Hip
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I always assumed that once you catch a persistent virus (the sort that afterwards remains latent in your body indefinitely, like herpes family viruses), and once you develop an antibody response to it, you will not be able to contract that same virus again, because your antibodies will protect you from catching it twice.
Well, it turns out that for cytomegalovirus, this is not true: even if you have cytomegalovirus in your body already (having acquired this virus earlier in life), and have antibodies that keep it under control, you can still catch a different strain of cytomegalovirus, and this different strain can cause an acute infection just like the original acute infection you may have experienced when you first caught cytomegalovirus. See this paper:
Human cytomegalovirus reinfection is associated with intrauterine transmission in a highly cytomegalovirus-immune maternal population
It also seems that individuals may be infected with multiple strains of Epstein-Barr virus (EBV):
This small study discovered that the same healthy individual can have latent infections with up to 5 different EBV genotypes. So that indicates you can catch more than one strain of EBV (but as to whether each time you catch a new strain of EBV you can experience the symptoms of acute infection again, I can't find any info on; though apparently getting EBV-induced mononucleosis more than once has never been observed).
And intriguingly, this study, which also observed multiple strains of EBV infection in healthy people, found that each strain of EBV infection tended to be compartmentalized, ie localized to specific locations in the body, although the strains found in any specific location of the body can change over time.
HHV-6 of course has two well-known variants: HHV-6B, the more common variant found in almost 100% of the population; and HHV-6A, the rarer and more neurotropic variant, observed at higher prevalences in ME/CFS and in multiple sclerosis. Refs: 1 2
However, HHV-6A and HHV-6B are sufficiently different from each other so as to be considered distinct viruses, rather than different strains of the same virus. And indeed, recently the International Committee on Taxonomy of Viruses finally classified HHV-6A and HHV-6B as separate viruses. Ref: 1
But presumably for a virus like HHV-6B, there will be multiple strains of HHV-6B; and likewise, there may be multiple strains of HHV-6A.
From the ME/CFS perspective, it should be borne in mind that some strains of the same virus can be more pathogenic than others. For example, multiple sclerosis has long been linked to EBV, but a recent study found that MS is strongly associated with certain genetic variants of EBV (this study actually provides a strong indication that EBV plays a causal role in MS, because it would be difficult to explain its observations otherwise).
So conceivably ME/CFS might also be associated with specific pathogenic strains of a virus like EBV, HHV-6A, HHV-6B or cytomegalovirus, and perhaps you only come down with ME/CFS once you catch a pathogenic strain.
Well, it turns out that for cytomegalovirus, this is not true: even if you have cytomegalovirus in your body already (having acquired this virus earlier in life), and have antibodies that keep it under control, you can still catch a different strain of cytomegalovirus, and this different strain can cause an acute infection just like the original acute infection you may have experienced when you first caught cytomegalovirus. See this paper:
Human cytomegalovirus reinfection is associated with intrauterine transmission in a highly cytomegalovirus-immune maternal population
It also seems that individuals may be infected with multiple strains of Epstein-Barr virus (EBV):
This small study discovered that the same healthy individual can have latent infections with up to 5 different EBV genotypes. So that indicates you can catch more than one strain of EBV (but as to whether each time you catch a new strain of EBV you can experience the symptoms of acute infection again, I can't find any info on; though apparently getting EBV-induced mononucleosis more than once has never been observed).
And intriguingly, this study, which also observed multiple strains of EBV infection in healthy people, found that each strain of EBV infection tended to be compartmentalized, ie localized to specific locations in the body, although the strains found in any specific location of the body can change over time.
HHV-6 of course has two well-known variants: HHV-6B, the more common variant found in almost 100% of the population; and HHV-6A, the rarer and more neurotropic variant, observed at higher prevalences in ME/CFS and in multiple sclerosis. Refs: 1 2
However, HHV-6A and HHV-6B are sufficiently different from each other so as to be considered distinct viruses, rather than different strains of the same virus. And indeed, recently the International Committee on Taxonomy of Viruses finally classified HHV-6A and HHV-6B as separate viruses. Ref: 1
But presumably for a virus like HHV-6B, there will be multiple strains of HHV-6B; and likewise, there may be multiple strains of HHV-6A.
From the ME/CFS perspective, it should be borne in mind that some strains of the same virus can be more pathogenic than others. For example, multiple sclerosis has long been linked to EBV, but a recent study found that MS is strongly associated with certain genetic variants of EBV (this study actually provides a strong indication that EBV plays a causal role in MS, because it would be difficult to explain its observations otherwise).
So conceivably ME/CFS might also be associated with specific pathogenic strains of a virus like EBV, HHV-6A, HHV-6B or cytomegalovirus, and perhaps you only come down with ME/CFS once you catch a pathogenic strain.
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