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.
Fenbendazole is a member of the benzimidazole group of anthelmintics, which also includes thiabendazole (TBZ), mebendazole (Telmin), and albendazole (Valbazen). This group of anthelmintics has been widely used in veterinary medicine because of their effect not only on mature worms but also on larval and even encysted stages. In addition to killing mature worms, the benzimidazoles exert an ovicidal effect, immediately decreasing egg production by effectively sterilizing the worm.
Although literally hundreds of benzimidazole derivatives have been formulated, fenbendazole is easily the drug of choice among the variations. While all medications in this group are considered to be very safe, fenbendazole is the safest of all of them. It was difficult to establish an LD50 (lethal dose at which 50 percent of test population will perish) in rodents. Even a dose greater than ten thousand times a normal dose would not consistently kill mice or rats. In addition to its excellent safety record, fenbendazole does not cause birth defects, unlike most of the other drugs of this group. The only contraindications for the use of fenbendazole are its uses in horses to be used for food and in lactating cattle. There are no proven reptile contraindications or known drug interactions.
Fenbendazole functions by inhibiting the uptake of glucose (sugar) in the nematode parasite. This block of glucose uptake is slow, so fenbendazole works better when given over a course of several days than in single or multiple doses that are given at spaced time intervals. Fenbendazole is the drug of choice for nematode parasites and is given orally at 25–50 mg/kg once daily for three to five days and then repeated in ten days, if warranted. If used for giardia infections, it should be given at 50 mg/kg daily for five days.
Upon oral administration, fenbendazole is minimally metabolized, and the majority of the drug is excreted unchanged. This helps explain why the cloacal administration of fenbendazole (Innis 1995) is effective only with parasites found in the cloaca and will do nothing for parasites in the remainder of the GI tract. In his report, Innis describes how he initially gave a tortoise fenbendazole at 100 mg/kg orally once and then again two weeks later, and the pinworms were not eliminated. He reported better results with dosing the fenbendazole with a syringe into the cloaca. We now know that had Innis administered the fenbendazole orally over a course of three to five days, it would have produced even better results as a result of the constant blockage of glucose uptake in the worms. In any case, it is interesting to have another route of administration because it isn’t always easy to get oral fenbendazole down a tortoise for three to five days.
The technique that Innis employed was to place the tortoise in dorsal recumbency (upside down) and, with the drug inside a tuberculin (1 mL) syringe with a lubricated tip, introduce the syringe into the cloaca. Although it is beneficial to place the medication as far into the rectum as possible, care should be taken to advance no more than half of the syringe and to stop immediately if resistance is noted. Leakage is prevented by manual pressure on each side of the cloaca. Innis further reported an almost immediate expulsion of pinworms due to the enema effect. The pinworms continued to be passed for another two to three days. He also suggested using a red rubber urethral catheter in larger specimens to prevent depositing the drug into the bladder. Again, although cloacal administration of fenbendazole will produce results, it is not as effective as the oral administration because the drug’s action is limited to the colon. Regardless of the route chosen for administration, the drug is best given over consecutive days, rather than in individual, spaced doses.
Some reptile veterinarians believe that albendazole is more effective in single doses than fenbendazole is. Even if single dose effectiveness is conceded, fenbendazole is preferable for several reasons. Although albendazole is considered to be safe, it is better absorbed than the other benzimidazoles are, which increases side effects with the liver and hematologic organ systems. Both drugs are more effective when administered daily for several days rather than in one dose, so single-dose effectiveness is a moot point. Albendazole is also teratogenic (causing birth defects) and embryotoxic (causing damage or death to the embryo), which would pose an undue risk for potentially gravid animals. Finally, albendazole interacts with praziquantel (Droncit) and dexamethasone, which would limit concurrent treatment of cestodes. Fenbendazole is simply safer than albendazole.
This author has been jokingly accused of owning stock in Panacur as a result of such enthusiastic promoting of fenbendazole. So to provide a balanced argument, let’s discuss some downsides of fenbendazole. As mentioned previously, the drug was implicated in the deaths of four Fea’s vipers. Stein and Wynne (G. Stein and J. Wynne 1993, pers. comm.) noted that tortoises treated with fenbendazole became anorexic but regained their appetites after a few days. The author has noted that bearded dragons can go off food dramatically when put through consecutive day dosing, with the longest period of anorexia being two weeks. As bearded dragons often have both pinworms and coccidia, it is not unusual for them to be treated with both sulfadimethoxine (Albon) and fenbendazole at the same time, leading to questions about which drug could be causing the appetite to be suppressed. In any case, both drugs have caused appetite suppression when used separately, so perhaps a noncritical bearded dragon should be treated in stages to minimize appetite disruption. For bearded dragons that won’t eat, force-feeding appears to kick-start most into eating again. Employ force-feeding early and continue force-feeding until the dragons feed on their own.
In some rare cases, parasites appear to be resistant to fenbendazole. In this situation, the author suggests using ivermectin, which may require more doses over time. A small study (Klingenberg 1993) demonstrated this in ball pythons. Fenbendazole eliminated nematodes in fewer doses than ivermectin did (see appendix III). Corwin (M. Corwin 1994, pers. comm.) reported that this was also true when treating nematode parasites of monitor lizards.
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.