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Myxomatosis in Rabbits

Myxomatosis refers to an often fatal disease that affects domestic and wild rabbit populations. This disease is caused by the myxoma virus, a species of the poxvirus family. Several strains of this virus exist today. The virus is most commonly spread through insect bites, as the insect transmits the virus through its mouthparts after feeding from an infected animal. Transmittal methods can include fly bites, fur mite bites, mosquito bites, thorns, animal bedding, and food.

The clinical signs an infected rabbit displays will determine how long the animal will survive, although most rabbits do not survive longer than two weeks once they have become infected with the virus. Outdoor rabbits are generally at a higher risk of infection than indoor rabbits.

Symptoms and Types

California strain in pet (domestic) rabbits

• Incubation period is usually 1-3 days
• In the acute form, eyelid edema (swelling) usually develops first
• Perioral swelling and edema (the tissue of the mouth)
• Perineal swelling and edema (the outer area between the anus and vulva or scrotum)
• Cutaneous (skin) hemorrhage
• Lethargy
• Anorexia
• Dyspnea (difficult breathing)
• Seizures or other central nervous system (CNS) signs - excitement, opisthotonos (spasm of the back muscles)
• Death typically occurs within 1-2 weeks

Wild/outdoor rabbits

• Cutaneous nodules at the site of transmission (insect bite, scratch) may be noticeable
• Young wild or feral rabbits may develop disease symptoms similar to pet rabbits

Causes

This disease is caused by the myxoma virus, a strain of leporipoxvirus. Outbreaks of it are more more likely when mosquitoes are numerous, in the summer and fall.

Diagnosis

Your veterinarian will perform a thorough physical exam on your rabbit, taking into account the background history of symptoms and possible incidents that might have led to this condition. A blood profile will be conducted, including a chemical blood profile, a complete blood count, and a urinalysis.

One of the obvious symptoms that will help your doctor to make a diagnosis will be the presence of nodules on the skin surface. However, in cases that are very sudden (peracute), there may be no lesions. Subcutaneous ecchymoses, or purple, bruise-like spots on the skin due to the rupturing of blood vessels, are sometimes associated with myxoma virus. An internal exploration may find ecchymoses in serosal surfaces (lining) of the gastrointestinal tract as well. In many cases, there is hepatic necrosis (death of the liver tissue), splenomegaly (enlargement of the spleen), infarcts (death of tissue due to deprivation of blood supply), or hemorrhage in the lungs, trachea (windpipe), and thymus (gland near the base of the neck).

Other findings include undifferentiated mesenchymal cells (the undetermined cells that are capable of transforming into many of the materials needed by the body (e.g., connective tissue, cartilage, blood), inflammatory cells, mucin (glycoproteins found in the mucous), and edema (swelling). If the rabbit is pregnant when it becomes infected, necrotizing lesions may be seen in fetal placentas.