Session #2002

Abstract

Theraphosid spiders (tarantulas) are a representative species for terrestrial pet invertebrates and a common pet throughout the world. They are the mainstay of many bug houses in zoos across the world and play an important educational role for the public. Theraphosids have fundamental anatomical differences that can make providing veterinary care challenging for clinicians that are more familiar with vertebrates. This masterclass will aim to provide a bird’s eye view of the veterinary approach to theraphosids, including the provision of the clinical exam, diagnostic tests, and treatment. Adaptations to the clinical exam, in the form of anaesthesia and magnification, will be discussed and examples of normal anatomy, as well as pathology, will be presented. The pitfalls and challenges of applying diagnostic tools such as histology, radiography, haematology, and microbiology to theraphosid species will be covered, alongside how to get meaningful and helpful clinical data to inform management change and treatment. Treatment can be achieved through similar methods to more conventional species, with adjustments made for their unique anatomy. Challenges in drug selection will be discussed. Veterinary procedures such as mite removal and autotomy will be covered. Finally, euthanasia can be performed via intracardiac or intra-ganglionic methods, or via immersion.

Introduction

Theraphosid spiders (tarantulas) are a prominent ambassador for invertebrates within zoology and are a common species kept in private collections throughout the world (Pearce-Kelly et al. 2007). Theraphosids represent more than 1,000 species, with a remarkable number reported in captivity, and are of increasing conservational concern (Branco, Cardoso 2020). There is a growing and active community of tarantula keepers in the United Kingdom, Europe, and the United States; thus, there is an increasing demand for veterinary opinion and intervention in these species. Theraphosids can represent a significant challenge due to their unique biology and pathophysiology, but the application of basic principles can often result in successful diagnosis and treatment of disease (Pizzi, Kennedy 2022).

Clinical Exam

Before a theraphosid presents to a clinic, all team members should be aware and comfortable with arachnids. Arachnophobia is a common fear among humans (Wiederhold, Bouchard 2014), and this should be considered both for those within the clinical team and the general public present within the clinic (for example, in the waiting room). The author recommends that theraphosids are brought into the clinic at the optimum temperature range for that species (using a heat pad or hot water bottle), and that they are double contained and covered. This both ensures that the animal is comfortable but also prevents aversive responses to their presence. Ultimately it is important that all members of the clinical team accept that theraphosids require clinical care and that provisions must be made to ensure they can be treated and considered within a professional environment.

Prior to examination, a detailed history should be taken. As a minimum, it is helpful to establish species (or the owner’s perception of the species), gender, feeding regime, enclosure design and management, and the date of the previous moult. Pictures should be taken prior to examination so species can be checked at a later point. The identification of species can sometimes be challenging as there have been multiple reclassifications of several prominent species as well as the prevalence of many ‘common’ names which can refer to different colour forms and hybrids. It is possible that spider species can be misidentified by an owner and thus inappropriate husbandry applied.

When first examining a theraphosid, a distance exam should be performed, and the animal should be stimulated with a long brush or probe to assess how docile it is prior to any form of handling. A good knowledge of normal in that species is invaluable as ‘docile’ behaviour in a normally aversive species can be a cause for concern. A docile animal can be handled for clinical examination with experience and confidence. Alternatively, a physical examination can be performed using the ‘cardboard and cup’ method.

Theraphosids are much smaller than the typical exotic patient, with the largest species being approximately 80–90 grams. Their small size means that clinical lesions are often microscopic; thus, magnification can be invaluable for visualisation of clinical lesions. In addition to this, many species are aversive to handling; thus, sedation or an anaesthetic can be helpful to ensure that the animal remains still enough for a successful clinical exam.

In terms of magnification, the author recommends 4–10 times magnification. This can be achieved either with a stereomicroscope or a rigid endoscope, both of which can provide effective magnification and therefore provide the ability to visualise lesions that would be difficult if not impossible to see with the naked eye.

In terms of anaesthetics, the author primarily uses inhalable anaesthetics (isoflurane or sevoflurane) within an enclosed fume cupboard. These are both effective and safe in many theraphosid species (Dombrowski et al. 2013; MacMillan et al. 2017; Zachariah et al. 2014). An induction chamber can be used to induce anaesthesia, but the ability to maintain anaesthesia is more challenging (especially when considering the exposure risks of inhalable anaesthetics). Injectable protocols have been reported in the literature but can be challenging in some species (Gjeltema et al. 2014).

When considering the clinical exam, using the principle of body systems, and developing a systematic approach is key for ensuring that all clinical information is gathered. Photography is invaluable so findings can be referred to at a later stage.

Investigations

Many of the same tools that can be used in companion animal and exotic animal medicine can be easily adapted to theraphosid species. Primarily though the mechanised and pathophysiology can be remarkably different between vertebrates and invertebrates, the same basic principles can often be applied.

Histopathology/Post-Mortem

Post-mortem of theraphosids can be fruitful but requires an intimate understanding of theraphosid anatomy and experience of the textural differences between organ systems. For this reason, the author recommends histopathology of the entire animal rather than post-mortem when investigating death or disease within an individual theraphosid or a theraphosid population (Pizzi, Kennedy 2022). Histology can be a useful tool for acquiring diagnostic information about a theraphosid as often multiple organs and body systems can be included within a single block.

There are some key considerations that need to be applied in order to acquire successful histology (Kennedy 2017). It is important to ensure that fixation occurs as soon after death as possible as theraphosid tissues, especially those within the opisthosoma, can autolyse rapidly. If an animal within a population is deteriorating quickly (and has minimal or no quality of life), then a clinician may want to consider euthanasia to ensure that diagnostic histology can be acquired (to obtain valuable information to dictate the further investigations and treatment of the remaining population). The author can attest that diagnostic material can, if necessary, be collected 12–24 hours following death, but ideally, material should be fixed within one hour of death. Immersion for 24–48 hours in an appropriate fixative such as Davidson’s or Kahle’s is very helpful for softening the external cuticle of theraphosids. Lastly, the author would recommend approaching a pathologist with experience in invertebrate histology and pathology to interpret invertebrate tissue histology. Additional stains, such as Masson’s trichrome, are often required to fully appreciate all anatomical features. The lack of reference histology, as well as the anatomical differences from vertebrate tissue, can make histopathology of theraphosid tissue challenging.

Plasma Biochemistry and Haematology

Theraphosid heamolymph can be collected easily from either the cardiac sinus or from one of the proximal limb membranes. Heamolymph can be helpful for determining more about the health of a theraphosid spider. Arachnids, in general, rely on haemocyanin free within the plasma for oxygen transport rather than cell-bound haemoglobin; thus, the cellularity of heamolymph is low, but the protein content is relatively high compared with vertebrates (B. Larouche et al. 2019; Benjamin Kennedy et al. 2019). This results in starkly different haematology than vertebrates. Immune cells can be visualised and counted. ‘Neutrophil’ like cells can be used to provide clinical data. Depending on the collection site, bacteria can be a variation of normal and must be interpreted carefully. The author would always recommend that culture and sensitivity be performed on heamolymph that has higher levels of bacteria than expected. Protein crystallisation will occur within heamolymph when dried on a slide; this finding appears significant but is a variation of normal in many theraphosid spiders. Biochemistry can be performed on heamolymph, but many of the values measured do not have clear invertebrate analogues, so must be interpreted carefully. Several reference intervals are available and these can be helpful for determining if a theraphosid has obvious biochemical differences from a normal population (Benjamin Kennedy et al. 2019; Eichelmann, Lewbart 2018; Zachariah et al. 2007).

Microbiology

Culture and sensitivity can be incredibly helpful for determining the presence of bacterial or fungal disease, but it is critically important that microbiology is informed and validated with concurrent cytology. Most laboratory conditions for culture and sensitivity testing are designed and validated for mammalian species; thus, it can be easy for false negatives and false positives to occur. Heamolymph or lesion cytology that identifies organisms that are consistent with the successfully cultured organism provide the clinician with more confidence that treatment will have a positive effect. A surface swab of the cuticle will only yield an assessment of the microbiome of the environment; thus, the author would instead recommend a culture of heamolymph, a distinct cuticular lesion, or of a discharge from the oral cavity.

Radiography and Ultrasonography

The advent of digital radiography and especially high-resolution dental plates have made radiography more fruitful in providing clinical data for theraphosids. Kilovoltage peak (kVp) and milliampere-seconds (mAs) need to be considered carefully as invertebrates are much smaller and do not have skeletons to be penetrated (Davis et al. 2008). The author has used a kVp of 40 and a mAs of 3.2 and finds these settings to be a good starting point (Kennedy, Naguib 2019), but undoubtedly adjustments to these values would be needed for smaller or larger spiders. The author can report that iohexol (Omnipaque™) can be administered orally and into the luminal heart with minimal incident (personal communication, Kennedy B). Barium has also previously been given orally (Davis et al. 2008). Radiography is particularly suited for determining more about the cuticle and for assessing opisthosomal granulomas or opacities.

Ultrasound can be very helpful for assessing opisthosomal structures, including the luminal heart. A high-frequency probe is required, and the author recommends removing any ultrasound gel immediately after. More advanced imaging such as CT and MRI can both be utilised to determine more about pathophysiology in theraphosid species, but this is often cost prohibitive for theraphosids (Pohlmann et al. 2007).

Endoscopy

Endoscopy is particularly helpful for evaluating the oral cavity and cloaca of theraphosids. Mites and nematodes can be visualised earlier in the disease process via endoscopy. The author would recommend a 30-degree 1.9 mm or 2.7 mm rigid endoscope for use in theraphosids. The anatomy of theraphosids does not make them good candidates for laparoscopy.

Treatment

Treatment Administration

Treatment can be administered via similar methods to other exotic species. Arachnids are surprisingly easy and simple to orally dose. This can be a good route for introducing medication and fluid therapy. Medication can be given ‘intravenously’ via the cardiac heart or via a joint membrane (though an anaesthetic may be required). Topical treatment may be helpful, especially with visible cuticular lesions.

Treatment Selection

It is common to have little to no data available on medical interventions with any theraphosid species. Though data is slowly becoming available, there is still much that we do not know with respect to pharmacokinetics. The author uses a series of different approaches during medication selection to mitigate the unknowns involved.

A hierarchy of evidence should be used when selecting a prospective medication. If a drug has already been established as being effective and tolerated in theraphosids, then this should be the first port of call. Honeybees, crustaceans, food invertebrates, and animal models have a significant amount of literature and thus present a logical next option. Using reptile or amphibian doses should be a last resort. The author recommends validating the medication use with genomic bioinformatics to ensure there is a theoretical basis for its use. Some medications have been used in theraphosids and invertebrates but have little to no theoretical basis for their use.

It is important to consider the pharmacokinetics of a particular medication and whether the relevant metabolism, pathways, and excretion routes are present in invertebrates. Albumin is not present in arachnid genomes (Kennedy et al. 2019), and although there are analogous proteins in plasma (haemocyanin), it cannot be assumed that a medication that binds to albumin will bind to haemocyanin in the same way.

Genomic Bioinformatics as a Tool in Drug Selection

Bioinformatics involving reported genomes can be useful for evaluating whether a medicine that uses a specific pathway in vertebrates has an analogous pathway that can be utilised in invertebrate species. Assemblies for several spiders and theraphosids are available publicly (Schwager et al. 2017). The basic local alignment search tool (BLAST) can be used to compare both the nucleotide and protein sequences.

Antimicrobial and Antifungal Use

Bacteria grown in theraphosid samples performed by the author tend to lean towards gram-negative environmental bacteria. Immunosuppression and inappropriate husbandry appear to increase susceptibility to bacterial overgrowth, and therefore, culture and sensitivity results may not indicate a primary infection. Multiple antibiotics have been reported in the literature and the author can recommend the invertebrate chapters in Carpenter’s Exotic Formulary as well as the Invertebrate Medicine textbook as a source of information about prospective antibiotics or antifungals (Lewbart 2018, 2012).

Analgesia

The capacity for invertebrates (and thus theraphosids) to feel pain and suffer has been a debated topic for many years (Elwood 2011). The author would support the assertion that an ‘argument of analogy’ is helpful, and thus, if a mammalian would feel pain or suffer in response to a given situation, then it would be reasonable to consider that a spider could have a similar response.

The COX-1 and COX-2 pathways do not appear to have an obvious equivalent in theraphosids. For this reason, the author does not recommend non-steroidal anti-inflammatory drugs in theraphosids.

The author will typically advise opioid and NDMA agonist use (as informed by genomic bioinformatics) as these tend to be better conserved between invertebrate and vertebrate species. The opioid pathway within invertebrates is distinct to vertebrates but does appear to have some sedative effects. Alternative analgesics such as ketamine could also be considered.

Procedures

Mite removal can usually be performed by an experienced owner with a docile spider. Veterinary involvement may be required for aversive spiders that would benefit from anaesthesia to perform mite removal. Mite removal can be performed by applying surgical spirit to individual mites with a small paintbrush. It is important to ensure that the oral cavity, joint membranes, the prosomal-opisthosomal junction, and the extremities are assessed carefully with magnification, as mites and mite eggs can be easily missed. Following treatment, the enclosure substrate should be changed.

Autotomy should be performed under minimal anaesthesia as it is an induced response. This can be done by firmly grasping the femur of the limb with forceps and inducing with a firm sharp motion ventrally. Haemostasis occurs in conscious spiders because the musculature ligates the vessels when this is induced but will not occur when performed under deep anaesthesia and can result in exsanguination of the spider.

Euthanasia

Euthanasia is a key tool to preserve animal welfare and alleviate suffering within veterinary medicine. General anaesthesia prior to euthanasia is strongly recommended. The author recommends the following methods for euthanasia of theraphosids: intracardiac administration of pentobarbiturates, intraganglionic administration of pentobarbiturates, or immersion in high percentage alcohol. Although freezing or drowning are common methods used to euthanise invertebrates, the author does not recommend these. Similarly, the use of carbon dioxide is also not recommended, as often invertebrates need to be exposed to high levels of carbon dioxide for long periods for carbon dioxide to have the desired effect.

References

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