Contagious means different things in different diseases . . . Print E-mail
News - Rubrieke
Tuesday, 26 February 2019 08:20
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Dr Liesel van der Merwe is a small animal medicine specialist. Send her your questions: This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

Dr Liesel van der Merwe

With the reports and scary messages about the distemper outbreak in Pretoria, I thought it might be a good idea to discuss the spread of disease and the role played by vaccination.

I am going to briefly, and hopefully using understandable terms, compare canine distemper and canine parvovirus according to contagiousness and the method and rate of spread between susceptible animals.

We vaccinate our pets against viral diseases – the aim being that prevention is better than cure. However, the various viral diseases spread and maintain themselves in the environment differently. Viral factors play an important role in how easily a disease spreads.

Canine parvovirus
Canine parvovirus, the virus responsible for ‘cat flu’ in dogs, is a very stable, tough and resistant virus. Infectious viruses survive for at least one year in the soil.

Dogs, usually puppies, will start shedding viral particles in their faeces within three to four days after being exposed – generally even before they start showing symptoms of diarrhoea and vomiting – and continue shedding for seven to ten days. By the time you have diagnosed them and removed them from the environment, the area is already contaminated.

The virus is spread by the oro-nasal route. Dogs walk in all sorts of mess and then lick their feet clean. The virus is also spread by fomites/vectors, which means that your dogs’ water bowls and bedding, clothing of carers and even the patient’s own hair coat can carry and contaminate other areas with viruses adhering to them. 

The parvovirus is sensitive to sodium hypochlorite solution, a household bleach (Jik). One part bleach in 30 parts of water, left on a surface for 10 minutes, will provide disinfection. Environmental control is vital to help prevent spread of parvovirus in an outbreak in kennels or breeding facilities.

Canine distemper virus
Compare the above to canine distemper virus (CDV). Dogs are the principle reservoir host for CDV and are the source of infection in wildlife. Infection occurs in the upper respiratory tract and then moves to the lungs and systemically to all epithelial linings and the nervous system.

The virus is spread by aerosol droplet exposure, much like influenza in people. Contact with infected animals is essential to maintain the virus. Infected animals do not always develop full blown disease, so owners are not always aware that shedding is occurring.

CDV is extremely susceptible to heat and drying out. It is destroyed at temperatures of 50– 60°C for 30 minutes. In moist secretions it survives for at least one hour at 37°C and for three hours at room temperature (20°C). In warm climates CDV will not persist in kennels once the dogs have been removed.

Although CDV is contagious, you need close contact between a susceptible and infected animal for transmission to occur. The prevalence of the CDV and parvovirus disease is very low where animals are vaccinated.

The distemper virus doesn’t gradually change over time. The virus particles you are vaccinating with are probably very close to the virus particles your dog is exposed to. Immunity is long lasting after vaccination with a modified live vaccination (MLV).

There are different types of MLV on the market. The kidney cell strain provokes a stronger immune response than the chick embryo strain. All MLV vaccinations provoke a stronger and more lasting immunity than inactivated vaccines.

Immunity to MLV vaccination is up to one year, and after the initial series of puppy vaccinations and the first year booster, vaccinating every three years will be protective. However, immunity after vaccination is not an absolute. If a dog is exposed to a highly virulent wild strain or an exceptionally high load of virus and is immune-compromised, disease may still develop.

Not all puppies affected with canine parvovirus will develop clinical disease. Those which recover will be immune for 20 months to life long. Attenuated vaccines provide a superior immunity. Their effect is seen in the blood within three days and the vaccinated animals will shed virus in the faeces, but these will be non-infectious and in lower quantities than in real disease. 

Susceptibility to wild strain virus is normally two to three weeks before puppies can be vaccinated.

Duration of immunity after vaccination with MLV is long lasting, up to seven years.

When born, puppies have a small amount of antibodies from their mother via the placenta, but they get the majority of their maternally derived antibodies (MDA) from drinking colostrum (‘biesmelk’), which is the antibody rich milk produced in the first few days after birth.

Thus a well-vaccinated bitch will have good levels of antibody in her colostrum. The puppies, however, can only absorb these large antibody proteins in the first 24 hours after birth, before the intestine lining seals over.

So, there is a definite time window for all optimum maternally derived immunity. If the puppy suckles late or not enough, it will not have adequate levels of immunity. If another puppy in the same litter suckles well, its blood will have lots of antibodies within the first 24 hours after birth.

You cannot vaccinate a puppy with lots of maternal antibodies as the antibodies will block the weakened /modified vaccine virus and remove it from the system. So, in one litter – as in the example above – some puppies need vaccination earlier and others at the average of about 10 weeks when maternal immunity is waning and will not interfere.

It is expensive to test levels of antibodies. So, in kennel situations one can vaccinate earlier than the six weeks. The earlier vaccination, and even the six-week vaccination, is often blocked in those puppies with good maternal immunity. But in those with weak maternal immunity, they will at least start developing resistance.

Thus there are viral factors, host factors and environmental factors which contribute to a disease being contagious. Equal attention should be given to the part played by each in a specific disease. There is no one approach to fit all.


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