Etiology

Snakes

• Crotalid venoms contain a wide variety of lytic enzymes and non-enzymatic proteins:
  • Effects include proteolysis → necrosis, hemorrhage, shock and hypotension.
  • Neurotoxic components are present in some Crotalid venoms; these cause ataxia and paralysis, especially of the respiratory muscles.
  • Disseminated intravascular coagulation Disseminated intravascular coagulation may occur.
  • Usually there is edema and severity of clinical signs depends on the site of the bite.
  • Bites to the head and neck are life-threatening.
  • Horses are obligate nasal breathers and severe edema may cause suffocation through nasal obliteration. 
• Coral snake venom is primarily neurotoxic → depression, weakness, paralysis, hypotension and respiratory distress:
  • Bites to the head and neck are more life-threatening because of proximity to the brain.
  • Lactrodectus venom is primarily neurotoxic, interfering with neurotransmission and neuromuscular function, through release of acetylcholine and adrenaline; this causes pain, ataxia, muscle tremors, muscle cramping and restlessness and paralysis.
  • Severity of signs depends on both the spider and the horse.
  • Older larger spiders are more likely to have more venom.
  • Location of bite is also another factor.
  • Young healthy victims are likely to be less affected than old predisposed victims. 
• Loxosceles bites typically → an indolent ulcer (necrotic arachnidism) that takes a long time to heal. Systemic effects are rare but may include hemolytic anemia Anemia: auto-immune hemolytic (AIHA) with hemoglobinuria, fever, nausea and weakness.

Scorpions

• Scorpion venoms are complex, containing a variety of neurotoxic proteins and non-proteins, eg amino acids, histamine and serotonin.
• Some contain enzymes others do not. 
• Envenomation causes release of acetylcholine.
• Reported effects in pets include parasympathomimetic syndrome, including skeletal muscle stimulation and hypertension. 

Ticks

• The toxin responsible for tick paralysis interferes with acetylcholine synthesis or release at neuromuscular endings. 

Lizards

• Heloderma venom is complex including hydrolytic enzymes (phospholipase A2, hyaluronidase) and gilatoxin, a neurotoxin.
• Heloderma lizards typically hang on and chew when they bite, causing local tissue damage.
• Although there is potential for envenomation in horses, currently there are no reports.

Bees/wasps/hornets/ants

• Hymenoptera venoms are a complex mixture of toxic chemicals, including formic acid, enzymes, dopamine, amino acids, etc.
• Locally there is pain and swelling.
• Systemic effects may include cardiovascular and respiratory collapse.
• Multiple bee stings cause frenzy in horses and could cause urticarial reaction Urticaria / angiedema, prostration and hemoglobinuria.

Timecourse

Snakes

• Crotalid bites are painful and onset of clinical signs is usually rapid but may be delayed for several hours. 
• Envenomation does not always occur in snakebite.
• If there is no swelling apparent within 2 h of a Crotalid bite, then envenomation has not occurred. 
• Coral snake envenomation may be painless, and onset of clinical signs may be delayed for up to 18 h. 
• Onset of clinical signs of Lactrodectus envenomation occurs within 8 h of the bite. 
• Loxosceles bites typically take several days to become evident but may take months to heal. 

Scorpions

• Stings are painful and onset of clinical signs is generally rapid. 

Ticks

• Paralysis is gradual in onset. 

Lizards

• Heloderma bites are intensely painful and local trauma is immediately apparent.
• It may be necessary to pry the lizard off the patient. 

Bees/wasps/hornets/ants

• Hymenoptera stings cause an immediate reaction.