Development and validation of a TaqMan-MGB real-time RT-PCR assay for simultaneous detection and characterization of infectious bursal disease virus

Advancing IBDV diagnostics: a one-step multiplex real-time qRT-PCR for discriminating between vvIBDV and non-vvIBDV viruses, including the newly emerged IBDV variant

The very virulent infectious bursal disease virus (vvIBDV) induces an acute, highly contagious and immunosuppressive disease in younger chicken causing massive economic losses globally. A major challenge in the field's clinical diagnosis is distinguishing gross lesions caused by vvIBDV from those induced by classic IBDV (cIBDV), commonly used as live attenuated vaccines. This study introduces a one-step multiplex real-time PCR assay designed to distinguish between vvIBDV and non-vvIBDV viruses. Via simultaneously targeting the VP2 sequence for vvIBDV detection and the VP1 sequence for non-vvIBDV identification, including classic, American variant and the recently emerged novel variant IBDV (nvarIBDV), the assay's specificity was validated against common avian viral diseases and nonspecific IBDV strains without any observed cross-reactions. It effectively differentiated between vvIBDV and non-vvIBDV field samples, including nvarIBDV, as confirmed by genotyping based on VP2 sequencing. The assay demonstrated a limit of detection ranging from 1.9×1010 to 103 DNA copies for vvIBDV-VP2, 9.2×1010 to 103 DNA copies for classic strains, and 1.2×1011 to 104 DNA copies for nvarIBDV in VP1 detection of non-vvIBDV. In conclusion, this study presents a specific, sensitive, and straight forward multiplex real-time PCR assay.

Genetic Sequence and Pathogenicity of Infectious Bursal Disease Virus in Chickens in Egypt During 2017-2021

The continued circulation of infectious bursal disease virus (IBDV) in Egypt, despite the use of various vaccines, is a serious problem that requires continuous detection of IBDV. In the current study, real-time reverse transcriptase polymerase chain reaction testing of 100 diseased chicken flocks during 2017-2021 revealed the presence of very virulent IBDV (vvIBDV) in 67% of the flocks, non-vvIBDV in 11%, and a mixture of both vvIBDV and non-vvIBDV in 4%. Twenty-nine IBDV isolates were submitted for partial sequencing of the viral protein 2 hypervariable region (VP2-HVR), and 27 isolates were confirmed to be genogroup A3 (vvIBDV) with 96.3%-98.5% similarity to the global A3 (vvIBDV) and 88.9%-97% similarity to genogroup A1 vaccine strains. The remaining two isolates were non-vvIBDV and showed 91.1% and 100% identity with classical genogroup A1 strains, respectively. Furthermore, the sequence and phylogenetic analysis of VP1 (amino acids 33-254) of two selected isolates of A3, 5/2017 and 98/2021, clustered them as B2, vvIBDV-like, strains with high similarity (99.5%) to four Egyptian, 99% to Chinese and European, and 97.7% to Chinese and Polish vvIBDV isolates. Experimental infection of commercial broiler chickens with two vvIBDV-A3B2 isolates (5/2017 and 98/2021) showed no mortality despite typical tissue lesions, clear histopathological changes, and strong ELISA antibody response. Isolate 98/2021 was more pathogenic, as confirmed by histopathology, whereas isolate 5/2017 induced a stronger serological response. In conclusion, vvIBDV (A3B2) strains with two amino acid (aa) substitutions in VP1 as V141I and V234I as well as VP2 as Y220F and G254S are still circulating in Egypt.

Isolation, Molecular, and Histopathological Patterns of a Novel Variant of Infectious Bursal Disease Virus in Chicken Flocks in Egypt

After an extended period of detecting classical virulent, attenuated, and very virulent IBDV, a novel variant (nVarIBDV) was confirmed in Egypt in this study in 18, IBD vaccinated, chicken flocks aged 19-49 days. Partial sequence of viral protein 2 (VP2) [219 aa, 147-366, resembling 657 bp] of two obtained isolates (nos. 3 and 4) revealed nVarIBDV (genotype A2d) and OR682618 and OR682619 GenBank accession numbers were obtained. Phylogenetic analysis revealed that both nVarIBDV isolates were closely related to nVarIBDV strains (A2d) circulating in China, exhibiting 100% identity to SD-2020 and 99.5-98.1% similarity to ZD-2018-1, QZ, GX and SG19 strains, respectively. Similarity to USA variant strains, belonging to genotypes A2b (9109), A2c (GLS) and A2a (variant E), respectively, was 95.5-92.6%. Also, the VP2 hypervariable region in those two, A2d, isolates revealed greater similarities to Faragher 52/70 (Vaxxitek®) at 90.4% and to an Indian strain (Ventri-Plus®) and V217 (Xtreme®) at 89.7% and 86-88.9% in other vaccines. Histopathological examination of both the bursa of Fabricius and spleen collected from diseased chickens in flock no. 18 revealed severe atrophy. In conclusion, further studies are required to investigate the epidemiological situation of this novel genotype across the country, and to assess various vaccine protections against nVarIBDV. Additionally, vaccination of breeders with inactivated IBD vaccines including this nVarIBDV is essential to obtain specific maternal antibodies in their broilers.

Development of a One-Step Real-Time TaqMan Reverse Transcription Polymerase Chain Reaction (RT-PCR) Assay for the Detection of the Novel Variant Infectious Bursal Disease Virus (nVarIBDV) Circulating in China

The novel variant IBDV (nVarIBDV, especially genotype A2dB1) mainly affects broilers in China. It causes an infection characterized by the atrophy of the bursa, a decrease in the level of lymphocytes, proliferation of fibrous tissue around the follicle, and severe atrophy of the follicle in the bursa. Poultry vaccinated with live IBDV vaccines do not have the challenge present with bursa atrophy, which is misdiagnosed for nVarIBDV because of the lack of other gross clinical symptoms. The present study sought to explore the potential and reliability of the real-time TaqMan analysis method for the detection and discrimination of the nVarIBDV genotype from that of the non-nVarIBDV, especially in live vaccine strains. This method will help monitor vaccinated poultry to control and manage infection with the nVarIBDV IBDVs. The nucleotide polymorphism in the 5'-UTR region and the vp5/vp2 overlapping region of the segment A sequences of IBDV were used to establish a one-step real-time TaqMan reverse transcription polymerase chain reaction (RT-PCR) method in this study. The results showed that the method accurately distinguished the nVarIBDV and non-nVarIBDV strains (especially live vaccine strains), and there were no cross-reactions with the infectious bronchitis virus (IBV), Newcastle disease virus (NDV), avian influenza virus (AIV), infectious laryngotracheitis virus (ILTV), fowlpox virus (FPV), Mycoplasma gallisepticum (M. gallisepticum), Mycoplasma synoviae (M. synoviae), and IBDV-negative field samples. The method showed a linear dynamic range between 10² and 10⁷ DNA copies/reaction, with an average R² of 0.99 and an efficiency of 93% for nVarIBDV and an average R² of 1.00 and an efficiency of 94% for non-nVarIBDV. The method was also used for the detection of 84 clinical bursae of chickens vaccinated with the live vaccine. The results showed that this method accurately distinguished the nVarIBDV and non-nVarIBDV strains (vaccine strains), compared with a strategy based on the sequence analysis of HVRs at the vp2 gene or the reverse transcription PCR (RT-PCR) for the vp5 gene. These findings showed that this one-step real-time TaqMan RT-PCR method provides a rapid, sensitive, specific, and simple approach for detection of infections caused by nVarIBDV and is a useful clinical diagnostic tool for identifying and distinguishing nVarIBDV from non-nVarIBDV, especially live vaccine strains.

Establishment of a TaqMan-MGB probe multiplex real-time PCR system for one-step levofloxacin and clarithromycin resistant Helicobacter pylori detection

Due to the abuse of antibiotics, the prevalence of antibiotic resistant Helicobacter pylori strains continues to increase. Therefore, antibiotic resistance assessment is now essential in addition to general H. pylori diagnosis in medical institutions to fulfill clinicians administering effective antibiotic regimens. However, the conventional antibiotic resistance assessment methods, such as in vitro antibiotic susceptibility test and E-test, are skilled-staff dependent and time-consuming. The aim of this study was to establish an easy-operating TaqMan-MGB probe multiplex real-time PCR system for one-step detection of levofloxacin and clarithromycin resistance mutations with concurrent H. pylori infection diagnosis. Through the optimization of primers, probes and reaction buffers, this proposed system could accurately distinguish the recombinant plasmids with different mutation markers. More importantly, the diagnosis results of this detection system exhibited excellent consistence with the gold standard of gastric biopsy and Sanger sequencing on the detection of H. pylori infection and relevant antibiotic resistant strains, the Kappa values of which all exceeded 0.90. In addition, the results of this detection system could also be applied for the prevalence statistics of antibiotic resistance patterns for patients by age, gender and geographical location. This simple and rapid system should be beneficial for clinicians issuing personalized treatments according to the patient's H. pylori strains and avoid the abuse of antibiotics.

Rapid differential detection of subtype H1 and H3 swine influenza viruses using a TaqMan-MGB-based duplex one-step real-time RT-PCR assay

Swine influenza is an economically important respiratory disease in swine, but it also constantly poses a threat to human health. Therefore, developing rapid, sensitive, and efficient detection methods for swine influenza virus (SIV) is important. By aligning the haemagglutinin (HA) gene sequences of SIVs circulating in China over a 10-year period, an H1 primer-probe set targeting both Eurasian avian-like H1N1 (EA H1N1) and pandemic 2009 H1N1 ((H1N1)pdm09) lineages plus a H3 primer-probe set targeting the prevalent human-like H3N2 (HL H3N2) subtype were designed. Subsequently, a TaqMan-MGB-based duplex one-step real-time RT-PCR (RT-qPCR) assay was established and evaluated. The duplex RT-qPCR has a detection limit of 5 copies/μL of HA plasmid for EA H1N1, (H1N1)pdm09, and HL H3N2 subtype SIVs, and its overall detection sensitivity of 100% and specificity of 91.67% matches that of traditional virus isolation through chicken embryo inoculation using experimentally infected mouse lung samples. The method showed high repeatability both within run and between runs, and there was no cross-reactivity against several other porcine viruses that are commonly circulating in China. Furthermore, the duplex RT-qPCR method revealed a higher prevalence of subtype H1 than subtype H3 in 166 nasal swabs from pigs collected from one slaughterhouse between October and December 2019. This assay could be very helpful in the rapid differential detection and routine surveillance of EA H1N1, (H1N1)pdm09, and HL H3N2 SIVs in China.

Detecting Infectious Bursal Disease Using a VP1 Gene-Based RT-qPCR Assay Compared to Standard Methods of Virus Isolation, ELISA, and Histopathology

Infectious bursal disease (IBD) is an immunosuppressive viral disease of chickens, associated with severe economic losses and major threats to poultry production worldwide. Disease prevention programs rely on unequivocal identification of the pathogen, as well as vaccination programs. This study developed a sensitive, one-step, real-time, quantitative reverse transcription polymerase chain reaction (RT-qPCR) assay using a hydrolysis probe system for infectious bursal disease virus (IBDV, VP1 gene) detection and quantification, which was compared to other routinely used diagnostic methods. The assay successfully detected IBD reference viruses and field isolates. The absence of cross-reactivity was detected with negative samples or with other avian viruses in the analytical specificity test. The detection limit of this assay was 70 RNA copies. RT-qPCR was more sensitive in the detection of serially diluted IBDV isolates compared to virus isolation. For clinical samples, the sensitivity and specificity values of RT-qPCR compared to enzyme-linked immunosorbent assay (ELISA) were 97.5% and 100%, respectively, and compared to histopathology, these values were 100% and 93.94%, respectively. RT-qPCR can provide a simple and reliable assay for IBDV surveillance programs and for evaluation of control strategies.

Origin and global spreading of an ancestral lineage of the infectious bursal disease virus

Infectious bursal disease virus (IBDV) is an economically relevant and widespread pathogen that produces immunosuppression in young chickens. IBDV is genetically classified into seven genogroups (G1-G7), where the traditional classic, variant and very virulent strains correspond to G1, G2 and G3, respectively. The G4 strains, also known as 'distinct' (dIBDV), have recently acquired increased relevance because of their prevalence and notorious impair to the poultry industry in South America. Here, worldwide dIBDV strains were studied using phylogenetic and phylodynamic approaches. The phylogenetic analyses performed using partial and complete sequences of both viral segments (A and B) consistently clustered the dIBDV strains in a monophyletic group. The analyses of the VP5, polyprotein and VP1 coding regions identified amino acid residues that act as markers for the identification of the entire dIBDV group or different sub-populations. The phylodynamic analyses performed using the hypervariable region of VP2 indicated that the dIBDV strains emerged in the early 1930s in Eastern Europe, shortly after the emergence of classic strains (1927) and before variant (1949) and very virulent strains (1967). The analysis of the migration routes indicated that after its emergence, the dIBDV strains spread to Eastern Asia around 1959, to Brazil around 1963, and to Argentina around 1990. These inter-continental migrations resulted in three sub-populations that are currently represented by strains from (a) Brazil, (b) Eastern Asia and Canada, and (c) Eastern Europe, Argentina and Uruguay. Taken together, our results highlight the complex evolutionary history of IBDV and the importance of new phylodynamic data to unravel and nearly follow the different evolutionary pathways taken by this important poultry pathogen.

Development of real-time PCR assays for single and simultaneous detection of infectious bursal disease virus and chicken anemia virus

Infectious bursal disease virus (IBDV) and chicken anemia virus (CAV) cause relevant immunosuppressive diseases in poultry. Clinical diagnosis of these viruses is challenging given the different disease presentations and the frequent occurrence of co-infections with other pathogens. Here, we standardized and validated simplex and duplex RT-qPCR assays for the straightforward detection of IBDV and CAV. The qPCR assays are based on primers and hydrolysis probes that target highly conserved regions of IBDV and CAV genomes. Analytical sensitivity tests on 10-fold serial dilutions containing 10⁰-10⁸ viral genomes indicated that the simplex assays have good determination coefficients and efficiency and detect a wide range of virus doses (10² to 10⁸ molecules copies/reactions). The relatively small values of intra- and inter-assay variability ensure the repeatability and support its reproducibility in different diagnostic and research facilities. The assays are also efficient tools for absolute quantification as indicated by the analytical performance analysis. The assays have an excellent specificity and absence of cross-reactivity with negative samples, or with other common avian viruses. The simplex IBDV and CAV assays use probes labelled with different dyes (FAM and HEX) and can be multiplexed for the simultaneous detection of both viruses. The determination coefficients, PCR efficiencies, and relatively small intra- and inter-assay variability were comparable to the simplex assays. This duplex assay is the first to simultaneously detect IBDV and CAV using the same RNA extraction from the bursa of Fabricius in a single and straightforward step. Therefore, this method is time saving, provides quantitative results for both targets without any cross-reaction, and reduces the risk of carrying-over contaminations. The qPCR assays here developed can be used in simplex and duplex formats for detection and quantification of large number of samples with reliable sensitivity and specificity. These tools are expected to improve surveillance and control of these ubiquitous viruses.

Molecular characterization and phylogenetic analysis of very virulent infectious bursal disease virus circulating in Morocco during 2016-2017

Very virulent infectious bursal disease virus (vvIBDV), the cause of significant economic losses in many poultry-producing areas, has been present in Morocco since 1991. In spite of the introduction of vaccination, disease outbreaks are frequently observed. To ascertain if vaccines failure may be due to the emergence of new strains, the aim of this study was to perform for the first time the molecular characterization of vvIBDV strains circulating in Morocco by focusing on the hypervariable region (HVR) of the VP2 protein, which is frequently used for molecular epidemiology and phylogenetic studies. Field samples of haemorrhagic bursae of Fabricius were collected for molecular characterization in different parts of the country during 2016-2017 from 48 chicken flocks showing symptoms of disease. In a phylogenetic tree, nucleotide sequences containing the VP2 HVR of 13 samples that were positive for vvIBDV formed a common branch with those of vvIBDV references strains published in GenBank, but they clearly grouped into a distinct subcluster. An alignment of the deduced amino acid sequences, in addition to confirming the presence of the "signature" typical of the vvIBDV HVR, also revealed the presence of substitutions in hydrophilic loops that are known to be involved in the elicitation of neutralizing antibodies. One of these substitutions is unique to the Moroccan isolates. These results represent the first molecular characterization of vvIBDV isolates in Morocco and may indicate that one of the causes of vaccine ineffectiveness is antigenic drift.

Multiplex one-step Real-time PCR by Taqman-MGB method for rapid detection of pan and H5 subtype avian influenza viruses

Avian influenza virus (AIV) can infect a variety of avian species and mammals, leading to severe economic losses in poultry industry and posing a substantial threat to public health. Currently, traditional virus isolation and identification is inadequate for the early diagnosis because of its labor-intensive and time-consuming features. Real-time RT-PCR (RRT-PCR) is an ideal method for the detection of AIV since it is highly specific, sensitive and rapid. In addition, as the new quencher MGB is used in RRT-PCR, it only needs shorter probe and helps the binding of target gene and probe. In this study, a pan-AIV RRT-PCR for the detection of all AIVs and H5-AIV RRT-PCR for detection of H5 AIV based on NP gene of AIV and HA gene of H5 AIV were successfully established using Taqman-MGB method. We tested 14 AIV strains in total and the results showed that the pan-AIV RRT-PCR can detect AIV of various HA subtypes and the H5-AIV RRT-PCR can detect H5 AIV circulating in poultry in China in recent three years, including H5 viruses of clade 7.2, clade 2.3.4.4 and clade 2.3.2.1. Furthermore, the multiplex detection limit for pan-AIV and H5-AIV RRT-PCR was 5 copies per reaction. When this multiplex method was applied in the detection of experimental and live poultry market samples, the detection rates of pan-AIV and H5 AIV in RRT-PCR were both higher than the routine virus isolation method with embryonated chicken eggs. The multiplex RRT-PCR method established in our study showed high sensitivity, reproducibility and specificity, suggesting the promising application of our method for surveillance of both pan AIV and prevalent H5 AIV in live poultry markets and clinical samples.

Development of an RT-qPCR assay for the specific detection of a distinct genetic lineage of the infectious bursal disease virus

The infectious bursal disease virus (IBDV) is a major health threat to the world's poultry industry despite intensive controls including proper biosafety practices and vaccination. IBDV (Avibirnavirus, Birnaviridae) is a non-enveloped virus with a bisegmented double-stranded RNA genome. The virus is traditionally classified into classic, variant and very virulent strains, each with different epidemiological relevance and clinical implications. Recently, a novel worldwide spread genetic lineage was described and denoted as distinct (d) IBDV. Here, we report the development and validation of a reverse transcription-quantitative polymerase chain reaction (RT-qPCR) assay for the specific detection of dIBDVs in the global poultry industry. The assay employs a TaqMan-MGB probe that hybridizes with a unique molecular signature of dIBDV. The assay successfully detected all the assessed strains belonging to the dIBDV genetic lineage, showing high specificity and absence of cross-reactivity with non-dIBDVs, IBDV-negative samples and other common avian viruses. Using serial dilutions of in vitro-transcribed RNA we obtained acceptable PCR efficiencies and determination coefficients, and relatively small intra- and inter-assay variability. The assay demonstrated a wide dynamic range between 10³ and 10⁸ RNA copies/reaction. This rapid, specific and quantitative assay is expected to improve IBDV surveillance and control worldwide and to increase our understanding of the molecular epidemiology of this economically detrimental poultry pathogen.

First outbreaks and phylogenetic analyses of avian influenza H9N2 viruses isolated from poultry flocks in Morocco

Background

H9N2 avian influenza viruses continue to spread in poultry and wild birds worldwide. Morocco just faced its first H9N2 influenza virus outbreaks early 2016 affecting different types of poultry production. After its introduction, the virus spread very rapidly throughout the country.

Methods

Samples were collected from 11 chicken flocks with high morbidity and mortality rates. Four viruses were successfully isolated from broiler chickens and one from broiler breeders and fully sequenced.

Results

Phylogenetic and molecular markers analyses showed the Moroccan viruses belonged to the G1 lineage and likely originated from the Middle East. As known for H9N2 viruses, the Moroccanisolates possess several genetic markers that enhance virulence in poultry and transmission to humans.

Conclusion

The present study demonstrated that under field conditions H9N2 could have a devastating effect on egg production and mortalities and highlighted a lack of surveillance data on the pathogen in the region.

Development of RT-qPCR assays for the specific identification of two major genotypes of avian infectious bronchitis virus

Infectious bronchitis virus (Gammacoronavirus, Coronaviridae) is a genetically variable RNA virus (27.6kb) that causes one of the most persistent respiratory disease in poultry. The virus is classified in genotypes with different epidemiological relevance and clinical implications. The present study reports the development and validation of specific RT-qPCR assays for the detection of two major IBV genotypes: South America I (SAI) and Asia/South America II (A/SAII). The SAI genotype is an exclusive and widespread South American lineage while the A/SAII genotype is distributed in Asia, Europe and South America. Both identification assays employ TaqMan probes that hybridize with unique sequences in the spike glycoprotein gene. The assays successfully detected all the assessed strains belonging to both genotypes, showing high specificity and absence of cross-reactivity. Using serial dilutions of in vitro-transcribed RNA we obtained acceptable determination coefficients, PCR efficiencies and relatively small intra- and inter-assay variability. The assays demonstrated a wide dynamic range between 10(1)-10(7) and 10(2)-10(7) RNA copies/reaction for SAI and A/SAII strains, respectively. The possibility to characterize a large number of samples in a rapid, sensitive and reproducible way makes these techniques suitable tools for routine testing, IBV control, and epidemiological research in poultry.

Genome Sequence of a Distinct Infectious Bursal Disease Virus

Infectious bursal disease virus is a relevant avian pathogen that affects poultry production. Here, we report the full-length coding sequence of the Uruguayan strain dIBDV/UY/2014/2202, isolated from a commercial broiler flock. The strain belongs to the distinct IBDV lineage that is widely distributed in South America.

Genetic characterization of South American infectious bursal disease virus reveals the existence of a distinct worldwide-spread genetic lineage

Infectious bursal disease virus (IBDV) is one of the most concerning health problems for world poultry production. IBDVs comprise four well-defined evolutionary lineages known as classic (c), classic attenuated (ca), variant (va) and very virulent (vv) strains. Here, we characterized IBDVs from South America by the genetic analysis of both segments of the viral genome. Viruses belonging to c, ca and vv strains were unambiguously classified by the presence of molecular markers and phylogenetic analysis of the hypervariable region of the vp2 gene. Notably, the majority of the characterized viruses (9 out of 15) could not be accurately assigned to any of the previously described strains and were then denoted as distinct (d) IBDVs. These dIBDVs constitute an independent evolutionary lineage that also comprises field IBDVs from America, Europe and Asia. The hypervariable VP2 sequence of dIBDVs has a unique and conserved molecular signature (272T, 289P, 290I and 296F) that is a diagnostic character for classification. A discriminant analysis of principal components (DAPC) also identified the dIBDVs as a cluster of genetically related viruses separated from the typical strains. DAPC and genetic distance estimation indicated that the dIBDVs are one of the most genetically divergent IBDV lineages. The vp1 gene of the dIBDVs has non-vvIBDV markers and unique nucleotide and amino acid features that support their divergence in both genomic segments. The present study suggests that the dIBDVs comprise a neglected, highly divergent lineage that has been circulating in world poultry production since the early time of IBDV emergence.

Real-time PCR detection chemistry

Real-time PCR is the method of choice in many laboratories for diagnostic and food applications. This technology merges the polymerase chain reaction chemistry with the use of fluorescent reporter molecules in order to monitor the production of amplification products during each cycle of the PCR reaction. Thus, the combination of excellent sensitivity and specificity, reproducible data, low contamination risk and reduced hand-on time, which make it a post-PCR analysis unnecessary, has made real-time PCR technology an appealing alternative to conventional PCR. The present paper attempts to provide a rigorous overview of fluorescent-based methods for nucleic acid analysis in real-time PCR described in the literature so far. Herein, different real-time PCR chemistries have been classified into two main groups; the first group comprises double-stranded DNA intercalating molecules, such as SYBR Green I and EvaGreen, whereas the second includes fluorophore-labeled oligonucleotides. The latter, in turn, has been divided into three subgroups according to the type of fluorescent molecules used in the PCR reaction: (i) primer-probes (Scorpions, Amplifluor, LUX, Cyclicons, Angler); (ii) probes; hydrolysis (TaqMan, MGB-TaqMan, Snake assay) and hybridization (Hybprobe or FRET, Molecular Beacons, HyBeacon, MGB-Pleiades, MGB-Eclipse, ResonSense, Yin-Yang or displacing); and (iii) analogues of nucleic acids (PNA, LNA, ZNA, non-natural bases: Plexor primer, Tiny-Molecular Beacon). In addition, structures, mechanisms of action, advantages and applications of such real-time PCR probes and analogues are depicted in this review.