Myxoma Virus (MYXV) (Myxomatosis) (Poxviridae, Chordopoxvirinae, Leporipoxvirus) – Molecular diagnosis (PCR). 

Information 16-11-2015.

The myxoma virus is a Leporipoxvirus, subfamily Chordopoxvirinae, family Poxviridae .This virus causes myxomatosis, an infectious disease that affects rabbits and that is characterized by the development of tumors (myxomas) in the connective tissues. It is a large enveloped virus (250 nm in diameter and 300 nm in length), which has an envelope membrane with lateral bodies. The genome is non-segmented and contains a single linear molecule of double-stranded DNA (dsDNA) with 160,000 nucleotides.

This disease was initially described in Uruguay (1898) and the virus was identified in 1927. There are two geographical types, the South American strain, whose natural host is the wild rabbit (Sylvilagus brasiliensis), and the Californian strain (West Coast of the USA) whose natural host is Sylvilagus brachmani. Both strains are the cause of a benign cutaneous fibroma, located in the place of inoculation, in their respective hosts. However, these strains were found to be highly contagious for European rabbits (Oryctolagus cuniculus) and hares (Lepus europaeus), in which it causes the disease called myxomatosis due to prominent mucinous skin lesions. In European rabbits it causes a very lethal disease, but in hares it is rare that it causes disease, since it seems to show some resistance to infection. For this reason, the South American strain was deliberately introduced, first in Australia and then in France, to reduce the population of harmful rabbits that caused local ecological alterations. Later this strain spread through most of Europe.

The transmission of the virus occurs mainly through the bite of insect vectors infected as fleas of rabbits (Spilopsyllus cuniculi) and mosquitoes, which act as mechanical vectors, transmitting the virus during feeding, and propagating over long distances. In addition, the virus can also be transmitted directly from an infected animal to an uninfected animal, by ingestion of water, contaminated food or by direct contact with contaminated fomites. Once in the animal´s organism, the virions encode extracellular and intracellular proteins. The extracellular proteins inhibit the immune response of the infected animal. The myxoma virus matures by budding through the cytoplasmic membrane of the host animal's cell.

The incubation period of the myxoma virus is generally 1 to 3 days. The severity of the infection depends on factors such as the affected rabbit species. For example, in rabbits of the genus Sylvilagus, infection with the myxoma virus usually causes the development of benign localized cutaneous and mucous membranes fibroma, particularly in the head and genitals. However, in the European rabbit (Oryctolagus cuniculus) infection by the virus causes a disease with more severe signs and symptoms and with a high mortality (close to 100%). In these rabbits, in addition to the development of myxomas (prominent mucinous skin lesions) on the head and face, the infection causes conjunctivitis, anorexia, languor and fever. Animals can die two days after the onset of symptoms. If the disease continues on a longer course, it is accompanied by depression and myxomas in the eyelids, nose, lips, ears, vulva and scrotum. The edematous ears become heavy and drop. Animals die within 1 to 2 weeks due to wasting and dyspnea. For this reason, the two strains from Brazil, the Moses strain in Australia (1950) and the Lausanne strain in Europe (1952) were introduced to control wild rabbits. In most cases, secondary bacterial infections that can cause pneumonia occur. Being a highly contagious virus, myxomatosis is a problem for rabbit breeders in Europe, causing considerable economic losses.

The coevolution between strains of introduced viruses and the population of rabbits has given rise to strains of attenuated virus and populations of wild rabbits resistant to the virus.

Recommended tests for diagnosis:

The diagnosis is based on the detection of antibodies by ELISA methods (Enzyme Immunosorbent Assay), or in molecular diagnostic methods (PCR).

The molecular diagnosis by means of PCR (Polymerase Chain Reaction), is the most recommended method at the moment.

Tests carried out in IVAMI:

• Molecular diagnosis (PCR: Polymerase Chain Reaction), to detect myxoma virus DNA.

Recommended sample:

• Biopsy of myxomatosis lesion (upper eyelid, nose, lip).

• Tissue samples (upper eyelid, nose, lip, lung, kidneys, rectum, or mammary gland) of dead or slaughtered rabbits.

Conservation and shipment of the sample:

• Refrigerated (preferred) for less than 2 days.

• Frozen: more than 2 days.

Delivery of results:

• Molecular diagnosis (PCR): 24 to 48 hours.

Cost of the test:

• Molecular diagnosis (PCR) for one sample test or for more than one sample, received simultaneously: Consult to ivami@ivami.com.