When and why direct transmission models can be used for environmentally persistent pathogens

Variants of the susceptible-infected-removed (SIR) model of Kermack & McKendrick (1927) enjoy wide application in epidemiology, offering simple yet powerful inferential and predictive tools in the study of diverse infectious diseases across human, animal and plant populations. Direct transmission models (DTM) are a subset of these that treat the processes of disease transmission as comprising a series of discrete instantaneous events. Infections transmitted indirectly by persistent environmental pathogens, however, are examples where a DTM description might fail and are perhaps better described by models that comprise explicit environmental transmission routes, so-called environmental transmission models (ETM). In this paper we discuss the stochastic susceptible-exposed-infected-removed (SEIR) DTM and susceptible-exposed-infected-removed-pathogen (SEIR-P) ETM and we show that the former is the timescale separation limit of the latter, with ETM host-disease dynamics increasingly resembling those of a DTM when the pathogen’s characteristic timescale is shortened, relative to that of the host population. Using graphical posterior predictive checks (GPPC), we investigate the validity of the SEIR model when fitted to simulated SEIR-P host infection and removal times. Such analyses demonstrate how, in many cases, the SEIR model is robust to departure from direct transmission. Finally, we present a case study of white spot disease (WSD) in penaeid shrimp with rates of environmental transmission and pathogen decay (SEIR-P model parameters) estimated using published results of experiments. Using SEIR and SEIR-P simulations of a hypothetical WSD outbreak management scenario, we demonstrate how relative shortening of the pathogen timescale comes about in practice. With atttempts to remove diseased shrimp from the population every 24h, we see SEIR and SEIR-P model outputs closely conincide. However, when removals are 6-hourly, the two models’ mean outputs diverge, with distinct predictions of outbreak size and duration.

ĐÁNH GIÁ IN VITRO KHẢ NĂNG KHÁNG VI-RÚT GÂY BỆNH ĐỐM TRẮNG CỦA CÁC LOẠI CAO CHIẾT TRÊN MÔ HÌNH TÔM THẺ CHÂN TRẮNG (Litopenaeus vannamei)

Vi-rút gây bệnh đốm trắng (WSSV) là tác nhân lây nhiễm và gây tử vong hàng loạt trên tôm nuôi nước lợ. Nghề nuôi tôm hiện nay thường sử dụng hóa chất và kháng sinh để điều trị và kiểm soát nhiều loại bệnh trong đó có bệnh đốm trắng, việc này gây nhiều trở ngại cho xuất khẩu và tiêu dùng. Có rất nhiều nghiên cứu đã cho thấy tác dụng của nhiều loại thảo dược giúp phòng trị bệnh đốm trắng trên tôm. Trong nghiên cứu này, 15 loại cao chiết thảo dược đã được đánh giá độ độc tính và khảo sát in vitro khả năng kháng WSSV trên mô hình tôm thẻ chân trắng bằng phương pháp tiêm. Kết quả khảo sát độc tính cho thấy, hầu hết các cao chiết là an toàn khi tiêm vào tôm ở nồng độ 0,25 mg/mL, trong đó 5 loại thể hiện độc tính cao, 4 loại thể hiện độc tính trung bình và 6 loại an toàn với tôm. Kết quả đánh giá in vitro cho thấy đưng, dà vôi, ổi, mấm trắng, cỏ mực, đước, cốc trắng và diệp hạ châu có hoạt tính kháng WSSV tốt nhất ở nồng độ khảo sát 0,0025 mg/mL. Những kết quả này tạo tiền đề cho các khảo sát in vivo khả năng kháng WSSV của 8 loại cao chiết thảo dược nhằm tìm ra các loại cây thảo dược tiềm năng trong phòng trị bệnh đốm trắng trên tôm. ABSTRACT White Spot Syndrome Virus (WSSV) is a highly infectious agent and causes mass mortality in farmed shrimp. In shrimp farming nowadays, farmers often use chemicals and antibiotics to treat and control many diseases, including white spot disease, thus resulting in many obstacles for shrimp export and consumption. Many studies have shown the effects of herbs on preventing white spot disease. In this study, 15 types of herbal extracts were evaluated for their toxicity and the in vitro resistance to WSSV in white leg shrimp injection models. The results of the toxicity showed that all the herbal extracts were safe when being injected into shrimp at a concentration of 0.25 mg/mL. Five of the herbs were highly toxic, four had medium toxicity, and six were safe for shrimp. The in vitro WSSV resistance testing showed that there were eight herbal extracts (Rhizophora mucronata, Ceriops tagal, Psidium guajava, Avicennia marina, Eclipta prostrata, Lumnitzera racemosa, Phyllanthus urinaria and Rhizophora apiculata) having the highest activity against WSSV at the concentration of 0.0025 mg/mL. These initial results suggest the in vivo investigations on the resistance to WSSV of these eight herbal extracts to find potential herbal plants for aquaculture use, instead of antibiotics, in the prevention and treatment of white spot disease in shrimp.

Non-specific immune response of Pacific white shrimp Litopenaeus vannamei by supplementation of sodium alginate of Sargassum collected from Lampung Indonesia

This study aims to determine the effectiveness of supplementation of sodium (Na) alginate Sargassum sp. from Lampung waters to enhance the non-specific immune response and the resistence of Pacific white shrimp (Litopenaeus vannamei) against white spot disease. This study used a completely randomized design with 3 treatments, namely feeding without alginate supplementation (A) or control, dietary Na alginate Sargassum supplementation at a dose of 2.0 (B), and 4.0 g kg−1 feed (C), each with four replications, for 14 days. Hemolymph sampling was performed on days 0, 7 and 14 to observe the parameter of shrimp hematology included total haemocyte count (THC), phagocytocyte activity (PA), phagocytic index (PI), and total plasma protein (TPP). The hepatopancreas histology profile and water quality were observed at the end of treatment. Data were analyzed by Anova at 95% of confidence interval and continued with Duncan test. Result showedthat the supplementation of Na alginat from Sargassum sp. significantly enhance several shrimp immune response namely THC, PA, and TPP. Meanwhile, the histology of the hepatopancreas of both shrimp treatment and control showed not significantly effect on tissue damage. The water quality was still in normal condition during the treatment. The results of this study indicate that the application of Sargassum alginate supplementation at a dose of 2gr/kg of feed is the best treatment to enhance the immune response of Pacific white shrimp.

Genotype Diversity and Spread of White Spot Syndrome Virus (WSSV) in Madagascar (2012–2016)

White Spot Disease (WSD) caused by the White Spot Syndrome Virus (WSSV) is the most devastating viral disease threatening the shrimp culture industry worldwide, including Madagascar. WDS was first reported on the island in 2012; however, little is known about the circulation of the virus and its genetic diversity. Our study aimed at describing the molecular diversity and the spread of WSSV in the populations of Madagascan crustaceans. Farmed and wild shrimps were collected from various locations in Madagascar from 2012 to 2016 and were tested for WSSV. Amplicons from positive specimens targeting five molecular markers (ORF75, ORF94, ORF125, VR14/15 and VR23/24) were sequenced for genotyping characterizations. Four genotypes were found in Madagascar. The type-I genotype was observed in the south-west of Madagascar in April 2012, causing a disastrous epidemic, then spread to the North-West coast. Type-II strains were detected in October 2012 causing an outbreak in another Penaeus monodon farm. In 2014 and 2015, types II and III were observed in shrimp farms. Finally, in 2016, types II and IV were found in wild species including Fenneropenaeus indicus, Metapenaeus monoceros, Marsupenaeus japonicus and Macrobrachium rosenbergii. Considering the economic importance of the shrimp industry for Madagascar, our study highlights the need to maintain WSSV surveillance to quickly take appropriate countermeasures in case of outbreak and to sustain this industry.

Global mRNA and miRNA Analysis Reveal Key Processes in the Initial Response to Infection with WSSV in the Pacific Whiteleg Shrimp

White Spot Disease (WSD) presents a major barrier to penaeid shrimp production. Mechanisms underlying White Spot Syndrome Virus (WSSV) susceptibility in penaeids are poorly understood due to limited information related to early infection. We investigated mRNA and miRNA transcription in Penaeus vannamei over 36 h following infection. Over this time course, 6192 transcripts and 27 miRNAs were differentially expressed—with limited differential expression from 3–12 h post injection (hpi) and a more significant transcriptional response associated with the onset of disease symptoms (24 hpi). During early infection, regulated processes included cytoskeletal remodelling and alterations in phagocytic activity that may assist WSSV entry and translocation, novel miRNA-induced metabolic shifts, and the downregulation of ATP-dependent proton transporter subunits that may impair cellular recycling. During later infection, uncoupling of the electron transport chain may drive cellular dysfunction and lead to high mortalities in infected penaeids. We propose that post-transcriptional silencing of the immune priming gene Dscam (downregulated following infections) by a novel shrimp miRNA (Pva-pmiR-78; upregulated) as a potential mechanism preventing future recognition of WSSV that may be suppressed in surviving shrimp. Our findings improve our understanding of WSD pathogenesis in P. vannamei and provide potential avenues for future development of prophylactics and treatments.

In Silico Prediction of Molecular Interaction Within PmCBP-VP24 Complex to Understand Initial Instigation of WSSV into Shrimps

White Spot Disease is one of the most devastating diseases of shrimps. Molecular interaction between shrimp receptor protein PmCBP (Chitin binding protein of Peneaus monodon) and viral envelop protein VP24 is obligatory for binding of the White Spot Syndrome Virus to the shrimp digestive tract, and failure of this anchoring leads to an ineffectual infection. This is a first study that throws light on the molecular interaction of PmCBP-VP24 complex and provides important clues for initial steps of ingression of the virus into shrimps.