Invertebrate anesthesia (Proceedings)

The field of invertebrate medicine is in its infancy. Traditionally veterinary interest in invertebrates has centered on the control of species that are involved in vertebrate disease processes. The husbandry and medical care of captive invertebrates has typically fallen to scientists and hobbyists, with very little veterinary involvement. Now, as many institutions and private individuals maintain large invertebrate collections, a genuine interest exists in advancing the quality of medical care available for these taxa. Many species are long-lived (exceeding 20 years in females of some species) and carry a significant monetary and/or conservation value, and are therefore not viewed as disposable specimens.

A key requirement in providing medical care for a species is the development of safe and reliable anesthetic protocols. Anesthesia is necessary for potentially painful or invasive/delicate procedures where patient motion is undesirable. Many species, both vertebrates and invertebrates, are impossible to manipulate safely without sedation or anesthesia. Simple examination and/or sample collection can be challenging with invertebrates. Many species are tractable and durable enough to allow manual restraint for such procedures, whereas others can pose a significant danger to handlers (bites, stings, urticating hairs, etc.). In addition to human dangers, some species are very fragile and are easily damaged with restraint. Even very short falls can result in ruptured abdomens in heavy-bodied terrestrial species. Finally, many invertebrates are fast moving and can quickly escape restraint and be difficult to recover. Hence, the development of anesthetic protocols is an important step in advancing the horizons of invertebrate medicine. Safe and effective anesthetic protocols are also important in research, where the need to maintain functional invertebrate specimens post-anesthesia is necessary.

Ideally an anesthetic agent should provide a safe, rapid, predictable response while allowing the clinician a reasonable degree of control. Post-anesthetic effects should be minimal.

Unfortunately there is very little information in the veterinary literature regarding anesthesia of invertebrate species. With a bit of digging, some information can be found in the journals of non-veterinary disciplines, though the protocols encountered in these areas are often not aimed at maximizing safety and keeping the completely functional post-anesthesia. The clinician should critically evaluate all suggested techniques and use common sense when deciding to employ them. The following information outlines some of the more commonly used anesthetic techniques used by laypersons, researchers and veterinarians.

Cold: Exposure to cold temperature has historically been used as a method of restraint for a variety of ectothermic animals including invertebrates. Cold exposure is undeniably an effective method of incapacitating invertebrate species; however it is unknown what long-term effects it may have on individual specimens. In addition, though analgesia of invertebrate species is poorly understood, the conscientious practitioner should make every reasonable attempt to provide relief in the event of a potentially noxious stimulus. Systemic hypothermia is not an acceptable method of analgesia in the "higher" animals and may even sensitize some species to painful stimuli.

Carbon Dioxide: Carbon Dioxide (CO2) has traditionally been the anesthetic of choice for invertebrate species. It can even be bubbled into water to anesthetize aquatic species. In my experience CO2 provides for very rapid inductions and rapid recoveries, and a deep plane of anesthesia. Some species seem to recover in a very disoriented state and can be extremely aggressive when they regain mobility. Previous studies have noted asystole in insects anesthetized with CO2, though they still recover when delivery of the gas is discontinued. Also, some agricultural invertebrates (honeybee, silkworms, etc.) or species used in research (fruit flies) have been noted to suffer long-lasting problems from episodes of CO2 anesthesia such as decreased growth, maturation and reproductive parameters. Obviously this is not desirable in animals intended for propagation.

Isoflurane/Sevoflurane: Both of the anesthetic gasses commonly found in domestic animal practices can be used effectively with invertebrate species. Typically induction times are slower than what is seen with CO2, and with typical vaporizer settings it is not uncommon for a tarantula or scorpion to never become completely immobilized and to remain at least minimally responsive to stimuli. Higher levels of anesthetic gas can be achieved by soaking a cotton ball in liquid anesthetic and placing it in an induction chamber with the specimen. A drawback to using gas anesthetics is that the respiratory openings are usually not on the head in inverts, but on the body, requiring that those areas be covered for continuous delivery of the anesthetic. Some practitioners devise chambers with built in gloves so all aspects of the animal are accessible, while anesthetic is delivered.

Injectable Anesthetics: There are occasional anecdotal reports of people using parenteral anesthetics in invertebrate species. Results are variable. In the author's experience with ketamine and propofol, neither elicited any worthwhile effect in rosehaired tarantulas when given intracoelomically at the low end of published mammalian doses. Interested practitioners with access to their own invertebrate specimens may elect to investigate further in an effort to identify useful doses or protocols using parenteral anesthetics.

Monitoring anesthesia in invertebrate species is a tricky proposition. First of all, at this stage we simply do not know what parameters are important to monitor in this group of animals. As previously stated, some species will experience asystole when anesthetized and recover without difficulty. Also, most do not actively respire, so there are no respiratory movements to monitor. It is the author's preference that specimens be kept at an anesthetic depth only deep enough to facilitate the necessary procedure, ideally so that some movement is present in response to stimuli. There is not much else to say in regards to monitoring short of the obvious; more research needs to be done to identify the important physiologic parameters that should be monitored during invertebrate anesthesia.

If an animal does not recover immediately from anesthesia; do not immediately assume it is deceased. It should be placed in a thermoneutral, oxygenated environment and not considered dead until it shows obvious signs of rigor or decomposition.