Parasite Biology and Parasite Molecular Typing
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Toxoplasmosis, a disease caused by Toxoplasma gondii, is a complex zoonotic disease. Infection with T. gondii amongst human populace is high considering its prevalence of approximately 15-85%(Howe 1995). The highest incidence rate of human Toxoplasmosis occurs in Europe specifically in France (occurs up to 55%). Approximately 22.5% of the United States population, 12 years old and above has been infected with this protozoon(CDC 2008). In various host species, the prevalence rate of T. gondii infection is between 30 to 40%.
These protozoan parasites are widely studied due to its ubiquitous nature; and, high morbidity and mortality of the disease it caused. The capability of T. gondii to evade the hosts’ immune mechanism enables this protozoan specie to cause chronic infections. It is for this reason that T. gondii is categorized as among the most successful protozoan parasites. Any warm-blooded animals as well as humans can be infected with this obligate intracellular parasite called T. gondii.
Toxoplasma gondii, the protozoan parasite that cause Toxoplasmosis belongs to kingdom Protozoa, phylum Apicomplexa (Chan 2007). It is a ~8-µm-long and 2-µm diameter banana-shaped organism. The size of the nuclear genome of this protozoon is approximately 80 Mb. The haploid genome of this protozoon specie has 14 chromosomes that totals to 65 Mbp in size and embodies approximately 7900 genes(Saeij 2007). In an electron microscope the single nucleus, mitochondrion, plastid, interconnected endoplasmic reticulum network, Golgi apparatus, and secretory organelles clustered apically can be visualized. T. gondii is not capable of reproducing outside the nucleated cell of its host but its tachyzoites stage can survive in the environment for long periods of time(Joiner 2002).
Cats serve as definitive hosts of T. gondii. In this host, their mode of replication is only sexual reproduction in the intestinal epithelial cells. Among the various secondary and intermediate hosts of this protozoan though, both sexual and asexual reproduction occur (Howe 1995). The life cycle of T. gondii is complicated with several stages despite having only the cats as the definitive host (Dubey 1998).
Toxoplasma gondii has three infectious stages: the tachyzoites usually in groups or clones; the bradyzoites found in tissue cysts; and the sporozoites in the oocysts. The commencement of T. gondii replication in their definitive host is the formation of infectious oocysts that are shed to the environment through feces. Then, the oocysts within 2-3 days in the feces will sporulate into sporozoites. Oocysts can be ingested by man and other animals through food and water contaminated with an infective cats feces. The oocyst will rupture in the intestine releasing the infective sporozoites that will penetrate the gut of its secondary host: man and other warm-blooded animals(Chan 2007).
Sporozoites undergo cycles of multiplication and after these cycles’ tachyzoites are produced. This infective stage (tachyzoites) is approximated to be 2 to 6µm, crescent shaped, and with conoidal anterior and rounded posterior. Despite being devoid of locomotion organelles, the tachyzoites can flex, glide, undulate, and rotate. These capabilities of the tachyzoites enable them to reach their target cells. Tachyzoites enter the circulation of the secondary host to infect the nucleated cells. Their replication and exit from cell to cell within the secondary host is rapid thus the parasite’s dissemination in the body of the host is rather quick (Chan 2007).
It is suggested that the function of the conoid, micropores, rhoptries, and micronemes present in the tachyzoites is for penetration in the host cell and establishment of an intracellular development needed by tachyzoites to survive. The apical complex of T. gondii wherein the conoid is a component is a big factor in the invasion of the infective tachyzoites in the host cells thus it is widely studied today. An objective in the specific studies of the T. gondii apical complex is to understand its composition (especially the proteins) and its function. Goals of studies regarding the T. gondii apical complex include the development a drug that will target this parasite component and be effective anti-toxoplasma medication(Hu 2006).
Entry of the tachyzoites into the host’s cell is through host cell plasmalemma penetration or phagocytosis. This is possible through the utilization of distinct surface proteins that facilitates the attachment and invasion. These proteins are recognized to be SAG1 and SAG2A. Due to the very immunogenic attribute of these proteins their expression is suggested to be attractants of the immunity of the host facilitating the regulation of virulence during infection(Jung 2003). Through repeated endodyogeny the tachyzoites multiply asexually within the host’s cell. The endodyogeny is a specific form of asexual reproduction in which inside the parent parasite two progeny form (Dubey 1998). The division of the tachyzoites occurs every 7 hours in a synchronous manner(Dzierszinski 2004).
After the endodyogeny, the tissue cysts that contain bradyzoites form in the brain and muscle. The cysts of bradyzoites divide slowly in the brain or muscle for years and will only have new cycle of tachyzoites if there is initiation in their proliferation(Chan 2007). Bradyzoites differentiation is an asynchronous replication combining endodyogeny and endopolygeny-schizogony. The division of the bradyzoites is slower than the tachyzoites because it asynchronously divides approximately every 12 hours whereas the tachyzoites are estimated to divide every 7 hours (Dzierszinski 2004).
The bradyzoites are resistant to gastric juices (stomach acids and pepsin). This property of the bradyzoites enables them to be infective orally because they are not killed in the stomach of their host. Bradyzoites can survive up to 2 hours in the stomach of their host whereas the tachyzoites are easily killed (within 1 hour) in the stomach of the host. The death of bradyzoites after two hours in the stomach of the host is not due to pepsin but rather to the gastric acids (Dubey 1998).
- gondii is not capable of growing in food substances and the environment outside a suitable host but the resting stages or oocysts can survive outside of the susceptible hosts. The oocysts can be infective for up to 400 days despite being suspended in water with temperatures ranging from 4 to 37°C. Freezing at -21°C will kill some of the sporulated oocysts but it will take up to 7 days for the unsporulated oocysts to be killed at the same temperature. Drying can inactivate the sporulated oocysts gradually. In 8% NaCl, the encysted T. gondii can survive up to 4 days(Lake 2002).
Various strains of Toxoplasma gondii exists around the world. The individual characteristics of the different strains were studied to elucidate the correlation between different manifestations of the disease and the different pathogenicity of the strains. Although Toxoplasma gondii has various strains those identified in Europe and North America were classified into three distinct clonal lineages: type I, type II, and type III. The strains were grouped into these three clonal lineages through the utilization of various methods of characterization such as restriction fragment length polymorphism (RFLP), isoenzyme electrophoresis, PCR, or random amplified polymorphism DNA(Fuentes 2001).
Classification and identification of the specific strains of T. gondii now involves utilization of the new diagnostic tools. The genetic typing methodologies for this protozoon for example are widely conducted across the globe. Amongst the tools utilized in the genetic typing of this protozoon is multilocus enzyme electrophoresis. The value of this tool for genetic typing purposes of T. gondii has been widely accepted. The limitation of this methodology though is the need for high amounts of purified T. gondii tachyzoites that are difficult to obtain due to the relative slow division characteristics of some T. gondii strains. The time required to arrive at an adequate amount of tachyzoites for electrophoresis analysis is between 1 to 2 months with repeated passages in cortisone-treated mice(Darde 2004).
A more direct approach in genetic typing of the T. gondii strains is restriction fragment length polymorphism (RFLP) analysis. The limitation though of this method is the same with that of the electrophoresis analysis in terms of the production and purification of the protozoon tachyzoites. The other limitations of this method are the utilization of 32P-labelled probes and the high level of difficulty in the interpretation of the data gathered from the test. Fingerprinting of the T. gondii isolates using the BS or TGR probes is the primary application of RFLP. The random amplified polymorphic DNA polymerase chain reaction (RAPD-PCR) using four arbitrary primers was observed to generate DNA fragments that distinguish the mice non-virulent and virulent strains of T. gondii. This methodology though is not widely used due to the greater preference of PCR typing methods with sequence-specific primers (Darde 2004).
Cost, availability of sequencer, and technical support are considerations in choosing the technique to be utilized in genetic typing of T. gondii. Detection of polymorphisms at different base pair level and detection of polymorphic endonucleases restriction sites for developing a PCR-RFLP method can be directly done using the DNA sequencing methods. Utilization of a single marker throughout the procedure of this method will enable the identification of more or less polymorphism. In GRA6 sequences analysis for example there was a high level of polymorphism detected in which 9 allellic sequences where identified among the 30 strains. The GRA6 PCR-RFLP technique could only distinguish three groupings of T. gondii strains. In typing of the T. gondii isolates, PCR-RFLP technique on single copy-genes is widely used (Darde 2004).
Continuous technological developments led to the discovery of microsatellites, short tamdem repeats of 2 to 6 nucleotides, as genetic markers. The length of variation of the repeats generated by these markers enables them to be highly polymorphic and manifest multiple alleles that expose diverse information useful in genetic studies. With only a small amount of DNA, PCR technique can evaluate the polymorphism of microsatellite. After the evaluation, sizing of alleles can be done with high assurance of reliability using automatic sequencing with fluorescent primers(Darde 2004).
Identification of the strains of T. gondii is based on their disease presentation in mice. The majority of the strains identified are non-virulent and these strains produce asymptomatic to chronic infections in the mice they infect. In some virulent strains of T. gondii even if there are less than 10 infective tachyzoites inside the body of the mouse, they are still capable of causing acute toxoplasmosis leading to the death of infected mouse (Darde 2004). The RH and BK strains which are commonly utilized as an antigen source during serological tests for routine diagnosis are the virulent strains of T. gondii. Cultures in mouse cells have been the proliferation method used to preserve these strains in the past decades. Strains of T. gondii that were recently isolated from infected humans are avirulent in mice though they produce chronic infections leading to the development of brain cysts in humans (Bohne 1993).
The utilization of the tachyzoites of the non-cyst forming RH strain (Type I) of T. gondii lead to the emergence of a new concern. This is whether the RH strain tachyzoites evolve during the continuous passage for diagnostic test reagents purposes. This concern is significant in diagnostics purposes because any change in the gene expression of the T. gondii RH strains will have a bearing in the T. gondii infection diagnosis. In line with this, a study was conducted by Marvin et al. in 2004. The results of the study showed that there was a consistency in the B and Q lineages produced tachyzoites and stability in their gene expression was observed despite the multiple passages. The tachyzoites that were produced though from the J lineage were observed to have an unstable and unsuitable growth as well as a changing gene expression during multiple passages. The study concluded that anomalies were existent in the different stocks of T. gondii thus they suggest that those lineages with continuous evolution in cell culture should not be utilized for diagnostic purposes(Mavin 2004).
Immunoprecipitation, isoenzyme analysis, molecular genetic techniques, and western blot with polyclonal antisera are used to demonstrate the differences in the strains of T. gondii. The virulent and avirulent strains can be differentiated by utilizing some serological and genetic markers. The RH and BK strains of T. gondii contain a virulence-associated 23-kDa antigen that was identified using a mouse monoclonal antibody (MAb). The non-virulent strains have polymorphic clonal lineage while the virulent strains posses a single restriction fragment pattern suggestive of a single clonal lineage(Bohne 1993).
The three types of T. gondii clonal lines have phenotypic differences such as persistence, induction of cytokine expression virulence factor, attraction of different cell-types, and migratory capacity. Phenotypic differences among the different genotypes of T. gondii were observed in studies of mice. The virulence capacities of the three clonal lineages were as follows: Type I- highly virulent, Type II- relatively non-virulent, and Type III- relatively virulent. The overstimulation of a Th1 immune response leading to the resultant pathology was partly a factor in the virulence improvement of Type I strains of T. gondii(Saeij 2007).
The different strains of T. gondii have varied antigenic characteristics. This has been demonstrated by the various genetic and serological techniques(Delibas 2006). The differences in the antigenic properties of the strains of this protozoon might have an effect in their virulence capacity as their varied pathogenesis. Aside from their differences in the antigenic characteristics of various strains there are other properties that vary. Amongst the three clonal lineages, polymorphism fluctuates from 1% to 3% at the amino acid level and restricted to only 2 allelic classes not considering the genetic locus. It is implicated then that the three distinct clonal lineages and rarer types of T. gondii were formed through the assortment of only two alleles (Kong 2003).
The correlation of T. gondii genotypes with the severity of toxoplasmosis in humans remains vague. The Type I strains of this protozoan were frequently associated with AIDS patients and those immunocompromised patients with recurrent ocular toxoplasmosis. The majority of the infections in humans and animals though were caused by Type II strains. The highly pathogenic strains and most likely to cause infection in immunocompromised individuals were type I strains whereas the most frequent causal agents of human toxoplasmosis either in AIDS patients or in infants was Type II strains. The most virulent and pathogenic strain amongst the various T. gondii strains is still uncertain(Khan 2005).
The map of virulence in the F1 progeny developed from crosses of the type II and the type III strains of T. gondii was made to identify the loci involved in the virulence of the strains of this protozoon. There were five loci identified and amongst them, two manifested a genetic composition having protein kinase as the key molecule (ROP18 and ROP16). These two proteins identified were hyperviable rhopty proteins which the protozoon secretes into the host cell during cell invasion. This protein kinases which are unique to the phylum Apicomplexa are correlated to this pathogen’s interaction with its host(Saeij 2007).
Saeiji et al (2006) also conducted linkage mapping of the loci involved in the protozoon’s modulation of the host gene expression. It is indicated in their study that the different strains of T. gondii have specific differences in their modulation of the host cell transcription which is mediated by ROP16. This polymorphic protein is injected into the cell of the host by the T. gondii protozoon upon their invasion of the cell resulting alteration of the host’s signal transducer and activator of transcription signaling pathways (Saeji 2006).
The T. gondii protozoon therefore has the capability to secrete protein kinases and inject it to the host cell and alters its signaling pathways. T. gondii has a wide storage of effectors that can intervene with the diverse signaling pathways of the host thus helping the protozoon evade the mechanisms of the hosts’ immune system (Saeiji 2006). These effectors might be factors in the virulence capacity of the T. gondii strains and might also explain their intricate capability to evade the host immune system.
A factor in the virulence characteristic of T. gondii is the growth rate. In T. gondii the correlation of the growth rate and virulence is said to be present. The number of parasites infecting a host is directly proportional to the amount of stimulation that can be induced by the parasites in the immune system of the host.
High the numbers of parasite in the body of the host cause greater chances of overstimulation of the host’s immune system. Eventually, high production of cytotoxic helper T cells (Th1), elevated level of cell death and greater damage in the organs of the host occurs. Due to the high virulence characteristic of type I strains, only one tachyzoite of this type will be enough to generate high level of parasite loads and high amount of Th1 cytokines. High numbers of T. gondii type II strains though will also generate the same high levels of Th1 cytokines and pathology with that of only one tachyzoite belonging to the type I strain. In spite the virulent characteristic of Type I strains, higher loads of the non-virulent type II and type III will still generate equal pathology and Th1 cells to only one tachyzoite type I infection. Thus, because of the high antigenic load due to the higher number of infective parasites present in the host, it can be expected that there will be higher immune pathology (Saeij 2005).
In the past, these phenotypic differences were only observed in the strains present in mice but recently these phenotypic differences were also recognized in T. gondii strains that cause human infections. In a study done by Kong et al (2003), the strains that cause infection in humans were identified using an enzyme-linked immunosorbent assay. Infection serum reacted with polymorphic peptides obtained from Toxoplasma antigens SAG2A, GRA3, GRA6, and GRA7 were also utilized in the procedure. The majority of the isolates identified belong to the three clonal lineages (type I, type II, and type III). It was observed in the study that polymorphic linear epitopes have strain-specificity. A common epitope was observed in type I and type III that was not detected in the type II strains.
The SAG2A, GRA3, GRA6, and GRA7 antigens of T. gondii were able to accurately identify the clonal lineage responsible for infection in mice. In tests using human serum samples, only the GRA6 was able to correctly identify the type II from the non-type II infections in the presence of adequate antibodies to T. gondii. The serotyping with the usage of the GRA6 antigens implies that most of the infections of the patients tested were caused by type II strains of T. gondii. The failure of assays using SAG2A and GRA6 to distinguish only between type II or non-type II strain is due to the identical alleles in type I and type III at SAG2A and GRA6. The type I and III strains also have an almost identical property at GRA6 and GRA7. The assay will be able to identify all the different types of clonal lineage in human infections if there will be an allele specific peptide obtained from various location (Kong 2003).
The ultimate goal of researches regarding T. gondii is to grasp the mechanism of varied capacity of T. gondii genotypes to elicit disease in humans. Being able to determine the severity of the disease elicited and immune responses of the hosts infected in relation to the genotype causing the disease will be of great significance to the treatment and prevention of T. gondii infections. The severity of the disease caused and the immune response of the infected host varies with the strain that infects the host. Due to this strain related variation, different studies have been conducted to characterize and identify strains that pose different health risks to humans and animals. Different strains of T. gondii possess different antigenic characteristics that were demonstrated through the utilization of mAb techniques, isoenzyme analyses, RFLP, random amplified polymorphic DNA, and Western Blotting(Songul Bayram Delibas 2006).
Toxoplasma gondii has various modes of transmission including ingestion of oocysts shed in the definitive host’s feces, ingestion of tissue cysts present in undercooked meat, and congenital or vertical transmission. Amongst the carnivores and omnivores, ingestion of infected meat from secondary hosts and direct transmission from the cat through the infective oocysts are the prime mechanism of parasite transmission(Hide 2006). In herbivores for instance like the sheep, the primary mode of transmission is through the cat whereas congenital transmission is insignificant (Duncan 2001). In a recent study though, the data gathered through PCR based detection assay implies that vertical or congenital mechanism of transmission is high in sheep. Due to the vegetarian diet of the sheep it is unlikely that they will acquire infection through ingestion of meat with T. gondii sarcocyst. It is then concluded that ovine infection with T. gondii is not only due to cats but the vertical mode of transmission is highly significant.
In humans, the major modes of transmission are: the ingestion of the parasite present in tissue cysts of uncooked meat; ingestion of food and water contaminated with oocysts from feces of infected cats; and transplacental or congenital transmission. Aside from these modes though, Toxoplasma gondii can also be transmitted through blood transfusion and organ transplantation(Singh 2003). Transplacental mode of transmission is considered to be relatively rare in humans and frequently correlated to severe pathology in the affected offsprings (Hide 2006). Except for congenital, organ transplantation, and blood transfusion modes of T. gondii transmission there were no other human to human modes of transmission observed(CDC 2008).
The primary modes of transmission mentioned above do not adequately explain the diversity of the hosts infected by Toxoplasma gondii. This led to theories that other modes of transmission such as infection through skin lesions and arthropod transmission is possible. Due to the other suggested routes of Toxoplasma gondii transmission, studies have been conducted to verify there are other significant mechanisms of transmission. Among these is the study by Sroka et al (2003) regarding the potential of the tick – Ixodes ricinus as an arthropod vector of this protozoan. Prior to this study there were associations of human toxoplasmosis to bites from ticks as well as studies that isolated Toxoplasma gondii from naturally infected ticks. The study by Sroka et al. (2003) confirms the potential of Ixodes ricinus ticks as vectors of Toxoplasma gondii. In the epidemiology of toxoplasmosis it is thus possible for Ixodes ricinus ticks to be a vector in the transmission of the causative protozoan(Sroka 2003).
The following are risk factors that were enumerated by epidemiologic studies: close proximity with seropositive cats; eating raw or undercooked meat; owning a cat; gardening; eating raw or unwashed vegetables; constant contact with soil; poor hand hygiene; infrequent washing of kitchen knives; and traveling outside of Canada, Europe, and United States. There were studies conducted though that shows that owning a cat even in pregnant women and those with compromised immune system is not correlated with any risk of T. gondii infection. In countries in which eating of undercooked meat is a practice, there is an observable increase in the T.gondii seroprevalence. Examples of areas where this eating habit is widely practiced and their seroprevalence is high are: sub-Saharan Africa, France, and tropical areas of Latin America. In a study done in France, the possibility of women being infected with T. gondii are equal to those of ages capable of child bearing and onwards(Jones 2001).
In the study by Jones et al. (2001), it was learned that the T. gondii seroprevalence in all ages is 22.5 percent and 15 percent amongst the childbearing women. Compared to France, countries in Latin America and sub-Saharan Africa the United States have a low seroprevalence of T. gondii. In a multivariate analysis, amongst the three racial and ethnic groups included in the study it was learned that immigrants have higher seroprevalence. The variations in the seroprevalence amongst different countries was concluded to be associated with the amount T. gondii in meat; exposure to the soil and cat feces; food preservation; and eating habits of the people(Jones 2001).
Since veterinarians and veterinary staff often handle cats which are the definitive hosts of T. gondii, it was suggested that they were of higher risk of infection with T. gondii. In 2003, a study was conducted by Shuhaiber et al. to verify this idea as well as determine if pregnant women owning cats are also at greater risk of infection with T. gondii. It was concluded however in the study that veterinarians and veterinary staff that are often exposed to cats have no increased risk of being infected with T. gondii. It was also concluded in the study that T. gondii is not correlated to owning a cat (Shuhaiber 2003).
In humans, Toxoplasma gondii infection in healthy adults will manifest as a mild or asymptomatic infection that led to the formation of cysts that are mostly situated in the infected individual’s brain. Immunocompromised patients such as those with AIDS on the other hand have fatal occurrences due to the reactivation of cysts which leads to cerebral toxoplasmosis. Congenital acute infection with this protozoan results in serious and often fatal illness in babies. Toxoplasmosis in infants usually causes cerebral and ocular damage.
Congenital human toxoplasmosis occurs in infants born to mothers that are infected with T. gondii prior to or during pregnancy. The T. gondii infection is passed to the fetus through placental transmission. In the United States, the estimated incidence of congenital toxoplasmosis is from 400 to 4000 cases annually(Jones 2001). The clinical manifestation of congenital human toxoplasmosis depends on the following factors: age at the time of primary infection, immune status of the host, and the virulence of the strain of T. gondii that infects the host. Toxoplasmosis when acquired congenitally is more severe in pathology than that of postnatally acquired infection. The severity of the infection in infants and the possibility that the infection shall be passed to the infant varies according to the trimester of pregnancy in which the mother is infected. During the course of the pregnancy, earlier infection of the mother corresponds to higher level of infection severity in the infant. Thus, an infant whose mother was infected with T. gondii during the first trimester of pregnancy shall have a more severe infection compared to an infant whose mother was infected with T. gondii during the third trimester of her pregnancy(Singh 2003).
Congenitally acquired toxoplasmosis does not manifest during early years of the infant’s life but can develop later on. The clinical conditions of congenital toxoplasmosis in children are: chorioretinitis, hydrocephalus, mental retardation, intracerebral calcification, hepatosplenomegaly, loss of hearing cholangitis, pancytopenia, and death. Other manifestations of this infection in congenitally infected children are: lymph node enlargement specifically in the cervical region, muscle aches, headaches, and sore throat. This disease condition is most often not diagnosed in children(Singh 2003).
Infection with T. gondii of patients that are immunocompromised can be life threatening. The manifestation of the disease does not necessarily mean a newly acquired infection because latent T. gondii infection is possible. The examples of immunocompromised patient that are at risk of acquiring toxoplasmosis are: transplant patients undergoing immunosuppressive therapy, cancer patients, and AIDS patients. It is estimated that amongst the AIDS patients 3 to 10% of them die due to toxoplasmosis. The predominant pathology in T. gondii infected AIDS patients is encephalitis but other organs may also be affected(Singh 2003).
In immunocompetent patents around the world, toxoplasmosis is the most frequent cause of posterior uveitis and intraocular inflammation. In United States alone, it is estimated that 30 to 50% of the posterior uveitis cases are caused by T. gondii infection. During subclinical infections there are no observable changes in the retina of the host. During times of host immune suppression, the cyst that is situated in the retina of the host will rupture thereby releasing the bradyzoites into the retina. This event can cause inflammation and then after healing a chorioretinal scar can be existent and within or adjacent to this the cyst will remain inactive(Wu 2007).
Neutropenia (depletion of the nuetrophils) occurs during cases of toxoplasmosis leading to the suppression of the immune system. The depletion of the neutrophils during T. gondii infections causes the development of multi-organ lesions that include the liver, spleen, brain, and lung. There is also diminished ability to manufacture early gamma interferon (IFN-γ), interleukin-12 (IL-12), and tumor necrosis factor alpha (TNF-α). The population of splenic helper T lymphocytes (Th) and natural killer (NK) cells also occur in T. gondii infected host. The neutrophils have a significant role in combating the tachyzoite replication thus depletion occurs(Bliss 2001). The neutropenia and the impairment of the host’s immune system enable the parasite to replicate uncontrollably in the body of the host and any secondary infection will be detrimental to the health of the host.
Asymptomatic manifestation of toxoplasmosis in adult humans and animals is due to the effective protection of the immune system that includes extracellular acting antibodies and intracellularly acting T lymphocyte factors. The inadequate protection from the immune system and late acquisition of immunity results into continuous parasite multiplication and destruction of the host’s cells that manifest as multi-organ lesions. Often the causes of death due to toxoplasmosis are pneumonia and encephalitis(Frenkel 1988).
Animals and humans have more or less the same manifestations of toxoplasmosis. In black-footed ferrets the clinical signs of acute toxoplasmosis include lethargy and anorexia whereas in chronic toxoplasmosis corneal edema, ataxia, glaucoma, and death were observed. The most predominant clinical sign in chronic T. gondii infections in ferrets involves the central nervous system wherein it is manifested as moderate to mild posterior weakness.(Burns 2003)
Diagnosis of Toxoplasma gondii infection in humans and animals involves serological tests. In the past the serology tools for the diagnosis of T. gondii infection in humans presented various problems thus various alternative methods emerged. The problems found when using T. gondii serological diagnostics included: expensive diagnostic tools, unavailability of the screening programs, slow and sometimes laborious method, and low sensitivity in diagnosis of early infection. The alternative diagnostic tools for T. gondii infection in 1980’s include the agglutination (AG) test. This methodology is very simple and commercial kits were then available that were made in France but this technique lacks sensitivity due lower titer generated compared to the dye test (DT) and the conventional immunofluorescent- antibody (IFA) test. Hence, most often false negatives were obtained using the AG test. Another drawback of the AG test is the lack of specificity wherein the sera that are negative in both the DT and IFA tests were positive on the AG test. The drawbacks of the AG test were due to the host’s immunoglobulin M (IgM) binding to the surface of the parasite.
Diagnostic tests utilized in the determination of infection in humans involve the measurement of the immunoglobulin G (IgG) with the employment of immunoflourescent antibody (IFA) assay and enzyme immunoassay (EIA) tests. In pregnant women though the approximation of the time of infection is significant thus the diagnostic test also involves the measurement of immunoglobulin M (IgM) combined with other tests such as the avidity test(CDC 2008).
Other diagnostic tests for T. gondii infection are: parasite observation in specimens from bronchoalveolar lavage material (immunocompromised patients) or lymph node biopsy; mouse inoculated with blood and other body fluids from patients, this demonstrates the parasite by the use of serological techniques on the inoculated mice at set time intervals post inoculation; and, genetic material detection with the use of PCR during congenital infections in utero (CDC 2008).
In infection, with T. gondii, in healthy and non-pregnant individual’s treatment is not necessary because the clinical signs usually resolve within a few weeks. Whereas, individuals that are pregnant or immunocompromised like AIDS patients need to be treated with drugs like pyrimethamine plus sulfadiazine(CDC 2008). The modes of action of these drugs are folic acid antagonism and inhibition of the dihydropteroic acid synthesis, respectively(Mui 2008).
In self limiting cases of systemic acquired toxoplasmosis, treatment is not often recommended. The cases of ocular toxoplasmosis though necessitate treatment involving the triple drug therapy composed of pyrimethamine, prednisone, and sulfadiazine. There is also a quadruple therapy wherein clindamycin is added to the triple therapy. The usual treatment duration ranges from 4 to 6 weeks depending on the patient’s response to the therapy. Photocoagulation or cryotherapy as surgical treatment of ocular toxoplasmosis are used but caution is employed due to the surgical complications that include vitreous hemorrhages, intraretinal hemorrhages, and detachment of the retina(Wu 2007).
Due to the high morbidity, mortality, and financial costs for healthcare in infections with T. gondii various studies have been conducted to come up with a drug that will kill the infective stages of this protozoon and cure toxoplasmosis around the globe. Among the products of these studies is the drug known as dihydrotriazine JPC-2067-B (4, 6-diamino-1, 2-dihydro-2, 2-dimethyl-1-(3′(2-chloro-, 4-trifluoromethoxyphenoxy)propyloxy)-1, 3, 5-triazine). The mode of action of this drug is the inhibition of the dihydrofolate reductase (DHFR) in T. gondii. Based on the effectiveness and toxicity testing using mammalian cells, dihydrotriazine JPC-2067-B is highly effective against T. gondii strains. Growth cultures of T. gondii were hindered by this drug by inhibiting the purified enzyme and it is more effective than pyrimethamine. Oral and parenteral administration of this drug is also effective against the tachyzoites of T. gondii cultured in vivo. It was concluded in their study that JPC-2056/JPC-2067-B has higher effectiveness compared to the medicines employed today for toxoplasmosis treatment. Aside from being potentially more effective, this drug is also potentially less toxic compared to the drugs used for treatment of T. gondii infections today (Mui 2008).
The immunocompromised people and those highly affected by the disease caused by T. gondii belongs to the third world countries. The necessity for medicines that are effective against this protozoon, without toxicity, and affordability is pressing. The cases of pregnant women being infected with this protozoon also necessitate the availability of drugs for toxoplasmosis therapy that are non-teratogenic. Another consideration in the formulation of drugs against T. gondii is increased capacity to penetrate the brain and the eye because these organs were the common infested by this protozoon(Mui 2008). These various factors need to be considered to be able to produce drugs that can combat this world wide protozoon problem. The search for the toxoplasmosis cure should not stop until it is eradicated in all the parts of the world.
Vast amount of information about T. gondii have been obtained from studies around the globe. The three clonal lineages do not have the same virulence and pathogenecity thus further studies regarding this matter need to be conducted. Studies regarding the pathogenecity of the different strains in different hosts also need to be studied further. The advocacy of various individuals in combating toxoplasmosis will eventually lead to the formulation of more effective drugs to eradicate this global zoonotic disease.
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