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By spring, koi have exhausted their energy reserves, and their immune systems are weak. Many small parasites are active in water temperatures between 45 and 55 degrees F. Most are known as debility parasites because they only harm weakened or stressed fish. Normal winter conditions have caused fish to be very slow and to huddle close together near the pond bottom. Under these conditions it is easy for parasites to move from fish to fish and to rapidly multiply. These parasites can only be seen with the aid of a microscope, but following are some symptoms you might note in their presence.
Costia – a flagellated, free-swimming protozoan that must attach to a host within 12 hours. It thrives best in over-crowded, poorly filtered conditions. Symptoms include a grayish film over most of the fish's body. The fish become listless and loners. Costia damage the gills and can cause suffocation.
Trichodina – a weak swimming, ciliated protozoan that often attacks the gills. Symptoms include sliminess, lethargy, fins held close to the body and difficulties in swimming. In extreme cases there may be bloody spots on the skin and tattered fins.
Chilodonella – a weak swimming protozoan that needs fish to fish contact to spread. It destroys the skin cells and can completely strip the gill epitheliums. Symptoms include a bluish-gray film, especially on the head and forward of the dorsal fin, and fish that hang near the surface or by running water or waterfalls.
Flukes – the worst of the larger parasites. Dactylogyrus species are .5mm to 1mm long and are egg layers. The rear half of the fluke attaches to the gills and skin with two large central hooks and several smaller hooks. The larvae are free swimming and multiply rapidly. Symptoms of gill flukes may not be obvious but the fish may be seen flashing, especially hitting its gill covers. There is severe irritation and fluke bites often are invaded by viruses causing white droplet formations on fins and lower body.
If you suspect any parasite infestation and you do not have access to a microscope, you can call your veterinarian or your Koi Health Advisors, Marlene Hyden or Greg Young who can examine the suspect under a microscope and advise on a course of action.
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