Canine distemper is a multi-systemic viral disease of dogs. Canine distemper is a highly contagious disease that attacks a large number of animal species. It is fatal in approximately half the cases. Canine distemper is most often transmitted through contact with respiratory secretions. Contact with fecal material and the urine of infected dogs can also cause infection. This disease was far and away the most common fatal disease of dogs during the first half of the twentieth century. It was a daily or weekly occurrence well into the 1950s in the Mexican border town where I grew up. In the 1940s, crude vaccines first became available to attempt to control this disease. In the 1960s sophisticated, weakened live virus vaccines took their place. These vaccines have made canine distemper a rare disease in the industrialized world.

Cause
The virus that produces canine distemper belongs to the paramyxovirus group. Human measles virus is also a member of this group. This virus is quickly killed by disinfectants and sunlight and heat. In the body, canine distemper virus attacks and grows within the white cells (lymphocytes and macrophages) of the blood and lymphatic system as well as the cells that line the intestinal tract. Canine distemper virus is very resistant to cold. A small majority of distemper cases occur in the fall and winter.

The Hosts Of The Disease
Canine distemper occurs worldwide. Young puppies, between three and six months of age, are most susceptible to the disease and are the most likely to die from it. However, non-immunized adult dogs are also highly susceptible to distemper. These older dogs often develop mild cases.
All species of dog-like animals are affected as well as weasel-like animals such as ferrets, mink skunk, badgers and raccoon and red pandas. Recently it was recognized that large cats such as lions also develop the disease. Wild hogs called javelinas also develop distemper. It has been reported that humans can contract an asymptomatic (no signs) canine distemper infection. Cats are not susceptible to canine distemper. Distemper of cats refers to a different virus, the virus of panleukopenia. . In wild meat-eating animals canine distemper can cause abnormal behavior and lack of fear that is suggestive of rabies. This is especially true in urban raccoons.

Course Of The Disease
Canine distemper virus is passed rapidly through coughed or sneezed droplets of saliva. It then invades the lymphatic tissue. Within two to five days lymphatic tissue throughout the body is infected. By the sixth to ninth day, the virus is present in the blood. It then spreads to the surfaces of the lungs, intestine and bladder and, on occasion, to the nervous system.

We see distemper most frequently in older puppies that are in the process of loosing their protective maternal antibody. Once an animal is infected, the virus is shed in all body secretions. The onset of virus shedding begins about a week after infection. This virus does not linger long in the environment. Once outside the host animal it dies rapidly. Animals that are infected incubate the virus without signs for 3-6 days. About the seventh day of infection, most dogs develop a fever of 103-106F and become depressed. During the next two weeks, dogs either develop antibodies against the disease that protect them and kill the virus or they gradually go downhill. There is much variation in the duration and severity of the clinical disease. Dogs that go downhill develop a cough, secondary bacterial pneumonia and inflammation of the intestines. Many of these dogs show infection and damage to the brain with central nervous system signs of tremor and convulsions. Labored breathing and an unkempt appearance are common. By the third week most dogs have either died from the infection or recovered. Exceptions are the cases in which damage to the brain is delayed for up to three months. These delayed cases often show no respiratory or intestinal signs before delayed nerve damage occurs. In a few cases, the virus persists in the eyes, footpads and nervous system.

Signs vary greatly from case to case. Mortality from the disease is about fifty percent with mild cases showing very few or no disease sign. Most dogs that die from distemper die from neurological complications. The fever that begins near day seven gradually drops only to peak again later at a time when nasal and ocular (eye) discharges become thick and tenacious. These dogs have little or no appetite. Blood samples tested early in the disease are always low in certain white blood cells (lymphopenia). These dogs soon develop diarrhea with loss of fluid, dehydration and depression. In dogs in which the brain is attacked, incoordination, stumbling, seizures and paralysis occur. Both the gray and the white matter of the brain are destroyed. Some dogs become blind during the course of the disease as the virus attacks the retina. Neurological disturbances that may be seen are aggressiveness, disorientation, convulsive movements of the head and paws, and aimless wandering. Others cases develop a clicking gait as the skin of the footpads thickens (hyperkeratosis). Many show a thickening of the skin of the bridge over the nose. If the disease occurs while teeth are still forming, defects in their enamel covering may occur. Early flawed vaccines sometimes actually enhanced the nervous and ocular form of the disease.

Immunity
Because the virus attacks the cells that produce immunity (T and B-lymphocytes) dogs are always immuno-suppressed early in the disease. As the disease progresses through days six to eight, dogs that are destined to recover produce a strong antibody responses that neutralize the virus. The virus-neutralizing antibody produced by the tenth to twentieth day protect the dog from reinfection for years and sometimes for life.

Laboratory Diagnosis
Typical disease signs accompanied by very low white blood cell counts (lymphopenia) are highly suggestive of canine distemper. Laboratory tests to positively confirm the disease include fluorescent antibody techniques, polymerase chain reaction, virus isolation and ELISA (Enzyme Linked Immunosorbent Assay) tests. The demonstration of heavily pigmented inclusion bodies in stained blood neutrophils or in smears from the eye also aids in diagnosis. In animals that die, pathologic changes in tissue samples diagnose the disease. These pathological lesions of canine distemper include congestion and inflammation of the lungs (focal pneumonitis). Characteristic red stained (eosinophilic) oval structures are found in the epithelial cells of the salivary glands, central nervous system, adrenal glands, bile duct, urinary tract, lymph nodes spleen and skin.

Treatment
No antiviral drugs exist that effect canine distemper virus. Because of this, we treat the disease symptomatically. We administer antibiotics because bacteria take advantage of the damaged lining of the intestines and lungs. Intestinal coatings, antispasmodic agents and emollients are given to minimize diarrhea. The dehydration, brought about by diarrhea, is corrected with intravenous electrolyte fluids. This one treatment is probably the most effective thing we can do. Debilitated dogs, unable to eat, benefit from injections of essential vitamins and nutrients. Once dogs develop nervous system signs we have no effective t reatment.

Prevention And Control
Excellent vaccines are now available to protect dogs from canine distemper virus. These newer vaccines are manufactured from living, weakened (attenuated) viruses that induce long-lasting immunity. These vaccines are produced in bird or dog cell cultures. Immunity lasts many years. Vaccinations must not be given too early to puppies. The shots must be given to puppies at a time when the level of circulating antibody that they received from their mothers is in decline or the vaccine’s effects are neutralized (6-10 weeks). Twenty percent of this maternal immunity crosses the walls of the womb into the puppy while eighty percent is absorbed from colostrum milk across the intestine. A method around this problem utilizes a vaccine against human measles , a similar virus, to protect young puppies against canine distemper. Vaccines produced in canine tissue culture are the most effective but also the most likely to produce a small number of vaccine-induced distemper cases.

When To Vaccinate
Dogs that survive natural infection with canine distemper virus remain immune for life. In ideal situations, puppies should receive a combination measles virus/canine distemper virus when they are six to eight weeks old. They should never receive a live virus distemper vaccine if they are under four weeks of age as these have proven fatal. Then, every three to four weeks they should receive a modified live virus distemper shot for an additional two vaccinations. Then, booster vaccinations every three to four years are sufficient in dogs with a normal immune system. The precise interval can be calculated based on a neutralizing antibody test run on the dogs serum.

Other Control Procedures
Dogs with this disease should be quarantined and scrupulously isolated from susceptible dogs. A good phenolic disinfectant or a 1:20 dilution of household bleach kills the virus instantly.
Several years ago there was speculation and supposed statistical evidence that canine distemper virus was in some way associated with multiple sclerosis of man. Several studies done over the last fifteen years have failed to show any such connection. ad

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