The Axolotl

It was once a common food source in Mexico, is commonly used in experimental studies, and popular as pets. They are now endangered in the wild due to habitat destruction, the species kept alive largely by the pet trade.

They are neotenic, remaining in the water-dwelling, larval state for their entire life cycle unless artificially induced by administering thyroxine. They come in various colours. They have poor eye-sight and mostly lie motionless on the tank floor. They normally move slowly with their primitive limbs, but can swim off quickly when disturbed. For respiration, they have rudimentary gills which are periodically flicked, primitive lungs and are able to exchange gas through their skin.

They are not territorial and can be housed in mixed-sex groups. This said, during breeding season, the males may become aggressive and there must be ample space and furnishings to avoid trauma. Aggressive individuals or poor-doers may be housed individually.

Clinical Signs of Disease

Axolotls can be stressed for many reasons and then succumb to disease. The majority of the time, it is due to the owner being unaware of their needs. For example, they should be kept in slow moving water and fast flows should be avoided. Axolotls are also often over-fed. When they are about one year old, they should only be fed 3–4 times a week, with no more than five pellets a day.

This presentation will cover the most common diseases faced by axolotls in captivity. However, before we progress, we need to know what the typical signs of illness are.

When sick, they present with rather non-specific signs of disease such as:

 Poorly responsive

 Gill degeneration

 Curling of the tail tip/tail membrane

 Anaemia (pallor to gill filaments)

 Ascites and subcutaneous oedema

 Floating

 Inappetance

 Skin lesions (excess mucus, ulcers, tumours, hyperaemia, haemorrhage)

 Eye lesions (exopthalmia, opacity)

The aetiology may not always be obvious and the cause is often reached after taking a complete history and running routine examination of water quality, the aquarium and filtration and of the animal.

Some of the Common Diseases

New Tank Syndrome

This occurs when there is biofilter insufficiency causing ammonia or nitrite to rise to toxic levels. Scenarios include inadequate time for the nitrifying bacteria in the biofilter to establish, damage of the biofilter microflora (e.g., exposure to chemicals, low dissolve oxygen, low pH), increased stocking density and over-feeding.

Management would involve a combination of multiple water changes, addition of nitrifying bacteria and addressing the underlying cause(s). Other more focused methods will be discussed.

Old Tank Syndrome

High levels of nitrate are an indicator of poor husbandry. By extrapolating from fish health, high levels of nitrate are suggested to cause depressed immunity. In axolotls, exophthalmia and corneal opacity can be a sign of prolonged exposure to high levels of nitrate.

Additionally, poor husbandry can indirectly lead to depletion of alkalinity and lead to acidic water. Axolotls will develop a diffuse film of mucus and present ill. Again, management will involve a combination of multiple water changes and addressing the underlying cause(s). Other more focused methods will be discussed using examples.

Hyperthermia

Axolotls are a cool water species, and where they occur naturally in the wild, their habitat is filled by springs and melting water from snow-capped mountains. If the water temperature rises and maintains above 24°C (e.g., during hot spells in summer), axolotls will present with clinical signs ranging from inappetance, ascites to uncontrollable floating. Emergency treatment by placing the axolotl in a dish in the refrigerator is helpful. Consider antibiotic therapy to prevent secondary bacterial infections. The owner is advised to install an aquarium chiller.

Gastric Foreign Body

Natural environment is in streams with rocky boulders. They should not be housed with gravel because it is common for axolotls to accidentally ingest the gravel, especially if they are underfed. They can regurgitate to expel the gravel; however, sometimes it may be necessary to manually remove it. This can be done in an anaesthetised axolotl where a lubricated pair of alligator forceps is gently passed into the stomach to retrieve the gravel. Surgical removal may be necessary if the foreign body moves beyond the stomach.

Bacterial

Septicaemia with Aeromonas spp. and Pseudomonas spp. are common. Axolotls will become inappetent, and in the advanced stage can develop ascites. The prognosis is poor in the advanced stage and euthanasia may be the best option. In the early stages, systemic antibiotics should be curative. If possible, obtain a sample of coelomic fluid or similar for bacterial culture and antibiotic sensitivity testing prior to therapy. Enrofloxacin, gentamycin and amikacin at 5 mg/kg IP, or trimethoprim sulphonamides PI have been successful. Note that tetracyclines are contraindicated in axolotls. Lowered water temperature appears to aid sick axolotls and so they should be maintained between 5–15°C for several weeks during treatment.

Mycobacteriosis shows no differentiating clinical characteristics to septicaemia. A combination of stress, ubiquitous microorganism and exposure to live foods predispose axolotls to mycobacteriosis. At necropsy, all coelomic organs will contain multiple granulomas. This can be confirmed by visualising acid-fast bacilli in smears or histologically. Bacterial culture may be done to identify the species of bacteria. There is no treatment for this. The owner must be notified of the zoonotic potential.

Parasites

It is not uncommon that axolotl owners feed live fish to their pet axolotls. From an animal welfare point of view, this practice is frowned upon. Moreover, live fish can pose a threat to axolotls because they can introduce harmful parasites.

Some common ectoparasitic diseases include Trichodina and Ichthyobodo (previously Costia). The key to identifying protozoal parasites is to examine wet mounts immediately, observing for mode of motility and morphology. Many protozoal ectoparasites can be treated with formalin at 0.025–0.050 ml/L for up to 8 hours.

Enteric parasites include the protozoa Hexamita and Opalina. These can be treated with metronidazole at 5 mg/g of food for 3–4 days.

Trauma

Axolotls have delicate skin and to avoid iatrogenic traumatic injuries, they should always be handled gently and in a soft mesh net.

It is common for animals to fight during the mating season (springtime). Axolotls that are housed in a mixed gender tank should have ample space and furnishings to break lines of sight. Sites of trauma should be treated to prevent secondary bacterial or fungal infection. These should be debrided (and sometimes limbs amputated) and a light topical treatment applied. If limbs are amputated, the wound should be left to heal by second intention. Suturing the site can inhibit limb regeneration.

Some antiseptic (e.g., 2–4 mg/L of mercurochrome to tint the water orange) can be added to the tank. Water is to be changed frequently. The axolotl will heal nicely, and the limbs/toes should regenerate. Regeneration actually occurs more quickly at the cooler end of its optimal temperature range.

Saprolegnia

This will appear as white cotton-like tufts that grow from the skin of the axolotl. The physician needs to address the primary cause (poor water quality, high organics, aggression) and treat the affected areas. If limbs are affected, they may be severed and then treated as per "Trauma."

Neoplasia

It is not uncommon for axolotls to develop tumours. These are usually benign and involve the pigment cells. It is not necessary to remove them unless they cause impediment or are at risk of traumatic injury.

Toxicosis

Some aquarium products are deemed unsafe in axolotls. Such products must be avoided include: malachite green, copper-based treatments and tetracycline. Excessive treatment with salt can also cause damage (they should only be given 10 minute salt baths at a dose of 10–15 g/L, once to twice daily). The products that are reportedly safe include some antiseptics (e.g., methylene blue, mercurochrome, betadine), a range of antibiotics (e.g., trimethoprim sulphonamide, kanamycin, neomycin, erythromycin, enrofloxacin, metronidazole) and some anthelmintics (e.g., levamisole, fenbendazole).

References

1.  Ambystoma. Sal-Site. http://www.ambystoma.org/. 2015. Accessed January 30, 2015.

2.  Bjorklund NK, Duhon ST. The Mexican axolotl as a pet and a laboratory animal. In: Ackerman L, ed. Biology, Husbandry and Health Care of Reptiles and Amphibians. Jersey City, NJ: Tropical Fish Hobbyist; 1996.

3.  Clare JP. Axolotls. http://www.axolotl.org. 1999. Accessed January 30, 2015.

4.  Loh R. Fish Vetting Axolotls. Perth, Australia: Richmond Loh Publishing; (in-prep).