Viral diseases are very common and very misunderstood relative to their response to medications and to their transmission and how they cause disease. This article has much information related to the “common cold” but the information can be applied to upper respiratory viral diseases in dogs, also. Some of the information below has been copied wholesale or modified somewhat from an article printed in The Week, 23 Oct 10, p. 52,53; titled “Ah-choo! Your duppy dose explaid”. Other information is pertinent to veterinary medicine; not human medicine.
There are at least 200 different cold viruses in the rhinovirus family. “Colds” or “the flu” are also caused by parainfluenza viruses and influenza viruses. Although the clinical symptoms of each “cold” infection may be similar, patients never have the same virus twice. If a patient is exposed to a strain of a “cold” virus to which he/she has never before been exposed (and therefore, to which he/she has no immunity), he/she may or may not become infected and have “cold” symptoms. Exposure causes infection in all patients (and therefore causes the patient to develop an immune response) but does not cause clinical signs of the disease in all infected patients. Why some patients do not develop clinical signs and why some do is still one of the great mysteries of the “common cold”.
As little as a single rhinovirus particle or influenza virus particle can cause infection. Once the particle is introduced into the nasal passages by normal respiration or other methods, it encounters the sticky mucus lining the nasal passages. This traps the virus(es) before it can enter the lungs. Cells lining the nasal passages have microscopic hairs that tend to move mucus (and the virus) to the back of the nasal passages. Once the virus reaches the lymphatic tissue in the back of the nasal passage (adenoids), the virus invades the lymphatic cells and releases a microscopic segment of RNA. This RNA then hijacks the metabolism of the cell and causes the cell to produce hundreds of copies of the virus. The invaded lympoid cell then ruptures and releases all of its copies of the virus and each copy then invades another cell. This process is very rapid and within a few hours millions of virus particles are active within the patient to cause disease.
The virus is not susceptible to antibiotics. Antibiotics may prevent secondary bacterial complications that can occur after the virus is established; but, antibiotics do not control nor eliminate the virus.
Depending upon the particular virus, some spread most easily by aerosol (coughing/sneezing) and others spread most easily by contamination and physical contact. The symptoms of a “cold” or the “flu” are not caused by the virus’ effect on the nasal mucosa but are caused by a very complex brew of chemicals collectively referred to as inflammatory mediators – the immune response. The virus causes the body to produce these mediators and these chemicals cause the clinical signs of the “cold” or of the “flu”, i.e., stuffy nose, coughing, sore throat, head aches, etc. Although mucus production is a common clinical sign of a “cold”, most of the respiratory difficulty is actually caused by the swelling of the mucous membranes of the nasal turbinates. This swelling then reduces the open airway spaces within the nasal passage.
Considering the great variety and number of upper respiratory virus and the similarity of the clinical signs that they produce, upper respiratory viral infections can be summarized as follows:
1. Most are highly contagious;
2. These viruses are not susceptible to antibiotics;
3. The patient is most contagious before the patient is even aware of the fact that he/she is sick or going to be sick;
4. The greatest contagion occurs early in the disease, not when the clinical signs are at their worst;
5. Although they may help the patient feel better, most over the counter (OTC) medications actually interfere with the body’s attempt to heal itself and therefore, actually prolong the recovery process.
6. Every patient exposed to any given virus will not develop clinical signs of the disease although that patient will produce antibodies against that particular virus.
7. The virus genus that produces the “cold” is separate and distinct from the several different genera of virus that produce the “flu”.
Upper respiratory viruses (cold and flu viruses) are a diverse group; but, all produce similar clinical signs. Secondary complications (pneumonitis and pneumonia) are quite common and, if ignored, can be fatal. Maintaining the body in a good state of health (nutrition, body weight, disease control, etc) is the best defense against these upper respiratory viruses.