Salmonid Rickettsial Septicemia
by Chad Russell
The purpose of this page is to provide
some basic information about the fish disease Salmonid Rickettsial Septicemia
and its causative agent Piscirickettsia salmonis. I have built this
page as part of my class project in "Diseases of Fish", at Oregon State
University. I have tried to include all of the relevant and interesting
information regarding this topic, so I hope you find it interesting as
well as informative. Welcome to the Salmonid Rickettsial Septicemia home
page!
Introduction
Rickettsial like infections have been
reported since as early as 1939 in Egypt and later in British Columbia
in 1970. However, because of the low mortality rate it was not considered
very important (at least in an economical sense) and was never researched.
This changed in the autumn of 1989 when, what was thought to be a new disease,
broke out in Calbuco, southern Chile. An estimated 1.5 million market sized
Coho salmon died from this unknown pathogen. Mortality rates ranged from
1 to 20% a month to 40% over a 10 week period. On certain farms mortality
rates rose as high as 90%. In the following years, major losses were also
attributed to the same organism among farmed Atlantic salmon. Because
of the unusual nature of this disease, it was initially named U.A. (Unknown
Agent), 'Coho salmon syndrome', or 'Huito disease'.
The causative organism of the this
Chilean outbreak has since been identified as Piscirickettsia salmonis,
which is a rickettsial organism belonging to the order Rickettsiales.
Today the most common names for this fish disease are known as Salmonid
Rickettsial Septicemia (SRS), and Piscirickettsiosis. This disease is probably
the most important disease affecting the Chilean salmon industry.
Over the past few years, reports of
ricketsial diseases infecting several different species of salmonids and
non-salmonids in several different countries have surfaced. This
has increased concerns about the possible impact of this virulent organism
in other fish cultures and wild fish stock.
Piscirickettsia salmonis
P. salmonis is a Gram negative,
acid fast, non-motile, non-capsulated, pleomorphic often spherical to coccoid
organism ranging in size from 0.5 to 1.8 um in diameter. It is also an
obligate intracellular organism and has been culture in 6 different cell
lines. Typically, growth is determined by the appearance of cytopathic
effect (CPE). Characteristic CPE include cell rounding and development
of one or more large vacuoles within the cytoplasm of the host. Four of
the six susceptible cell lines come from salmonid species. The other two
lines come from warm water fish species. P. salmonis is very temperature
sensitive, having a maximum range of 4 - 25 C and optimal growth at 15
- 18 C. Click here to see picture
Pathology
External signs of SRS include: lethargy,
anorexia, darkening of the skin, surface swimming, and respiratory distress.
External skin lesions are also common and include: hemorrhagic ulcers and
white nodules measuring up to 1 cm in size. However, it has been reported
that many heavily infected fish show no external lesions. Internal lesions
are very characteristic of the disease. Commonly infected organs are liver,
spleen, intestine, and hematopoietic tissue of the kidney. The most characteristic
internal lesions are off-white to yellow nodules measuring up to 2 cm in
diameter throughout the liver. Click
here to see picture
P. salmonis a wide variety
of cells including macrophages. Within the cell they replicate in membrane
bound intracytoplasmic vacuoles of variable size. It has also been found
extracellularly, apparently as a result of cell lysis. The mechanisms by
which it infects and survives in the host cell is still not clear.
Transmission
The mode of infection and transmission
are still unclear. Experimentally, fish have been infected via the intraperitoneal,
oral, and gill routes. Natural horizontal transmission seems to occur a
few weeks after smolts are transferred to sea water. Infectivity of P.
salmonis is greatly increased in sea water. It has been shown to survive
up to 28 days in sea water at 5 C, while only surviving less than 30 min
in fresh water at the same temperature. However, infections have occurred
in fresh water tanks. Infected cells have been found in both the intestine
and renal tubules, suggesting that the bacteria could be released into
the water and survive to infect other fish.
Intermediate vectors may also play
a role in the natural transmission of the organism similar to terrestrial
rickettsial diseases that are mainly transmitted by ticks to the host mammal.
Preliminary studies show that invertebrates, non-salmonids, and salmonid
parasites are able to carry the disease, and presumably act as vectors
or reservoirs for the pathogen.
Vertical transmission of P. salmonis
has been considered unlikely. However, one study showed that 98.3% of fingerlings
from positive brood stock were also positive. In contrast, only 26.7% of
fingerlings from negative brood stock were positive. Due to the rick of
vertical transmission, Chilean farmers have been screening brood stocks.
Many different stains have been used to detect the organism in smears.
Although this technique is not very specific, it is very fast and economical.
Other more definitive techniques including cell line cultures, serotypes,
PCR, and microwave radiation have been developed but are more costly.
Control
P. salmonis has proven very
sensitive to a large range of antibiotics used in vitro. However,
results are far less encouraging in vivo. One possible explanation
for this is that it is likely that the antibiotics can not reach the organism
because of its intracellular nature. Another possibility is that the antibiotics
are disabled by the ionic nature of the sea water. However, some
success has been made with antibiotics to control outbreaks of the disease.
Among the antibiotics currently used, oxolinic acid and flumequine given
orally are the most popular. Strategies to decrease transmission include
injection of brood stock with antibiotics 30 to 60 days before spawning,
and incorporation of antibiotics in the water while the eggs are hardening
after fertilization.
Other methods of control aside from
antibiotics have proven quite successful as well. Decreases in unnecessary
stress, quick elimination of dead or diseased fish, appropriate disposal
of blood, and decreases in stocking densities have proven successful in
limiting outbreaks.
Conclusion
Piscirickettsia salmonis has
continued to cause large losses in the Chilean salmon industry and reports
continue to increase of rickettsial infections around the world. Due to
the intracellular nature of the organism, the control of this disease has
been difficult with antibiotics. Further research and information on the
transmission, pathogenic mechanisms, and intracellular survival are needed
to establish better preventative strategies.
Bibliography
MB
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If you have comments or suggestions, please email me at russelch@ucs.orst.edu
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