During the last five years, reported cases of anemia in farm-raised channel catfish from the Mississippi Delta have risen dramatically. Farmers refer to this condition as "no blood" or "white-lip" disease. The anemia appears to affect larger 1.5 - 2.0 lb. catfish, and the etiology is unknown. Severe anemia occurs bimodally in October-November and in April-May. Last year, severe anemia (packed cell volume C 10%) was documented in approximately 5% of cases at the Delta Branch Experimental Station, but it is thought to be more prevalent due to unreported cases. In addition, the anemia may run concurrent with other disease conditions (winter kill), thereby masking the prevalence.

In an effort to document seasonal fluctuations in critical blood values of channel catfish, a survey was conducted from September 1992 through January 1994. Packed cell volume (PCV), total protein (TP), and osmolarity were recorded in 1633 fish from 6 different production ponds. Approximately 20 fish were sampled from each pond at each sampling date. In addition, water temperature and fish weight were recorded.

Total protein and osmolarity remained fairly constant throughout the year. However, catfish displayed a sinusoidal seasonal fluctuation in PCV. PCV was highest in mid-summer (June, July, August) and lowest in mid-winter (December, January, February). Mean values ranged from > 34% in mid­summer to approximately 20% in mid-winter. However, beginning in November, many fish displayed PCV values < 15% and several dropped to < 10%. Fish were still alive and displayed no noticeable external lesions with the exception of extremely pale gills. It is unknown why, in the same pond, some fish became severely anemic while others did not.

In a concurrent study, a complete blood cell count (CBC) was run on 640 fish from October 1993 through January 1994 (the time of year when PCVs were lowest). Sixteen channel catfish from four Delta production ponds were sampled bimonthly. The following indices were recorded for each fish: PCV, TP, total red blood cell count (RBC), hemoglobin (Hb), mean corpuscular volume (MCV), mean corpuscular hemoglobin concentration (MCHC), mean corpuscular hemoglobin (MCH), total white blood cell count (WBC), percent and absolute differential WBC count. Mean PCV was 22.7% with RBC 1.99 xl06cells/uL in October while in December mean PCV dropped to 14.4% with RBC 1.49 xl0⁶cells/uL. Mean MCV went from 112 fL in October to 97 fL in December. The anemia was classified as microcytic, normochromic and regenerative. Numerous polychromatic red blood cells (which in fish, are smaller than mature RBCs) could be seen in all anemic fish, especially the most severely anemic ones.

There are three general pathophysiological mechanisms of anemia, 1. reduced or defective erythropoiesis, 2. accelerated erythrocyte destruction, and 3. hemorrhagic blood loss. Possibilities for reduced erythropoiesis include temperature or seasonal effects (hormonal) on hematopoietic tissue, nutritional deficiencies (folic acid, vit B12, iron etc.), or anemia of chronic disease. Accelerated erythrocyte destruction may be caused by infectious agents, blood parasites, chemical toxins in the water, or immune-mediated disorders. Hemorrhagic blood loss may be due to heavy parasite loads, gastrointestinal ulcers, or hemostasis defects including disseminated intravascular coagulation or other clotting disorders (Duncan and Prasse, 1986).

Seasonal fluctuations in PCV have been documented in other species, e.g., trout and flounder (Demon and Yousef, 1975; Audet, and Claireaux, 1992; Mahoney and McNulty, 1992). However, the fluctuations in these species are not as severe as those recorded from channel catfish in our study. Denton and Yousef showed that seasonal changes in rainbow trout hematology were not related to temperature or diet. They suggest that other seasonal effects on activity and metabolic adaptations probably play an important role in altering blood values. Our studies suggest that there may be a possible correlation between temperature fluctuations and PCV fluctuations, and we are currently designing experiments to further explore this relationship.

In conclusion, our study indicates that channel catfish in production ponds undergo a sinusoidal seasonal fluctuation in PCV. During the time of year when PCV is lowest, individual fish will display moderate to severe anemia. The anemia is characterized as microcytic, normochromic and regenerative. Since it is regenerative we know that hematopoiesis is taking place. Whether erythropoiesis is optimal during this time is unknown. We have no evidence to support other pathophysiological mechanisms of anemia at this time, but we certainly can not rule them out.

References

1.  Audet, C. and Claireaux, G. 1992. Diel and seasonal changes in resting levels of various blood parameters in brook trout (Salvelinus fontinalis). Can J Fish Aquat Sci, 40:870-877.

2.  Duncan, R.J. and Prasse, K.W. 1986. Veterinary Laboratory Medicine, Clinical Pathology. 2nd ed. Iowa State University Press, pp. 3-30.

3.  Denton, J.E. and Yousef, M.K 1975. Seasonal changes in hematology of rainbow trout, Salmo Gairdneri. Comp Biochem Physiol, 51A:151-153.

4.  Mahoney, J.B. and McNulty, J.K. 1992. Disease-associated blood changes and normal seasonal hematological variation in winter flounder in the Hudson-Raitan estuary. Trans Amer Fish Soc, 121:261-268.