Withering Syndrome of Abalone
Category 3 (Host Not in Canada)
Common, generally accepted names of the organism or disease agent
Withering syndrome (WS), Withering disease, Foot withering syndrome, Abalone wasting disease, Withering syndrome - an intracellular Rickettsiales-like prokaryote (WS-RLP).
Scientific name or taxonomic affiliation
'Candidatus Xenohaliotis californiensis', a proposed new genus and new species of intracellular prokaryote, with morphological characteristics of the class Proteobacteria, order Rickettsiales and family Rickettsiaceae, in the epithelium of the intestinal tract (Gardner et al. 1995, Friedman et al. 2000b to d). Initially, heavy infections of coccidia in the kidney were thought to cause the disease but, a correlation between coccidial infection and withering syndrome was not found (Steinbeck et al. 1992; VanBlaricom et al. 1993; Kuris et al. 1994; Friedman et al. 1997).
Coast of California, USA, south of Point Conception and on the west coast of Baja California, Mexico. In Diablo Cove, California (70 km north of Point Conception), disease and mortalities were limited to the immediate vicinity of a warm-water discharge. In 1996, there was evidence that this disease was progressing northward from Point Conception (Altstatt et al. 1996) and possibly as far north as San Francisco, California (Finley and Friedman 2000). In addition, the bacterium (but not withering syndrome) was detected at two locations in northern California (Crescent City and Van Damme) (Friedman and Finley 2003). In surveys of abalone from Baja California, Mexico, this pathogen was detected in high prevalences in the digestive tract of symptomatic and non-symptomatic cultured and natural populations of H. rufescens, H. fulgens and H. corrugata (Cáceres Martínez and Tinoco Orta 2000, Cáceres Martínez et al. 2000, Caceres-Martinez and Tinoco-Orta 2001, Álvarez-Tinajero et al. 2002). Recently, Xenohaliotis californiensis was reported in H. rufescens cultured in Iceland (unofficial report from Gisli Jonsson in an e-mail dated November 8, 2004) and in H. tuberculata cultured in Ireland in 2006 (records of the OIE) and France (Balserio et al. 2006) with no associated mortalities in these cases. However, H. tuberculata experimentally grown in Galicia (NW Spain) from stocks originating in Ireland experienced high mortality (45% to 100%) within 14 months of importation (most mortalities occurring during the spring and summer months) and were infected with X. californiensis but the associated mortalities were attributed to a co-infection with a protistan pathogen (Balserio et al. 2006).
A similar looking disease of unknown etiology has been reported from cultured Haliotis discus hannai on the northern coast of China (Guo et al. 1999). Rickettsia-like organisms have also been reported in the digestive tract of abalone (Haliotis midae) from culture facilities in South Africa with no associated pathology (Mouton 2000).
Disease most evident in Haliotis cracherodii, however, the disease and pathogen also occurs in Haliotis rufescens, Haliotis corrugata, Haliotis fulgens, Haliotis sorenseni and Haliotis tuberculata and possibly in Haliotis discus hannai and Haliotis midae.
Impact on the host
A lethal disease that affects all sizes of abalone and causes lethargy, retracted visceral tissues, atrophy of the foot muscle (thereby adversely affects the ability of the abalone to adhere to the substrate) and is lethal. Elevated temperatures accelerated disease progression and decreased survival. At 18 to 20 °C, death usually occurs within one month of the appearance of the clinical signs. Diseased abalone consumed 4.4 times less kelp, 1.2 times less oxygen and excreted 3.8 time more ammonia per gram wet weight than did healthy abalone (Kismohandaka et al. 1993). Severe metabolic alterations were detected in abalone before visible atrophy of the foot occurred. Haemocyanin concentration in the blood decreased, glycogen in the foot muscle was depleted, haemocyte abundance was reduced and haemocytes with abnormal morphology increased in wasted abalone (Friedman 1996; Shields et al. 1996). In addition, haemocytes were more chemotactically active but the capability of the stimulated cells to engulf and destroy foreign particles appeared to be compromised and may contribute to mortality associated with the disease (Friedman et al. 1999, 2000a). Mass specific ammonia excretion was observed in affected abalone indicating protein from the foot muscle was being used as an energy source. This conclusion was also suggested by Kismohandaka et al. (1995) who observed severe foot muscle fibre depletion in samples examined using histology. However, no pathogens were found in the muscle or blood tissues.
This disease is associated with mass mortalities of H. cracherodii. Withering syndrome progressively spread throughout the California Channel Islands causing population crashes on six of the eight Channel Islands by 1992 (95 to 100 percent of the H. cracherodii were lost) and closure of the California black abalone fishery in 1993. A dramatic increase in the number of cultured H. rufescens with foot withering syndrome was noticed in conjunction with El Niño - Southern Oscillation (ENSO) elevated seawater temperatures (Moore et al. 1999). However, differences in susceptibility and tissue changes were noted between species with H. cracherodii being more susceptible than H. rufescens and survivors appear to be relatively resistant to the disease (Friedman et al. 2003b).
Gross Observations: Body mass relative to shell size is smaller than normal. Affected abalone were discoloured (pale) and weakened, and the soft tissues were atrophied and non-responsive to stimuli. In the field, affected abalone can be detached from the substrate by hand and do not attempt to right themselves when turned upside down.
Squash Preparations: Minced pieces (about 2 mm square) of gastrointestinal tract from the posterior portion of the esophagus to the posterior end of the crop were places on a microscope slide, gently pressed into the slide with a second slide and dried with low heat from a blow dryer for 20 min. Dried samples can be prepared for examination immediately or held indefinitely at 4 °C with desiccant. To prepare for examination, the tissue was flooded with a 10 µg per ml solution of Hoechst 33258 (bisBenzimide, Sigma, St. Louis, MO, USA) in distilled water, covered with a coverslip, incubated in the dark for several minutes and viewed at 100 to 400X magnification with a epifluorescent ultraviolet light and filters appropriate for the spectra of 356 nm excitation and 465 nm emission. This staining technique caused the large inclusions of Xenohaliotis californiensis, which are usually difficult to detect in unstained tissue, to fluoresce a bright blue against a black to dull red background. Although the abalone cell nuclei were also fluorescent, they were small ( about 5 µm in length) in comparison to the inclusions (about 50 µm in length). An alternate nucleic acid-specific fluorochrome, propidium iodide (10 µg per ml in distilled water, Sigma), viewed with ultraviolet light and 530 nm excitation and 615 nm emission filters gave similar results (for further details see Moore et al. 2001a).
Histology: Severe foot muscle fiber depletion. Occurrence of extensive infections of Gram-negative intracellular prokaryotes in the epithelium of the intestinal tract, especially in the enzymes secreting cells of the digestive diverticula. The prokaryotes had morphological characteristics of the order Rickettsiales. They were accumulated into intracellular colonies within epithelial cells. Infection of the digestive diverticula is accompanied by a loss of digestive enzyme granules from epithelial cells and apparently by a metaplasia of enzyme secretory cells to cells morphologically similar to epithelial cells lining the gut (Gardner et al. 1995). This pathology in heavily infected abalone is speculated to be the cause of muscle tissue catabolism resulting in the withering disease.
Electron Microscopy: Observation of rod-shaped, ribosome-rich prokaryotes with trilaminar cell walls accumulated into intracellular colonies within membrane-bound vacuoles in the cytoplasm of gastrointestinal epithelial cells.
DNA Probes: The 16S rDNA was amplified, cloned and sequenced. A polymerase chain reaction (PCR) test was developed that specifically amplifies a 160 base-pair segment of the Rickettsia-like pathogen but not four other microbial species isolated from the gut of abalone. Apparently, this PCR test greatly increases the ability to detect the pathogen (Andree et al. 2000). Also, an in situ hybridization test has been developed (Antonio et al. 2000).
Methods of control
Experiments indicate that the pathogen can be transmitted via the water column and did not require direct contact between infected and uninfected abalone (Moore et al. 2000a and 2001b, Friedman et al. 2002). Above normal temperatures seem to have a synergistic effect on the disease (Cáceres Martínez et al. 2000, Moore et al. 2000a, Raimondi et al. 2002). Results of experiments by Friedman et al.(1997) and Moore et al. (2000a and b) indicated that H. cracherodii and H. rufescens, respectively, held at elevated temperatures (20 °C and 18.5 °C, respectively) had higher mortality, more severe signs of WS and more severe infections with the Rickettsia-like prokaryote than those held in cooler waters (13 °C and 14 °C, respectively). Also, the recovery of black abalone populations affected by mass mortalities from foot withering syndrome seemed to be closely linked with temperature. In affected culture facilities, the severity of the disease may be curtailed if water temperatures could be reduced to about 15 °C or less (Moore et al. 1999). Results of subsequent long-term (447 days) experimentation employing fed and starved abalone indicated that the high morbidity and mortality exhibited by infected abalone is a consequence of disease and not direct thermal stress (Braid et al. 2005).
Oceanographic factors that result in elevated seawater temperatures (i.e., ENSO) had a strong negative impact on the recovery of black abalone populations in southern California (Tissot 1995). These elevated temperatures were also associated with a dramatic increase in the number of red abalone with foot withering syndrome in culture facilities in California (Moore et al. 1999). Despite the devastation caused to black abalone populations, a few large, old individuals can still be found and some small juveniles have been seen (Haaker 1997). Also, the research of Tissot (1995) suggests that black abalone populations in southern California may recover with the subsidence of ENSO oceanographic conditions. Genetic structure of black abalone populations in the California islands and central California coast was assessed in order to identify patterns of recruitment in surviving populations (Chambers et al. 2006).
Evidence indicated that the occurrence of Xenohaliotis californiensis in H. rufescens at two new locations in northern California were associated with out-plants of hatchery-reared abalone, suggesting a link between restoration efforts and the present distribution of this pathogen (Friedman and Finley 2003). The detection of the pathogen outside the previous known distribution highlights the need for careful assessment of animal health before restocking depleted populations or transplanting animals for aquaculture.
Intramuscular injection and oral administration of an antibiotic was effective in reducing the losses of infected abalone (Friedman et al. 2003a) and tissue retention of this therapeutant in the digestive gland remained high for a prolonged time (at least 38 days post treatment) (Braid et al. 2005, Friedman et al. 2007). However, other antimicrobials had no measurable affect on the disease (Friedman et al. 2000b).
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Bower, S.M. (2009): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Withering Syndrome of Abalone.
Date last revised: July 2009
Comments to Susan Bower
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