred pigs fed with a single T. gondii oocyst developed toxoplasmosis as determined by seroconversion, the presence of parasites in host tissues, and/or the ability of excised tissues (e.g. tongue, brain, or heart) to transfer disease to mice ( Dubey et al. 1996). In rats and mice, a single oocyst is also sufficient to cause disease ( Dubey 1996b). In cats, a higher innoculum dose of at least 100 oocysts is required to cause seroconversion and oocyst shedding ( Dubey 1996a). More recently, a severely compromised immunodeficient (SCID) mouse model was used to determine infectious dose of T. gondii oocysts in vivo. In this study, a logistic regression of likelihood of infection along with flow cytometrically enumerated oocysts were used to develop a dose-response curve that resulted in an ID50 of 24 oocysts (Figure 2) ( Ware et al. 2010).
Figure 2. SCID mice infected with T. gondii oocysts. Mice were injected intraperitoneally with 5, 10, 50, 500, or 5,000 sporulated T. gondii oocysts (VEG). Control uninfected mice were mock exposed with PBS , inoculated with 5,000 unsporulated oocysts, or 5,000 heat killed sporulated oocysts. Mice were monitored for moribundity for at least 40 days. (Ware and Villegas, unpublished)
Efforts to monitor T. gondii oocysts in drinking and recreational waters are limited by the lack of adequate and standardized methods. Studies have described using nominal porosity filtration ( Isaac-Renton et al. 1998), membrane filtration centrifugation and chemical flocculation ( Kourenti et al. 2003; Kourenti and Karanis 2004; Kourenti and Karanis 2006), or sucrose gradient purification ( Isaac-Renton et al. 1998; Sroka et al. 2006) approaches as the initial oocyst concentration procedure. However, results were highly variable and recovery and precision data were limited. Recent technological developments using high volume water collection devices and molecular detection techniques may allow for the development of a complete method that can be used towards determining levels and genotypes of T. gondii in water. While there have been many molecular-based detection assays developed to detect T. gondii in clinical samples, only a few have been adapted for environmental monitoring ( Su et al. 2010). Below is a selection of techniques that may be applied towards developing a standardized method for environmental detection of T. gondii oocysts.
Sample concentration and purification procedures:
Continuous separation channel centrifugation. Borchardt and colleagues evaluated the use of continuous separation centrifugation as a multi-pathogen concentration technique for Cyclospora cayetanensis and T. gondii oocysts ( Borchardt et al. 2009). In their study, T. gondii oocysts were efficiently recovered from 10-100 L of tap water with a mean recovery rate ranging from 73 % to 99.5 %. Recoveries from raw surface waters were slightly lower, ranging from 68.5% to 100% with the lowest recovery from turbid water (33.6 NTU). Overall, this study demonstrated the potential application and utility of continuous separation centrifugation as a multi-pathogen concentration technique for water samples. However, there are certain limitations to this technique including the relative centrifugal force being too low to concentrate viruses, and because all particulates are retained, a secondary method of separation or purification, such as immunomagnetic separation, is necessary. Another limitation is the availability of these types of centrifuges for this method, which are not typically found in a microbiology lab. But, there are also several advantages with using this concentration approach: membrane fouling is unlikely to occur when analyzing turbid samples, loss of the target organism is minimal since all of the material is retained, and higher recovery efficiency associated with this method compared to other concentration methods. In addition, this technique offers a multi-pathogen concentration capability that can simultaneously capture bacteria, parasites, and other larger microorganisms.
Envirochek® HV filters. The Envirocheck HV is a 1μm absolute porosity membrane filter that is currently used for concentrating Cryptosporidium oocysts and Giardia cysts as described in USEPA Method 1623 (USEPA, 2005). Shapiro and colleagues recently evaluated the ability of membrane filtration to concentrate spiked T. gondii oocyst in 10 L of tap, fresh, and sea water (Shapiro et al., 2010). Their results revealed that as low as 100 spiked oocysts can be detected by microscopy or nested polymerase chain reaction (nPCR). The detection rates ranged from ~2-25%, suggesting that Envirocheck HV filters can be used for initial concentration of indigenous T. gondii oocysts in various water matrices and that the concentration procedure appears to be compatible with microscopic and PCR detection assays. However, additional studies evaluating the limits of detection and robustness of the filter for capturing oocysts are warranted.
Hollow fiber ultrafiltration. Hollow fiber ultrafilters have recently become very attractive concentration procedures because of their ability to simultaneously and efficiently capture waterborne microorganisms ranging from viruses to protozoa. Ultrafiltration uses size-based exclusion membranes in which particles larger than the pore size of the filter are captured in the rententate while smaller particulates pass through. Studies by Morales-Morales and others evaluated the ability of ultrafilters to concentrate T. gondii oocysts in water. Their results indicated that T. gondii oocysts can be detected (detection rates ranged from 2 - 30%) in 10 L of tap or sea water spiked with at least 100 oocysts by either microscopy or PCR ( Morales-Morales et al. 2003; Shapiro et al. 2010). However, no oocysts were detected in spiked fresh water samples with relatively high turbidity (> 100 NTU) using PCR.
Immunomagnetic separation. Immunomagnetic separation (IMS) of T. gondii oocysts can be an extremely useful technique, especially concerning environmental water samples, which typically contain factors that could inhibit downstream detection methods, such as PCR. IMS binds the target organism using monoclonal antibodies that are attached to magnetic beads. When applied to a magnetic field, the bound organisms are separated from debris and potential inhibitors. Dumetre and Darde initially developed monoclonal antibodies produced against the outer wall of sporulated T. gondii oocysts ( Dumetre and Darde 2005). Indirect and direct IMS procedures were then performed to enrich oocysts from water samples. One of the clones tested reacted to either the inner oocyst wall or sporocyst wall components, and the other stained intact unsporulated and sporulated oocysts. Initially, this indirect enrichment procedure resulted in the better recovery of T. gondii oocysts when compared to the direct IMS. However, in later studies, the monoclonal antibodies used for IMS procedures also cross-reacted with H. hammondi, H. heydorni, and N. caninum ( Dumetre and Darde 2007). Although IMS may still be useful in detecting T. gondii oocysts in the environment, molecular detection assays capable of distinguishing T. gondii from other cross-reactive species must be used in conjunction with the antibody-based IMS procedure or alternative antibodies must be developed that do not cross-react with closely related species.
Bioassays. Animal bioassays are typically considered the gold standard for detecting infectious T. gondii. A bioassay consists of inoculating animals by gavage with a test sample ( Isaac-Renton et al. 1998) or feeding membrane filters containing the test sample to animals (de Moura et al. 2006) and allowing sufficient time for the disease to develop. The infected animal is then examined for the presence of parasites either by seroconversion or the detection of tissue cysts. Although the bioassay is the most accurate method for detecting infectious oocysts, animal models of infection are not quantitative and cannot determine the total levels of T. gondii oocysts present in a given sample. However, previous studies in our laboratories have successfully demonstrated the use of SCID mouse model of infection as a quantitative assay to determine oocyst infectivity ( Ware et al. 2010). Nevertheless, animal bioassays are expensive, labor intensive, and require specialized skills and facilities in order to accurately and safely perform these assays.
Single Round Polymerase Chain Reaction (PCR). PCR provides a rapid and sensitive assay for detection, which is necessary when dealing with the need to quickly identify potential sources of waterborne outbreaks of toxoplasmosis. Gene targets for amplification of T. gondii DNA typically include multi-copy genes such as B1 (35 copies), small subunit rRNA (110 copies), and the 529 bp repeat element (200-300 copies).
Nested PCR. To increase sensitivity of the PCR assay described above, nested PCR can be performed. This process involves an initial amplification of the gene target using conventional PCR followed by a re-amplification of the same target region using primers designed to amplify internal regions of the primary PCR product. Kourenti and Karanis used nested PCR to detect T. gondii oocysts that had been spiked into various water matrices including river water, well water, and seawater. Their nested PCR assay had detection limits of 100 hundred oocysts in river water and 10 oocysts in well water and sea water. This method was then applied to numerous environmental water matrices including river water, well water, reservoir untreated water, sewage water, recreational water, spring water and tap water ( Kourenti and Karanis 2006). Although nested PCR approaches can provide increased detection sensitivities, the potential for cross-contamination with free DNA (i.e. false positives) are much more likely to occur than in single round PCR. Nevertheless, cross-contamination can be minimized by having designated work flow areas where separate steps of the amplification process can be performed. Another approach recently developed to minimize accidental cross-contamination is the single-tube nested PCR. This PCR reaction contains both primary and secondary primers, which are designed to have different melting points, so that the first and second PCR reactions can be conducted in the same tube. The first and second rounds of PCR use different annealing temperatures, and since it is contained within the same tube, this helps minimize contamination commonly occurring with nested PCRs ( Minarovicova et al. 2009). While this approach has not been tested for T. gondii, it has the potential to improve PCR sensitivity.
Quantitative real-time PCR (qPCR). QPCR has increasingly become the preferred method for DNA amplification due to its ability to simultaneously detect and quantify the presence of target DNA in real time. qPCRs targeting the B1 or the 529 bp repeat elements have been used in T. gondii detection assays. Water sample concentrates seeded with T. gondii DNA were assayed by qPCR, and as little as 1 oocyst equivalent in 0.5 ml packed pellet was detected ( Yang et al. 2009). In another study, environmental water samples were seeded with T. gondii oocysts, filtered, and then analyzed by either qPCR or mouse bioassay. Results from this comparison indicated that qPCR was more sensitive than the mouse bioassay with detection occurring in 8 % of the samples as opposed to 0 %, respectively ( Villena et al. 2004).
Loop-mediated isothermal amplification (LAMP). LAMP is a relatively new technique that rapidly amplifies DNA with high specificity and efficiency similar to qPCR except that no temperature cycling is necessary-all reactions are conducted at one temperature ( Notomi et al. 2000). The LAMP assay can amplify DNA from just a few copies up to 109 copies in under an hour. LAMP is routinely used in clinical applications for diagnoses of bacterial, protozoal, viral, and fungal infections ( Karanis et al. 2007). Recently, assays have been developed to detect T. gondii using LAMP. Sotiriadou and Karanis evaluated the detection of T. gondii in spiked environmental water samples by LAMP, nested PCR, and immunofluorescent microscopy. Using primers that amplify the B1 gene, LAMP was able to detect T. gondii in 48% of samples examined, and nested PCR was able to only detect 13.5% of samples examined, while immunofluorescence microscopy was negative for all samples examined (Sotiriadou and Karanis, 2008).
Genotyping assays. Molecular tools to genotype T. gondii in clinical and environmental samples have been extensively used for outbreak investigations as well as to describe its global population structure. Techniques include end-point or nested PCR, restriction fragment length polymorphism (RFLP), single locus sequencing, and multi-locus sequence typing (MLST). Even though the T. gondii genome is highly conserved, there are three distinct genotypes (Types I, II, and III) that have been characterized by virulence and differences in molecular markers. Recently, another genotype, Type X, has been identified and has mainly been associated with California sea otter infections ( Miller et al. 2008). It is important that these genotypes be differentiated so valuable information on source tracking, human infection, and the emergence of new strains can be identified. For the purposes of this review, only a few selected studies will be described. Su and colleagues have provided a more comprehensive review of this topic ( Su et al. 2010).
Image 1. Developing oocysts observed under a differential interference contrast microscope (A). Autofluorescence properties of the oocyst and sporocyst walls are observed under fluorescence microscopy using a DAPI filter set (B). A magnified image of an unsporulated oocyst. Arrow indicates the oocyst wall (C). A magnified image of the infective sporulated oocyst. The white and red arrows indicate the oocyst and sporocyst walls, respectively. Arrowhead points to a residual body. Asterisks indicate individual sporozoites within a sporocyst. Scale bar on all images is 10μm in length. Photos by: Eric N. Villegas.
Image 2. Tachyzoites. A) A single parasitophorous vacuole (PV) containing two tachyzoites in an infected fibroblast. B) Two PVs within an infected fibroblast cell. Each (PV) contains approximately 8 tachyzoites. Note the formation of rosettes due to the way parasites divide. C) Two PVs within an infected fibroblast cell, each containing hundreds of tachyzoites. Images taken using differential interference (DIC) microscopy. White arrows point to individual tachyzoites. Red arrow points to the PV membrane, not visible at this magnification. Photos by: Eric N. Villegas.
Links to useful external sites can be found in the following:
CDC Safety Sheet
USDA fact sheet on Toxoplasma gondii
Toxoplasma Research Institute
ToxoDB.org (Genomic database)
Popular press about toxoplasmosis
|Last Updated on Wednesday, 04 April 2012 06:44|