Image Gallery: Fish Diseases

The normal appearance of fish can vary greatly. Both scaled and scaleless fish are typically coated in a mucus layer that acts as a natural protection from infection. This protective mucus layer is considered immunologically active⁴; thus, disruption of this layer and underlying skin can make the fish more susceptible to skin lesions and infection.

Hyperemia is caused by vasodilation of capillaries in the epidermal layer of the skin and along the fins and can occur in response to inflammation, infection, or generalized stress. The degree of hyperemia should be noted prior to handling, as handling can artificially worsen hyperemia. Thus, careful inspection of the fish through the tank or from the water’s surface is recommended prior to catching the fish for any physical examination.

Ulcerations are erosions through the epidermis and dermis and can be severe enough to expose underlying muscle or bone. Ulcerations can occur as a result of primary infection (eg, parasitic, bacterial, fungal, viral) or can be complicated by secondary infections (eg, bacterial, fungal or oomycote, parasitic). As with ulcerations or wounds in other species, care should be taken to properly identify the causative agent through cytology and culture and/or biopsy before starting appropriate treatment.²

Fin rot is the common term for any erosion that destroys the normal architecture of the fins. Fin rot can be associated with infectious (eg, bacterial, fungal, parasitic) or noninfectious (eg, trauma, poor water quality) etiologies. In minor cases, the tissue between rays of the fin may be eroded. In severe cases, infection may reach the base of the fin and expose underlying muscle tissue. Diagnostics should be aimed at identifying the etiology of the underlying infection for proper treatment. The fish with fin rot in Figure 4A can be compared with the healthy fish in Figure 4B. In Figure 4A, fin rot is affecting the fish’s distal tail but not eroding skin or muscle.

Saprolegnia spp, Achlya spp, and other water molds are common secondary infections of fish and other aquatic species. Although oomycota are similar to true fungi in many ways, they are more closely related to protists. Infections present as focal or generalized white-to-gray lesions with a cotton-like appearance on the external surface of the fish. Infection causes local inflammation and may extend into the muscle layers.⁵ Treatment can be difficult and is directed at improving water quality and reducing environmental stressors.¹

HLLE refers to erosion and scale loss originating around the eyes and extending caudally down the lateral line. An exact cause of these erosions has not been identified and may be multifaceted, but there is a strong correlation between the use of activated carbon and development of lesions.⁶ HLLE has been reported in multiple species of freshwater and marine fish. Erosions are rarely fatal but diagnostic testing should be performed to rule out secondary infection. Proper medical management and husbandry correction may stop the progression of these erosions and, in some cases, may cure them.

Lepidorthosis, commonly referred to as pine-coning or dropsy, is a common syndrome in many species of freshwater fish. The pine cone-like appearance of the scales occurs when the epidermis becomes edematous, causing the scales to fan out at right angles. This edema is caused by an osmotic imbalance in the fish, which may be caused by disruption of gill function, renal disease, or hepatic disease.⁷ Treatment is aimed at correcting any environmental issues and identifying the underlying cause of the edema.

Several types of neoplasia have been noted in various fish species. Skin tumors are most commonly presented for veterinary care, as they are more noticeable to fish owners. Skin tumors are generally benign local lesions (eg, papillomas, spindle cell tumors, neurofibromas).⁸ Neoplasia of the coelomic viscera (eg, hepatobiliary, renal, gonadal) may present as coelomic swelling or bloating. Methods of tumor removal and treatment include surgical excision, cryotherapy, and debridement. Biopsy is recommended for all excised lesions.

Dactylogyrids are monogenean parasites most commonly found on the gills of freshwater fish. Dactylogyrids are distinguished from gyrodactylids by their distinct eyespots and scalloped head region, visible in Figure 9. The eggs of dactylogyrids are resistant to all known treatments.¹ Proper treatment should be performed at regular intervals and aim to kill the parasites after hatching.

In contrast to dactylogyrids, which are typically found on the gills of freshwater fish, gyrodactylids are monogenean parasites most commonly found on the skin. Signs of infection include flashing (ie, rubbing or rapidly scratching against items in the enclosure) or ulceration, but gyrodactylids can also be found on clinically normal fish. Gyrodactylids attach to their host with a large opisthaptor, a hooked adhesive organ, and give birth to live young, which are often visualized as an extra opisthaptor in the uterus of the adult parasite.¹ Because there is no resistant egg stage, duration of treatment for gyrodactylids is often shorter than for dactylogyrids (Figure 9).

Trichodina are unicellular protistan parasites often found in freshwater and marine fish. They can be found along the skin, gills, or, in some species, in the urogenital tract. In low numbers, these parasites are rarely pathogenic. In poor water quality conditions or in the presence of high levels of organic material in the water, these parasites can be present in large enough numbers to cause host morbidity (eg, flashing, ulcerations, thickening to mucous layer producing a bluish or dull appearance).¹ Although trichodina are sensitive to some therapies, treatment should be aimed at improving overall water quality.