Norwalk Virus and Hepatitis E Virus (HEV) – Caliciviruses

00:01 Caliciviridae viruses.

00:04 The Caliciviridae are very small, non-enveloped, icosahedral capsids, such as you see on the electron micrograph in front of you.

00:13 They have a linear, single-stranded, positive-sense RNA genome, which, because it is positive sense, can function as a messenger RNA.

00:22 There are only a couple medically relevant species in this genus.

00:26 However, they are quite important.

00:29 One is the hepatitis E virus, which, as you see there, is the Orthohepavirus A.

00:35 It's very similar to the Caliciviridae, and therefore it is discussed in this family.

00:41 And then the Norwalk virus, which previously may have been called the Norwalk agent, or Norwalk-associated virus.

00:49 Infection, and let's start with the Norwalk virus first because it is perhaps the most prominent.

00:54 Infection with Norwalk virus starts as a primary exposure to the virus, fecal, oral, many times.

01:01 And then entrance, eventually, to the small intestine where the virus attaches to the brush border of the intestine, and it is those cells that initially have a function affected by the lysis of the host cell.

01:16 As that happens, those cells lose their ability to absorb water nutrients, which allows for a malabsorption diarrhea to develop.

01:26 However, the principal component or principal symptoms of Norwalk virus are coming from its effect of delaying gastric emptying, and it is that specific issue which creates hyperemesis -- and we're talking explosive hyperemesis because the stomach is unable to empty, and that is what drives a lot of the infamous outbreaks of Norwalk virus.

01:50 The picture you see in front of you is of a cruise ship.

01:54 And this should suggest to you or help you remember that most of the large outbreaks in Norwalk virus infection have all occurred on the closed space in a cruise ship.

02:06 Incubation period, 24-60 hours, so just within the perfect time of a cruise.

02:11 Transmission is via the fecal-oral route.

02:14 However, this is one of the times when the emesis, the vomiting, is so explosive that it aerosolizes the vomitus.

02:22 And so, even if one is trying to wash one's hands on a cruise ship, one may be walking through a cloud of virus, a cloud of Norwalk virus and get infected that way.

02:33 This is not to bash the cruise ship industry, whatsoever.

02:36 They do a wonderful job of trying to minimize this of requiring onboard hygiene, but yet, this is still a very prominent mechanism of being exposed to Norwalk virus.

02:47 The clinical manifestations.

02:49 The watery diarrhea because it is a malabsorptive process due to poisoning or lysis of the brush border cells, but then the very prominent nausea and vomiting.

02:59 Sometimes, there's low grade fevers, but the whole process thankfully, is self-limited.

03:05 So, incubation period, 12-60 hours.

03:08 Total length of symptoms, 12-60 hours, but that's just enough time to get severely dehydrated.

03:15 The Hepatitis E virus, then, is best discussed in combination or comparison, actually, to the other hepatitis viruses which are not in the same Caliciviridae family.

03:26 In fact, if you look down the columns on the far left, hepatitis A virus, and the far right, hepatitis E virus, you will note that these are very similar.

03:36 In fact, the hepatitis, the clinical hepatitis caused by hepatitis E is very similar to that caused in hepatitis A.

03:45 Both are transmitted in a fecal- oral route.

03:49 Both have a very short incubation period.

03:52 Both are mostly asymptomatic, but when they do develop acute disease, it's quite fulminant.

03:58 Hepatitis A can cause acute disease in anyone.

04:01 Hepatitis E is especially fulminant and even tragic in women who are pregnant, in part, due to their immune susceptibility, and in part, due to increased volume of delivery for the hepatitis virus.

04:15 The mortality rate in hepatitis A, very low.

04:18 Almost all people recover, unless they have a specific immunodeficiency.

04:23 Unfortunately, as just mentioned, the mortality or death rate in hepatitis E is very high in women who are pregnant because they have a transient immunosuppression as part of their pregnancy.

04:35 There is no carrier state for either and there are no other associated or relevant or even extra-hepatic manifestations.

04:42 So, this particular group of viruses is most notable for norovirus or Norwalk virus as it is commonly known, which is a very pronounced cause of a vomiting, especially gastroenteritis, and most especially due to closed environments such as a cruise ship, closed meetings, etc.

05:02 And then hepatitis E virus, which doesn't cause vomiting, but can cause a icteric or jaundiced hepatitis, which in most cases resolves spontaneously.

05:14 The moral to this story for both viruses, though, is wash your hands and don't breath when walking through a cruise ship.