Animal Care - Rabbit Care - Myxomatosis


Myxomatosis - by Dr. Micheal Murray
Myxomatosis is a viral disease caused by a poxvirus in rabbits. The pathogenicity and subsequent morbidity and mortality are influenced by the virulence of the particular viral strain, but also the host infected by the organism. Following its initial recognition in Uruguay in 1896, it has spread, frequently with man's intentional assistance, to four continents, North America, South America, Europe and Australia. The virus was intentionally intorduced in Australia in 1926 in an effort to control the rampant pest European rabbit, Oryctolagus cuniculus. The effort was relatively successful, decimating the rabbit population by 1953. Since then it has become endemic, with periodic outbreaks associated with high mortality. As genetic resistence has risen, the mortality has dropped from 90% to 25%.

Its spread into Europe was again an intentional introduction. While governments were debating the wisdom of bringing an infectious disease to the continent, an ambitious French land owner, imported the virus to aid in the control of the rabbit population upon his lands. It spread rapidly, and within one year had been identified not only in France, but also in Belgium, the Netherlands, Germany, Luxembourg, Spain and England. The first North American case of myxomatosis was dioagnosed in San Diego in 1928. It is believed that the virus originated in infected domestic rabbits imported from Mexico.

Myxomatosis is now found in four continents. It has been a classic model of an infectious disease whose etiologic agent has modified over time concurrently with the host species to a state of equilibrium. It is now endemic in wild rabbits of the genus Oryctolagus in South America, Europe, Australia and New Zealand. Rabbits of the genus Sylvilagus are endemically infected in both North and South America.
Etiologic Agent
Myxomatosis is caused by one of several strains of a myxoma virus of the poxvirus family. These strains have varying degrees of virulence with the California strain being one of the most devestating. California strains have been known to cause over 99% mortality. The neuromyxoma and Nottingham strains of Europe have both been associated with substantially less mortality and hence less virulence.
Epidemiology
 In the western hemisphere, the myxoma virus is generally endemic in the wild rabbits of the genus Sylvilagus. In particular, the California brush rabbit, (Sylvilagus bachmani). It is from close indirect contact with this native rabbit species that most pet domestic rabbits are infected in California.

The disease is transmitted by blood sucking arthropod vectors, typically mosquitos and fleas. Transmission is mechanical upon the mouth parts of the vector, therefore the species of anthropod is not significant. Generally, the virus is acquired from the superficial layers of the skin, often around the eyes and ears. Other anthropods may act as mechanical vectors; biting gnats, mites, lice, and flies are capable of carrying myxoma virus. It has even been postulated that the talons of carniverous birds and even thorns may act as agents of transmission.

The female mosquito has a life span, and therefore transmission potential, of only 2-3 weeks. It can, however, range for up to 14 miles. The flea, on the other hand, has a limited range, but may feed actively for over a year. The virus has been found to be active in fleas without rabbit contact for 105 days.
Clinical Signs
The clinical signs of myxomatosis vary dependant upon the strain of virus involved and the species of rabbit infected. For the practitioner, however, the signs of the disease will be quite consistent within an area. The most important area affected in North America appears to be in California, in which significant outbreaks are often seen coinciding with the peak mosquito months of the summer and fall.

The Sylvilagus spp. appear to be relatively resistent to infection, as they seem to be the natural host for the virus. Development of firm swellings, fibromas, at the site of the insect inoculation is the most common clinical finding in affected individuals. Young animals many succumb to the generalized disease, similar to that seen in Oryctolagus spp. Hares, (Lepus spp , are similarly resistant to infection with occasional observation of generalized infections in certain individuals.

Of particular importance is the effect the myxoma virus has upon the domestic, pet rabbit, Oryctolagus cuniculus. As the California strain is one of the most commonly seen in North America, the signs will generally be attributed to this viral strain. In general, as the virus is exeptionally virulent, the clinical signs seen by the clinician are directly related to the survival time of the infected rabbit.

The peracute form of the disease will typically cause death within 7 days of infection. Minimal signs are seen in these individuals. Lethargy and edema of the eyelids are frequently the only signs seen in these individuals. Pyrexia is noted occasionally.

Rabbits with the acute form tend to survive for 1-2 weeks. Edema of the eyelids, typically seen 6-7 days post-infection, gives the rabbit a sleepy appearence. Shortly thereafter, edema and swelling around the nose, anus, mouth, and genetalia is noted. Fevers of 104-107° are not uncommon. Most rabbits die by the tenth day with cutaneous hemorrhage and seizures often occurring.

Only rarely is a chronic form of this strain of virus noted. Those individuals may develop a mucopurulent blepharoconjuntivitis and swelling around the base of the ears. The swelling may proceed to resemble that of the less virulent strains of the virus. The classically described myxoma is only rarely seen.

Less virulent forms of the disease will take a more protracted course. These individuals may develop generalized tumors in addition to the edema and blepharoconjuntivitis. Dyspnea may occur and most die within two weeks. The predominant European strains tend to cause tumors only, with a marked decrease in mortality.
Diagnosis
In most cases, diagnosis of myxomatosis is difficult and generally done postmortm. The classically described myxoma nodule with its homogenous mucinous material in the center is easily described in the European cases. Definitive diagnosis in the California strain affected rabbits is much more difficult. The rapid onset and death of the rabbit with minmal premonitory signs is suggestive. Eccymosis of the subcutis and subserosa of the gastro-intestinal tract are also suggestive. Often, it is the postmortem change that is most diagnostic. Virus isolation is easily performed on embryonated chicken eggs or cell culture.
Treatment and Control
The viral etiology and rapid progression of the disease tend to make attempts at treatment frustrating. Administration of paliative remedies, fluids, vitamins, and forced feeding are typically not rewarded. To this date, the use of anti-viral medication have shown no promise in the reversal of the typical clinical progress of the infection.

Most efforts are directed towards the prevention of myxomatosis. This is particularly important in area where the virus is endemic in wild rabbit populations. Vector control, particularly fleas and mosquitos is typically employed. Screening is the most effective mechanism for exclusion of the vector. Use of appropriate flea control products is also valuable in eliminating vectors. Rabbits should not be allowed to remain outdoors during peak mosquito activity periods of dusk and dawn. Obviously, quarantine of new arrivals and ill animals is important in preventing entry through an infected host.

The use of the fibroma virus live vaccine had not been consistently protective. An attenuated myxoma virus vaccine has provided an immunity for 9 months, but it is not available within the U.S.

While the effects of myxomatosis may seem to affect only those in California, even practioners in the midwest and east should remain vigilant. The mechanical nature of the vector and the ease of transmission imply that the virus may be introduced to any rabbit-bearing population with ease. The rapid movement of rabbits from coast to coast has increased commensurate with the increase in the popularity of the animal as a house pet. It may only be a matter of time before other areas are beset with the tragic losses of treasured pet rabbits.
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