Avian Lymphomatosis

B. R. BURMESTER AND NELSON F. WATERS.

LYMPHOMATOSIS, an infectious disease, leads to tumorous accumulations of lymphoid cells in various parts of the body. Three forms of the disease complex reflect the location of the pathologic alterations visceral (internal organs), neural (nerve), and ocular (eye) lymphomatosis.

The disease is widespread in flocks of chickens throughout the United States and the world.

An estimate of its importance was had in results of the New York Random Sample Test, in which many poultry breeders from different sections of the country entered their stock from 1950 to 1954. The yearly average total mortality of the hens, which were hatched and reared from eggs received at the test, was 34 percent. Lymphomatosis was responsible for 13.4 percent; the visceral form caused three-fourths of this mortality. If that death rate were applied to the total number of chickens on farms, it can be estimated that in 1954 more than 59 million mature chickens died of it.

Heavy losses from neural lymphomatosis occurred on many farms in the 1920's. The visceral form became more prevalent after 1930 and has been responsible for two-thirds to three-fourths of all mortality caused by this disease complex.

At no time has the ocular form caused so high a death rate as the other two forms.

Soon after this disease complex became a definite economic hazard in poultry production, many State experiment stations and other laboratories began investigations for the purpose of developing measures for its prevention and control. Because of the many difficulties encountered in this research, certain agricultural experiment stations sponsored the establishment of a central laboratory. The United States Regional Poultry Research Laboratory, at East Lansing, Mich., approved on December 23, 1937, by the Secretary of Agriculture. Its purpose was designated as the improvement of viability in poultry. Since its inception, most of the work has been devoted to investigations on lymphomatosis and associated diseases. The work at the laboratory and that done at the State agricultural experiment stations has resulted in much information about many phases of this disease, but a great deal more research will be needed before adequate prevention or controls can be developed.

The three forms of lymphomatosis are related to several other neoplastic (cancerous) conditions, all of which are grouped in "the avian leukosis complex." These associated diseases of minor or no economic importance--include osteopetrosis (bone disease), the true leukemias, erythroblastosis, granuloblastosis, and myelocytomatosis (blood diseases). The names are based on the pathologic alterations observed without regard to their causes.

lymphomatosis is limited primarily to chickens. We have indications that its occurrence in turkeys has been increasing. It has been reported in pheasants. Symptoms and lesions typical of neural lymphomatosis have been noted in ducks.

Pathologic manifestations of the disease are varied. The common indication is the uncontrolled accumulation of blood-forming cells and closely related cells.

THE VISCERAL LYMPHOMATOSIS also known as big liver disease, lymphocytoma, or lymphomatic leukosis may occur at any age after 4 weeks. Most deaths usually occur soon after the pullets are in heavy production. Mortality then drops gradually but continues as long as the chickens are kept.

Outward signs of the disease are variable. It may be acute or chronic. Fast-growing pullets may become listless, droopy, and succumb within a few days. Laying hens may cease laying abruptly and soon die. Other birds may show similar symptoms over a long period, becoming emaciated and dying after a long illness.

As the disease advances, the comb becomes shriveled and pale or bluish. A white, pasty deposit of uric acid alone or in combination with greenish bile pigments often is found on the abdominal feathers.

Certain internal organs of birds that are not excessively fat may be felt; enlargement of the liver may be recognized in that way by its projection beyond the keel and the margin of the ribs.

The lesions characteristic of visceral lymphomatosis are found oftenest in the liver, spleen, and kidneys, but all organs, including the skin, at times are affected. The gross alterations may be a general enlargement, with a variation in color from dark red to light gray in organs such as the liver, spleen, and kidneys. These represent the diffuse type of involvement and often require microscopic examination for diagnosis. More frequently discrete tumor nodules, one-sixteenth to one-half inch or more in diameter, are located in the organs. These nodules are spherical. When they are on a surface, they are flattened or merge with other nodules. They are grayish, and the cut surface is uniform in color; similar nodules seen in tuberculosis are white and usually contain yellowish centers.

The microscopic picture is comparatively uniform and is characterized by massive accumulations of lymphoid cells, which are of two types. The small round cells, resembling lymphocytes, usually make up the tumors of chronic cases. The large cells, like lymphoblasts, occur in the acute type. Intermediates and mixtures of the two types are often found.

In the absence of large accumulations, difficulties often arise in the interpretation of microscopic changes. Birds, with few exceptions, are without organized lymph nodes, but microscopic lymphoid foci are present in the glandular organs and the digestive tract. Because these foci respond to the presence of several disease agents, it often is difficult to state whether a given lymphoid focus represents normal, reactive, or neoplastic tissue. All such foci should be considered abnormal; they may represent a predisposition toward lymphomatosis.

THE NEURAL LYMPHOMATOSIS also known as neurolymphomatosis, fowl paralysis, or range paralysis occurs oftenest in birds 2 to 6 months old, although the rate of its appearance reduces only slowly after 6 months and it has been seen in birds as early as 3 weeks of age.

The sign of this disease most familiar to the poultryman is a spraddling position, in which the chicken may lie on its side with one leg extended forward and the other backward. Less advanced cases may show a weakness or lack of coordination of the legs, wings, or neck, so that the bird has trouble in standing or walking, and a drooping of wings and head.

Nerves supplying the digestive tract may be affected, so that there are loss of weight and appetite, dilation of the crop, and diarrhea.

Respiratory distress, such as gasping, may result when one or both vagus nerves are involved. Only one of several nerves of the same bird may be affected. Survival depends largely on the extent, location, and function of the nerve or nerves affected.

Partly paralyzed birds may live many months and sometimes show partial recovery when placed in individual cages with access to feed and water.

Similar symptoms may occur in chickens with other diseases, such as those due to riboflavin deficiency or avian encephalomyelitis. A microscopic study of the affected nerves is the basis of a differential diagnosis.

The gross pathologic alterations of the affected nerves are characterized by localized or extensive soft swellings of the peripheral nerve trunks, which are yellowish or slightly grayish. In less obvious cases, the affected nerves may show only a loss of cross-striations, which are seen in normal nerves. Microscopic examination is often necessary for an accurate diagnosis. The nerves most commonly affected are the sciatic (large nerve in thigh) and its roots, the vagus (easily seen in the neck), and the brachial (shoulder and arm), celiac, and lumbar (viscera) plexuses.

Microscopic examination of affected nerves shows an infiltration with the small, round lymphocyte with a few large cells not unlike the lymphoblast. Occasional infiltrations are made up predominately of cells of the latter type.

OCULAR LYMPHOMATOSIS, formerly known as gray eye, pearly eye, or iritis, causes impairment of vision or complete blindness in one or both eyes. That is because lymphoid cells enter the iris and other vascular parts of the eye. This infiltration occurs over a period of several weeks and finally stops the normal dilating and constricting movements of the iris. The iris becomes fixed, usually in a restricted state, and the pupil becomes small. The pupillary border may become irregular, show indentations, and lose its regular circular shape.

These changes in form and loss of movement are perhaps more diagnostic than the change in iris color, which is a more commonly observed symptom.

The infiltrating cells also mask the pigments of the iris and cause a change from an orange or reddish brown to a pearly gray. The amount of pigment in the iris is influenced by the diet, the rate of egg production, and the age of the chicken; furthermore, some breeds and crosses naturally have a gray iris. It is important to consider these normal variations in the color of the iris, arriving at a diagnosis of ocular lymphomatosis.

OSTEOPETROSIS, commonly known as marble bone or thick leg disease, afflicts an occasional bird in some flocks or a sizable number in other flocks, but it is not so widespread as to be of major economic importance. An enlargement of the shanks, giving a convex appearance, is usually the first symptom in chickens.

As the disease progresses, other bones, especially the long bones of the wing and of the leg, become enlarged. Articulation becomes restricted, and the chicken walks in a stiff, stilted manner. The histopathologic picture varies with the stage of the disease in the bone examined. The excessive deposition of hard bone appears to be due to a greatly increased proliferation and activity of the bone-forming cells of the inner and outer surfaces of the shaft and a reduced activity of the bone-dissolving cells.

THE TRUE LEUKEMIAS in chickens are of two types.

In erythroblastosis, the immature forms of the erythrocyte (the red blood cell) are dominant in the blood. When the leukemia is due to an overabundance of the immature forms of the granulated white cells, it is known as myeloblastosis or granuloblastosis. Often both cell lineages are involved. This condition is referred to as erythromyeloblastosis.