Invertebrates
Arachnids
Although tick bites are nowadays most notorious as transmitters of bacterial infections (Borrelia causes Lyme disease), several tick species, such as Ixodus holocyclus (Australia) and Dermacentor andersoni (North America) produce holocyclo toxins, which can result in a lethal paralysis through acetylcholinesterase inhibition at the neuromuscular junction. A serious bite from the black widow spider Latrodectus mactans produces a-latrotoxins. These are proteins of about 1000 amino acid residues that bind irreversibly with presynaptic cell membranes, to produce cation-sensitive channels and interfere with the endocytosis of vesicle membranes. Local clinical signs are negligible; systemic clinical signs range from pain and muscle cramps to hypertension, pulmonary oedema and CNS involvement. Specific antivenom and muscle relaxants are administered in treatment (Piek and Leeuwin 1995; M.E. Peterson, personal communication, 2002). The Australian funnel web spider Atrax robustus, found around Sydney, is far more dangerous, with a bite capable of causing death within 15 minutes. The toxin, robustotoxin, acts by opening sodium channels in axons of the victim. Administration of an antivenom has been shown to be lifesaving and, since its intro duction, no further fatalities have been reported. The related species Hadronynche versutus, which is found in the southern regions of Queensland, Australia, produces versutoxin, the effects of which can also be treated with At. robustus antivenom (Hawdon and Winkel 1997). A different symptomatology is caused by the brown recluse spiders of the genus Loxosceles, of which five species are known in the southern US. Their venom contains a range of enzymes which are responsible for necrotic arachnidism at the biting site and a wide range of systemic symptoms, from fever and arthralgia to diffuse intravasal coagulation and renal failure (M.E. Peterson, personal communication, 2002).
Scorpions constitute a serious public health concern in warm climates. Worldwide, 30 species produce potentially fatal toxins; it is estimated that yearly 5000 human fatalities result from scorpion stings. Examples of scorpion species are Centruroides infamatus, which is responsible for the highest envenomation rate in Mexico; Tityus serrulatus, which causes thousands of envenomations each year in Brazil; and Buthus occitanus, the most venomous scorpion, which is common in North Africa. Scorpion venom contains at least 85 different peptide neurotoxins. The sting site is usually very painful; pain may migrate proximally and hyperaesthesia may occur. Antivenom administration and treatment of hypertension is the recommended treatment.
