Myxomatosis

Myxomatosis is a disease caused by the Myxoma virus, a type of poxvirus. It naturally affects tapeti rabbits in South and Central America and brush rabbits in North America, where it causes only mild illness. However, in European rabbits—the kind many people keep as pets or raise for food—the disease is very severe and usually fatal.

This disease is important because it shows what happens when a virus moves from animals that are used to it to animals that have never encountered it before. Scientists have studied it a lot to learn more about how diseases spread.

In the 1950s, people deliberately released the virus in Australia, France, and Chile. They did this to help control large populations of wild European rabbits, which were causing damage to crops and the environment.

Cause

Myxomatosis is a disease caused by the Myxoma virus, which belongs to the Leporipoxvirus genus. This virus is a type of large DNA virus and lives naturally in tapeti rabbits in South and Central America, and in brush rabbits in North America. In these rabbits, the disease is usually mild, causing only small bumps on the skin.

However, in European rabbits, which are often kept as pets or raised for food, the same virus causes a very serious illness that can often be deadly. There are different versions, or strains, of the virus. Some are extremely harmful, while others are less so. Over time, wild rabbits in some places have become a little more resistant to the disease, but pet rabbits usually do not have this protection.

Transmission

Myxomatosis is spread mainly by tiny insects that bite rabbits. These insects, like mosquitoes and fleas, carry the virus from one rabbit to another without getting sick themselves. In North America, mosquitoes are the main carriers, while in Europe and Australia, rabbit fleas play a bigger role. The virus can survive for several months inside these fleas.

The virus can also spread through direct contact. Infected rabbits release the virus in their tears, nose secretions, and from damaged skin. It can also be found in other body fluids. The virus stays alive in the environment for a while and can move from objects like water bottles, feeders, or even hands. However, certain cleaning products can kill it.

Pathophysiology

Scientists studied how the myxoma virus affects European rabbits by giving them small amounts of the virus under the skin. They watched what happened next. At first, the virus grew in the skin where it was placed. After about two days, it moved to nearby parts of the body that help fight infections. By the third day, the virus was in the blood and some inner organs. Around the fourth day, it could also be found in other skin areas and in the testes. By the fifth day, the rabbits’ eyelids started to get a little swollen, and the virus was in fluids around the eyes. By the sixth day, the testes began to swell up too.

Clinical presentation: South American strains

Myxomatosis shows different signs depending on the virus strain and the rabbit’s health. In the classic form, rabbits develop lumps under the skin, swelling around the eyes and genitals, discharge from the eyes, fever, and loss of appetite. A specific strain called the Lausanne strain causes large purple lumps.

Rabbits with very strong strains of the virus may die quickly, sometimes within just a few days, with few other signs besides eye swelling. With weaker strains, rabbits may live longer and have milder symptoms, such as scabs and scars on the skin. Even rabbits that seem to recover can later have breathing problems and fluid in the lungs. Since the 1970s, a milder form of the disease has appeared in Europe, mainly affecting farmed rabbits, with fewer skin lumps but still causing swelling and eye discharge.

Clinical presentation: North American strains of myxomatosis

Myxomatosis in North America is found mainly where brush rabbits live, from western Oregon and California to Baja California in Mexico. Brush rabbits are the only animals in North America that can carry the myxoma virus, which causes the disease. Other rabbits and hares cannot spread it.

In California, scientists have studied cases of the disease. Rabbits with myxomatosis often have swelling around their eyes, nose, lips, and ears, along with fever and tiredness. Some rabbits also develop small bumps on their skin. Outdoor rabbits are more likely to get the disease, especially in late summer and early fall. In Oregon, similar signs were seen, including swelling and tiredness. In Baja California, affected rabbits had skin bumps and trouble breathing.

Diagnosis

Myxomatosis can often be diagnosed by looking at the rabbit's symptoms, which are usually very clear. If more proof is needed, scientists can use special tests to check for the virus.

One way to test is by studying tiny pieces of the rabbit’s tissues under a microscope. Another quick and accurate method is using a lab technique called polymerase chain reaction, or PCR, on samples from the rabbit. Other tests, like looking at the virus under an electron microscope or growing the virus in a lab, can also help confirm the diagnosis.

Treatment

Currently, there is no special medicine to cure myxomatosis. Because this disease can be very deadly for rabbits, some people choose to gently end a sick rabbit's life to prevent suffering. In places where the disease is not as strong and vaccines exist, doctors can give care to help rabbits fight the infection. It is important to watch the rabbit closely and decide to stop treatment if the rabbit stops eating, drinking, loses a lot of weight, or gets very cold.

Prevention

Vaccines are available in some countries to protect rabbits from myxomatosis. These vaccines help keep rabbits safe, even if they are kept indoors, because the virus can be carried inside by tiny insects or things like clothing and tools. It’s important for rabbit owners to keep up with regular vaccinations to make sure the protection stays strong.

In places like Europe and the United Kingdom, there is a special vaccine that protects rabbits from both myxomatosis and another serious disease called rabbit hemorrhagic disease. This vaccine works for rabbits that are at least 5 weeks old and gives protection for about a year, so it’s recommended to give the vaccine every year. However, in Australia, using the vaccine is not allowed because there is a concern it might help wild rabbits resist the disease, which could be bad for the environment. Many people in Australia are working to change this rule so pet rabbits can be protected too.

To keep rabbits safe where vaccines aren’t available, it’s important to avoid contact with wild rabbits and keep pets indoors or behind screens. Keeping rabbits free from fleas, lice, and mites with safe medications is also helpful. If a new rabbit might have been near the virus, it should be kept apart for 14 days before meeting other pets. If a rabbit shows signs of myxomatosis, it should be isolated right away. Cleaning anything that might have touched the virus with bleach, ammonia, or alcohol can help stop it from spreading.

Reporting

Myxomatosis is a reportable disease in California, the United States, and Mexico. Veterinarians and pet owners who notice this disease should report any cases to the right agencies. This helps make sure that everyone has the right information about how common the disease is, which can encourage vaccine makers to create vaccines when they are needed.

Use as a population control agent

Myxoma virus was the first virus intentionally introduced into the wild to control the population of European rabbits in Australia and Europe. In Australia, rabbits were brought by settlers in 1788 and became a big problem. In 1950, a special strain of the virus was released, which was very effective at first. Over time, however, rabbits became more resistant, and the virus became less harmful.

In Europe, the virus was first used in 1952 in France and quickly spread. It caused big drops in rabbit numbers, which also affected animals that rely on rabbits for food. Like in Australia, the virus eventually became less harmful, and rabbits grew more resistant. Efforts to stop the spread in Europe were not successful. In New Zealand, the virus did not work because the necessary carriers were not present. In South America, the virus was successfully used to control rabbit populations in Tierra del Fuego.

Use as an evolutionary model

Scientists have studied the myxoma virus closely because it helps us understand how diseases change over time. When the virus was first introduced to rabbit populations in Australia and France, it caused very serious illness. However, over time, less harmful versions of the virus appeared. These versions allowed rabbits to live longer, which helped the virus spread more easily.

As scientists learned more about the virus's genes, they discovered exactly which parts caused the disease to become weaker or stronger. Even though the virus today is not as harmful as it once was, it still helps control rabbit populations. Some newer versions of the virus let it live longer in the rabbits, which helps it spread further.

In culture

Myxomatosis is mentioned in the novel Watership Down (1972) by Richard Adams. In the story, a rabbit chief drives out rabbits who appear to have the disease. The book also includes a folk tale where the hero learns how the disease spreads.

In 2003, the British rock band Radiohead released a song named Myxomatosis on their album Hail to the Thief.