By Roger J. Klingenberg
Nematodes are among the most common parasites diagnosed in reptiles, with more than five hundred reptile forms identified. Larval forms migrate through the body. Adult nematodes are typically thought of as worms. Because of their ubiquitous nature, several drugs have been adapted to treat them.
Ivermectin is an avermectin anthelmintic, formed by a combination of two macrolytic lactones derived from a mold, Streptomyces avermitillis. Ivermectin has proven to be a very versatile drug in that it can be administered orally, topically, and by injection with good effects against nematode and arthropod parasites.
Ivermectin works by enhancing the release of gamma amino butyric acid (GABA) at presynaptic neurons. This stimulation of GABA release causes a paralytic effect on the parasite. It is interesting to note that flukes and cestodes (tapeworms) don’t use GABA, and thus ivermectin is useless against these parasites.
Like fenbendazole, ivermectin is safely used in gravid females. Ivermectin is well distributed in all tissues except the CNS. Unfortunately, there are some species in which this drug does cross the blood-brain barrier, and they can suffer severe CNS toxicity ranging from depression, stupor, tremors, ataxia, seizures, and death. Chelonians appear to be the only reptiles susceptible to toxicity from routine doses of ivermectin, and the use on these reptiles is strictly contraindicated. Dumonceaux (2004) reported on the toxic effects of ivermectin administered orally to five Nile crocodiles through prey rats. One died, and the others recovered with routine supportive care. The doses given were based on estimated weights, and the crocodiles were given doses ranging from 0.24 to 0.375 mg/kg, which is just over the recommended reptile dose of 0.2 mg/kg. Again, this is an example of the need to calculate accurate doses from accurate weights.
Deaths have also been reported in Central American skinks (Stein 1993, pers. comm.) and prehensile-tailed skinks (T. Boyer 1993, pers. comm.), so use ivermectin with caution in skinks. Barten (S. Barten 1993, pers. comm.) reported a transient paresis in a Parson’s chameleon, and Klingenberg (1994) noted CNS signs in a panther chameleon, but it recovered within twelve hours. It should be noted that some chameleon breeders have used ivermectin extensively without problems. DeNardo (D. F. DeNardo 1993 pers. comm.) reported using ivermectin in more than one thousand lizards (species not stated) with only discoloration at the injection site, lethargy, and inappetance (loss or lack of appetite) that lasted no more than forty-eight hours. One juvenile boa constrictor died acutely after being sprayed with ivermectin spray, and pathology determined this to be an idiosyncratic reaction to the ivermectin (Klingenberg 1996a). However, in general, if dosed carefully in reptiles other than chelonians and skinks, ivermectin appears to be an effective and safe anthelmintic. Ivermectin is absolutely contraindicated in turtles and tortoises, even as a spray.
Ivermectin has also been employed against mites and ticks by injecting the reptile, by giving oral doses, or by doing both concurrently. There has been no work done to indicate whether injectable ivermectin is more effective than when given orally or applied topically. When treating mites, the topical application appears to be more effective and less likely to expose the reptile to cumulative doses. However, it is a great advantage to be able to inject a reptile not amenable to handling, spraying, or oral administration.
The current recommended dosages are 0.2 mg/kg or 0.02 mL/kg (of a 1 percent solution) once every two weeks until negative fecals are obtained or mites are gone. Using products such as Ivomec or Double Impact, which are 1 percent ivermectin solutions (10 mg/mL), means we need to give approximately 0.01 mL per pound (0.45 kg). This translates roughly into a small drop of the undiluted solution per pound (0.45 kg). For use with mites, 10 mg (1 mL) of 1 percent ivermectin solution is mixed with 1 quart (946 mL) of water, and the reptile and cage are lightly but thoroughly sprayed every three to five days until resolved (usually six to eight weeks). The use of this spray will be discussed in detail in the section on treating mites.
Excerpt from the book Understanding Reptile Parasites by Roger Klingenberg with permission from its publisher, Advanced Vivarium Systems, an imprint of BowTie Press. Purchase Understanding Reptile Parasites here.