I have heard many graduate students (and some undergraduates) casually talk about the concept of virus virulence. They mistakenly believe that after the virus is inoculated into cells, it can quickly cause cell lesions and eventually reach a higher titer, which is a highly virulent strain, and vice versa. In essence, this is completely wrong.
The virulence of animal viruses (only animal viruses are discussed in this paper) can be defined as the pathogenicity of viruses to host animals. There is an extremely important keyword here, which is "animal". In other words, the so-called virus with high or low virulence is only aimed at animals (including chicken embryos) and has nothing to do with virus cell culture. For example, it is easy to understand that the commercial vaccine strain usually causes obvious and rapid cytopathic effect after inoculation, but it is a weak strain.
Therefore, the concept of "virulence" must be based on the animal level. Moreover, if we want to accurately determine the virulence of the virus, we usually need to do animal experiments (including chicken embryo experiments). In other words, only scientific animal experiments can correctly judge the virulence of the virus. This is very important and often overlooked. For example, blue ear disease broke out in a pig farm, causing a large area of pigs to die. The virus isolated from dead pigs can't jump to conclusions, and blue ear disease virus is a virulent strain. In order to judge whether the isolated virus is a virulent strain, strict animal attack experiments must be passed.
The toxicity of viruses can usually be quantified. The main methods are to determine the lethal dose of the virus to animals and the paralyzing dose to chickens. The lower these values, the stronger the toxicity. At the same time, it can also monitor the clinical symptoms of infected animals, such as body temperature changes, neurological symptoms, diarrhea, diet and so on. The more obvious the clinical symptoms, the stronger the virus virulence. It is also worth emphasizing that the so-called virulence is relative, and there is no absolute level. Therefore, when doing animal experiments, strict control should be set to facilitate comparison.
So, what factors are related to the level of toxicity? There are two main points: one is the virus, and the other is the host. If we discuss these two factors, it is estimated that a complete textbook will be written. As far as the virus is concerned, the differences of genes and protein among different strains and the complexity of quasi-species are all important factors affecting the virulence. As far as animals are concerned, different species (races), ages, sexes, physical conditions, etc. Will show different susceptibility to viruses. Therefore, it seems that the virulence of the virus is not determined by a single factor, but by the correlation of various factors.
Finally, it should be pointed out that virus, as an intracellular parasitic microorganism, has a completely different pathogenic mechanism from common toxins (such as arsenic). For example, arsenic is a super toxic substance, but if people just ingest it, it will not kill them. But rabies virus, if a person is bitten by a mad dog, even if only one virus enters the human body and escapes immune killing, and finally enters the nervous system and begins to replicate, then this person will definitely die 100%. In this way, the semilethal dose of virology seems to be less meaningful than that of toxicology.
To sum up, the determinant of virus virulence is at best determined by the virus itself and the host animal; It is determined by virus gene, protein, quasi-species, host species, age, sex and other factors. Therefore, the COVID-19 epidemic, as a virologist, can't judge the virulence of COVID-19 arbitrarily, because: you didn't do animal experiments! 2020-03-3 1
F. Liu (surname)