There are many venomous snakes around the world. Australia is notorious for having more than its fair share of highly venomous reptiles. The two major families of snakes of clinical importance to humans and horses are the Elapidae, and the Viperidae with sub-family Crotalinae. The Elapids are found in many regions of the world and are very common in Australia. The Crotalids are also common in many regions, with the exception of Australia.
Horses are curious animals and consequently many bites occur around the muzzle. This is particularly true for rattlesnake bites in North America. The other major site of envenomation are the limbs, particularly when horses do not see the snakes or fail to avoid aggressive snakes.
In Australia snake envenomation is commonly used as an explanation for the unexpected death of apparently healthy horses. This is often without physical and laboratory evidence of snake bite.
The signs of envenomation do vary slightly between these 2 major families of snakes.
Elapids
This group includes tiger snakes (Notechis species), brown snakes (Pseudonaja species), black snakes (Pseudechis species), death adders (Acanthophis antarcticus), cobras (Naja species), coral snakes and mambas (Dendroaspis species).
Common features of elapid envenomation include altered mentation, generalized weakness, high heart rates, and muscle inflammation/damage. Some horses experience haemolysis.
Crotalids
This group includes rattlesnakes (Crotalus species), cottonmouth (Agkistrodon piscivorus) and copperhead (Agkistrodon species), and pigmy rattlesnakes (Sistrurus species).
Crotalid envenomation is typically characterized by severe local swelling (within 12-24 hours of the bite), red blood cell destruction (haemolysis) and spontaneous bleeding due to coagulation dysfunction. Many rattlesnake bites occur on the muzzle, leading to muzzle swelling, oedema of the nasal bones, and airway obstruction. This airway blockage can be severe enough to cause death from asphyxiation, necessitating the emergency placement of a tracheal tube.
Diagnosis
The diagnosis of elapid envenomation can be difficult. Although suspected the attack is rarely witnessed and bite marks may be hard find on legs. Treatment may be initiated on the basis of signs. Although most bites occur in warmer months they are known to occur in any month. Ideally a snake venom detection kit is used. Samples include urine, blood, or fluid from bite sites. The venom is rapidly cleared from blood so urine may be a better sample.
Principles of therapy
For snake envenomation for the primary treatment is the use of antivenom (aka antivenin). In an ideal world the identification of the offending snake is known and therefore monovalent antivenom can used. In Australia most horses receive polyvalent antivenom. For example, the product in Australia contains antivenom against black snakes, taipans, death adders, tiger snakes, and brown snakes. In the US there are several polyvalent antivenom products directed against pit vipers, including rattlesnake, copperhead, and cottonmouth bites. The required number of vials does vary, and can range between 1 and 5 vials, although some horses have received more. A positive response to antivenom is more obvious after elapid bite, as the severe generalized weakness and altered mentation is often improved.
An important feature of therapy after crotalid envenomation is maintaining an airway. As you can see from the images above the swelling after a bite to the muzzle is substantial with obstruction of the nasal passages. An airway is provided by insertion of a tube into the trachea. I have witnessed a vet place two cut-off stomach tubes through the nostrils into the nasopharynx prior to referral. This act saved the horse’s life. It is also important to keep the head elevated so as not to worsen the head swelling.
Antibiotics are often used as local infection (including Clostridial species) after the bite is common. Fluid therapy should be used cautiously in horses that have haemolysis.
Further reading: Nick Bamford and others, 2018 doi:10.1111/evj.12735
Image Credits: Perth Zoological Gardens; Meg Jarrod (Unsplash);
Tags: Toxicities