About half of dogs infected with distemper fight the infection so quickly that they do not appear ill. Some dogs will have a few days of listlessness. The other half of dogs infected with canine distemper develop symptoms. Among those with symptoms, about half will die unless they receive medical care.
Often, the first signs of a canine distemper infection include a runny nose, eye discharge, and fever. These respiratory symptoms typically develop within 24 hours of infection. Dogs with weak immune systems develop pneumonia. Most dogs have no appetite and some vomit and have diarrhea. Because vomiting and diarrhea are typical of parvovirus infection, these symptoms can make it difficult to determine whether a dog has parvovirus or distemper. Respiratory and gastrointestinal symptoms cause dehydration.
Some dogs with canine distemper develop thick, hard paw pads. Dogs infected before their adult teeth are formed develop permanent teeth that are weak and misshapen because the enamel doesn't form. Dogs can have eye problems as the nerves to the eye become inflamed. Almost all dogs experience a drop in the immune system's white blood cells (lymphopenia), and as a result, other infections can invade and make them even sicker.
If the distemper virus infects the brain, dogs can develop seizures and paralysis within days. Other dogs are slow to develop brain and nervous system symptoms and nothing is apparent until they are in their senior years. These dogs are said to have "old dog encephalitis." Symptoms of brain and nervous system infection include fine muscle tremors and jerky muscle spasms (myoclonus) that are more obvious when dogs are sleeping. Some dogs snap at imaginary flies in the air or act as through they are chewing imaginary gum. These "fly snapping" and "gum chewing" behaviors are actually focal seizures caused by brain damage. Brain damage can also make dogs become uncoordinated (ataxic).
Brain damage is caused by two factors: the distemper virus itself and the white blood cells' response to the virus. Unfortunately, as the white blood cells try to kill the virus, they release enzymes that damage the myelin which covers nerves. This allows nerve signals to short circuit. In addition, the white blood cells release free radicals and destructive materials called cytokines. Free radicals are molecules with an unbalanced number of electrons that grab for electrons and cause cell damage. Cytokines cause harm because they stimulate inflammation, and the heat and swelling damages the brain and nerves. Omega 3 fatty acids can help promote normal brain and white blood cell immune function.
Despite all the white blood cell activity within the brain, often the virus remains safe and hidden. Dogs that have the distemper virus remaining within the brain are said to have a persistent infection.
Diagnosing canine distemper can be difficult. Finding that white blood cells are very low (lymphopenia) suggests a diagnosis of distemper, but lymphopenia also occurs with parvovirus infection. Your veterinarian can do special blood tests and examine cells from your dog's bladder, brain, and eyelids to help make a diagnosis. Even when the virus is present, much of the time it won't be found with these methods.The usual method used to diagnose a disease is to look for evidence that the white blood cells have produced special antibodies to fight it. But with distemper, looking for antibodies does not work well. In young dogs that do have antibodies, their presence can mean either your dog was vaccinated and is actually safe from infection, or that your dog has an active infection. Senior dogs with "old dog encephalitis" caused by distemper do not make antibodies.
A test of the fluid surrounding the brain and spinal column (cerebral spinal fluid or CSF) can diagnose distemper, but CSF analysis is an expensive, somewhat dangerous test. Another type of blood test can look for the virus itself by mixing fluorescent antibodies with the blood to see if they attach to distemper virus. If they do, the blood will glow. Even when dogs have distemper, this test is not always positive because the virus may hide within cells and not be present in blood.