Fish Diseases and Management

2.2.20 Brain

The brain or encephalon is placed inside the cranial cavity or skull.

Pathology

a) Enchephalitis / Encephalopathy

The brain is covered by a thick layer of fat and meninges which protects the brain from any traumatic damage as well as protects from the invasion by pathogens. Brain is richly supplied with blood by a network of blood vessels which at times may also act as a source of entry for pathogens to the brain. Encephalopathy is a term referring to any of the various diseases of the brain while encephalitis refers to the inflammatory response in the brain. In response to an infection eosinophilic granular cells migrate into the brain and cause allergic reaction called as encephalitis. This may sometimes cause disfunctioning of brain cells and partial damage to the brain.

b) Vacuolating Encephalopathy

The brain cells i.e nerve cells get destructed by means of vacuolation which disrupts its function. It is mainly caused by viral infections such as (EHN) Epizootic haematopoietic necrosis in rainbow trout and noda virus infection in Sea Bass.

a) Hydrocephalus

Fluid accumulation in brain. The blood vessels in brain under go haemorrhage there by leading to an increase in volume of fluid in brain.

2) Spinal cord

Pathology is caused due to damage to spinal vertebrae. In case of whirling disease spinal vertebrae may get distorted along with compression and lesions in the spinal cord.

Meningitis- Pathological changes involving the meninges are more frequently observed in teleosts than encephalitis. Meninges is normally subdivided into two categories, depending on the particular layer of meninges involved.

Pachymeningitis:- refers to inflammation of the pia and arachnoid

Leptomeningitis:- refers to inflammation of the pia and arachnoid

Leptomeningitis is more frequently seen than pachymeningitis.

3. Sense organ - @semicircular canals (Buoyancy organ)

a) In Whirling disease

caused by a parasitic protozoan Myxosoma cerebralis, the parasite is present in the semicircular canals in the internal ear (which normally regulates the buoyancy). This parasite normally infects the juvenile fish where the ossification / calcification is not yet complete. The parasite gets migrated into the semicircular canals causing damage and leads to loss of buoyancy and eventually death.