Can be attributed to two types of sources :

A) Inorganic         
B) Organic

A) Inorganic Sources

Excessive calcium intake in broiler chicks results in urolithiasis with urate deposits on the abdominal viscera and in the joints. Tetanic convulsions can also be seen in chicks consuming excess calcium. Calcium levels >2% will induce these lesions in broilers. Feeding calcium in excess of 3% before the onset of egg production will induce the same lesions in egg-type or meat-type pullets.
Carbon Monoxide:
This poisoning commonly happens because of exhaust fumes when chicks are being transported by truck or improper ventilation in hatchers. Mortality may be high, fresh air helps immediately. At necropsy, the beak is cyanotic, and a characteristic bright pink color is noted throughout the viscera, mainly lungs.Can be diagnosed by spectroscopic analysis of  blood.
Copper sulfate in a single dose of >1 g is fatal. The signs are watery diarrhea and listlessness. A catarrhal gastroenteritis and burns or erosions in the lining of the gizzard, accompanied by a greenish, sero-mucous exudates throughout the intestinal tract.
Diazinon is an organic phosphate that is commonly used for the control of a variety of insects around poultry houses. Some producers have used diazinon in poultry houses for the control of fire ants, thoughIt should not be. Chickens consume the diazinon crystals, which results in lacrymation, diarrhea, dyspnea, and death. Necropsy lesions include lung edema, fatty livers, and severe enteritis. The diazinon crystals can be seen in the crop and gizzard contents.
Lead poisoning usually is caused by paint or orchard-spray material. Metallic lead in amounts of 7.2 mg/kg body wt is lethal. Signs are depression, inappetence, emaciation, thirst, and weakness. Greenish droppings are commonly seen within 36 hr. As poisoning progresses, the wings may be extended downward. Young birds may die within 36 hr after ingestion. Acute lead poisoning may be diagnosed from the history and necropsy findings of a greenish brown gizzard mucosa, enteritis, and degeneration of the liver and kidney. Chronic poisoning results in emaciation and in atrophy of the liver and heart. The pericardium is distended with fluid, the gallbladder is thickened and enlarged, and urate deposits are usually found in the kidneys. Ingestion of lead shot often occurs in wild waterfowl on heavily gunned feeding grounds. Retention of only a small quantity of lead in the gizzard can kill a duck.
Poisoning occurs from mercurial disinfectants and fungicides, including mercurous chloride (calomel) and bichloride of mercury (corrosive sublimate). Clinical findings are progressive weakness and incoordination. Diarrhea may occur, depending on the amount ingested. The caustic action of the chemical may produce gray areas in the mouth and esophagus, which usually ulcerate if the bird lives >24 hr. Catarrhal inflammation of the proventriculus and intestines may occur; if a large amount of mercury is ingested, extensive hemorrhage may occur in these organs. The kidneys are pale and studded with small, white foci. The liver shows fatty degeneration.
The addition of 0.5% salt (NaCl) to the ration of chickens and turkeys is recommended, but amounts >2% are usually considered dangerous. Rations for chicks have contained as much as 8% without injurious effect, but in poults, rations containing 4% were harmful and levels of 6-8% have resulted in mortality. The addition of 2% NaCl to the feed, or 4,000 ppm in the water, depresses growth in young ducks and lowers the fertility and hatchability of the eggs in breeding stock.
Salt levels high enough to produce poisoning may be reached when salty protein concentrates (eg, fish meal) are added to rations already fortified with salt or when the salt is poorly incorporated in the feed. Sporadic poisoning also has been reported from accidental ingestion of rock salt or salt provided for other livestock. Necropsy findings are not diagnostic; enteritis and ascites are common. Watery droppings and wet litter often are suggestive of a high salt intake. Edema of the testicle is pathognomonic of salt toxicity in young birds.
Ingestion of feeds containing >5 ppm of selenium decreases the hatchability of eggs due to deformities of the embryos, which are unable to emerge from the shell because of beak anomalies. Eyes may be unilaterally hypo-plastic or aplastic, and feet and wings may be deformed or underdeveloped. Selenium at 10 ppm, as in seleniferous grains in the laying ration, usually reduces hatchability to zero. Young laying hens entering egg production are more susceptible than older hens.

Mature birds seem to tolerate more selenium in their feed than do pigs, cattle, or horses and do not exhibit signs of poisoning other than poor hatchability of their eggs. Starting rations containing 8 ppm selenium have reduced the growth rate of chicks, but 4 ppm had no noticeable effect. Rations containing as little as 2.5 ppm have resulted in meat and eggs with concentrations of selenium in excess of the suggested tolerance limit in foods. Sodium arsenite and some of the organic arsenicals, when administered to laying hens with selenium, have increased hatchability.
Sulfur is often used in broiler houses in an attempt to improve growth rate and feed conversion and to minimize bacterial disease. The compound is applied to the floor after the litter has been removed. If the amount of new litter placed in the house is inadequate, young chicks will come in contact with the sulfur, resulting in conjunctivitis and cutaneous burns, especially under the wings and on the legs. Clinically, the birds appear cold and tend to huddle; in many instances, death will occur due to the birds piling-up, causing overheating and suffocation. When sulfur comes into contact with moisture, sulfuric acid is produced, which results in the burns.

B)Organic Sources

Various organic chemicals are dangerous, especially those used to treat seed grain.

Coffee Weed Seed:
Cassia obtusifolia seeds are frequently found in corn and soybeans. When present at ≥2%, they reduce feed intake and lower body weights, increase feed conversion in broilers, and significantly depress egg production in laying birds. Necropsy lesions are absent.
Seeds of many species are toxic to chickens. Concentrations >0.05% in the feed produce signs of toxicosis. At 0.2%, weight gain is reduced markedly; 0.3% causes death in 18 days. Lesions consist of ascites, swelling or cirrhosis of the liver, and hemorrhages. Resistance to the toxin increases with age.
Cottonseed meal contains appreciable amounts of gossypol, which produces severe cardiac edema that results in dyspnea, weakness, and anorexia. When fed to laying hens, gossypol also causes egg-yolk discoloration.

Lasalocid is an anticoccidial compound that has been used in hot summer months because it increases water consumption. When used at other times of the year, the level of salt in the ration is reduced to prevent excessive water elimination and wet litter problems. If the salt level is reduced too much, it will result in stunting, increased lameness, and a characteristic clinical picture in broilers manifested by the bird walking on its toes. This clinical syndrome has been called lasolocid toxicity when, in reality, it is due to low levels of salt in the feed.

This ionophore coccidiostat is widely used in the broiler industry. At levels >120 ppm, it reduces feed intake and weight gain; in layers, egg production is reduced. Signs of toxicity include a characteristic paralysis in which the legs are extended backward. Mortality occurs in naive turkeys if they are switched to a feed that contains monensin. They become paralyzed with the legs extended backward; no lesions are seen at necropsy.
This coccidiostat is used in broilers. It should not be fed to layers, as it can cause discoloration and reduced hatchability of eggs (although the effect is reversible once the nicarbazin is withdrawn). It also may result in reduced heat tolerance in birds exposed to high temperature and humidity.
This has been used to treat several bacterial diseases in poultry, but is no longer approved for use in the USA. When fed at 0.022%, it causes hyperexcitability manifest by rapid movements, loud squawking, and frequent falling forward. In turkeys, which are more sensitive to nitrofurazone than are chickens, it produces cardiac dilatation, ascites, and when fed at 0.033%, death.
3-Nitro-4-hydroxyphenylarsonic Acid:
When this compound, widely used in feed to improve weight gain and feed efficiency, is improperly mixed or fed at a level 2-3 times higher than normal, it induces a high-pitched chirp and a “duck-walking” stance. Cervical paralysis is frequently seen in chickens that consume excessive amounts. Clinical signs are usually reversible in a few minutes. Chronic exposure may produce intrahepatic cholangitis.
Polychlorinated Biphenyls (PCB):
Residues have been reported in the fatty tissue of chickens and turkeys in excess of the 5 ppm permitted in edible tissue, and in egg products in excess of the permitted 0.5 ppm. PCB depress egg production and hatchability, and levels of 50 ppm result in cirrhosis of the liver and ascites in broilers and a drop in egg production and hatchability in hens.
Quaternary Ammonia:
Quaternary-ammonia-based compounds are widely used as disinfectants. Turkeys are very sensitive; levels of 150 ppm result in substantial mortality. Clinical signs include reduced water intake, nasal and ocular discharge, facial swelling, and gasping. Necropsy lesions include caseous ulcers at the base of the tongue and commissures of the mouth.
Salinomycin is commonly used as an anticoccidial compound in the broiler industry. When used at 60 g/ton of feed, the compound is safe. Toxicities occur when broiler feed containing salinomycin is accidentally fed to naive breeder hens. Clinical signs in these hen flocks include paralysis with the legs extended backward and decreased feed consumption, egg production, and hatchability. Levels of salinomycin >10 g/ton in breeder-hen feed are sufficient to produce these clinical signs. Necropsy lesions are absent in birds with this clinical picture.
Sulfonamides are widely used for treatment of several bacterial and protozoal infections in poultry. Sulfaquinoxaline, when fed at 0.25%, results in severe pancytopenia. Hemorrhages are common on the legs, breast muscle, and in virtually all abdominal organs. The bone marrow is pale, and the blood is slow to clot. Toxicity is frequently seen in hot weather when sulfaquinoxaline is provided in drinking water. Water consumption increases rapidly as the temperature increases, which leads to increased drug intake. This toxicity usually is responsive to vitamin K therapy.
Thiram is used to treat seed corn. It is toxic to chicks at 40 ppm and to goslings at 150 ppm; it causes leg deformities and weight loss. At 10 ppm, it causes soft-shelled eggs, and at 40 ppm, egg production and hatchability are reduced. Turkey poults tolerate up to 200 ppm.
Toxic Fat:
A crystalline halogen has been identified as the “toxic fat” factor in some feeds. In young pullets, it reduces growth, retards sexual development, and increases mortality. Hatchability is decreased. Turkeys and ducks are less susceptible than chickens. Signs of intoxication include ruffled feathers, droopiness, and dyspnea. Lesions include ascites and hydropericardium, liver necrosis, subepicardial hemorrhage, and bile duct hyperplasia. Although the amount of toxin varies in feeds from different sources, 0.25-0.5% fed for 35-150 days produces typical lesions.
Reference :- Merck Veterinary Manual
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