Introduction

Invertebrates are becoming increasingly popular as private pets and exhibit animals. Zoological institutions commonly promote insectariums and butterfly houses or gardens. While knowledge of the husbandry and medical care of these animals is important, awareness of the threats that they potentially pose to staff and guests is also meaningful. The following is a brief primer on the human health hazards of terrestrial invertebrates.

Arachnids

Spiders

There are approximately 44,000 known species of spiders in the world.²⁰ Of these, very few present significant health concerns for humans, and even fewer can potentially cause fatality. Though envenomation is often the primary focus when spiders are considered, lesions from urticating hairs, spines, and secondary infections should also be noted.

Spider venoms are complex mixtures of components that have evolved over 300 million years for the purpose of subduing invertebrate prey.¹⁸ Of the species for which venom has been analyzed, six main types of components (and their functions) have been identified: low molecular mass compounds (various), cysteine-rich mini-proteins (neurotoxins), enzymes (membrane/tissue destruction), large proteins (membrane destruction), acylpolyamines (neurotoxins), and linear peptides (membrane destruction).¹⁷ The former three components are found in most spider venoms, and synergistic relationships between the various molecules are common.¹⁷

Despite their impressive size, the venom toxicity of tarantulas (family Theraphosidae) is relatively minor.^8,13 The most common symptoms are local pain and swelling, though systemic effects (e.g., pyrexia, muscle cramps) are possible.^5,8 There is anecdotal evidence that tarantulas from the Eastern hemisphere and those that are arboreal (e.g., Poecilotheria spp.) possess more potent venom than those that are from the American continents or are terrestrial.⁸ However, no confirmed case of human fatality from a tarantula envenomation has ever been documented.⁸

Of more clinical consequence is latrodectism, envenomation by spiders of the genus Latrodectus (widow spiders; family Theridiidae). These spiders are famous for the perceived danger that they represent, as well as the distinctive black and red color patterns of most species. The primary component of widow spider venom affecting vertebrates is the neurotoxic large protein α-latrotoxin, which causes massive neurotransmitter release.^17,31 Common clinical signs include severe pain (local, radiating, regional), local erythema, muscle rigidity (particularly abdominal), diaphoresis, and nonspecific systemic effects (e.g., nausea, emesis, headache, malaise).^6,13,29,31 Hypertension, agitation, pyrexia, and cardiac effects are also possible.^6,29 Clinical signs of brown widow spider (L. geometricus) envenomation are typically similar, but less severe than, those of other widow spiders.^6,29 Latrodectism treatment varies considerably.¹³ Recommended treatments include some or all of the following: local wound care of the bite site, tetanus prophylaxis, analgesics, calcium gluconate, and antivenom.^6,31 Most symptoms last less than one week, and associated fatalities are uncommon.^6,18

Loxoscelism is the name given to the clinical condition resulting from envenomation by spiders of the genus Loxosceles (recluse spiders; family Sicariidae). The brown recluse (L. reclusa), with the dark brown "violin" pattern on its carapace, is the best known of this group. The primary component of recluse spider venom is the enzyme phospholipase D.¹⁷ Four clinical categories of loxoscelism have been described: 1) minimal to no damage, self-healing; 2) mild reaction of erythema, pruritus, slight lesion, self-healing; 3) dermonecrotic lesion, with vasoconstriction, edema, inflammation, ischemia, pain, bleb or blister followed by eschar that eventually sloughs, and possibly a "bull's-eye" lesion; 4) systemic/viscerocutaneous reaction in which hemolysis, disseminated intravascular coagulation, sepsis, rhabdomyolysis with secondary renal failure, and death are possible.²⁸ It should be noted that the majority of bites result in the former two categories and typically do not need medical intervention, while < 1% of loxoscelism cases result in systemic symptoms.²⁸ The risk of developing dermonecrotic lesions produces the most fear of recluse spiders, yet two-thirds of these cases heal without complication.²⁸ However, such lesions can take several months to heal and potentially leave large scars of up to 40 cm.²⁸ Treatments for loxoscelism are controversial, and a variety of therapies have been recommended, including rest, ice, compression, elevation, antibiotics, debridement, and antivenom.²⁸

In South America, another threat of envenomation comes from the armed spiders of the genus Phoneutria (family Ctenidae). These spiders are only found in South America, where they may also be referred to as "banana spiders".⁶ They are relatively large and have distinctive reddish hairs on their chelicerae.^18,31 Armed spiders are somewhat unusual in that they are quick to aggressively defend themselves.³¹ Their venom is a mixture of components that is neurotoxic and affects both the central and peripheral nervous systems.⁶ All armed spider envenomations are accompanied by local and radiating pain, as well as swelling and hyperemia.¹⁸ Most cases (90%) have only mild systemic effects, principally tachycardia.¹⁸ Moderate cases (9%) will also display nausea, emesis, and diaphoresis, while severe cases (1%) can include hypotension, bradycardia, arrhythmias, and pulmonary edema.¹⁸ Treatment for all armed spider envenomations involves symptomatic and supportive care, especially analgesia.^6,18 For more severe cases, antivenom can be administered, and fatalities are rare.^6,18

The most dangerous spiders in the world are widely considered to be the Australian funnel-web spiders (FWS) of the genera Atrax and Hadronyche (family Hexathelidae).^6,13,29 The FWS are large and dark brown to black.¹⁸ Their venom contains neurotoxic polypeptide atracotoxins, which stimulate both the sympathetic (drenergic) and parasympathetic (cholinergic) portions of the autonomic nervous system.^6,13,18 In the majority (~ 70%) of envenomation cases, only mild to moderate symptoms occur, including local pain, paresthesia, and fasciculations, as well as nonspecific systemic effects (e.g., headache, nausea, lethargy).^13,29 Severe FWS envenomation also includes more generalized fasciculations and paresthesia, piloerection, diaphoresis, ptyalism, hyperlacrimation, emesis, diarrhea, brady- or tachycardia, miosis or mydriasis, hyper- or hypotension, pulmonary edema, and dyspnea.^6,13,29 Prior to the development of antivenom in 1981, further consequences included neuromuscular paralysis, secondary coagulopathy, coma, multi-organ failure, and death.¹³ However, since the advent of antivenom, no fatalities have been recorded.^13,18 Initial treatment should consist of a pressure bandage or tourniquet to slow the spread of the venom to systemic circulation.^13,31 This is followed by immediate removal of the patient to a medical facility for antivenom and supportive therapy and monitoring.^6,13

Spider envenomation in humans is affected by both intrinsic and extrinsic factors. Pediatric, geriatric, pregnant, or other patients with pre-existing conditions are at greater risk of more severe effects or complications and should be monitored closely. It is a myth that all spider bites necessarily include the application of venom. Venom is a product that is metabolically costly for spiders to produce; it is known that spiders can control the amount that is applied depending on the situation.¹⁰ Thus, "dry" bites are possible and occur at a particularly high rate among the Australian FWS.¹³

It should also be noted that there is a high rate of over-diagnosis of spider bites among the human medical community, particularly with regard to the recluse spiders.^26,28,29 This can be attributed to the commonness of arachnophobia, difficulty in diagnosis of the condition, misidentification of suspected spiders, and misinformation regarding the biology and natural history of spider species. For example, diagnoses of loxoscelism in the Southeastern United States are made far more often than is probable, even in areas where recluse spiders are known to be rare or non-existent.²⁷ Also, Vetter²⁹ lists nearly 50 medical conditions that could be differential diagnoses for a necrotic skin lesion other than loxoscelism. One of the more important of these is methicillin-resistant Staphylococcus aureus (MRSA), with multiple reports of misdiagnosis as spider bites.^7,23,30

Envenomation is not the only risk associated with spider bites. Mechanical trauma can occur, especially with the larger specimens. The Australian FWS are known for leaving conspicuous bite marks, as are tarantulas.¹³ In the largest spider species, the goliath birdeater tarantula (Theraphosa blondi), the fangs may reach up to 3 cm in length.³³ Tarantulas are also relatively strong; their fangs can pierce through human fingernails.¹³ It should also be noted that because a spider bite disrupts the integrity of the epidermis, it could potentially lead to secondary infection. In one study of the tarantula Aphonopelma anax (reported as from the genus Dugesiella, which is non-viable²⁰), multiple bacteria were culture from the mouthparts: Bacillus subtilis, B. cereus, Pseudomonas aeruginosa, P. fluorescens, P. cepacia, P. spp., Aeromonas hydrophila, Acinetobacter calcoaceticus, Micrococcus varians, Staphylococcus salivarius, S. aureus, and unidentified coryneforms and Enterobacteriaceae¹⁶. Three of these (S. aureus, A. hydrophila, P. aeruginosa) are known human pathogens, while others are potential opportunistic pathogens.

Aside from bites, spiders have others means to potentially harm humans. Many tarantulas endemic to the American continents possess urticating hairs on their opisthosomas (abdomens) that are used for defense.^3,4 These hairs are up to 1.5 mm in length, contain multiple barbs, and occur in densities of approximately 10,000 per mm², resulting in a total of greater than 1 million per animal.^3,4 When threatened, tarantulas use their caudal pair of walking legs to rapidly flick the hairs toward predators. Urticating hairs may also be incorporated into silk that lines burrows or retreats, mats, and egg sacs.^3,4,15 They can also contaminate the substrate and environment of a tarantula, particularly in captivity.³³ There are four described types of urticating hairs, of which type III is primarily of concern.^3,4 When in contact with human skin, urticating hairs can penetrate farther than 2 mm, causing their namesake inflammatory reaction, which is characterized by severe pruritus of up to two or three weeks duration.^3,4 If inhaled, the hairs can cause a similar condition within the respiratory mucosa.^3,4 A more serious condition occurs if urticating hairs come into contact with the cornea. The subsequent chronic, granulomatous inflammatory condition is termed ophthalmia nodosa.¹¹

Finally, some spiders possess spines on their exoskeletons that can cause mechanical damage to human skin and possibly introduce secondary infections. Symptoms typically include minor pain, swelling, erythema, and pruritus at the site that were transient. This human health hazard has most commonly been documented in huntsman spiders (family Sparassidae).¹²

Scorpions

There are approximately 1200 known scorpion species.²¹ Similar to spiders, the majority of scorpion species pose no significant health threat to humans. However, determining which of those that do can be somewhat difficult. A general trend is that the larger the animal and more robust the pincers, the less likely that the scorpion's venom is harmful; yet, species within the same genera may have vastly disparate venom toxicities.¹⁴ The most dangerous scorpions are found in the family Buthidae and belong to the genera Buthus, Androctonus, Leiurus, Buthotus, Centruroides, Tityus, Parabuthus, and Mesobuthus.¹⁴ An additional genus to include is Hemiscorpion (family Scorpionidae).¹⁴

Scorpion venoms are complex mixtures of components, and interestingly, they have minimal to no enzymes.²⁴ The primary toxins in scorpion venom are peptides that affect voltage-gated ion channels of nerve cells.²⁴ Organs particularly affected after envenomation include the cardiovascular system, lungs, various glands, and smooth and skeletal muscle.²⁴

Despite any differences in venom toxicity, symptoms associated with stings are quite similar across species.^14,24 Mild envenomations often involve local pain, swelling and edema, erythema, pruritus and paresthesia, and possibly ecchymosis, nausea, emesis, pyrexia, and regional lymphadenopathy.^14,24 Symptoms typically subside within 24 hours without medical attention other than first aid wound care.¹⁴

Severe scorpion envenomations involve systemic effects that may be delayed by minutes to more than 24 hours.^14,24 Symptoms may include diaphoresis, pallor, ptyalism, tachypnea, dyspnea, brady- or tachycardia, hypo- or hypertension, pharyngeal spasms and dysphagia, muscle fasciculations and spasms, ataxia, emesis, diarrhea, poor temperature regulation, seizures, angina, cyanosis, and death.^14,24 Not all these symptoms are directly attributable to scorpion venom. Some may be secondary to other conditions that develop, including disseminate intravascular coagulopathy, myocarditis, myocardial necrosis, and pulmonary edema.¹⁴ Death is usually attributed to cardiovascular and/or respiratory failure.²⁴

There is no consensus regarding treatment of severe scorpion envenomations. Antivenom administration can be effective, though it is not available for every species of dangerous scorpion.¹⁴ Other therapies mentioned in the literature include analgesics, steroids, antibiotics, diuretics, and calcium supplementation.^14,24 The use of opioid and barbiturate analgesics is sometimes discouraged, as these drugs may potentially be synergistic with the venom from certain scorpion species or may exacerbated respiratory distress.¹⁴ Relapses of symptoms can occur suddenly during recovery, particularly respiratory difficulties, so patients should be monitored carefully.¹⁴ As with spider envenomations, people of extreme young or old age, or with other pre-existing health conditions are more susceptible to scorpion venom. Children suffer greater morbidity and mortality.²⁴

Myriapods

Diplopods

There are approximately 10,000 species of millipedes, almost the entirety of which are slow-moving detritivores.²¹ As such, they pose little threat to human health. Most possess a pair of repugnatorial glands in each body segment. These glands exude or spray secretions that are used in defense. The secretion components vary by species, but aldehydes, quinones, phenols, iodine, chlorine, and hydrogen cyanide have been identified.²¹ Brownish staining of human skin is not uncommon, but blistering or chemical burns are also possible. If first aid wound care is applied, lesions typically heal without incident.

Chilopods

There are approximately 2800 species of centipedes, and unlike the related diplopods, they are fast-moving carnivores.²¹ Attached to the first body segment behind their head is a pair of modified appendages bearing fangs, called forcipules. Despite the fearsome reputation and predatory skills of centipedes, fatalities from envenomation are rare.^19,25 Most species present minimal to no harm to human health, and many of the most dangerous species are in the genus Scolopendra.

Like most arthropod venoms thus far described, centipede venom is a complex mixture of components. The most common factors are proteins, both enzymatic and not.²⁵ Most centipede envenomations involve local pain, swelling and erythema, though paresthesia, headache, dizziness, lethargy, nausea, emesis, local necrosis, and lymphadenopathy are possible.^19,25 More serious envenomations may involve myocardial ischemia and infarction or rhabdomyolysis and subsequent renal damage.^19,25 Secondary infections are common and often the cause of serious complications and fatalities.²⁵

Treatment of centipede bites involves icing or cold compresses, wound care, analgesics, tetanus prophylaxis, and monitoring for secondary infections.¹⁹

Insects

Lepidopterans

Many moth and butterfly larvae possess urticating hairs on their bodies.¹ These have antipredatory functions and cause dermatologic, ocular, and respiratory conditions similar to those of certain tarantula species.¹ However, the hairs of Lepidopteran larvae are not actively employed as they are in tarantulas.

A few species of Lepidopteran larvae possess spines by which they can envenomate potential predators. The most notorious of these are Lonomia obliqua and L. achelous in South America. The venom of these animals causes severe coagulopathies with subsequent hemorrhagic syndrome, and possible fatalities due to acute renal failure or intracerebral bleeding.²

Cockroaches

Cockroaches do not pose a direct threat to human health. However, they do serve as vectors for a number of bacterial, fungal, and, parasitic pathogens of humans.^9,22,32 These findings can be true even for species commonly kept in captive settings, such as the Madagascar hissing cockroach (Gromphadorhina portentosa).³²

References

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2.  Carrigo-Carvalho, L. C., and A. M. Chudzinski-Tavassi. 2007. The venom of the Lonomia caterpillar: an overview. Toxicon. 49: 741–757.

3.  Cooke, J. A. L., F. H. Miller, R. W. Grover, and J. L. Duffy. 1973. Urticaria caused by tarantula hairs. Am. J. Trop. Med. Hyg. 22: 130–133.

4.  Cooke, J. A. L., V. D. Roth, and F. H. Miller. 1972. The urticating hairs of theraphosid spiders. Am. Mus. Novitates. No. 2498: 1–43.

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19. Ozsarac, M., O. Karcioglu, C. Ayrik, F. Somuncu, and S. Gumrukcu. 2004. Acute coronary ischemia following centipede envenomation: case report and review of literature. Wild. Environ. Med. 15: 109–112.

20. Platnick, N. I. 2013. The world spider catalog, version 14.0. American Museum of Natural History, online at http://research.amnh.org/entomology/spiders/catalog/ DOI: 10.5531/db.iz.0001.

21. Ruppert, E. E., R. S. Fox, and R. D. Barnes. 2004. Invertebrate Zoology. Brooks/Cole, Belmont, California.

22. Salehzadeh, A., P. Tavacol, and H. Mahjub. 2007. Bacterial, fungal and parasitic contamination of cockroaches in public hospitals of Hamadan, Iran. J. Vect. Borne Dis. 44: 105–110.

23. Segarra-Newnham, M. 2006. Skin infections with methicillin-resistant Staphylococcus aureus presenting as insect or spider bites. Am. J. Health. 63: 2046–2048.

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25. Undheim, E. A. B., and G. F. King. 2011. On the venom system of centipedes (Chilopoda), a neglected group of venomous animals. Toxicon. 57: 512–524.

26. Vetter, R. S. 2009. Arachnids misidentified as brown recluse spiders by medical personnel and other authorities in North America. Toxicon. 54: 545–547.

27. Vetter, R. S. 2009. The distribution of brown recluse spiders in the southeastern quadrant of the United States in relation to loxoscelism diagnoses. S. Med. J. 102: 518–522.

28. Vetter, R. S. 2008. Spiders of the genus Loxosceles (Araneae, Sicariidae): a review of biological, medical and psychological aspects regarding envenomations. J. Arachnol. 36: 150–163.

29. Vetter, R. S., and G. K. Isbister. 2008. Medical aspects of spider bites. Annu. Rev. Entomol. 53: 409–429.

30. Vetter, R. S., B. B. Pagac, R. W. Reiland, D. T. Bolesh, and D. L. Swanson. 2006. Skin lesions in barracks: consider community-acquired methicillin-resistant Staphylococcus aureus infection instead of spider bites. Military Med. 171: 830–832.

31. Vetter, R. S., and P. K. Visscher. 1998. Bites and stings of medically important venomous arthropods. Internat. J. Dermatol. 37: 481–496.

32. Yoder, J. A., B. D. Glenn, J. B. Benoit, and L. W. Zettler. 2007. The giant Madagascar hissing-cockroach (Gromphadorhina portentosa) as a source of antagonistic moulds: concerns arising from its use in a public setting. Mycoses. 51: 95–98.

33. Zachariah, T., and M. A. Mitchell. 2009. Invertebrates. In: Mitchell, M. A., and T. N. Tully, Jr. (eds.). Manual of Exotic Pet Practice. Saunders, St. Louis, Missouri. Pp. 11–38.