Osteoporosis or Bighead

By John R. Mohler, V. M. D., Assistant Chief, Bureau of Animal Industry.

Osteoporosis is a general disease of the bones which develops slowly and progressively and is characterized by the absorption of the calcareous or compact bony substance and the formation of enlarged, softened, and porous bone. It is particularly manifest in the bones of the head, causing enlargement and bulging of the face and jaws, thereby giving rise to the terms "bighead" and "swelled head," which are applied to it. The disease affects horses, mules, and asses of all ages, classes, and breeds, and of both sexes, and is found under all soil, dietetic, and climatic conditions. It may occur in sporadic form, but in certain regions, such as South Africa, Australia, Madagascar, India, Hawaii, and in this country it seems to be enzootic, several cases usually appearing in the same stable or on the same farm, and numerous animals being affected in the same district. In the United States the disease has been found in all the States bordering the Delaware River and Chesapeake Bay, in some of the New England States, and in many of the Southern States, especially in low regions along the coast. In Europe the disease appears to be quite rare, and is usually described as a form of osteomalacia, a disease which is not uncommon among cattle of that continent. The opinion that bighead is only a form of osteomalacia, however, can not be accepted, nor can the infrequency of the former among European horses and the frequency of the latter among other live stock be conceded on the argument which has been presented, namely, that the better care which horses receive prevents them from becoming affected. In the Southwest, where osteomalacia, or creeps, has not infrequently been observed among range cattle by the writer, no case of osteoporosis of the horses using the same range has been noted, although the latter animals are given no more attention than the cattle.

The appropriate treatment of osteomalacia in cattle is so effective that if osteoporosis were a similar manifestation of disease a similar line of treatment should prove equally efficacious. However, this is not the fact. On the other hand, the occurrence of osteomalacia on old, worn-out soil, or on land deficient in lime salts, or from eating feed lacking in these bone-forming substances, or drinking water with a lime deficiency, is in perfect accord with our knowledge of the disease. But osteoporosis may occur on rich, fertile soil, in the most hygienic stables, and in animals receiving the best of care and of bone-forming feeds with a proper amount of mineral salts in the drinking water.

Cause.—The cause of this disease still remains obscure, although various theories have been advanced, some entirely erroneous, others more or less plausible; but none of them has been established. Thus the idea that feeding fodder and cereals poor in mineral salts and grazing in pastures where the soil is poor in lime and phosphates will cause the disease has been entirely disproved in many instances. Others have considered that the disease starts as a muscular rheumatism which is followed by an inflammatory condition of the bones, terminating in osteoporosis. The idea that the disease is contagious has been advanced by many writers, although no causative agent has been isolated. Numerous experiments have been made by inoculating the blood of an affected horse into normal horses without results. A piece of bone taken by Pearson from the diseased lower jaw of a colt was transplanted into a cavity made for it in the jaw of a normal horse, but without reproducing the disease. Pétrone believes that the Micrococcus nitrificans causes osteomalacia in man as a result of its producing nitrous acid, which dissolves the calcareous tissues, and when injected into dogs in pure culture a similar disease is produced. It is probable that if this work is confirmed a somewhat similar causative factor will be discovered for osteoporosis.

Elliott considers the latter disease to be of microbic origin, the result of climatic conditions, and divides the island of Hawaii into two districts, in one of which the rainfall is 150 inches annually, where bighead is very prevalent, and the second of which is dry and rarely visited by rain, where the disease is unknown. Removal of animals from the wet to the dry district is followed by immediate improvement and frequently by recovery. In the wet district horses in both good and bad stables take the disease, but in the dry districts no unfavorable or unhygienic surroundings produce the affection. As both native and imported horses are equally susceptible, there is no indication of an acquired immunity to be observed.

Theiler has recently stated that his experiments in transfusing blood from diseased to normal horses were negative, and has suggested that the causative agent may be transmitted by an intermediate host only, as in the case of Texas fever. He draws attention to this method of spreading East African coast fever, although blood inoculations, as in osteoporosis, are always without result. We know that coast fever is infectious, and that it can not be transmitted by blood inoculations, but is conveyed with remarkable ease by ticks from diseased cattle. That the cause has not been observed may be accounted for by its being invisible even to the high magnification of the microscope.

On some farms and in some stables bighead is quite prevalent, a number of cases following one after another. On one farm of Thoroughbreds in Pennsylvania all the yearling colts and some of the aged horses were affected during one year, and on a similar farm in Virginia a large proportion of the horses for several years were diseased, although the cows and sheep of this farm remained unaffected.

Symptoms.—The commencement of the disease is usually unobserved by the owner, and these symptoms which do develop are generally not well marked or are misleading unless other cases have been noted in the vicinity. Until the bones become enlarged the symptoms remain so vague as not to be diagnosed readily. The disease may be present itself under a variety of symptoms. If the bones of the hock become affected, the animal will first show a hock lameness. If the long bones are involved, symptoms of rheumatism will be the first observed, while if the dorsal or lumbar vertebræ are affected indications of a strain of the lumbar region are in evidence. Probably the first symptom to be noticed is a loss of vitality combined with an irregular appetite or other digestive disturbance and with a tendency to stumble while in action. These earlier symptoms, however, may pass unobserved, and the appearance of an intermittent or migratory lameness without any visible cause may be the first sign to attract attention. This shifting and indefinite lameness, involving first one leg and then the other, is very suggestive, and is even more important when it is associated with a tendency to lie down frequently in the stall and the absence of a desire to get up, or the presence of evident pain and difficulty in arising.

About this time, or probably before, swelling of the bones of the face and jaw, which is almost constantly present in this disease, will be observed. The bones of the lower jaw are the most frequently involved, and this condition is readily detected with the fingers by the bulging ridge of the bone outside and along the lower edge of the molar teeth. A thickening of the lower jawbone may likewise be identified by feeling on both sides of each branch at the same time and comparing it with the thinness of this bone in a normal horse. As a result mastication becomes difficult or impossible and the teeth become loose and painful. The imperfect chewing which follows causes balls of feed to form which drop out of the mouth into the manger. Similar enlargements of the bones of the upper jaw may be seen, causing a widening of the face and a bulging of the bones about midway between the eyes and the nostrils. In some cases the nasal bones also become swollen and deformed, which, together with the bulging of the bones under the eyes, gives a good illustration of the reason for the application of the term bighead.

Other bones of the body will undergo similar changes, but these alterations are not so readily noted except by the symptoms they occasion. The alterations of the bones of the spinal column and the limbs, while difficult of observation, are nevertheless indicated by the reluctance of the animal to get up and the desire to remain lying for long periods of time. The animal easily tires, moves less rapidly, and if urged to go faster may sustain a fracture or have a ligament torn from its bony attachments, especially in the lower bones of the leg. An affected horse weighing 1,000 pounds was seen by the writer to fracture the large pastern bone from rearing during halter exercise.

The animal becomes poor in flesh, the coat is rough and lusterless, and the skin tight and harsh, producing a condition termed "hidebound," with considerable "tucking up" of the abdomen. The horse shows a short, stilted, choppy gait, which later becomes stiffer and more restricted, while on standing a position simulating that in founder is assumed, with a noticeable drop to the croup. The animal at this stage usually lies down and remains recumbent for several days at a time. Bed sores frequently arise and fractures are not uncommon in consequence of attempts to arise, which complications, in addition to emaciation, result in death.

The disease may exist in this manner for variable periods extending from two or three months to two years. The termination of the disease is uncertain at best, but is likely to be favorable if treatment and a change of feed, water, and location is adopted in the early stages of the malady.

Lesions.—As has been stated, the bones are the principal tissues involved. The nutrition of the bone is disturbed, as is indicated by the diminished density or rarefaction of the bony substances, the increase in the size or widening of the Haversian canal and the medullary cavity, and the enlargement of the network of spaces in the spongy tissue, the absorptive changes following the course of the Haversian system. In this process of absorption there are formed within the substance of the bone areas of erosion, indentations, or hollow spaces of irregular shape. These spaces increase in size and become confluent, causing an appearance resembling some varieties of coral. The affected bone may be readily incised with a knife, the cut surface appearing finely porous. This porous area is soft, pliable, and yields easily to the pressure of the finger. It has been shown by chemical analysis that the bone of an osteoporotic horse, when compared with that of a normal horse, shows a reduction in the amount of fat, phosphoric acid, lime, and soda, but a slight increase in organic matter and silicic acid. The bones lose their yellowish-white appearance, becoming gray and brittle. The affected bones may be those of any region or portion of the body. Besides the change already noted in the bones of the face, the ends of the long bones, such as the ribs, are involved, and may be sectioned, though not so readily as the facial bones. The bones of the vertebræ are also frequently involved, necessitating great care in casting a horse, as the writer has seen several cases of broken backs in casting such animals for other operations. The marrow and cancellated tissue of the long bones may contain hemorrhages and soft gelatinous material or coagulated fibrin. The internal organs are usually normal, but a catarrhal condition of the gastrointestinal tract may be noted as the result of the improper mastication, resulting from the enlargement of the jaws and soreness of the teeth.

Treatment.—The affected animal should be immediately placed under new conditions, both as to feed and surroundings. If the horse has been stable fed, it is advisable to turn it out on grass for two or three months, preferably in a higher altitude. If the disease has been contracted while running on pasture, place the animal in the stable or corral. In the early stages of the disease beneficial results have followed the supplemental use of lime given in the drinking water. One peck of lime slaked in a cask of water and additional water added from time to time is satisfactory and can be provided at slight expense. This treatment may be supplemented by giving a tablespoonful of powdered bone meal in each feed, with free access to a large piece of rock salt, or the bone meal may be given with four tablespoonfuls of molasses mixed with the feed. Feeds containing mineral salts, such as beans, cowpeas, oats, and cottonseed meal, may prove beneficial in replenishing the bony substance that is being absorbed. Cottonseed meal is one of the best feeds for this purpose, but it should be fed carefully. The animal should not be allowed to work at all during the active stage of the disease, nor should it be used for breeding purposes.