Canine Distemper And Your Dog – CDV

Canine Distemper And Your Dog – CDV

Ron Hines DVM PhD

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Tell Me Some General Facts About Canine Distemper

Canine distemper (CDV) is a serious, highly contagious viral disease. Although it affects many animal species, most pet owners think of it as a disease of dogs. The virus is not related to the “distemper” of cats (panleukopenia). Dog distemper is an RNA virus ; cat distemper on the other hand is a DNA virus.

The distemper virus affects dogs all over the world. Some historical records suggest distemper was once confined to the Americas and that was imported from Peru to Spain in the 17th century. (read here)  Others believe the evidence points to the disease moving the other direction, from Spain to America. With the coming of the Spaniards, the dog population of native Americans almost completely disappeared  (read here & here)

The dog distemper virus has a strong affinity for the lymphocyte cells that are a critical part of your dog’s immune system. Lymphocytes are the first cells that distemper attacks after it enters the body. The number of circulating lymphocytes in an distemper-infected dog often drop sharply soon after infection. With their immune system crippled, those dogs become more susceptible to secondary bacterial infections.

Very effective, long lasting, vaccines have made canine distemper a rare disease in the United States and everywhere that dogs receive preventative distemper vaccinations early in their life. Read when here. But canine distemper is still a major cause of canine death and disability wherever the population is too poor to afford those vaccinations, where dogs roam unattended, in animal shelters and at disreputable pet stores. In prosperous parts of the United States where most dogs are vaccinated, it is the urban raccoon population, not dogs, that are the primary reservoir of dog distemper virus. (read here)

You Mentioned Raccoons. Are Other Animals Susceptible To Canine Distemper?

Yes, quite a few are, and the link at the top of this page will show you who they are. 

Fox, coyotes and wolves are just as susceptible as dogs. All weasels-like animals (mustella) such as ferrets, skunk and mink are as well. So are all large wild felines (lions, tigers, etc.) as well as bears. Even monkeys can catch canine distemper. (read here) Although research scientists have identified antibodies against canine distemper in house cats and CDV has been shown to grow in cat tissue, it remains unclear if the CDV virus is able to cause disease when a house cat is exposed to it. 

How Might My Dog Catch Distemper – How Does This Virus Spread?

The distemper virus is fragile when it is not inside an animal. It is thought to survive less than a day in a typical outside environment. Sunlight, summer-day warmth and drying rapidly inactivate CDV. It is generally a sneeze or cough from an infected dog that spreads the virus to another dog. Since the distemper virus is present in a dog’s lungs in huge numbers early in infection and before the pet feels ill, a single sniff, a greeting, a lick on the muzzle is all that is required. Once the disease is in full bloom (systemic) the virus is present in all secretions, urine, feces, tears and saliva. (read here) So kennel workers can track the virus from dog to dog on their shoes, on utensils, on their hands.

How Long After My Dog Is Exposed To The Distemper Virus Might It Take For It To Become ill?

It generally takes 4-7 days after exposure for a dog to begin to feel ill. It might begin with a loss of appetite, a slightly depressed mood and a low fever that progresses to eye and nasal discharges and a soft cough. Canine distemper is a bi-phasic, two-wave disease. During the first portion, the virus is reproducing throughout the dog’s body. During the second, the dog’s immune system is severely depressed and the pet has little defense against secondary bacterial infections. Many dogs show mild or no signs of illness during the first phase, only to become severely ill during the second phase when the accumulated immunosupression allows for severe diarrhea, dehydration and pneumonia. 

Some dogs also skip the traditional second phase and are only left with varying amounts of viral nerve damage (twitching, seizures, “chewing gum”, snapping at imaginary flies ). Mental impairment and occasional foot pad involvement (“hard pad disease” are late symptoms occasionally associated with canine distemper. A dog is really not out of the woods (safe) until a little over a month has passed since symptoms began or exposure occurred.  Occasionally, neurological signs appear even later (3-4 months). Because the course of distemper disease can be so extended, administering anti-distemper vaccines after the virus is already in a dog’s body can easily be misinterpreted as vaccine failure. Vaccines were never meant to be given after or just before exposure. Immunity is not an instant event.

How distemper progresses depends on what degree of immunity to distemper your dog might already have had. In puppies, that immunity is the residual immunity of their mothers (=maternal immunity) , passed on to them through the first milk (colostrum). Once those maternally-obtained antibodies against distemper have dissipated (faded), your dog is again susceptible to distemper. Disease severity can also be influenced by the strain of distemper virus your dog has been exposed to – some “wild” strains are more virulent (worse) than others. Your dog’s nutritional status, stress level, intestinal parasite load and other concurrent health issues influence the severity of canine distemper disease as well.

If My Dog Caught The Distemper Virus What Signs Might I See?

The early symptoms of canine distemper in dogs are not sufficiently unique for your veterinarian to make a CDV diagnosis. Kennel cough, or adenovirus-1 and a large number of other virus and bacteria that infect dogs can all cause the same early symptoms.  It is also not that unusual for young or stressed dogs to be facing several of these diseases virus and bacteria all at the same time. (read here)

Since a dog’s lungs and airways are one of the first areas that the distemper virus colonizes, a nasal discharge and cough often appear early in CDV disease. The surface cells of the eye are affected early by CDV as well. So matter or a greenish ocular discharge often occur as well (lots and lots of less serious health issues including dehydration can also cause moderate eye mattering).

Later in the disease, after the virus has crippled the dog’s immune system, bacteria commonly take advantage of the situation. That can result in pneumonia, gasping and severe diarrhea. Dogs at that point have no interest in food. Sometimes the deeper structures of the eye resulting in blindness occurs as well ( uveitis,   keratoconjunctivitis,   retinitis ). With time, those symptoms often subside and dog owners think their pet is on the road to recovery. But unfortunately many of those dogs then begin to show signs of  permanent nervous system damaged. Tremors, twitching (tics) , chorea, seizures, head tilt, chewing gum “fits”, aimless circling, snapping at imaginary flies and loss of mental acuity frequently occur late in distemper. They are all signs of nervous system damage that is usually permanent. A subset of dogs do not show any of the early signs of distemper. Owners and veterinarians only realizes the extent of damage the virus has caused when these neurological signs finally begin to occur. Hardening and cracking of the skin of the paws (“hard pad disease”/hyperkeratosis) and of the nose are also two late signs of distemper that occasionally appear. In puppies that survive distemper, their permanent teeth sometimes erupt with tooth enamel defects.

There is currently no way that your veterinarian to predict the course of an individual case of distemper. Some vets believe that particular distemper viral strains might be more likely to produce one set of symptoms than another and many believe that certain viral strains (highly virulent ones) are more likely to be fatal or cause lasting neurological damage. 

Canine distemper tends to be most severe in 3-6 month old puppies. By 3 months of age, the immunity the puppy inherited from their mother through drinking her first milk (colostrum‘s  passive immunity) begins to wane (decrease). Yet the pup’s immune system has not yet reached its full defensive potential. Many dogs of this age reach animal shelters in a very nutritionally deprived condition with high internal and external parasite loads. That also weakens their ability to survive a CDV infection. Most receive their first vaccination against distemper at the door. Those vaccinations can be helpful – but vaccines take some time to be protective. No one has determined how many days it takes for the initial distemper vaccination to provide some protection to dogs.   (perhaps 4-7 days)  (sometimes longer). By 9 month of age, incoming shelter dogs are often already survivors of mild cases of canine distemper which imparts them with a solid immunity against reinfection.

Old Dog Encephalitis

Occasionally, an older dog will develop neurological signs that are very characteristic of distemper (= “old dog encephalitis”). Those dogs were often appropriately vaccinated against distemper in their youth and many had received periodic booster vaccinations throughout their lives. To the best of my knowledge, no one has ever been able to isolate living distemper virus from these older dogs, but hints at the virus’ presence have been found. (read here & here) We do not know why this sometimes occurs or what triggers it. Some think that certain wild strains of distemper have the ability to persist silently in a dog’s brain. Others believe that wild CDV strains might fuse within the dog with vaccine strains to create mutant virus with enhanced ability to cause nerve and brain damage. That is similar to what is known to be possible in human measles. (read here) Still others believe that nerve cells are innocent bystanders – injured when the dog’s immune system cells attack incomplete viral CDV remnants adjacent to them that persist in recovered dogs.  (read here)     

How Will My Veterinarian Decide If  Distemper Is The Cause of My Dog’s Illness?

I mentioned earlier that the early symptoms of canine distemper are not sufficiently unique to make the diagnosis (not pathognomonic). But the history you give, the lack of a properly administered canine distemper vaccination at 16-20 weeks of age, low lymphocyte/thrombocyte counts (lymphopenia thrombocytopenia) during the first week of illness, neurological signs, and sign that suggest many body organs and systems are being affected (multi-systemic symptoms) are all suggestive of canine distemper.

Tests that positively identify CDV as the cause of your dog’s illness are complicated and are best performed at a central veterinary laboratory. Those include tests that show a rising antibody titre (ELISA tests) against the distemper virus and tests that look for the presence of the living distemper virus itself ( eg RT-PCR,  Quant RealPCR™ ,  Antech PCR ,  RT-RPA ) These tests all have their drawbacks. Recent distemper vaccination or samples obtained from the dog too late in the disease can both cloud  test result significance. If a dog does not survive, properly preserved tissue samples, sent to a veterinary pathologist, can confirm that distemper was the cause of its death.

How Can I Reduce The Risks That My Dog Or Puppy Will Be Exposed To The Distemper Virus?

Current vaccines against canine distemper are highly effective.The biggest errors is beginning your puppy’s three-shot series when it is too young and failing to give the crucial 16-18 week booster. One cannot count on the earlier-administered vaccinations being effective – no matter how many shots were given – until a puppy is about 16-18 weeks of age. By that time, its mother’s interfering immunity should be gone from its system – or low enough to not interfere with the production of long-lasting vaccine immunity. It is true that many puppies reach the point where there is no maternal antibody interference earlier (~14 wks) – but not all of them do. The problem is the creation of the veterinary industry which still suggests routine vaccination at a time when the majority of puppies do not benefit from it. After multiple vaccination trips to the vet with their young puppy, many dog owners are so tired of the visits and the expense that they forgo the most important shot of all – the one at 14-18 weeks of age. I believe that isolation of your new puppy from sources of distemper and other virus until one can be sure the shots will be effective is a much-preferred way to protect your new dog. Read more about my thought on that here. If you can’t do that, if you do not know the vaccination status of the dog’s mother, if this is a puppy from an animal shelter or distressed situation, then the the vaccine schedule suggested for a puppy and available on the vaccine manufacturer’s website is the one to follow.

How Long After Infection Can A Dog Still Spread The Distemper Virus?

Many dogs overcome a distemper virus infection with few signs to alert their owners or caregivers that the dog was ill. Those dogs are still quite active and can spread the virus for many weeks. Some say for up to 4 months – but I do not believe that that has been scientifically confirmed. Shelters are the source of much canine distemper chatter these days. In North America, animal shelters are still common hotbeds for distemper exposure. It can be very difficult for these good people to determine the source of continuing new infections. If a dog develops distemper 4 month after their last case was diagnosed, that is not sufficient evidence for me that a new distemper virus did not enter their facilities in the interim. Few, if any animal shelters would have the financial resources to methodically track down how the CDV virus moved from dog to dog in their facility and virtually none have the resources to pay for the complicated laboratory tests that would indicate a recovered dog did or did not still harbor distemper virus. Even those sophisticated tests often miss low virus loads (numbers). 

If My Dog Catches Distemper What Treatments Might Help?

Veterinarians have not reported any specific medications or treatments that kill distemper virus within a dog’s body. So we are limited to “supportive care”. That is, seeing to it that your dog is properly hydrated, administering antibiotics to combat secondary bacterial infections, providing warmth as required, lowering fever (antipyretics) when required and the like. Nebulization of antibiotics to combat pneumonia is sometimes helpful. Some young dogs will recover, some will not depending on the strength of their immune system and, perhaps, the strain of distemper virus that they were exposed to and perhaps the size of the initial virus dose. We really do not know. 

Italian veterinarians reported that a combination of two human antiviral drugs, ribavirin and boceprevir, prevented the growth of distemper virus in their laboratory setting. (read here) If these drugs have a place in treating dogs that have been exposed to or fighting distemper has not yet been determined. The canine distemper virus and the human measles virus are very close relatives. If effective treatment options are discovered for CDV, they will most likely be the result of newly discovered treatment options for measles. (read here)  Measles is a close cousin to the distemper virus. There is some evidence from measles exposure reports, that vaccinating exposed children within 3 days after they were exposed to the virus lowered the number that later developed measles. (read here & here)  So in a distemper outbreak, at-risk unvaccinated dogs might (perhaps) benefit from a post-exposure distemper vaccination. We really do not know.

When diarrhea accompanies CDV, medications that slow your dog’s intestinal motility and soothe and coat its intestinal lining are often indicated. Many of the young dogs that arrive with distemper have concurrent health problems – intestinal parasites such as hookworms, other parasites, skin and ear infections, and vitamin deficiencies. Those issues need to be attended to by your veterinarian to improve the dog’s chances of recovery. 

Veterinarians currently have no way to lessen the chances of lasting neurological involvement occurring. But anti-seizure medications (anticonvulsants and neuromuscular relaxing agents eg diazepam or phenobarbital) will sometimes make those dogs more comfortable. In desperation, vets have even injected Botox®. (read here) Read about all medications that might be helpful when neurological involvement is involved here

Might There Be Strains Of Distemper Virus That Are Resistant To Our Current Vaccines?

Reports, often from China, document cases of canine distemper in fur-farmed animals that had been previously vaccinated against the canine distemper.  Another report from Mexico also appeared to document distemper vaccine failure. (read here) But to the best of my knowledge, these are very rare events. There are plausible explanations  other than a new resistant strain of distemper  that could account for what appeared to be vaccine failures.  

What Will Kill Distemper Virus In The Environment?

Unlike parvovirus, the canine distemper virus does not survive long when it is not in a dog (less than 24 hours on a warm, sunny day). It is rapidly inactivated by sunlight, drying, heat and all commonly used household disinfectants. When using disinfectants, heavy grime needs to be mechanically removed first.

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