External & Internal Signs

Clinical Diagnosis
The clinical signs of an acute infection with E. ictaluri in channel catfish are typical of a Gram negative enteric bacteremia. Diseased fish typically stop feeding, become lethargic and hang near the surface of the water and sides of the pond in a “tail-down” position. Prior to death, they will often be observed to swim in circles to exhaustion.

On clinical examination, clinical signs can vary greatly but diseased fish characteristically exhibit pale gills, bilateral exopthalmia and a distended abdomen filled with fluid. Petechial haemorrhage of the skin is often apparent in the non-pigmented areas under the jaw, belly and base of the fins.

Acute Septicaemic form

  • Starts as an enteritic infection. The vent may appear congested, with mucoid and haemorrhagic intestinal exudate.
  • In advanced cases, fish become lethargic.

Chronic Meningo-Encephalitis

  • Progresses slowly from the olfactory sacs, through the nasal route, causing a granulomatous inflammation in brain tissues.
  • Listlessness and erratic swimming are resultant behavioural signs. Affected fish may display rigor or uncoordinated muscle twitching when handled.
  • Many cases of Meningo-Encephalitis occur after an acute septicaemic form has occurred in the population.

Lesions

  • Petechia and haemorrhages around the mouth, on the throat, abdominal region and fin bases.
  • Anaemia, exophthalmia, abdominal distension or dropsy in advanced cases.
  • Gills often appear irritated with petechia.
  • 1-2 mm raised lesions scattered on the body. These lesions progress in to shallow ulcers.
  • Internally, haemorrhages and necrotic foci in the liver and other internal organs. Enteritis, systemic oedema, accumulation of yellow- or blood-tinged ascitic fluid in the body cavity and enlargement of the spleen.
  • Concurrent infection with Flavobacter columnare is common.

Laboratory tests
ESC is typically diagnosed by culturing E. ictaluri from the internal organs and/or brain on an enriched media such as tryptic soy agar (TSA) supplemented with sheep’s blood (5% v/v) or brain heart infusion (BHI) agar.

Primary cultures should be incubated at 25 to 30 °C for five days. E. ictaluri is typically seen as numerous but very small (1 –2 mm diameter) smooth, round, entire and non-pigmented white colonies at 48 hours or more following inoculation.

If non-enriched media or lower incubation temperatures are used, E. ictaluri is often overgrown by other organisms and not detected. Therefore, when mixed infections are expected or the diagnostician is not familiar with the pathogen, a selective or differential media such as Shotts and Waldman’s EIM media is recommended.

E. ictaluri is a short Gram negative bacilli (0.8 X 1 to 3 um), typically occurring singly or in very short chains. It is weakly motile at 25 to 28 C., oxidase negative, O/F on glucose, and K/A on triple sugar iron agar. E. ictaluri is easily differentiated from E. tarda on the basis that it does not produce H2S on TSI media and is negative for indole in tryptone broth. If the API 20E biochemical identification system (BioMerieux Vitek Inc.) is used at 30 °C, E. ictaluri will result in the code number 4004000.

Confirmed identification is typically based on biochemical characteristics or serological tests. Slide agglutination, indirect fluorescent antibody and enzyme-linked immunoassay (ELISA) tests using polyclonal or monoclonal antisera are routinely used. No serological cross reactions have been reported.

Post Mortel Diagnosis

  • Petechiation is also often seen throughout the visceral cavity and musculature.
  • The kidney and spleen are usually swollen or hypertrophied.
  • The liver is characteristically mottled and pale in colour and can show white spots of focal necrosis.
  • The intestine is usually flaccid, void of food and filled with a bloody fluid.

In more chronic infections, often seen in larger fish or when water temperatures are outside of the optimal range for the disease, diseased fish will often exhibit numerous small (1–3 mm diameter) necrotic (white) or haemorrhagic (red) skin lesions on the sides and back of the fish.
One very unique and possibly pathonomonic sign is a single raised induration or “pimple” above the median cranial foramen over the brain and between the eyes. This induration can progress into open lesion directly into the brain.

This condition is commonly referred to as “hole in the head” disease and E. ictaluri can be readily isolated from the brain, but not necessarily from the visceral organs.

Histopathology
The histopathology of of ESC disease is typical of agram negative enteric bacteraemia. It is characterised by colonisation of capillary beds in all major organs and tissues resulting in inflammation, haemorrhage and necrosis.

One unique observation is the appearance of viable organisms inside of macrophages. This is due to the fact that E. ictaluri is capable of surviving and even replicating within macrophages for extended periods of time following phagocytosis.


Disease reprinted courtesy of OIE Diagnostic Manual for Aquatic Animal Diseases, OIE (World Organisation for Animal Health), Paris, France.