Canine Distemper And Your Dog – CDV

Ron Hines DVM PhD

Kennel Cough 

Can You Tell Me Some General Facts About Canine Distemper?

Canine distemper is a serious, highly contagious viral disease. Although it affects many animal species, most of us 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 (single strand) ; cat distemper on the other hand is a DNA virus (double strand).

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

Today, the distemper virus affects dogs throughout 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. (ref Others believe evidence points to the disease moving from Spain to the Americas. (ref)

Very effective, long lasting, vaccines have made canine distemper a rare disease in countries where advanced veterinary care is the rule. But distemper is still a major cause of canine death and disability wherever dogs run loose, in areas where the population is too poor to afford vaccinations and in animal shelter and pet store situations. In prosperous areas of the United States where most dogs are vaccinated, it is urban raccoons that are a common reservoir of the dog distemper virus. (ref)

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

Yes, quite a few are.

Fox, coyotes and wolves are just as susceptible as dogs. Weasels-like animals (mustella) such as ferrets, skunk and mink are as well. So are large wild felines (lions, tigers, etc.) and bears. (ref1, ref2, ref3)

Although domestic cats sometimes have antibodies against canine distemper, and the virus has been shown to grow in cat tissue in the laboratory, it remains unclear if the CDV virus is able to cause disease when house cats are exposed to it. (ref1, ref2, ref3)      

Canine distemper virus has caused disease in monkeys as well. (ref1, ref2) If the virus might some day pose a threat  to humans is unknown. (ref1, ref2)  But since the CDV is such a close relative of the human measles virus, anyone vaccinated for measles, or having had a prior case of measles would probably not be at risk. 

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

The distemper virus is fragile when not living in a host. It is thought to survive less than a day in the environment – when exposed to sunlight, summer-day warmth and drying. It is generally a sneeze or cough from an infected dog that spreads the virus from animal to animal. Since the virus is present in the lungs in huge numbers very early on, respiratory secretions from an infected dog that appears outwardly healthy can still spread the virus – perhaps a single sniff, a greeting, a lick on the muzzle. Once the disease is full-blown, the virus is present in all secretions, urine, feces, tears and saliva. (ref) So kennel workers can track the virus from dog to dog on their shoes and clothes.

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 body. During the second, the dog’s immune system is severely compromised and it is open to secondary bacterial infections. Many dogs show few 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 also skip the traditional second phase and are only left with varying amounts of nerve damage (twitching, seizures, “chewing gum”), mental impairment and occasional foot pad involvement (hard pad disease). One is really not out of the woods until a bit over a month has passed.  Occasionally, neurological signs appear even later (3-4 months). Because the course of distemper can be so extended, administering anti-distemper vaccines after the virus is already in a dog’s body can be misinterpreted as a vaccine failure.

How the disease progresses depends on what degree of immunity to distemper the dog might already have had. In puppies, that immunity is the residual immunity of their mothers passed on through the first milk (colostrum). Disease severity can also be influenced by the strain of distemper virus the dog has been exposed to – some “wild” strains appear worse (more virulent) than others. Nutritional status, stress, intestinal parasites and other concurrent diseases influence the severity of 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 to make a CDV diagnosis. Kennel cough, adenovirus and a large number of other virus and bacteria that affect dogs can cause the same early symptoms. (ref) It is also not that unusual for young or stressed dogs to be facing several of these diseases all at the same time. (ref)

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 consequential things cause moderate eye mattering).     

Later in the disease, as the virus cripples the dog’s immune system, bacteria often take advantage of the situation. That can result in pneumonia, gasping and severe diarrhea. Dogs at that point have no interest in food.

Damage to the deeper structures of the eye as time goes by sometimes occurs as well (uveitis, keratoconjunctivitis, retinitis).

With time, those symptoms often subside; but unfortunately they are sometimes followed by signs that the dog’s nervous system has been damaged. Tremors, twitching (tics [=twitches], chorea), seizures, head tilt, chewing gum “fits”, aimless circling, snapping at imaginary flies and loss of mental acuity that frequently occur late in distemper are all signs of nervous system damage. A subset of dogs do not show the early signs of distemper. One only realizes the extent of damage the virus has caused when these neurological signs finally occur.

Hardening 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 the disease, their permanent teeth sometimes erupt with tooth enamel defects.

There is currently no way that your veterinarian can 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. 

Canine distemper tends to be most severe in 3-6 month old puppies. By 3 months of age, the immunity they 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 reached its full defensive potential. Many dogs of this age reach animal shelters in a deprived condition. That also weakens their ability to survive CDV infection. Most receive a vaccination against distemper at the door. Those vaccinations can be helpful – but they take some time to be fully protective. By 7-9 month of age, incoming pups are often already survivors of mild cases of 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. To the best of my knowledge, no one has ever been able to isolate living distemper virus from these older dogs. 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, 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. (ref) 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 that persist in recovered dogs.   (ref1, ref2, ref3   

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 titer (ELISA test) to the distemper virus (ref1 , ref2) and tests that look for the presence of the living distemper virus itself (RT-PCR, Quant RealPCR™, Antech PCR, RT-RPA, etc.) These tests have their drawbacks, recent distemper vaccination or samples obtained from the dog too late in the disease can both cloud the test result significance. If a dog does not survive, 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 very effective. One of the biggest errors is giving these vaccines when the dog is too young. One cannot count on the vaccine being effective – no matter how many shots are given – until a pup is about 18 weeks of age. By that time, its mother’s interfering immunity should be gone from its system. Many pups reach the point where there is no maternal interference earlier (~14 wks) – but not all of them do. The problem is a partial creation of the veterinary industry in beginning vaccinations too young. After multiple vaccination trips to the vet with young pups, many dog owners are so tired of the visits and expense that they forgo the most important shot of all – the one at 18 weeks of age. Isolation of puppies from sources of distemper and other virus until one is sure the shots will be effective is a much-preferred way to protect your dog. Read more about my thought on that here. If you can’t do that or you do not know the vaccination status of the dog’s mother, the vaccine schedule suggested for a puppy and available on the 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 that the dog is ill. Those dogs are 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 adequately confirmed. In North America, animal shelters are common hotbeds for distemper exposure and observation. It can be very hard for them to determine the source of continuing new infections. If a dog developed distemper 4 month after their last case was diagnosed, that is not sufficient evidence that a new virus did not enter their facilities in the interim. Few, if any shelters would have the financial resources to methodically track how the CDV virus moved from dog to dog and virtually none have the resources to pay for complicated laboratory tests that would indicate a recovered dog did not still harbor distemper virus. Even the sophisticated tests often miss low virus loads.

If My Dog Catches Distemper What Treatments Might Help?

Through 2019, veterinarians have not reported specific treatments that kill distemper virus within a dog’s body. We are limited to “supportive” care. That is, seeing to it that the 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 the virus dose.

In 2017, some Italian veterinarians reported that a combination of two human antiviral drugs, ribavirin and boceprevir, prevented the growth of distemper virus in a laboratory setting. (ref) If these drugs have any place in treating dogs that have been exposed to distemper remains to be seen. Let me know if you have further information.

When diarrhea occurs, medications that slow intestinal motility and soothe and coat the intestinal lining as much as possible are often indicated. Many of the young dogs that arrive with distemper have concurrent health problems – intestinal parasites such as hookworms, skin and ear infections, and vitamin deficiencies. Those issues need to be attended to to improve the chances of recovery. 

We 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®. (ref)

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 developed measles. (ref1 ref2)  In a distemper outbreak at-risk unvaccinated dogs might, perhaps, benefit from a post-exposure distemper vaccination. We really do not know.

Might There Be Newer Treatment Options?

The canine distemper virus is a close relative of our own human measles virus. Ribavirin®, a drug sometimes found effective in hepatitis C treatment plans, was also found to inhibit the measles virus. (ref)  A number of laboratory studies have found that ribavirin, alone or in combination with other antiviral drugs, destroys the canine distemper virus too. (ref1, ref2, ref3)   

Generic ribavirin is widely available and not very expensive; but I do not know of anyone who has incorporated this drug the treatment of field cases of canine distemper in dogs. If you do, let me know.

Other antiviral medications have also been found to destroy canine distemper virus in the laboratory. (ref1, ref2, ref3)

Vaccinations To Protect My Dog From Distemper

Excellent vaccines against canine distemper have been available since the 1950s. These vaccines are manufactured from living-but-weakened (attenuated) distemper virus. A unique one, Merial’s Recombitek® contains only a small portion of the distemper virus linked to a harmless canary pox virus for transport. Given at the right time, they all induce long-lasting solid immunity. There is no agreement as to the best age to give a puppy its first distemper shot. You can read my suggestions through the same link I gave earlier. There is also no agreement as to how frequently, if ever, a dog needs a booster vaccination. Tests exist to measure the amount of antibody that your dog has in its blood against distemper. But those tests do not measure the amount of immunological memory your dog retains to fight distemper. Immunological memory persists long after antibody levels are too low to measure and immunological memory is a key factor in preventing disease. No creature could carry the amount of antibodies necessary to fight every infection it ever experienced. But they carry the recipe book for the ones that worked. That is their immunological memory.

Animal shelter situations are unique. Their personnel have developed extreme vaccination coping methods to deal with the virus, bacteria and parasites that constantly arrive at their doorstep along with their wards. Dogs at shelters generally receive a distemper shot when they arrive (>4-6 wks of age) and frequently thereafter because exposure to the virus is so likely and the stressed animal’s immune defenses so compromised.

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 disease. (ref)  Another report from Mexico also appeared to document distemper vaccine failure. (ref) But to the best of my knowledge, these are very rare events. There are plausible explanations – other than resistant strains 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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