Remarks
Analyzing the epidemiology and the clinic of colds, the first question
is, why they appear above all in the cold seasons, or in correspondence
of periods of sudden thermal decrease and why they affect the upper
respiratory tract, and initially , the mucosa of the nasal and paranasal
cavities in particular and/or of the oral cavity and the larynx.
That is, because for the most part they replicate better at temperatures
of around 33-35°C or lower, so they could be defined as ”cryophilic”
(cold loving ).
Therefore they prefer the mucous membranes of the ”colder” or ”more
easily cooled” territories, whose average temperature is lower than the
average basal endocorporeal human standard (37°C).3,5,7
Rhinoviruses replicate at a temperature of 33°C, which corresponds to
the temperature of the human nasal mucosa. 6
Rhinoviruses replicate more easily at slightly lower temperatures than
the body temperature (around 32/33°C, compared to 37°C in our body),
which is why they attack the upper airways, and in particular the mucosa
of the nose, where they find the ideal climate to
multiply.,6,8
Hyperthermic treatment at 43°C for 20 min is effective in reducing the
symptoms of common colds by 50%. 8
A 20-min hypertermic treatment at 45°C is effective in suppressing human
Rhinoviruses multiplication by more than 90% when applied at specific
stages of the virus replication cycle. 9
Coronavirus dried on smooth surfaces retains its vitality for over 5
days at temperatures of 22-25°C and a relative humidity of 40-50%,
which is typical for air-conditioned rooms. 10
Conversely, the viability of the virus is rapidly lost (> 3
log10) at higher temperatures and higher relative humidity (for example:
38°C and a relative humidity of > 95%).11
The improved stability of the SARS-CoV at low temperature and low
humidity environment can facilitate its transmission in communities in
the subtropical area (such as Hong Kong) in air-conditioned
environments. 12
In a experimental 2012 study Xiangyug Q. characterized viral
(bacteriofage λ ) structure changes upon heating. 13
Moreover, innate immune defenses have a temperature-dependent efficacy
in limiting the replication of common cold viruses: it increases with
warm temperature 6
In fact, it is documented that hyperthermia induces endogenous
production of γ-interferon (IFNγ), with a consequent antiviral effect.14,15,16
It should certainly be mentioned, that a Cochrane review of 2017 does
not detect any difference in results in subjects with acute rhinitis in
progress, therapeutically using the treatment with humidified and heated
air at 40-47°C administered with the RhinoTerm device, compared
to the group of control. 17
These conclusions are also reflected in a subsequent 2018 BMJ paper.18
However, these assessments consider the effect of humidified and heated
air as a therapy of a disease in progress and not as a preventative
treatment.
Vice versa, the Yamaya’s 2019 study on the effect of high temperature on
pandemic and seasonal human influenza viral replication19 and the very recent work (February 2020) published
by Wang M. are particularly interesting. In this last paper the
difference in the diffusion of the Coronavirus SARS-CoV-2 in different
chinese cities is detected, correlating with different relative
temperatures. As a result, a minimum temperature increase of 1°C
correlates with the reduction in the cumulative number of cases by a
factor of 0.86 and at a temperature of 38°C the virus quickly loses its
activity. 20
Although the spread of the viral airway diseases, in particular of the
more serious, such as the recent pandemic from Coronavirus SARS-CoV-2,
cannot be influenced only by ambient temperature, the correlations
between the spread of virosis and the environmental temperatures are
documented. 21
These epidemiological evaluations are consistent with the potential use
of hyperthermic treatment not so much as therapy for acute pathologies,
but for prophylactic/preventive purposes.