Symptoms and Pathological Features of Snake Venom Poisoning in Animals

Snake venom poisoning is an acute toxic disease caused by venomous snake bites in animals, in which the venom enters the body through the wound and causes hemolysis, paralysis of sensory nerve endings and shock as its characteristic features. It can occur in all kinds of animals, and is commonly seen in grazing animals and dogs.

1. Clinical symptoms

The sites where animals are bitten by venomous snakes are mostly the lips, nose tip, face and lower parts of the limbs. In horses, cattle and sheep, the bite sites are mostly near the hock joint or fetlock joint, while in pigs and dogs they are mostly on the limbs and nose tip. The closer the bite site is to the central nervous system and to areas rich in blood vessels, the faster the onset and the more severe the symptoms. Different species of animals also differ in their response to snake venom after being bitten; equines are the most sensitive, followed by ruminants, while adult pigs are the least sensitive because their abundant subcutaneous fat affects the speed at which the venom enters the blood, whereas piglets often die of poisoning within 1 hour after being bitten. After an animal is bitten by a venomous snake, because the snake's teeth are very small and the wound is not large, it is not easy to detect, but the symptoms appear relatively quickly and the condition progresses rapidly. The severity of the symptoms is also related to the species of venomous snake and the amount of venom injected. After poisoning, animals mainly show both local symptoms and systemic symptoms:

(1) Local symptoms: redness, swelling, pain and bleeding at the site of the snake bite. When the head is bitten, the lips, nose tip, cheeks and submandibular glands become extremely swollen, the animal is restless, and the conjunctiva is congested. In severe cases, the upper and lower lips cannot close, the nasal mucosa is swollen, the nasal passage is narrowed, breathing is difficult, and the conjunctiva is swollen. When the limbs are bitten, there is local swelling, heat and pain, the affected limb cannot bear weight, and when standing the animal rests on the tip of the hoof, or even lifts the diseased limb; when moving there is lameness, and sometimes the animal lies down and cannot rise.

(2) Systemic symptoms: because the toxins contained in venomous snakes act differently, the symptoms are mainly those caused by neurotoxins, hemotoxins and mixed toxins.

(3) Neurotoxic symptoms: the many-banded krait, the banded krait and sea snakes all belong to the neurotoxic type. After being bitten by this type of venomous snake, the local reaction is not obvious, there is little bleeding, and local symptoms such as redness, swelling, heat and pain are mild; however, after being bitten by a cobra, there is local tissue necrosis and ulceration, and the wound is difficult to heal for a long time. When the toxin spreads through the blood and lymph, acute systemic symptoms usually appear within several hours after the bite. First, the muscles of the limbs become paralyzed and weak, the animal lies down and cannot rise, and moans in pain; then swallowing becomes difficult, there is foaming at the mouth, dilation of the pupils, arrhythmia, irregular pulse, difficulty breathing, vomiting and generalized sweating. Finally there is generalized muscle tremor, even convulsions, a drop in blood pressure, shock and coma, and death ultimately occurs due to asphyxia caused by paralysis of the respiratory muscles.

(4) Cardiac damage and coagulation disorders: vipers, the five-paced pit viper, the bamboo leaf green pit viper, the turtle-shell snake and others can all produce blood-circulation toxins. After animals are bitten, the local symptoms are severe, with obvious swelling, intense pain, continuous bleeding, extreme edema, and darkening of the local color; the regional lymph nodes are enlarged, there is subcutaneous hemorrhage, and in some cases blisters and blood blisters occur, even tissue ulceration and necrosis. The swelling spreads rapidly upward along the limb, and generally after 6-8 hours it can spread to the entire head and neck, or spread to the forelimbs and the lumbar and dorsal regions. After the toxin is absorbed, there is generalized tremor, tachycardia, accelerated pulse, and then fever; massive hemolysis causes hemoglobinuria. In severe cases there is a drop in blood pressure, arrhythmia, difficulty breathing, inability to stand, and finally the animal falls to the ground and dies of circulatory failure and cardiac paralysis.

(5) Mixed toxin symptoms: the venom of the cobra and the king cobra mostly belongs to the mixed toxin type. After animals are bitten, local symptoms such as redness, swelling, heat, pain, infection or necrosis are obvious. After the toxin is absorbed, the symptoms caused by neurotoxins and blood-circulation toxins coexist, and the systemic symptoms are severe and complex. The direct cause of death is usually asphyxia caused by paralysis of the respiratory center and respiratory muscles, or shock caused by paralysis of the vasomotor center and cardiac weakness.

2. Pathological changes

Necropsy shows swelling at the bite site, and only in the center of the swelling can the fang marks of the snake bite be found; there is serous infiltration under the skin, and nearby muscles have a boiled-meat appearance. In deaths caused by blood-circulation toxins, hemorrhage of the skin, mucous membranes and internal organs is commonly seen. There is myocardial hemorrhage and necrosis.