Thursday, March 31, 2011

Total Eclipse of the...Virus

Terminology can get a bit confusing when discussing different stages or "periods" of viral infections in relationship to transfusion-transmitted infections. I've tried to simplify some things in this post, to help you understand the differences between "windows" and "eclipses!

In October 2010, Morbidity and Mortality Weekly Report contained an alarming report of a case of transfusion-transmitted HIV. In reading this article (which was of very great interest to me because it involved a transfusion given in the state in which I live; though the blood product was not from the blood center where I work), I was struck by the author's use of the terms "eclipse phase" and "window period." These two terms are defined in the article, but they seemed really similar at first glance. (NOTE: I will use HIV as an example in the following discussion, but the terms apply generally)

According to the MMWR article, the eclipse phase of an HIV infection is defined as "the interval between infection and the development of detectable concentrations of HIV RNA in plasma," while the window period is defined as " the interval between infection and development of detectable HIV markers in blood." These definitions sound similar, but there is a very important distinction: A person may be infectious at points during the window period, but most people would say that person is not infectious, by definition, during the eclipse phase. In fact, it is probably easiest to think of the eclipse phase as being a part of the window period (some have even designated it as such, as in: "the eclipse phase of the window period") as shown in the figure below:

Example of the relationship between the window period and the eclipse phase
As you can see from the diagram, the part of the window period that we really worry most about is after the eclipse phase and before the end of the window period, since that is the only portion during which the donor is infectious for a particular organism.
I will admit, however, that you will find different definitions for these two time periods in published literature. Some of the definitions say that the eclipse phase is an infectious period, while others disagree. The MMWR report cited, in fact, seems to favor that definition. However, the majority of what I have been able to read cites the eclipse phase as noninfectious.

Into that mix we throw another term that can be confusing: Incubation period. The incubation period may or may not correlate with the window period. This time frame, unlike the window period, is not defined by positive laboratory tests. Rather, the incubation period is simply the time from exposure to an organism until the patient displays clinical symptoms. It can be shorter than the window period (good news for blood safety, as their symptoms manifest before they are infectious) or longer (which is typical for HIV).

Finally, the latent period/phase has been variably defined in the literature. It has been used to indicate the period after the eclipse phase and before the end of the window period (the time marked as "Infectious" on the chart above), but it is more commonly used in HIV infections as a term for an asymptomatic dormant period after infection, possibly after a mild manifestation of initial symptoms. The latent period may extend for years in some cases; that is certainly common in HIV.

I hope that helps a little bit. You can read more about each term by clicking the link, which will take you to the glossary section of the BBGuy web site. Here is a summary of the above:
  1. Window Period: Period from infection until laboratory detection
  2. Eclipse Phase: Period from infection cell until infectious virions assembled
  3. Incubation Period: Period from infection until clinical symptoms
  4. Latent Phase/Period:
    1. After eclipse until infectious particles present outside of cell, or 
    2. Asymptomatic phase after infection when virus is still present and may be multiplying, or 
    3. Dormant period after initial infection when manifestations are not seen (HSV or VZV)