ETIOLOGY AND PATHOGENESIS

Nearly 90% of venomous snakebites in horses take place between April and October. This is also the time of year associated with the maximum venom yield per bite; however, pit vipers can determine the amount of venom released based on their perception of danger. Rattlesnake venom is typically the most toxic of the common pit vipers, but this varies among species.

A number of enzymes, including phospholipase-A2, lactate dehydrogenase, and arginine ester hydrolase, are present in snake venom, as are numerous nonenzymatic proteins and peptides. These components are responsible for the variety of clinical signs observed in horses and other species, including tissue destruction, weakness, and coagulation disorders. Death rates as high as 25% have been reported for rattlesnake envenomation in horses, which is much higher than that reported in people (0.5%).

CLINICAL SIGNS

The most common clinical sign in horses is swelling at the bite site, usually the muzzle or limb. In one study, more than 90% of snakebites were reported to have occurred on the head of horses. Affected animals may have difficulty in prehending feed; be febrile; and have tachycardia, epistaxis, stridor, and tachypnea. Fang marks cannot always be identified because of the severe local swelling and pain associated with inspection; snakebite should not be ruled out if bite marks cannot be definitively identified. The local edema is typically rapid in onset and marked, making upper airway obstruction and respiratory distress a potential complication and warranting immediate intervention.

DIAGNOSIS

Rattlesnake bites are usually not challenging to diagnose. Differential diagnoses for local swelling and edema should be considered. The time of year, location of the swelling (frequently the muzzle), and identification of a bite wound are strongly supportive of the diagnosis (Figure 205-1).

Figure 205-1 Photograph of a horse bitten on the muzzle by a Western Diamondback rattlesnake. Notice the severe swelling and near occlusion of the nostrils, which necessitated tracheostomy.

Hematologic abnormalities include thrombocytopenia, anemia, and variable degrees of leukocytosis. Prolongation of clotting times (activated prothrombin time and prothrombin time) has also been reported, and the severity of coagulopathy depends on the degree of envenomation. High serum activities of muscle enzymes (creatine kinase and aspartate aminotransferase) are common.

An envenomation scoring system has been developed in human medicine, and a modification of this scoring system for horses is proposed in Table 205-1. The maximum score is 13 points, and horses should be reassessed every 6 hours. This scoring system was applied to a group of eight horses I treated for rattlesnake bite. Of those, all horses with a score less than or equal to 7 survived. Three horses with scores from 10 to 12 died or were euthanized because of clinical deterioration in the face of aggressive supportive treatment.

Table 205-1 Scoring System for Rattlesnake Envenomation^∗

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