Fenbendazole, Albendazole, and Oxibendazole

Are We Seeing Negative Side Effects?

by Cat Logsdon

Note: This article is intended only to alert caretakers and vets of a potential problem that they should be aware of. There is no clinical proof at this time of a relationship between the rabbit deaths or illnesses and the drugs mentioned.

Within the last six months three unrelated spayed female rabbits from different households have gone into bone marrow failure. Dr. Sari Kanfer, Zooh Corner’s medical director, believes that all three cases may have been related to administration of albendazole, fenbendazole or oxibendazole, the general drugs of choice used to treat rabbits with positive titres for e. cuniculi.

The presenting signs were acute onset of anorexia and GI stasis. Blood work showed that all three [rabbits] had EXTREMELY low numbers of white blood cells, red blood cells and platelets (all cells produced by the bone marrow). Says Dr. Kanfer, “These rabbits were not just anemic secondary to e. cuniculi-related kidney problems, note that they also were lacking platelets and white blood cells.” The additional lack of platelets and white blood cells is strongly suggestive of bone marrow failure.


A young female rabbit, excellent husbandry and diet, ‘medium’ e. cuniculi positive titre. Near the end of her six week treatment with albendazole this rabbit went into acute GI stasis, began drinking large amounts of water and became lethargic. The rabbit was treated for GI stasis (fluids, motility drugs, other supportive care) at the emergency clinic, then transferred to Dr. Kanfer, who did x-rays and a full blood work-up. There was no obstruction and the blood work showed strong evidence of a rabbit in bone marrow failure (low red & white blood cells, low platelets and liver enzymes through the roof). Dr. Kanfer felt there was no other course but a blood transfusion at this point. It took two whole blood transfusions, the second one including the administration of Epogen (a drug that encourages red blood cell production) as well as various antibiotics and GI supportive care – but this rabbit is alive and well today (several months later) and her blood work is nearly back to normal. (More details available, e-mail author.)

Dr. Kanfer has no clinical proof that these drugs did indeed cause the bone marrow failure, she has only strong suspicion, supported by the fact that higher doses of these drugs have been known to cause similar problems in a percentage of dogs, cats, birds and reptiles.

While it is true that there are thousands of rabbits currently being treated with [one of] these drugs, with no apparent side effects, we still feel that the question needs to be asked: If the ‘bendazoles could cause bone marrow failure in a [percentage] of various species of other animals, could it also have adverse effects on a small percentage of rabbits?

It is important to note that while these various treatments for e. cuniculi are not cures, practical experience has shown them to have the beneficial effect of decreasing signs or slowing organism spread in many rabbits. The author herself has used all three of these drugs at various times in various rabbits and believes she has seen positive effects; further, she will continue to use them, [now] with the addition of frequent blood panels to monitor each rabbits’ blood levels.

Conclusion: How Concerned Should You Be and What you Can Do

There is certainly no need to panic. Even if side effects are being seen, it is most likely only a very small percentage of rabbits that would be so affected. We simply suggest that you and your vet monitor your rabbits while they are being treated with these drugs (a good idea with any drug). We suggest that all rabbits who are being treated with one of the bendazoles (Albendazole, Oxibendazole, Fenbendazole) have blood work (a CBC – Complete Blood Chemistry) done before, during and after treatment.

More research and a more official article is pending.

 What is E. Cuniculi?

Encephalitozoon Cuniculi (e. cuniculi) is an obligate protozoal parasite. Little is known about its biology, but it is thought to be transferred from mother to offspring prior to birth, and possibly shed into the urine of infected rabbits. The organism is thought to travel through the body in white blood cells, the cells that normally fight disease, and may infect the tissues of the rabbit’s brain, kidney, spinal cord, heart, liver and lungs. It has also been known to cause damage to the eyes (uveitis).

The general drugs of choice to used to help control e. cuniculi, Fenbendazole, Oxibendazole and Albendazole, all benzimidazole derivatives, are de-wormers that are used on various species of animals (cats, dogs, horses) to rid them of intestinal parasites. They do their job by blocking the transport and uptake of glucose. Though e. cuniculi is a protozoal parasite, the thought is that it will have a similar effect.

A positive tire for e. cuniculi means only that your rabbit has been exposed to the parasite at some point. Many rabbits who test positive for e. cuniculi never show any clinical signs.


If your vet feels that this may be happening with your rabbit and wishes to consult with Dr. Sari Kanfer, she can be reached at Exotic Animal Care Center (626) 405-1777.

We would like to learn more. We are currently doing further research on this topic. You can help us out. If your rabbit becomes suspiciously ill or dies while on one of these medications (or shortly thereafter) you can help us gather more info by 1) asking your vet for a necropsy and histopathology and then sending us a copy of the results; 2) Sending us copies of all related medical records (blood tests, vet’s diagnosis and logs, and so on) along with any info you personally wish to add (please be concise).

If you wish to do this, or you have any general questions about the article or the various cases, please feel free to contact us.

Upcoming Events