Clinical Signs
Equine infectious anemia should be among the differentials in individual horses with weight loss, edema and intermittent fever. It should also be considered when several horses experience fever, anemia, edema, progressive weakness or weight loss, particularly when new animals have been introduced into the herd or a member of the herd has died. (2) Some differentials include illnesses which display the same clinical signs: equine viral arteritis, purpura hemorrhagica, leptospirosis, babesiosis, severe strongyliasis or fascioliasis, phenothiazine toxicity, autoimmune hemolytic anemia, parasitic infection and other diseases that cause fever, edema and/or anemia. (2,9) Diagnostic tests should be performed to eliminate these differentials (see diagnosis). All of the clinical signs seen are eventually related to the progressive anemia (e.g. dependent edema, accelerated breathing, loss of appetite, thirst, depression and muscular weakness) (9)

The clinical signs of Equine Infectious anemia can vary considerably. The symptoms can be broken down into 3 categories of differentiation:

Acute: The acute form is a apparent onset of the disease. Symptoms include: high fever, anemia, weakness, swelling of the lower abdomen and legs, weak pulse, heart arrhythmias, thirst, perspiration, debility and depression. Thrombocytopenia has been described as the earliest and most consistent abnormality observed in febrile horses infected (8). Death is even a possibility. In the acute stage, fever attacks are severe, often reaching 105;  The possibility of death is apprent when/if there are no, or few, remissions in temperature.(9) Signs of anemia are not  prominent at the onset and pallor of the mucous membranes occurs later in the course of the disease.  Those that do not die during the acute phase of the disease recover or progress into the subacute or chronic phases. (9)  Infected horses may continue to ooze blood from fly bites, needle punctures or other wounds for some time after the injury has been inflicted. The anemia associated with the virus usually coincides with the first day of fever and the hematocrit may return to normal upon the return to normal body temperature. (9)

Subacute: Subacute occurs after "recovery" from the acute condition. It is a slower and less severe progression of the infection. Symptoms include recurrent fever, weight loss, an enlarged spleen, anemia, and swelling of the lower chest, abdominal area, perianal area and legs. Horses may display a repeated intermittent fever interdispersed with periods of normal temperature. The duration of this subacute phase may be weeks, or even months (9)

Chronic: Horse tires easily and is unsuitable for work. May have a recurrent fever and anemia, may relapse to the subacute or acute form even several years after the original attack. A horse in the chronic phase of the disease has milder temperature rises (101.5 to 1020 F.) occurring at intervals of one to three months or more, over a period of months to years. Horses in the latter phase of the disease may be apparently healthy to  all intents and purposes (the so-called carrier animal), or the disease may progress, resulting in  a chronically ill horse having emaciation, edema of dependent parts, debility and a progressive anemia.  In the latter phase the signs are less spectacular  but the anemia is progressive and the animal is debilitated. The animal which poses the real threat to other horses is not the one which is acutely ill and thus isolated from other horses, but the chronic case which shows negligible or no clinical signs. Such animals have a constant viremia and thus are able to infect other horses by inoculation of infected blood by means of biting insects, or hypodermic needles (9) The anemia seen in infected horses is a normocytic, normochromic anemia; that is, the erythrocytes are normal in size, shape and staining qualities. It does not appear to be a hemolytic anemia, at least of the intravascular type, but rather a sequestration and destruction of red blood cells in the liver and spleen. (9)

Horses remain carriers of the virus for life, and unlike the majority of viral infections, the presence of serum antibodies is therefore indicative of an infection rather than exposure to the virus at some time (8)