KARL W. HAGEN, JR.
THE INFECTIOUS agents that are responsible for diseases in the rabbit belong to the same major groups that are found in man bacteria, fungi, viruses, and protozoa.
High on the list of the infections, which are steadily growing in importance as more and more rabbits are raised commercially, is pasteurellosis.
The acute infection by Pasteurella multocida is marked by a septicemia and is like the hemorrhagic septicemia of farm animals. It is most common in adult females. It can cause death in 24 to 48 hours. Autopsy reveals a congestion of blood vessels. The surface of the intestine and tissues just under the skin show hemorrhagic areas. Often the spleen is enlarged.
The chronic form is not necessarily fatal, but it can detract from the animal's appearance and value. Breeders refer to it as "a cold" or "snuffles." A nasal discharge may persist and may spread to a large proportion of the stock if conditions favor it. The animal may become a carrier if the mucous discharge subsides.
Abscesses under the skin on various parts of the body are also typical of the chronic form. The abscesses contain a thick, creamy pus.
Pasteurellosis of the reproductive tract is usually a chronic infection. It produces a purulent discharge from the sex organs. The infection may be spread among other animals in the process of mating.
The introduction of a suitable or virulent bacteria into the uterus will not produce this reproductive infection. The change that takes place in the pregnant or falsely pregnant uterine tissue is necessary before the bacteria can become established in the uterus.
Since the doe has two uteri, it is possible that one uterus is infected while the other uterus is healthy and carrying young. The animal becomes sterile if both uteri are infected.
When the infection is not well established in the uterus, it is possible for the purulent material (pus) to be evacuated from the reproductive tract. When it is free of this material, the uterus can carry a normal litter.
The prevention of pasteurellosis depends mainly on the degree of sanitation maintained in the rabbitry, careful isolation and quarantine of all newly acquired animals, and prompt disposal of sick or diseased animals. Regular cleaning and disinfecting of hutches are measures that help to reduce all infectious diseases.
A clean water supply and properly stored feed are important.
Isolation of new animals will prevent exposure to potential carriers and reduce the possibility of introducing disease into a clean rabbitry. All animals that show symptoms of disease should be isolated immediately or be slaughtered and properly disposed of by burning or deep burial.
No vaccine is available. Antibiotic preparations, such as streptomycin and penicillin, and sulfaquinoxaline are effective, but the cost of such therapy must be weighed against the value of the individual animal.
STATISTICS of the United States Rabbit Experiment Station, at Fontana, Calif., show that pneumonia is the greatest single cause of death in mature rabbits. It was present alone in 53 percent of all losses in mature rabbits in 1947-1949. Among the developing rabbits, 10 percent showed primary pneumonia at death. One-third of all autopsied cases showed pneumonia either alone or with other pathology, such as enteritis. Primary pneumonia was found in 5 percent of the unweaned young. The highest death rates from pneumonia alone were during the third week, when the young were emerging from the nestbox, and during the 11th week.
The major predisposing factors to pneumonia in mature does were pregnancy, parturition, and heavy lactation. In the 6- to 12-week-old rabbits, abscesses, and long teeth were the main factors. In the nestbox babies, exposure and improper care were the main causes.
Rabbits with pneumonia lack appetite and breathe with difficulty. The head is thrown back, and the nostrils are elevated. They literally gasp for air during the later stages. There may be a watery or a purulent discharge from the eyes and nose. The color of the ears and eyes is not pink, but bluish, and the body temperature is elevated above the normal (102.5 F., with a variation of one-half degree).
When such animals are opened after death, the lobe or lobes of the lungs appear mottled with patches of gray and red. The tissue in advanced cases may have the consistency of liver.
Frequently the bronchi and bronchioles fill with a thick pus. An entire lobe in advanced cases may be replaced by this cheeselike matter, which is characteristic of a Pasteurella infection.
In primary pneumonia cases, P. multocida was isolated in 65 percent of the animals; when the pneumonia was secondary to enteritis, the Pasteurella isolates were only 15 percent and were not found in cases of infant pneumonia. Alcaligenes bronchisepticus and a Streptococcus species were found infrequently in primary pneumonia, but in secondary pneumonia they each accounted for 35 percent of the isolates In infant pneumonia the two species occupied equal importance.
Rabbits should be housed in well ventilated hutches that provide protection from dampness, drafts, and extremes of heat and cold. Units should have good head space and allow some sunlight to enter. Overcrowding should be avoided.
Intramuscular doses of penicillin are effective against many cases of pneumonia if given early enough. Late cases with advanced pathology are fatal. Reducing feed concentrates and providing fresh greens often will help the animal resist the infection.
INFECTIOUS MYXOMATOSIS (mosquito disease) is a specific, rapidly fatal disease that is caused by a filterable virus, Molitor myxomae. It was first discovered in domestic rabbits in Mexico when some laboratory animals were shipped from Lower California to San Diego, Calif., in late 1927. By 1937 it had spread along the entire west coast. It is called mosquito disease by some rabbit growers, because it is found frequently in low, marshy areas where mosquitoes are numerous.
The disease is highly infectious among rabbits, but it does not appear to be transmissible to other animals or to man. Mosquitoes transmit the virus by carrying blood or exudate from the eye or nostril of an affected animal to a noninfected rabbit. Direct contact between animals also spreads the infection. Biting flies and fleas can transmit the virus, but the insect vector is purely mechanical, as no life cycle is required in the vector.
Myxomatosis is characterized by tumors, which appear in the subcutaneous tissues around the eyes, mouth, nose, and sex organs. Conjunctivitis, edema of the eyelids, and a nasal discharge occur. The ears swell and become pendulous and heavy a characteristic symptom.
The central nervous system is invaded by the virus from the blood stream, breathing becomes labored, and the animal enters coma. Artificially infected animals show symptoms in about 10 days and die 4 or 5 days later. Mortality may be as high as 95 percent in animals more than 60 days old. The young are affected less.
Internal changes at autopsy are not characteristic. If the soft tumors are cut, the surfaces are white, gelatinous, and glistening. The lungs are usually congested. The spleen is dark, enlarged, and pulpy. Infected epithelial cells in or near the tumors contain cytoplasmic inclusion bodies.
Rabbits recovering from myxomatosis are immune to the disease, but vaccine protection has not proved to be efficient. Antibiotics have no effect on the course of the infection.