HHV-6B is more dominant than HHV-6A in serum testing of hospitalized infants in Zambia
A group led by Matthew Bates through University College London and the University of Zambia has reported that 20.5% of hospitalized infants were positive for HHV-6B, second only to CMV (24.3%). Surprisingly, HHV-6A was found in only 0.3% of patients. The group used Taqman PCR to screen 202 pediatric patients under the age of two. There was a significant correlation between HHV-6B and CMV viremia, with HHV-6B infection increasing the odds of CMV viremia.
These results were surprising, as a previous paper from the same group showed a high prevalence of HHV-6A through the utilization of a qualitative PCR assay. In that study, led by Dr. Ursula Gompels and co-authored by Dr. Bates, 86% of the 56 samples genotyped were HHV-6A, compared to only 1% with HHV-6B and 13% with co-infections (Bates 2009).
Differences between the two studies include 1) the material tested, 2) the methods used (qualitative PCR vs TaqMan), 3) the patient cohort of sick versus healthy infants, and 4) the method used to type the virus species. While the 2009 Bates study sequenced at two loci U46(gN) and U47 (gO), the present study used a single DNA polymerase gene (U38) to differentiate between HHV-6A and HHV-6B. One explanation raised by the authors is the use of conserved primers, which could mask the species with the lower copy number and thus increase the risk of false-negativity.
Another possibility is that some HHV-6A positives could have fallen below the level of detection in the new study. It is known that HHV-6A generally carries a lower viral load than HHV-6B. The median viral load for HHV-6B in these patients was quite low at 91 copies/ug DNA, a full log lower than that of CMV. HHV-7 produced the highest viral loads of the four beta herpesviruses tested at 12,218 copies/ug DNa, followed by CMV at 2993 copies/ug.
For more information, read the full paper.