2. BACTERIAL AND VIRAL FISH PATHOGENS

2.1 Bacterial pathogens in salmonids
2.2 Viral pathogens in salmonids
2.3 Bacterial pathogens in shrimp
2.4 Viral pathogens in shrimp

2.1 Bacterial pathogens in salmonids

Most bacterial pathogens in salmon are spread through excretions of urine or faeces from one host to another although the survival rate for most pathogens is limited in a marine environment. The major bacterial pathogens in farmed salmon are listed in Table 2.1. The important Gram negative pathogens are from the genera Aeromonas, Pseudomonas and Vibrio, while the important Gram positive organisms are from the genera Clostridium, Renibacterium and Streptococcus.

Table 2.1 Major Bacterial Salmonid Diseases and Organisms Responsible

Aeromonas hydrophila and Pseudomonas are common in freshwater and can survive and multiply in that environment provided there is enough organic matter and suitable growth temperature. Most bacteria in this group are not usually capable of multiplying or causing disease below 10-12�C, although some strains are pathogenic at temperatures as low as 5�C. They are capable of producing a variety of diseases, which predominate during summer months when water temperatures and organic loadings are high. Skin infections may appear resulting in bright red blotches around the vent, back and sides. Haemorrhages may appear in the internal organs and in advanced stages, the kidney appears liquified.

Aeromonas salmonicida is the causative agent for furunculosis. This organism is more cold tolerant than some of the other Gram negatives and may produce disease at any time of the year although is most dangerous at times when temperatures are high. In some, but not all fish, large, red, swollen, boil-like lesions (furuncles) are formed and may burst with the release of fluid containing viable organisms. Aeromonas salmonicida produces a brown pigment, which diffuses into the media when grown on tryptone-soya-agar. This pigment is not produced by Aeromonas hydrophila. Vibrio is a genus, which often causes disease in salt-water salmonids. Vibrios are Gram negative bacteria, which infect the host and produce toxins which attack the circulatory system and in particular, the red blood cells. Vibrio anguillarum multiplies on the skin but the toxins produced cause severe anemia. Vibrio salmonicida is often associated with salmon reared in salt water at low temperatures causing "Hitra"disease or cold water vibriosis. Symptoms may include anemia with paleness and swelling of the liver, skin ulcerations, which develop into small openings which exude a pale red infective exudate.

Gram positive bacterial pathogens include Clostridium, Renibacterium and Streptococcus spp. Renibacterium salmonarum causes bacterial kidney disease and can grow only within the infected host. It is fastidious and difficult to cultivate in artificial media. It can be transferred from one fish to another or vertically through fertilized eggs, thus being passed from one generation to another. This disease results in white modular lesions on kidney and spleen tissue.

Clostridium botulinum is an anaerobic organism which is sometimes associated with putrefied fish. The vegetative cells produce a deadly neurotoxin in putrefied moist feeds and as such do not cause infection. However consumption of putrefied feed or mortalities laying at the bottom of the aquaculture enclosure can cause intoxication, resulting in the fish lying motionless on the bottom, then repeatedly rising and falling between the surface and the bottom. Death usually occurs within a few hours of consumption of toxic material.

For a detailed review of salmonid diseases see Roberts and Shepherd (1997).

2.2 Viral pathogens in salmonids

The major viruses affecting the health of fresh water and seawater salmonids are the infectious haemopoietic necrosis virus (IHN), the infectious pancreatic necrosis virus (IPN), the infectious salmon anemia virus (ISA), the pancreas disease virus (PD), salmon pox and viral haemorrhagic septicemia virus (VHS).

IHN is a viral disease primarily confined to Pacific salmon species in North America and Japan. The virus results in pale gills, swollen bellies and occasionally infected fish will trail a ribbon of faeces from the vent. It normally occurs in salmon and trout in colder water (<15�C).

The IPN virus can be transmitted through the eggs, milt or faeces. It can be transmitted by birds as well. It may affect salmonids in both fresh and salt-water environments and causes necrosis of the intestine and pancreas. It often affects young fry but adult fish can carry the virus particularly in the reproductive organs for some time without exhibiting symptoms. This disease is of great economic importance and has been found in farmed fish all over the world.

Infectious salmon anemia (ISA) was first reported in 1984. It was the cause of a massive disease outbreak in Norwegian farmed salmon in the early 1990's and most recently in a major outbreak in Eastern Canada in 1997. Affected fish are reluctant to feed, become anemic and eventually die. The ISA virus is highly communicable through seawater and has been shown to often spread through the dumping of infectious waste or blood water from the processing plants.

Pancreas disease (PD) is sometimes found in salmon reared in seawater cages. It is particularly prevalent in young salmon during the first few months in seawater. Infected individuals lose their appetite, making them more susceptible to other diseases although PD is not necessarily fatal.

Salmon pox results in the formation of grey, circular, flat, wart-like spots on the fish skin. The skin lesions lead to scale loss, exposing individuals to secondary infections which may be fatal.

VHS is primarily a disease of fresh water salmonids. Red haemorrhagic spots often occur around the eye and on the gills. Blood clots appear in the body fat, viscera and muscle. Outbreaks are most common in cold water environments and VHS has been transmitted through the feed.

2.3 Bacterial pathogens in shrimp

The major bacterial pathogens affecting farmed shrimp include necrotizing hepatopancreatitis (NHP), caused by a poorly understood pleomorphic Gram negative bacterium, vibriosis which is caused by a variety of organisms from the genus Vibrio, and luminescent bacterial infections which are caused by organisms from the genus Photobacterium and Vibrio.

NHP is a bacterial disease which is difficult to control and can lead to mortalities ranging from 20 to 95%. The organism is yet to be fully characterized or named. Part of the problem is that this bacterium can assume two distinct morphologies. One is the form of a small rod while the other is a longer helical rod. The bacterium has complex growth requirements and therefore efforts to grow the organism on artificial culture media have been fraught with difficulty (Loy et al., 1996). This organism infects and destroys the hepatopancreas which is the major digestive organ in shrimp and other crustaceans. Outbreaks have been reported in Texas, Latin America and South America with the most devastating outbreaks occurring in Texas and Peru (Jory, 1997).

A number of species of Vibrio have been associated with shrimp diseases. In a review article, Panchayuthapani (1997) referred to V. alginolyticus, V. costicola, V. harveyi, V. splendidus, and V. parahaemolyticus as being the causes of bacterial diseases in shrimp in India. Pathogenic Vibrio species may or may not be luminescent, affect shrimp at any stage of growth and development and can cause symptoms such as septicaemia, localized infections or nodules in a variety of organs including heart, gills, hepatopancreas, epidermis or the lymphoid organ.

2.4 Viral pathogens in shrimp

The greatest threat to the future of world shrimp aquaculture is disease, in particular the virulent, untreatable viruses (Lotz, 1997). The major viral pathogens are listed in Table 2.2. It is perhaps interesting to note that for each commercially important shrimp species, there is at least one devastating viral disease associated with it (Lotz, 1997). Thus switching farmed shrimp species in order to avoid one viral pathogen may well lead to the increased susceptibility of another.

The "Taura" syndrome virus (TSV) affects pond reared Penaeus vannamei. Outbreaks have occurred in recent years in a variety of geographical locations including Ecuador, Peru, Columbia, Honduras, Brazil, the U.S., Guatemala and Mexico. The name "Taura" was derived from the Taura River in Equador where the syndrome was first recognized (Hasson et al., 1995). TSV causes lesions which often appear "pepper like" or "buckshot laden"and may be located on the surface or below the surface of the cuticle, in the gills, mouth, aesophagus, stomach, hindgut or appendages.

Penaeid shrimp are also affected by a baculovirus which causes a so-called "white spot" syndrome which is manifested as white spots on the exoskeleton, legs and other parts of the body. Mortalities of 100% have been reached within 2 to 7 days (Chou et al., 1995).

The yellowhead virus (YHV) causes as the name suggests, yellow discoloration of the carapace of shrimp.

Table 2.2 Common Viral Pathogens in Shrimp

According to Karunasagar and Karunasagar (1995) the YHV affects the gills, lymphoid organ, haemocytes and connective tissue. Cumulative mortalities may reach 100% within 3 to 5 days.

The infectious hypodermal and hematopoietic necrosis virus (IHHNV) is highly virulent and attacks the cuticular hypodermis, the haemocytes, the haemopoietic organs and connective tissue.

Other viral pathogens have been identified in shrimp farms around the world and have caused devastation to the industry because of their highly contagious nature. There are no known cures for viral pathogens making prevention the only means of reducing the economic impact.