I am trying to understand how and why the frequency of non-genital ("boxer shorts" area) hsv2 recurrences differs from the frequency of hsv2 genital recurrences. In particular, I need help interpreting this study: Frequency and reactivation of nongenital lesions among patients with genital herpes simplex virus.
To summarize the relevant findings, the study followed 384 patients with initial hsv2 genital infection for an average of 63 weeks. The study separated oral outbreaks from non-genital outbreaks. So, when I say non-genital, I mean non-genital, non-oral outbreaks. The authors explain 3 possible ways to get non-genital hsv2 outbreaks: direct inoculation of a non-genital site, auto-inoculation of a non-genital site, or viremic spread within an infected nerve group.
Of the 384 people, 15 had a non-genital site included in the initial outbreak. The location was typically somewhere nearby on the trunk or legs that was inoculated in addition to the genitals. Of those 15, 9 (60%) had non-genital recurrences, with the location being the same as the point of inoculation (ie., somebody with initial infection on the genitals and left thigh was likely to have recurrences at both the genitals and left thigh). I will call this the "direct inoculation" group.
Of the other 369 whose initial infection was limited to the genitals, 20 (5.4%) went on to have non-genital recurrences. The location was typically the buttocks, which I assume is because it is in the same dermatome as the genitals. I will call this the "viremic spread" group.
To compare the frequency of genital hsv2 recurrences to the frequency of non-genital recurrences, the authors considered the "direct inoculation" group and the "viremic spread" group separately.
For the "direct inoculation" group, the authors found that the median time to first genital recurrence was 67 days while the median time to first non-genital recurrence was 725 days. The frequency of genital recurrence was also much higher than the frequency of non-genital recurrences in these people.
For the "viremic spread" group, the authors looked only at people who experienced recurrences on both the genitals and the buttocks. They found that genital recurrences occurred more than twice as often as buttocks recurrences in these people.
So to summarize the results, about 10% of people with genital HSV2 went on to have non-genital recurrences, due either to direct or auto inoculation of non-genital sites during the initial infection, or to viremic spread from the genitals (usually to the buttocks). Regardless of the cause, people with non-genital recurrences get them with much less frequency than genital recurrences.
I am trying to understand why the difference in frequency. The authors give the following explanation:
"Perhaps the greater distribution of peripheral nerve endings per millimeter of skin and the longer distance required to travel may be factors in these differences."
I want to get your thoughts on this study -- does my summary jive with your understanding of non-genital hsv2 recurrences? Regarding the authors' explanation for the difference in frequency, is there some way to quantify the different density of nerve endings at genital vs. non-genital sites? It makes sense to me that less nerve endings could lead to less paths to the surface and thus less recurrences. I don't understand what "longer distance" they are talking about. Does the virus have to travel further to get to say the buttocks than the genitals? Does the thickness of skin somehow affect this? Any other possible explanations for the difference in frequency?