Imaging schistosomes in vivo

A comprehensive exploration of schistosomiasis: Global impact, molecular characterization, drug discovery, artificial intelligence and future prospects

Schistosomiasis, one of the neglected tropical diseases which affects both humans and animals, is caused by trematode worms of the genus Schistosoma. The disease is caused by several species of Schistosoma which affect several organs such as urethra, liver, bladder, intestines, skin and bile ducts. The life cycle of the disease involves an intermediate host (snail) and a mammalian host. It affects people who are in close proximity to water bodies where the intermediate host is abundant. Common clinical manifestations of the disease at various stages include fever, chills, headache, cough, dysuria, hyperplasia and hydronephrosis. To date, most of the control strategies are dependent on effective diagnosis, chemotherapy and public health education on the biology of the vectors and parasites. Microscopy (Kato-Katz) is considered the golden standard for the detection of the parasite, while praziquantel is the drug of choice for the mass treatment of the disease since no vaccines have yet been developed. Most of the previous reviews on schistosomiasis have concentrated on epidemiology, life cycle, diagnosis, control and treatment. Thus, a comprehensive review that is in tune with modern developments is needed. Here, we extend this domain to cover historical perspectives, global impact, symptoms and detection, biochemical and molecular characterization, gene therapy, current drugs and vaccine status. We also discuss the prospects of using plants as potential and alternative sources of novel anti-schistosomal agents. Furthermore, we highlight advanced molecular techniques, imaging and artificial intelligence that may be useful in the future detection and treatment of the disease. Overall, the proper detection of schistosomiasis using state-of-the-art tools and techniques, as well as development of vaccines or new anti-schistosomal drugs may aid in the elimination of the disease.

Small animal in vivo imaging of parasitic infections: A systematic review

Non-invasive small animal in vivo imaging is an essential tool in a broad variety of biomedical sciences and enables continuous monitoring of disease progression in order to develop and improve diagnostic, therapeutic and preventive measures. Imaging parasites non-invasively in live animals allows efficient parasite distribution evaluation in the host organism and objective evaluation of parasitic diseases' burden and progression in individual animals. The aim of this systematic review was to summarize recent trends in small animal in vivo imaging and compare and discuss imaging of single-cell and multicellular eukaryotic parasites. A literature survey was performed using Web of Science and PubMed databases in research articles published between 1990 and 2018. The inclusion criteria were using any imaging method to visualize a range of protozoan and helminth parasites in laboratory animals in vivo. A total of 92 studies met our inclusion criteria. Protozoans and helminths were imaged in 88% and 12% of 92 studies, respectively. The most common parasite genus studied was the protozoan Plasmodium followed by Trypanosoma and Leishmania. The most frequent imaging method was bioluminescence. Among the helminths, Schistosoma and Echinococcus were the most studied organisms. In vivo imaging is applicable in both protozoans and helminths. In helminths, however, the use of in vivo imaging methods is limited to some extent. Imaging parasites in small animal models is a powerful tool in preclinical research aiming to develop novel therapeutic and preventive strategies for parasitic diseases of interest both in human and veterinary medicine.

Clinical and Preclinical Imaging of Hepatosplenic Schistosomiasis

Schistosomiasis, a neglected tropical disease, is a major cause of chronic morbidity and disability, and premature death. The hepatosplenic form of schistosomiasis is characterized by hepatosplenomegaly, liver fibrosis, portal hypertension, and esophageal varices, whose rupture may cause bleeding and death. We review currently available abdominal imaging modalities and describe their basic principles, strengths, weaknesses, and usefulness in the assessment of hepatosplenic schistosomiasis (HSS). Advanced imaging methods are presented that could be of interest for hepatosplenic schistosomiasis evaluation by yielding morphological, functional, and molecular parameters of disease progression. We also provide a comprehensive view of preclinical imaging studies and current research objectives such as parasite visualization in hosts, follow-up of the host's immune response, and development of noninvasive quantitative methods for liver fibrosis assessment.

Spirorchiidiasis in marine turtles: the current state of knowledge

Blood flukes of the family Spirorchiidae are important disease agents in marine turtles. The family is near cosmopolitan in distribution. Twenty-nine marine species across 10 genera are currently recognized, but taxonomic problems remain and it is likely that more species will be discovered. Spirorchiids infect the circulatory system, where they and their eggs cause a range of inflammatory lesions. Infection is sometimes implicated in the death of the turtle. In some regions, prevalence in stranded turtles is close to 100%. Knowledge of life cycles, important for control and epidemiological studies, has proven elusive until recently, when the first intermediate host identifications were made. Recent molecular studies of eggs and adult worms indicate that a considerable level of intrageneric and intraspecific diversity exists. The characterization of this diversity is likely to be of importance in exploring parasite taxonomy and ecology, unravelling life cycles, identifying the differential pathogenicity of genotypes and species, and developing antemortem diagnostic tools, all of which are major priorities for future spirorchiid research. Diagnosis to date has been reliant on copromicroscopy or necropsy, which both have significant limitations. The current lack of reliable antemortem diagnostic options is a roadblock to determining the true prevalence and epidemiology of spirorchiidiasis and the development of effective treatment regimes.

Live imaging of collagen deposition during experimental hepatic schistosomiasis and recovery: a view on a dynamic process

Hepatic fibrosis is the central cause of chronic clinical pathology resulting from infection by the blood flukes Schistosoma japonicum or S. mansoni. Much has been elucidated regarding the molecular, cellular and immunological responses that correspond to the formation of the granulomatous response to trapped schistosome eggs. A central feature of this Th2 response is the deposition of collagen around the periphery of the granuloma. To date, traditional histology and transcriptional methods have been used to quantify the deposition of collagen and to monitor the formation of the hepatic granuloma during experimental animal models of schistosomiasis. We have investigated the dynamic nature of granuloma formation through the use of a transgenic mouse model (B6.Collagen 1(A) luciferase mice (B6.Coll 1A-luc⁺)). With this model and whole-animal bioluminescence imaging, we followed the deposition of collagen during an active schistosome infection with Chinese and Philippines geographical strains of S. japonicum and after clearance of the adult parasites by the drug praziquantel. Individual mice were re-imaged over the time course to provide robust real-time quantitation of the development of chronic fibrotic disease. This model provides an improved method to follow the course of hepatic schistosomiasis-induced hepatic pathology and effectively supports the current dogma of the formation of hepatic fibrosis, originally elucidated from static traditional histology. This study demonstrates the first use of the B6.Coll 1A-luc⁺ mouse to monitor the dynamics of disease development and the treatment of pathogen-induced infection with the underlying pathology of fibrosis.

Early Detection of Acute Drug-Induced Liver Injury in Mice by Noninvasive Near-Infrared Fluorescence Imaging

Hepatocellular and cholestatic forms of drug-induced liver injury (DILI) are major reasons for late-stage termination of small-molecule drug discovery research projects. Biochemical serum markers are limited in their ability to sensitively and specifically detect both of these common DILI forms in preclinical models, and tissue-specific approaches to assessing this are labor intensive, requiring extensive animal dosing, tissue preparation, and pathology assessment. In vivo fluorescent imaging offers noninvasive detection of biologic changes detected directly in the livers of living animals. Three different near-infrared fluorescent imaging probes, specific for cell death (Annexin-Vivo 750), matrix metalloproteases (MMPSense 750 FAST), and transferrin receptor (Transferrin-Vivo 750) were used to measure the effects of single bolus intraperitoneal doses of four different chemical agents known to induce liver injury. Hepatocellular injury–inducing agents, thioacetamide and acetaminophen, showed optimal injury detection with probe injection at 18–24 hours, the liver cholestasis-inducing drug rifampicin required early probe injection (2 hours), and chlorpromazine, which induces mixed hepatocellular/cholestatic injury, showed injury with both early and late injection. Different patterns of liver responses were seen among these different imaging probes, and no one probe detected injury by all four compounds. By using a cocktail of these three near-infrared fluorescent imaging probes, all labeled with 750-nm fluorophores, each of the four different DILI agents induced comparable tissue injury within the liver region, as assessed by epifluorescence imaging. A strategy of probe cocktail injection in separate cohorts at 2 hours and at 20–24 hours allowed the effective detection of drugs with either early- or late-onset injury.

Automated imaging and other developments in whole-organism anthelmintic screening

Helminth infections still represent a huge public health problem throughout the developing world and in the absence of vaccines control is based on periodic mass drug administration. Poor efficacy of some anthelmintics and concerns about emergence of drug resistance has highlighted the need for new drug discovery. Most current anthelmintics were discovered through in vivo screening of selected compounds in animal models but recent approaches have shifted towards screening for activity against adult or larval stages in vitro. Larvae are normally available in greater numbers than adults, can often be produced in vitro and are small enough for microplate assays. However, the manual visualization of drug effects in vitro is subjective, laborious and slow. This can be overcome by application of automated readouts including high-content imaging. Incorporated into robotically controlled HTS platforms such methods allow the very large compound collections being made available by the pharmaceutical industry or academic organizations to be screened against helminths for the first time, invigorating the drug discovery pipeline. Here, we review the status of whole-organism screens based on in vitro activity against living worms and highlight the recent progress towards automated image-based readouts.

The use of imaging to detect schistosomes and diagnose schistosomiasis

Several imaging modalities have been employed to examine schistosomes and monitor schistosome-induced pathology. Ultrasound is a noninvasive imaging method that has long been used in the laboratory and in the field to evaluate pathological changes, notably fibrosis, that arise as a consequence of the host response to schistosome eggs lodging in a variety of tissues. Ultrasonography has been widely used to monitor changes in the extent of fibrosis and in spleen/liver enlargement following chemotherapeutic treatment for schistosomiasis. Imaging methods to monitor schistosomes themselves in vivo (as opposed to detecting schistosome-induced pathology) include positron emission tomography and fluorescence molecular tomography. Both approaches rely on schistosome uptake of tracers that are introduced into infected animals and that can be detected externally. These methods have been used to successfully detect schistosomes in vivo and to monitor their elimination following chemotherapeutic treatment. Direct monitoring of live schistosomes in vivo has been achieved using intravital microscopy, when the infected tissues of anaesthetized animals are exposed. Finally, schistosome eggs have been visualized by confocal laser scanning microscopy in infected mice as well as in a human patient with schistosomiasis hematobium. Further advances in imaging technologies seem likely to provide greater insight into disease progression and into the biology of schistosomes in the most relevant setting-within a live animal.

Identification and characterization of a Mef2 transcriptional activator in schistosome parasites

Myocyte enhancer factor 2 protein (Mef2) is an evolutionarily conserved activator of transcription that is critical to induce and control complex processes in myogenesis and neurogenesis in vertebrates and insects, and osteogenesis in vertebrates. In Drosophila, Mef2 null mutants are unable to produce differentiated muscle cells, and in vertebrates, Mef2 mutants are embryonic lethal. Schistosome worms are responsible for over 200 million cases of schistosomiasis globally, but little is known about early development of schistosome parasites after infecting a vertebrate host. Understanding basic schistosome development could be crucial to delineating potential drug targets. Here, we identify and characterize Mef2 from the schistosome worm Schistosoma mansoni (SmMef2). We initially identified SmMef2 as a homolog to the yeast Mef2 homolog, Resistance to Lethality of MKK1P386 overexpression (Rlm1), and we show that SmMef2 is homologous to conserved Mef2 family proteins. Using a genetics approach, we demonstrate that SmMef2 is a transactivator that can induce transcription of four separate heterologous reporter genes by yeast one-hybrid analysis. We also show that Mef2 is expressed during several stages of schistosome development by quantitative PCR and that it can bind to conserved Mef2 DNA consensus binding sequences.

Schistosome parasites infect more than 200 million people worldwide and cause human schistosomiasis. Free-swimming schistosome larvae are highly mobile and invade and penetrate the host's skin to perpetuate their lifecycle in their human host, growing from 90–215 micrometers in length as a schistosomulum to a 7–20 millimeter long adult worm. Few molecular pathways have been identified in schistosome worms that are important for parasite early development. The myocyte enhancer factor protein 2 is a major regulator of muscle and nerve development in mammals and insects and is highly conserved from bread yeast to vertebrates. Here we identify and characterize the Mef2 activator from parasitic schistosome worms, the first described in any parasitic worm, and delineation of its function may be important to further understanding the basic biology of schistosome early development. Additionally, since schistosomes developed early evolutionarily, an investigation of schistosome Mef2 regulatory mechanisms could lead to a greater understanding of the development of early muscle and neurogenic development in animals.

Tracing of the Bile-chemotactic migration of juvenile Clonorchis sinensis in rabbits by PET-CT

Background

Adult Clonorchis sinensis live in the bile duct and cause clonorchiasis. It is known that the C. sinensis metacercariae excyst in the duodenum and migrate up to the bile duct through the common bile duct. However, no direct evidence is available on the in vivo migration of newly excysted C. sinensis juveniles (CsNEJs). Advanced imaging technologies now allow the in vivo migration and localization to be visualized. In the present study, we sought to determine how sensitively CsNEJs respond to bile and how fast they migrate to the intrahepatic bile duct using PET-CT.

Methodology/Principal Findings

CsNEJs were radiolabeled with ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG). Rabbits with a gallbladder contraction response to cholecystokinin-8 (CCK-8) injection were pre-screened using cholescintigraphy. In these rabbits, gallbladders contracted by 50% in volume at an average of 11.5 min post-injection. The four rabbits examined were kept anesthetized and a catheter inserted into the mid duodenum. Gallbladder contraction was stimulated by injecting CCK-8 (20 ng/kg every minute) over the experiment. Anatomical images were acquired by CT initially and dynamic PET was then carried out for 90 min with a 3-min acquisition per frame. Twelve minutes after CCK-8 injection, about 3,000 ¹⁸F-FDG-labeled CsNEJs were inoculated into the mid duodenum through the catheter. Photon signals were detected in the liver 7–9 min after CsNEJs inoculation, and these then increased in the whole liver with stronger intensity in the central area, presenting that the CsNEJs were arriving at the intrahepatic bile ducts.

Conclusion

In the duodenum, CsNEJs immediately sense bile and migrate quickly with bile-chemotaxis to reach the intrahepatic bile ducts by way of the ampulla of Vater.

Clonorchis sinensis adults habituating in the bile duct cause clonorchiasis endemic in East Asian countries, in which about 15–20 million people are supposedly infected. It has previously been reported that C. sinensis metacercariae excyst in the duodenum and that the juvenile flukes migrate to the bile duct through the ampulla of Vater in 4–7 hours. Recently advanced imaging technologies have enabled visualization of movements and localizations of parasites in mammalian hosts. From present study, we found the following: newly excysted C. sinensis juveniles (CsNEJs) were efficiently in vitro radiolabeled with ¹⁸F-FDG since CsNEJs have glucose transporters; CCK-8-induced gallbladder contraction was various rabbit to rabbit; CsNEJs promptly recognized bile and migrated up the duodenum to reach the intrahepatic bile ducts by way of the ampulla of Vater and the common bile duct as early as 7–9 minutes after inoculation. Some CsNEJs responding slowly to the bile delayed arriving at the distal bile capillaries. It was visualized for the first time that the CsNEJs migrate quickly within 10–20 minutes from the duodenum to the intrahepatic bile duct. These findings provide fundamental information on the migration of parasites living in the biliary passages of mammals.

Noninvasive biophotonic imaging for studies of infectious disease

According to World Health Organization estimates, infectious organisms are responsible for approximately one in four deaths worldwide. Animal models play an essential role in the development of vaccines and therapeutic agents but large numbers of animals are required to obtain quantitative microbiological data by tissue sampling. Biophotonic imaging (BPI) is a highly sensitive, nontoxic technique based on the detection of visible light, produced by luciferase-catalysed reactions (bioluminescence) or by excitation of fluorescent molecules, using sensitive photon detectors. The development of bioluminescent/fluorescent microorganisms therefore allows the real-time noninvasive detection of microorganisms within intact living animals. Multiple imaging of the same animal throughout an experiment allows disease progression to be followed with extreme accuracy, reducing the number of animals required to yield statistically meaningful data. In the study of infectious disease, the use of BPI is becoming widespread due to the novel insights it can provide into established models, as well as the impact of the technique on two of the guiding principles of using animals in research, namely reduction and refinement. Here, we review the technology of BPI, from the instrumentation through to the generation of a photonic signal, and illustrate how the technique is shedding light on infection dynamics in vivo.

In vivo imaging of schistosomes to assess disease burden using positron emission tomography (PET)

BACKGROUND

Schistosomes are chronic intravascular helminth parasites of humans causing a heavy burden of disease worldwide. Diagnosis of schistosomiasis currently requires the detection of schistosome eggs in the feces and urine of infected individuals. This method unreliably measures disease burden due to poor sensitivity and wide variances in egg shedding. In vivo imaging of schistosome parasites could potentially better assess disease burden, improve management of schistosomiasis, facilitate vaccine development, and enhance study of the parasite's biology. Schistosoma mansoni (S. mansoni) have a high metabolic demand for glucose. In this work we investigated whether the parasite burden in mice could be assessed by positron emission tomography (PET) imaging with 2-deoxy-2[(18)F]fluoro-D-glucose (FDG).

METHODOLOGY/PRINCIPAL FINDINGS

Live adult S. mansoni worms FDG uptake in vitro increased with the number of worms. Athymic nude mice infected with S. mansoni 5-6 weeks earlier were used in the imaging studies. Fluorescence molecular tomography (FMT) imaging with Prosense 680 was first performed. Accumulation of the imaging probe in the lower abdomen correlated with the number of worms in mice with low infection burden. The total FDG uptake in the common portal vein and/or regions of elevated FDG uptake in the liver linearly correlated to the number of worms recovered from infected animals (R(2) =0.58, P<0.001, n = 40). FDG uptake showed a stronger correlation with the worm burden in mice with more than 50 worms (R(2) = 0.85, P<0.001, n = 17). Cryomicrotome imaging confirmed that most of the worms in a mouse with a high infection burden were in the portal vein, but not in a mouse with a low infection burden. FDG uptake in recovered worms measured by well counting closely correlated with worm number (R(2) = 0.85, P<0.001, n = 21). Infected mice showed a 32% average decrease in total FDG uptake after three days of praziquantel treatment (P = 0.12). The total FDG uptake in untreated mice increased on average by 36% over the same period (P = 0.052).

CONCLUSION

FDG PET may be useful to non-invasively quantify the worm burden in schistosomiasis-infected animals. Future investigations aiming at minimizing non-specific FDG uptake and to improve the recovery of signal from worms located in the lower abdomen will include the development of more specific radiotracers.

Schistosomiasis in the People's Republic of China: the era of the Three Gorges Dam

The potential impact of the Three Gorges Dam (TGD) on schistosomiasis transmission in China has invoked considerable global concern. The TGD will result in changes in the water level and silt deposition downstream, favoring the reproduction of Oncomelania snails. Combined with blockages of the Yangtze River's tributaries, these changes will increase the schistosomiasis transmission season within the marshlands along the middle and lower reaches of the Yangtze River. The changing schistosome transmission dynamics necessitate a comprehensive strategy to control schistosomiasis. This review discusses aspects of the epidemiology and transmission of Schistosoma japonicum in China and considers the pathology, clinical outcomes, diagnosis, treatment, immunobiology, and genetics of schistosomiasis japonica together with an overview of current progress in vaccine development, all of which will have an impact on future control efforts. The use of synchronous praziquantel (PZQ) chemotherapy for humans and domestic animals is only temporarily effective, as schistosome reinfection occurs rapidly. Drug delivery requires a substantial infrastructure to regularly cover all parts of an area of endemicity. This makes chemotherapy expensive and, as compliance is often low, a less than satisfactory control option. There is increasing disquiet about the possibility that PZQ-resistant schistosomes will develop. Consequently, as mathematical modeling predicts, vaccine strategies represent an essential component in the future control of schistosomiasis in China. With the inclusion of focal mollusciciding, improvements in sanitation, and health education into the control scenario, China's target of reducing the level of schistosome infection to less than 1% by 2015 may be achievable.

Exposed proteins of the Schistosoma japonicum tegument

The ability of the mammalian blood fluke Schistosoma japonicum to survive in the inhospitable environment of the mammalian bloodstream can be attributed, at least in part, to its host-exposed outer surface, called the tegument. The tegument is a dynamic organ and is involved in nutrition, immune evasion and modulation, excretion, osmoregulation and signal transduction. Given its importance for parasite survival, proteins exposed to the host at the surface of the tegument are ideal targets for the development of vaccines and drugs. By biotinylating live adult worms and using a combination of OFFGEL electrophoresis and tandem mass spectrometry 54 proteins were identified as putatively host-exposed in S. japonicum. These included glucose transport proteins, an amino permease, a leucine aminopeptidase and a range of transporters, heat shock proteins and novel immune-active proteins. Members of the tetraspanin protein family and a homologue of Sm 29, a tegument membrane protein from Schistosoma mansoni, both effective vaccine antigens in S. mansoni, were also identified. The fate of labelled surface proteins was monitored over time using electron microscopy and revealed that biotinylated proteins were rapidly internalised from the surface of the tegument and trafficked into the cytoplasmic bridges that connect the distal cytoplasm of the tegument to the underlying cell bodies. The results reported herein dramatically increase the number of S. japonicum proteins known to be exposed to the host and, hence, those of interest as therapeutic targets. The ability of the parasite to rapidly internalise proteins at its surface has implications for the development of vaccines and may explain how these parasites are able to avoid the host immune system for long periods of time.

Immune effector mechanisms against schistosomiasis: looking for a chink in the parasite's armour

A recombinant antigen vaccine against Schistosoma mansoni remains elusive, in part because the parasite deploys complex defensive and offensive strategies to combat immune attack. Nevertheless, research on rodent and primate models has shown that schistosomes can be defeated when appropriate responses are elicited. Acquired protection appears to involve protracted inhibition of larval migration or key molecular processes at the adult surfaces, not rapid cytolytic killing mechanisms. A successful vaccine will likely require a cocktail of antigens rather than a single recombinant protein. In addition, ways need to be found of keeping the immune system on permanent alert, either to achieve adequate inhibition of protein function in adults, or because a trickle of incoming parasites does not amplify the secondary response.