COMMON PARASITES OF FRESHWATER AQUACULTURE SPECIES

Jessie M. Sanders, DVM, CertAqV*
 
 Aquatic Veterinary Services
 4061B Soquel Dr
 Soquel, CA 95073
 cafishvet@gmail.com
 

Unfortunately, fish exist in an environment that is constantly contaminated. Just from their own wastes alone, no fish can exist in a disease-free environment. This includes the parasites that opportunistically feed on fish from the outside and inside. When a fish is healthy, their immune system is able to keep parasite levels low, but they are never fully eradicated except in isolated systems. However, due to stress from handling, transport, water quality or numerous other causes, parasites can take advantage of a pause in a fish's immune function and proliferate rapidly.

Fish parasites exist both externally and internally. Depending on your species and aquaculture setup, you may see one type more often than the other. All parasites require early diagnosis in order to prevent spread and possible fish morbidity and mortality. Physical signs of parasitic disease can include flashing, scale loss, bruising or decreased appetite with external parasites and failure to thrive and increased morbidity and mortality with internal parasites.

External parasites are very common in all fish species. Most are small enough to only be seen with a microscope on low power (40x). Larger parasites, such as Argulus spp (fish lice) and Lernaea spp (anchor worms), are large enough to be seen with the naked eye. Simple skin/mucus scrapings are sufficient to diagnose an ectoparasite infection. Common external parasites include Ichthyophthirius multifiliis, Trichodina, Ichthyobodo, monogenean treamatodes (flukes), Chilodinella, and others. Many of these parasites require easy water-based treatment. Some systems where infections are common may consider prophylactic parasite treatment.

Internal parasites are more common with aquaculture species in contact with wild populations. It is not uncommon to find parasites in wild-caught fish with no physical symptoms. However, in fish bring raised for consumption, internal parasites may result in decreased body condition and failure to thrive. Both internal and external parasites may show increased morbidity and mortality, and increased secondary bacterial and fungal infections. Internal parasites will usually require a feed-based or injectable medication for treatment. Diagnosis can be made upon necropsy or fresh fecal samples.

No matter the parasite, fish can become stressed, leading to decreased immune function. This can open the door to many more infections and disease spread. If you suspect a parasite infection, take immediate steps to correctly diagnosis the problem and start treatment early. Early diagnosis and treatment is the best thing to protect your fish investment.