Cause of the Disease
Infectious pancreatic necrosis (also known as Acute catarrhal enteritis) is one of the first described and most extensively studied diseases of fish. There are strains and substrains of IPN virus that can be differentiated by serological, biochemical and genetic means. The VR-299 (type 1) strain was originally isolated in North America and has since been found almost worldwide. The Ab (type 2) and Sp (type 3) strains were originally isolated in Europe, have been widely disseminated but have not yet been isolated in North America. Relative virulence and host specificity can var from one isolate or strain to another.
Host range, distribution, and occurrence
IPN is a highly contagious disease of trout and salmon. High mortality is typically experienced, especially in fry and fingerling rainbow, brook and brown trout. No species of salmonid fish is completely resistant to the virus. IPN virus also causes mortality in striped bass, sea bass, menhaden, halibut, yellowtail and eel. IPN and IPNlike viruses have been isolated from over 65 species of aquatic vertebrates and invertebrates in both freshwater and marine environments. IPN virus occurs in many parts of the world including North, Central and South America, Europe (including the UK), Scandinavia, Japan and Southeast Asia. It has not yet been described in Africa, Australia and New Zealand. The virus tends to become endemic to most watersheds in which it is found.
IPN virus can infect fish at all stages in the life cycle. Smaller fish tend to be more sensitive to the virus and mortality is rarely seen in fish over 5 g. (90 fish per pound). The one exception is Atlantic salmon, where IPN virus can cause significant chronic mortality in larger fish, particularly after the stress of smoltification and saltwater introduction. The disease can occur over a wide temperature range but tends to be most acute at 10 to 15 C. Mortality typically begins 3 to 10 days following infection and peaks in 10 to 20 days. Total mortality from acute infection may reach 70% or more over 2 to 4 weeks. Under certain conditions the course of the disease may be more chronic with lower mortality. Up to 90% of survivors carry the virus and shed it in their feces for the remainder of their life.
Clinical Signs, Diagnosis and Confirmed Identification
The clinical signs of IPN can vary with the size of the fish. Smaller fish tend to quit feeding, darken in color and be very excitable. When startled, they often chase their tails in a tight circular spinning motion or swim in a lateral rolling or “corkscrew” manner. They often exhibit exopthalmia (eyes bulge out) and a grossly distended abdomen. Acutely infected fish often trail a long white cast from the anus. The fish usually have hemorrhages on the ventral abdominal surface and fins. The gills are typically pale and spotted with hemorrhagic lesions. The stomach and intestine are void of food but grossly distended with mucus or fluid. The pyloric caeca are hemorrhagic. Prior to death, fish tend to become lethargic and congregate on the bottom of the pond or screens. In larger fish, the course of the disease may be more chronic and the gross clinical signs less apparent or even absent.
Diseased fish should be submitted to a competent laboratory for diagnosis. Presumptive diagnosis is usually made on the basis of virus isolation and the characteristic cytopathic effect of the virus in tissue culture. Tests may take three to five days for diagnosis. The identity of the virus is usually confirmed by means of neutralizing its activity with diagnostic antiserum specific for IPN virus. ELIZA, FAT, IFAT, DNA probes and several other tests can also be used for presumptive and confirmed diagnosis and often take less time than tissue culture. Isolation and identification is usually sensitive and accurate, and IPN virus is seldom confused with other pathogens. However, the disease can be found concurrently with other diseases so a complete and differential diagnosis should always be conducted.
Reservoirs of Infection and Transmission
IPN virus is reservoired in clinically diseased fish as well as in asymptomatic carriers which shed high levels of virus via infected feces. The virus can survive for several days, particularly in freshwater, and is relatively tolerant of freezing, warm temperatures and drying. It can remain infective for extended periods of time in frozen fish, viscera, faeces or mud. The primary site of infection seems to be the gut, following ingestion of infected feces, fecal casts or dead fish. Free virus shed into the water can also infect fish across the exposed epithelia of the gills and fins.
IPN virus can be vertically transmitted from infected parents to their progeny. IPN virus is sensitive to iodiphore disinfectants but these are not effective in preventing vertical transmission of the disease when the virus is inside of the egg and their use alone does not eliminate risk of vertical transmission. To completely avoid the introduction of IPN virus it is essential that eyed eggs come only from inspected and certified IPN virus free broodstocks with a long history of being certified IPN virus free over several generations such as those found at all Troutlodge operations.
A variety of fish eating birds and mammals are known to serve as vectors, disseminating IPN virus in their feces, particularly when moving from one infected farm or pond to another. Since IPN and IPN-like viruses have been isolated from invertebrate hosts including insects, crayfish and other aquatic invertebrates, these organisms may also reservoir the virus. In addition, it has been shown that some biological materials, such as lyophilized fish pituitaries, can transmit IPN virus if not disinfected.
Prevention, Treatment and Control
If IPN virus has never been found on your farm, every precaution should be taken to prevent its introduction. Stock your farm only with inspected and certified IPN virus-free stocks, preferably as eyed eggs obtained from reputable broodstock operators like Troutlodge. Since IPN virus can be transmitted by birds and mammals, it is strongly recommended that virus free farms erect complete exclusion enclosures around their fry and fingerling operations. To limit spread of disease, younger fish should always be held above older fish in serial reuse systems.
IPN virus is readily inactivated by chemical disinfectants such as chlorine, iodine, formalin and ozone. However, it appears to be more resistant to UV light, sunlight and drying, compared to other fish viruses. UV filters may not adequately disinfect water supplies or protect recirculating systems against IPN virus. Ozone disinfection or chlorination/declorination may be more effective methods.
Another alternative for the prevention of IPN virus is vaccination. There are several experimental IPN virus vaccines for fry or fingerlings in development, but they are not yet licensed or available on a commercial basis. The only licensed and commercially available vaccines against IPN are injectable products suitable for larger and more valuable fish such as Atlantic salmon smolt.
There are currently no registered treatments for viral diseases. Prevention is the best method of control. Once IPN virus is found on a farm, it can be very difficult to eliminate, particularly if there are infected fish in the water supply above the farm. A farm may also become reinfected if frequented by birds and mammals that have recently fed on IPN virus infected fish from nearby farms. Therefore, many infected or high risk farms must learn to manage around the disease. Since the virus is most virulent for fry and fingerlings, mortality can usually be avoided by purchasing certified IPN virus-free eggs and rearing the stock for the first three to six months (8 to 13 cm or 3 to 5 in.) on a virus free water supply.
Certain selected strains of fish have been shown to be somewhat resistant to IPN virus infection. Troutlodge has made genetic selection for IPN virus resistance a major priority in their line breeding programs. If losses due to IPN virus are a chronic problem in your operations, be sure to discuss this opportunity with your Troutlodge representative.
Regulatory Status & Public Health Importance
IPN virus is listed as a certifiable disease by most regulatory authorities. At this time, regulations do not differentiate between strains of the virus. All Troutlodge broodstocks are inspected for IPN virus at least five to six times before they are spawned and all Troutlodge operations have a long established history of being inspected and certified free from IPN virus. All inspections are done by an independent laboratory approved by the U.S. Department of Agriculture, using the latest approved and most sensitive methods of detection available. All eggs produced by Troutlodge are certified free of IPN virus and meet all international standards and requirements.
IPN virus poses no known health risk to humans or any animals other than fish and invertebrates. There are no restrictions on processing or selling infected fish for human consumption and no particular care needs to be taken from a public health perspective.
Wolf, K. 1988. Fish viruses and fish viral diseases. Cornell University Press, Ithaca, NY. 476 p.
Thoesen, J. (Ed). 1994. Suggested procedures for the detection and identification of certain finfish and shellfish pathogens. 4th ed., Fish Health Section, American Fisheries Society, Bethesda, MD, USA