A variety of causes may be involved: water quality,bacterial and parasite infections,nutrition problem.In my practice I've met cases with unilateral exopthalmia in bass,tilapia and carp.Carp  injures caused by gas bubble disease.Bass and red tilapia had Streptococcus infection,that was very intresting becouse a more common sign fo rstreptococcosis is bilateral exophthalmia.And in black tiliapia with unibilateral exophtalm Digenea  was found.

The reasons for this symptom can be a lot. I suggest you check the virus test - Rhabdoviridae (VHSV, IHNV, SVCV / PFR)

I had often observed that effect in several species when exposed to rapid pressure changes due to explosion procedures in underwater rock removals. Sometimes in both eyes. I agree with positive internal pressure as mechanism for exophthalmia, as a gas bubble. So, if there isn't a source of pressure changes, internal responses to infections are likely. 

One possible pathogen to consider is Weissella ceti.  Typically, it causes bilateral exophthalmia with conjunctival and scleral hemorrhage.  However, early in infection it can cause unilateral exophthalmia.  Try isolation from brain/spleen on TSA agar supplemented with 5% sheep blood.  It does not form visible colonies when plated on BHI, TSA or TYES.

Article Acute Mortality, Bacterial Load, and Pathology of Select Lin...

I worked with warmed water species before and most of them are caused by pathogenic bacteria and occasionally by gas bubble disease.

you should have to perform some tests for confirmation of exact type of pathogen responsible for this

For unilateral exophthalmia you could be consider as differential diagnoses: Viral infections (IPN, IHN, VHS), parasitisms of digenean trematode (Diplostomum),  streptococcosis, proliferative kidney disease, trauma, gas bubble disease, neoplasia (retinoblastoma, teratoid medulloepithelioma, lymphosarcoma), idiopathic cystic degeneration of choroid or thyroid tissue in the choroid.

I suggests to consider mechanical damage, when only one eye is affected.

www.marcosgodoy.com

Marcos,

Dear respected colleagues,

Thanks a lot for your valued responses and time spent.

Lethargy and anorexia, darkening in skin, septicemia, Internal and external haemorrhages, distension of the eyes were typical clinical signs in moribund fish.

For more details please see my other question in site as follows:

https://www.researchgate.net/post/What_is_your_diagnosis_for_this_recent_mortality_in_Rainbow_Trout_Fish?_tpcectx=profile_highlights

Kindly regards

A unilateral exophthalmia may be caused by strong circulation in a round tank, perhaps a tank with a rough concrete or fiberglass surface. If fish are swiming against the current or swept along with it in one direction a unilateral condition can occur.  We have seen this with tilapia and other species as well.

Suspicion of infectious disease or a non-infectious disease should be considered. (Ichthyophthirius, Tuberculosis or .....?)

Unilateral or bilateral exophthalmia is also associated with bacterial infections, such as Yersiniosis, as described by Busch (1978) "...Exophthalmos is brought about tissue edema. This condition is noted to often start unilaterally and develop to a bilateral involvement. Histopathological examination demonstrates an edematous type of lesion of the choroid gland of the eye with an intraocular accumulation of fluid. This edema and increased intraocular fluid pressure induces the exophthalmic condition and eventually results in rupture of the eye with ensuing lens opacity and blindness...". This sign would not be typical of Yersinia ruckeri, but a frequent sign associated with various pathogenic aquaculture bacteria.

Reference: Busch RA. 1978. Enteric redmouth disease (Hagerman strain). Mar Fish Rev 40: 42–51.

Dear Fernando,

Thanks for your valued response.

Regarding unilateral exophthalmia that mentioned in your response(First line). are you sure that it can be related to systemic bacterial septicemia?

Because it would be general septicemia and should be effected on both eyes.

Thanks again and wish you all the best.

Jalil

Poor water quality accompanied by higher temperatures than optimum can be the cause.

Dear Ashkan,

Thanks for your valued response but it occurred as Unilateral exophthalmia.

What do you think??

Thanks again!

Dear Mohammad Jalil Zorriehzahra, you are very welcome. Apart from poor physico-chemical conditions of water and taking into consideration that you spotted the symptom during "an infectious mortality", exophthalmia, both unilateral and bilateral, can be associated with viral or bacterial infections. In such cases, as Fernando referred to a reference, exophthalmia can start or even remain unilaterally.