Daniel P. Shaw Gamebirds can be afflicted with musculoskeletal and neurologic diseases that are similar to those found in domestic poultry species. The condition tends to be seen mainly in meat birds, such as broilers and turkeys, during the growing period. It can, however, occur in any class of gamebirds and poultry. The condition develops when bacteria enter the bloodstream and lodge in the tiny capillaries of the bone, particularly in the growth plate. In this location, they can grow and damage bone by inducing inflammation. Bone tissue is difficult for the body to defend effectively. Once injured by infection, the damaged bone breaks down. This results in lameness if a leg bone is involved, or impairment of the use of the wings. Lameness becomes noticeable when body weight increases because the damaged bone bulges and stimulates the pressure receptors in the periosteum. This is interpreted as pain and causes limping. There are several bacteria that can cause the infection. At necropsy, the ends of the affected long bones, such as the proximal tibia, should be sectioned sagittally to reveal the growth plate area. The gross changes can be subtle but if several birds are examined, there will be at least a few that have obvious damage, such as accumulation of caseous tan material in the center of the growth plate area. Bacterial culture of the damaged bone is required for identification of the causative organism. Extensive bone damage may occur in chicks from a flock that had septicemia from infection with Salmonella sp. Infection with Escherichia coli is a common cause of osteomyelitis in birds of various ages. There is a poor prognosis for affected birds. Broad‐spectrum antibiotic treatment should be administered to the flock to try to protect the birds that do not have yet osteomyelitis. It may also help some of the less severely affected birds in the flock. Prognosis is poor for birds that have bone damage. Even if the bacteria are killed by the antibacterial therapy, it is unlikely that the damage can be repaired. Maintaining good sanitation is important in preventing this disease. Mycoplasma synoviae (MS) is a unique, fragile bacterium that lacks a cell wall. It is very fastidious in its growth requirements on artificial media and can be difficult to isolate from infected birds. It can be transmitted vertically, from an infected hen to her chick through the egg. It can also be transmitted horizontally, between birds. Natural infection occurs in ducks, geese, guineafowl, pigeons, Japanese quail, pheasants, red‐legged partridge, chickens, and turkeys [1]. There is variation in pathogenicity of different strains of the bacterium, but infection can cause synovitis and arthritis. The organism can be carried between groups of birds on shoes, hands, and clothing. Lame birds with swollen hocks and foot pads will appear. The swollen hocks and foot pads contain inflammatory exudate. The affected birds are reluctant to move, have a limping gait, and develop breast blisters. Younger birds are more severely affected and may develop bone deformities. Some of the infected chicks may also develop airsacculitis. Several different agents may cause synovitis and swollen foot pads. Laboratory workup is required to identify the cause. Many veterinary diagnostic laboratories offer a very sensitive and specific PCR assay that is performed on preparations of tracheal or pharyngeal swabs. Affected joints from at least a few birds should be cultured to determine if some other bacterium is present. Serologic tests for detection of antibody are available specifically for chickens and turkeys but should function well enough for gamebird species. These include the enzyme‐linked immunosorbent assay (ELISA), serum plate agglutination (SPA), and hemagglutination inhibition (HI). The detection of antibody indicates that the bird has been exposed to the organism at least 1.5 weeks previously. The primary immune response requires 10–14 days for development. This is a chronic condition and infected birds are carriers for life. Administration of an appropriate antibiotic in the water or feed often decreases the severity of clinical signs but is unlikely to eliminate the organism from an infected flock. Prevention requires that chicks be purchased from breeding flocks that are free of the organism. The National Poultry Improvement Program (NPIP) promotes and manages a program aimed at prevention and elimination of this organism in poultry species. Chicks specifically certified by NPIP against the presence this organism are derived from breeding stock that is free of it. Pododermatitis is also known as bumblefoot and affects the bottom of the footpad [2]. It appears as varying degrees of irritation, up to and including the formation of a large thick ulcerated area. It can affect any species of gamebird at any age. The cause can be obvious, such as injury from overly rough bedding or excessively moist litter, or obscure. The source of the problem, however, is not always apparent. Affected birds may be reluctant to walk. While walking, affected gamebirds may display lameness. The footpad is often reddened and swollen in the early stages. Without intervention, this lesion can progress to formation of an ulcerated sore that is covered by a mat of necrotic exudate. There is often an underlying cavity filled with thick exudate. It can progress deeper into underlying tissue and involve the tendon sheaths. The condition is readily identified with physical examination. Several birds should be examined to determine the extent of the problem in the flock. It is helpful to find the cause of the irritation which often arises from wet litter or overly rough flooring surfaces. Jumping from high perches to rough flooring may also be suspected as a cause. Sometimes, dietary deficiencies can cause weakness in the skin of the ball of the foot and predispose it to secondary infection. The lesion can be cultured to identify the causative bacterium, but the area is usually so contaminated that it is difficult to isolate the original inciting agent. Broad‐spectrum antibiotics can be administered through the water or feed to help prevent the infection developing in other members of the flock. The poor blood circulation in the foot impairs healing and delivery of antibiotics to the affected area. Topical administration of antibacterial agents and bandaging may be helpful in treating individual cases. Steps should be taken to improve the condition of the flooring to prevent injury to the feet of other birds in the group [3]. Marek disease (MD) is a neoplastic disease of the lymphocytes caused by infection with a specific herpesvirus. It is primarily a disease of chickens but it can also infect pheasants, quail (coturnix and bobwhite), domestic turkeys, and crested partridges [4–9]. The neoplastic lymphocytes proliferate and form tumors in different organs, including liver, spleen, proventriculus, and kidneys. Tumors may also form in the skin (skin leukosis). The neoplastic lymphocytes from MD also typically infiltrate peripheral nerves and the central nervous system (CNS) and cause impairment of their function. The incubation period can be as short as 5 days. In chickens, outbreaks usually occur after 8–9 weeks of age. The disease in quail tends to produce tumors in visceral organs and nerves are not as commonly involved. Pheasants appear to be more susceptible to the neurologic form of the disease but also develop tumors in visceral organs. Signs of the neurologic form include paralysis of a limb or torticollis (twisted neck), depending on which nerves are affected by the infiltrating neoplastic lymphocytes. Other signs are dependent on which internal organs are involved. Diagnosis is based on gross and microscopic changes in tissues. Grossly, the tumors in visceral organs are soft, tan‐white, and variable in size and shape. In addition, the neoplastic lymphocytes of MD invade the nervous system. Peripheral nerves may be grossly enlarged and have a yellowish discoloration. The eyeball may be involved, causing the iris to be discolored and the pupil misshapen. Microscopically, the neoplastic lymphocytes are visible invading nerves, brain, or eye [9, 10]. In the internal organs, the neoplastic lymphocytes form sheets that invade the structures of the affected organ. There is no treatment for the disease. Vaccines are available to prevent the development of tumors in chickens. The vaccine, however, does not prevent infection with the virus. There are no vaccines available for gamebirds. Typical of herpesviruses, once infected, a bird is a carrier for life and can shed the virus periodically. The virus itself is delicate. It is transmitted via the airborne route in the feather follicle dander which protects it. It can remain viable in poultry house dust for several months at 20–25 °C and for years at 4 °C. It is, therefore, important to clean and disinfect between broods to eliminate the virus or, at least, reduce the level of exposure for new chicks. Occasionally, gamebirds have been observed to have crooked toes. The condition is also known as curly toe paralysis or curled‐toe paralysis. It is caused by a deficiency of vitamin B2 (riboflavin) in young chicks [3, 11]. Any species of gamebirds or poultry can be affected [9]. The deficiency causes damage to the nerves that control the legs and toes. Affected chicks are reluctant to walk. When forced to walk, they may walk on their hocks. When examined, the toes of affected chicks curl medially (inward). One or both legs may be affected. Many of the chicks may have paralysis of the legs. In advanced stages, they lie with their legs sprawled out. Depending on the severity of the deficiency, the flock may be unthrifty and the chicks stunted. Diarrhea may develop by 2 weeks of age. Breeding hens fed a deficient diet will have decreased egg production, poor embryo survival, and decreased hatchability. The embryos that fail to hatch are dwarfed and have “clubbed” down. “Clubbed” down results from failure of the down feathers to rupture the feather sheaths, causing them to coil in a characteristic way. The clinical signs are typical for crooked toes. Microscopic examination will reveal the damage to the sciatic, cervical, and lumbar spinal nerves characterized by axonal swelling and demyelination [9]. There is also atrophy of the muscles in the legs. If this condition is suspected, it is important to add a water‐soluble B vitamin supplement that contains riboflavin to the drinking water and supply new fresh starter feed. It is uncommon for feed to be deficient in vitamin B2 due to errors in mixing or formulation. The vitamin is, however, susceptible to degradation when stored for long periods of time or in adverse conditions of heat and humidity. Hence, it is important to provide fresh feed for baby chicks. It is also necessary to feed a balanced, age‐appropriate, and species‐specific formulated ration as gamebird species require higher levels of several nutrients than chickens. Gamebirds do not do well on rations formulated for chickens. If the deficiency has been of long standing, the damage will be permanent. Perosis is also known as splay leg or slipped tendon. It is associated with a deficiency of manganese, choline, or other B vitamins [3, 9]. It is usually seen in young growing birds. Most species of poultry and gamebirds are susceptible. Due to the nutritional deficiency, the groove at the end of the tibiotarsus (back of the hock) is malformed and shallow. This defect allows the gastrocnemius (Achilles) tendon to slip out of the groove. When this occurs, the muscle attached the tendon loses its fulcrum and cannot extend the leg. The condition can affect one or both legs. Affected chicks are unable to extend the affected leg and cannot support weight on that leg (Figure 12.1a) [3]. This condition is easily detected on physical examination of live birds or during necropsy of dead birds (Figure 12.1b) [9]. There is a poor prognosis for affected birds. The addition of water‐soluble vitamins and minerals in the drinking water for 1 week to prevent this disease in the rest of the flock is recommended. It is important to feed a balanced ration appropriate for the species. The only species that does well on chicken feed is chickens. Gamebird species have higher nutrient requirements as supplied in specialized rations or in turkey feed formulations. Hence, it is crucial that gamebirds be fed diets formulated specifically for gamebirds. Figure 12.1 Perosis. (a) White pheasant chick cannot extend right leg due to lateral dislocation of the gastrocnemius tendon. (b) Partially dissected legs. On the left, the abnormal left hock has a gastrocnemius tendon (marked in blue) that is dislocated laterally from groove (bordered with black dots) of the distal tibiotarsus. On the right, the gastrocnemius tendon (marked in blue) of the normal leg extends over the caudal aspect of the hock joint (groove in distal tibiotarsus is marked as the space between the two lines of black dots). Rickets is caused by a deficiency of calcium, phosphorus, or vitamin D, or an imbalance in the calcium/phosphorus ratio in the feed [9, 10]. It is seen in young growing birds and is associated with weakness, inability to stand, and excessive death loss in the flock. Adults develop soft bones (osteomalcia) [9]. The joints of affected chicks are enlarged, especially noticeable in the hocks. The chicks are reluctant or unable to stand. The bones may be soft and rubbery. In fact, these soft bones can often be flexed with minimal discomfort to the affected chick. The diagnosis may be obvious on physical examination of live or dead chicks. At necropsy, the long bones and ribs are softer than expected or rubbery. There may be thickening of the junctions of the ribs with the sternum and spinal column (beading). Deformities of the long bones, sternum, or spinal column may also be present. The long bones can be sectioned sagittally and the growth plate will appear as a wide zone of cartilage (Figure 12.2). The accumulation of cartilage is due to failure of mineralization of the cartilage model in the growth plate. The diagnosis can be confirmed by microscopic examination of the growth plate area [9, 10]. Figure 12.2 Rickets. The proximal tibiotarsal bones have been sectioned sagittally. The normal bone (upper) has a growth plate (arrows) of normal length. The rachitic bone (lower) has a growth plate (arrows) that is abnormally elongated. Treatment includes adding water‐soluble vitamin D to the drinking water as prescribed in the label directions. The feed can be top dressed with a calcium supplement. Because of their tiny size, baby chicks do not eat very much feed very quickly. It is possible for the ration, if improperly stored, to become stale and the vitamin D degraded. It should also be noted that baby chicks are kept inside to facilitate brooding and thus they do not have exposure to sunlight. Sunlight activates the sterols in the dander and the bird ingests the vitamin when it preens. Birds that have access to direct sunlight, therefore, are unlikely to develop a vitamin D deficiency. Bacterial infection of the brain is often manifest as meningitis with perivascular infiltration by inflammatory cells in the encephalon. It can be caused by infection with several different bacteria including E. coli, Pasteurella multocida, Salmonella sp., Riemerella anatipestifer (ducks and turkeys), and Listeria monocytogenes [9, 12–15]. It may occur as a primary infection or as part of a general infection. It may also be secondary to airsacculitis. Affected birds display listlessness, ataxia, torticollis, tremors of head and neck, and depression.
12
Gamebird Musculoskeletal and Neurologic Diseases
12.1 Diseases Affecting the Extremities
12.1.1 Bacterial Osteomyelitis
12.1.1.1 Clinical Signs
12.1.1.2 Diagnosis
12.1.1.3 Treatment and Prevention
12.1.2 Mycoplasma synoviae Infection
12.1.2.1 Clinical Signs
12.1.2.2 Diagnosis
12.1.2.3 Treatment and Prevention
12.1.3 Pododermatitis
12.1.3.1 Clinical Signs
12.1.3.2 Diagnosis
12.1.3.3 Treatment and Prevention
12.1.4 Marek Disease
12.1.4.1 Clinical Signs
12.1.4.2 Diagnosis
12.1.4.3 Treatment and Prevention
12.1.5 Crooked Toes
12.1.5.1 Clinical Signs
12.1.5.2 Diagnosis
12.1.5.3 Treatment and Prevention
12.1.6 Perosis
12.1.6.1 Clinical Signs
12.1.6.2 Diagnosis
12.1.6.3 Treatment and Prevention
12.1.7 Rickets
12.1.7.1 Clinical Signs
12.1.7.2 Diagnosis
12.1.7.3 Treatment and Prevention
12.2 Diseases Affecting the Head
12.2.1 Bacterial Infection of the Brain
12.2.1.1 Clinical Signs
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