Shrimp disease is a major concern for the farmers. It not only causes crop loss but also cripples their economy.
Aquaculture, especially shrimp farming, has witnessed tremendous growth in the last decade. This has been achieved through expansion, intensification and diversification. Major transformation in shrimp production has been witnessed in India, after switching over from native species, Penaeus monodon to the non-native species, white leg shrimp, Penaeus vannamei which was introduced during 2009-10.
Ever since the introduction of P. vannamei, the area under farming has increased remarkably. However, as happened in the case of P. monodon, P. vannamei farming also has been experiencing large-scale production losses due to a variety of diseases.
These diseases have been either due to infectious aetiology, poor management measures, spurious seed, water pollution. Diseases are the outcome of imbalance between environment, host and pathogen. The impact of disease is a combination of several factors such as mass mortality of shrimp, reduction in feed acceptance or morbidity. Morbidity and production losses due to multiple disease conditions of obscure aetiology have been a common phenomenon and running mortality syndrome (RMS), slow growth / retarded growth caused by multiple causes, white gut and white faeces syndrome and vibriosis etc. have been of common occurrence. Although shrimp farming in the neighbouring Asian countries and Latin America have been severely affected by Acute Hepato Pancreatic Necrosis Disease (AHPND), popularly known as Early Mortality Syndrome (EMS), Indian shrimp farming system has not been affected by this serious problem.
Concomitant with the expansion and intensification of shrimp aquaculture, large number of diseases, especially those caused by viral infection, have been recorded in farmed shrimp. Disease such as white spot syndrome virus (WSSV) has devastated the shrimp aquaculture world over due to its high virulence, wide host and geographical range. Some of the other serious viral diseases include Taura Syndrome Virus (TSV), Yellow Head Virus (YHV), infectious hypodermal and haematopoietic necrosis virus (IHHNV), infectious myonecrosis virus (IMNV) etc.
In addition, a number of pathogens or disease conditions have emerged in farmed shrimp during the past several years and caused significant production loss to the industry. Further, some of the existing / endemic diseases have shown increased virulence and a few of them also showed distribution in new host species and geographical areas. Some of the reasons attributed to these phenomena include high intensification of culture practices, introduction of non-native species for farming, unregulated transboundary movement of aquatic animals and products together with changing climate and the increased stress imposed on the farmed animals. Although many pathogens results in heavy mortality in farmed shrimp, some of the emerging disease conditions which are reported recently resulted in severe retarded growth or other chronic conditions. Nevertheless, these chronic conditions are often resulted in heavy production losses and significant economic impact in shrimp farming.
Globally, disease emergence is recognized as an important constraint to the expansion of aquaculture. An “emerging disease” is defined as a new disease, a new presentation of a known disease or an existing disease that appears in a new geographical area (Brown, 2000). According to Walker and Winton (2010), emerging disease can be categorized into four: ‘i) new or previously unknown diseases; ii) known diseases appearing for the first time in a new species (expanding host range); iii) known diseases appearing for the first time in a new location (expanding geographical range); and iv) known diseases with a new presentation (different signs) or higher virulence due to changes in the causative agent’. Further, Walker and Winton reported that emerging new diseases are challenging problems because of the following reasons: ‘a) difficulty or delay in developing confirmatory diagnostic tools; b) poor knowledge on the host susceptibility and host-range; c) lack of knowledge on the epidemiological factors such as mode of transmission, reservoirs and carrier hosts etc’.
Followings are some of the diseases which can be categorized under emerging diseases in farmed shrimp:
1. Hepatopancreatic microsporidiosis (HPM):
Hepatopancreatic microsporidiosis, caused by the microsporidian parasite, Enterocytozoon hepatopenaei (EHP), is an emerging pathogen of cultured penaeid shrimp. Incidence of EHP has been reported from cultured shrimps of many Southeast Asian Countries including Vietnam, Thailand, Malaysia, Indonesia and China. Recently, the pathogen has been recorded in the farmed shrimp in India (Rajendran et al., 2016). EHP is reported to be associated with growth retardation in farmed shrimp.
It was first discovered from Penaeus monodon in Thailand in 2004 (Chayaburakul, et al., 2004), and reported as an undersigned microsporidian. Subsequently, the parasite was identified and characterized from P. monodon from Thailand in 2009. It infects only the epithelial cells of the hepatopancreatic (HP) tubules of shrimp. The EHP infection affects the digestive and absorptive functioning of the hepatopancreas leading to poor growth and immunity (Fig. 1). Although there are reports indicating that EHP would cause growth retardation in farmed shrimp, a statistical correlation between the parasite and the clinical signs has not been proved so far. However, the parasite will not cause any mortality or obvious clinical signs and symptom.
Nevertheless, the micropsoridian had been identified as one the pathogenic agents associated with white faces syndrome (WFS) in cultured P. monodon in Vietnam. Similar indications have also been reported from Thailand.
2. Early Mortality Syndrome (EMS) / Acute Hepatopancreatic Necrosis Syndrome or Disease (AHPNS or AHPND):
Recently, a new emerging disease known as Early Mortality Syndrome (EMS) or Acute Hepatopancreatic Necrosis Disease (AHPND), has been a major issue of concern for economic loss in the shrimp farming industry in many Asian countries and Latin America. Incidence of AHPND has been reported from China in 2010 and subsequently in Vietnam, Thailand, Malaysia and Mexico (Tran et al., 2013; Soto – Rodriguex et al., 2015). However, there are no confirmed reports of EMS from India.
It affects both Penaeus monodon and P. vannamei (P. chinensis was also reported to be affected in China). The EMS / AHPND disease typically affect shrimp post larvae within 20-30 days after stocking and frequently causes up to 100% mortality. Recently, the aetiology of AHPND has been identified as a unique strain of V. parahaemolyticus that is infected by a phage, which causes it to release a potent toxin (Lightner 2014). Gross clinical signs of the disease include lethargy, slow growth, corkscrew swimming, loose shells and pale colouration. Other signs include an empty stomach and midgut, and a pale to white atrophied hepatopancreas, and ultimately dead animals can be seen at the bottom of the pond. The most distinctive histological feature of AHPND is extensive rounding and sloughing of hepatopancreatic tubule epithelial cells in the absence of any detected causative pathogen (Tran et al., 2013).
3. Infectious myonecrosis (IMN):
Outbreaks of infectious myonecrosis (IMN) due to infectious myonecrosis virus (IMNV) were first reported from Brazil in whiteleg shrimp, Penaeus vannamei, in 2002. Infectious myonecrosis disease is characterized by extensive necrosis of skeletal muscle, especially in the distal abdominal segments and tail fin. This disease resulted in cumulative mortality rate of up to 70% throughout the production cycle. The species of shrimp P. vannamei, P. stylirostris and P. monodon are susceptible to IMNV and among these species P. vannamei is the most susceptible.
IMNV has been reported to be transmitted from shrimp to shrimp by cannibalism. Transmission via water and vertical transmission from broodstock to progeny probably occurs. The causative agent has been identified as an RNA virus known as infectious myonecrosis virus (IMNV), belonging to the Totiviridae family. Severely infected shrimp become lethargic during or after stressful events such as capturing using cast-netting, feeding, sudden changes in temperature and drop in salinity. However, shrimp will have full gut. In acute infection, animals show extensive whitish necrosis in skeletal muscles, especially in the distal abdominal segments and tail (Fig. 2).
Lymphoid organ shows excessive hypertrophy. Necrotic areas may appear reddened in some animals. Severe infection results in morbidity and high mortality which continues for several days. The disease has not been reported from India.
4. Muscle necrosis of Penaeus vannamei caused by Penaeus vannamei nodavirus (PvNV):
Viral muscle necrosis is an emerging disease of Penaeus (Litopenaeus) vannamei caused by Penaeus vannamei nodavirus (PvNV). The disease was first reported in Belize in 2004 and caused 50% production losses in affected shrimp ponds (Tang et al., 2007). PvNV is a member of the family Nodaviridae and genus Alphanodavirus (Tang et al., 2011). Infected shrimp show clinical signs of white, opaque lesions in the tail muscle thus resembling infectious myonecrosis virus (IMNV) infection. Histopathological changes include multifocal necrosis and haemocytic fibrosis in the skeletal muscle. Basophilic cytoplasmic inclusions in strained muscle, lymphboid organ and connective tissues are also observed. The disease also causes lymphoid organ spheroids. Although the virus does not cause serious mortality, the infection impacts survival in grow-out ponds. Sporadic mortality of infected shrimp has been recorded when they are under environmental stress such as crowding (stocking density (>50 m-2) and high temperature (>32oC) and survival decreased to 40% and increased food conversion ratio. The disease has not been reported from India.
5. Running mortality syndrome (RMS): The term running mortality syndrome (RMS) has been used to describe prolonged chronic mortality during a crop. The mortality starts 1-2 months after stocking and become severe during later part of summer crop. This condition has been noticed in farmed P. vannamei in India, since 2001. The farmed shrimp in the affected ponds showed different mortality patterns which are result of unusual symptoms with no correlation to any other reported diseases. Reports indicate that some farmers have lost up to four crops, with mortality percentage reaching 50-70% in most of the cases (Fig. 3). The exact cause of the disease has not been identified; however, reports indicate the involvement of multiple causes such as covert mortality disease caused by covert mortality nodavirus (CMNV), white muscle syndrome, white gut / faeces syndrome and white patch disease.
6. White gut / faeces syndrome (WFS):
This has been observed in farmed P. monodon and P. vannamei. Gross signs of WFS in shrimp include the appearance of faecal string-like bodies in the gut. It appears like vermiform bodies that resemble gregarines within the hepatopancreatic tubules, at the hepatopancreas- stomach – midgut junction and in the midgut (Fig. 4). It generally occurs approximately from 2 months of culture and initially the causative agent has been reported as gregarines and the condition is described as white faeces syndrome (WS). The disease has been reported to cause 10-15% production loss due to decreased survival and smaller harvest size of shrimp. Although the causative organisms has not been identified, latest report shows that WFS arises from transformation, sloughing and aggregation of hepatopancreatic microvilli into vermiform bodies, which superficially resembles like with protozoan, gregarines (Sriurairatana et al., 2014) and this will result in retarded growth and may predispose shrimp to opportunistic pathogens.
Vibrio species have been found in the faecal analysis from infected shrimps. Incidence of WFS in shrimp has been reported to be associated with stressful conditions including high stocking densities, poor water quality, poor pond bottom, high plankton blooms and bad feed management and high pollution in pond water. White faeces syndrome has been reported to be a serious problem in P. vannamei culture ponds of Andhra Pradesh and Tamilnadu (Mastan, 2015). Although direct correlation between the microsporidian EHP and WFS has not been established, spores of the microsporidian have been recorded in the faecal samples of WFS infected / slow growing shrimp (Rajendran et al., 2016; Tang et al., 2016).
7. White patch disease
This condition has been witnessed in the semi-intensive P. vannamei farming in Andhra Pradesh and Tamilnadu. Initially (before 2012), the outbreak was observed in summer season only, however, in 2014, the disease outbreak could be noticed both in the winter and summer seasons also (Velmurugan et al., 2015). The major symptoms include focal to extensive necrotic areas in striated tail muscle tissues and abdominal muscles tissues.
Necrotic areas appear white opaque patches and in later stage the white patches changed into black spot or splinter. Other symptoms include whitish blue discoloration of infected shrimp body, loss of appetite, roughness on whole surface of infected shrimp with or without red discoloration and pale white muscles. The mortality rate was observed up to 70%. The causative agent responsible for this condition has been identified as Gram-positive bacterium, Bacillus cereus. The isolates exhibited high degree of proteolytic, haemolytic and lipase activity sufficient to lyse the cuticle of shrimp.
The prevalence of Vibrio parahaemolyticus and V. harveyi has caused the occurrence of vibriosis and resulted in severe crop losses to the farmers. It easily affects the shrimp cultured in saline waters and known as “bacterial septicaemia”. The shrimp are affected at any stage. Environmental stresses aggravate the disease and cause huge loss to farmers.
The major symptoms include the exhibition of septicaemia conditions followed by loss of reflex and cuticular fouling. The gills appear brown in color and the body becomes red. Antennal cut has also been observed. The affected shrimps do not eat and hence stomach appears empty and at times white watery liquid oozes out, sometimes luminescence has also been observed in ponds. In serious conditions mortality is observed. Blackening or whitening of the basal part of the antenna, the oviduct and edges of the abdominal segments. The shrimps may exhibit either one of the symptoms or all the symptoms based on disease severity. The preventive measures like drying between production cycles, strict biosecurity measures and good management practices are to be adopted (Fig. 5).
Shrimp diseases caused by either Viral or bacterial diminishes the production and productivity of the farm and increases the usage of probiotics, and chemicals, thereby impacting the economic condition of the farmers. Stringent biosecurity protocols and right identification of diseases will help reduce the occurrence of spread of diseases from one farm to another.
(The author is the Principal Scientist & Head , Fisheries Research Station, Sri Venkateswar Veterinary University, West Godavari, AP. Views expressed are personal.)