Traveling together in packets is what makes some viruses so infectious

New discovery changes our understanding of how viruses are transmitted.

Noroviruses and rotaviruses, viruses that cause severe stomach distress, travel to new hosts in membrane-bound clusters, new research reveals. This increases the severity of symptoms and may help the viruses dodge our immune systems.

The viruses were previously thought to be solo travelers. In fact, until a recent study showed that polio viruses can cluster together, all viruses were thought to be spread individually. After the polio study revealed this wasn’t the case, researchers still weren’t sure if viruses could be transmitted in packets in both animals and humans. Or whether this clustering makes infections hit harder and spread faster. The new study confirms: it does.

We spoke with senior investigator Nihal Altan-Bonnet to learn more.

ResearchGate: Can you tell us a bit about the viruses you focused on in this study?

Nihal Altan-Bonnet: Rotaviruses and noroviruses are enteric viruses. They both cause very severe gastroenteric disease (vomiting and diarrhea) and can be fatal, especially in the young and elderly. Both viruses replicate themselves in the intestine and transmit themselves to the next host via the fecal-oral route.

RG: How were these viruses previously thought to have spread?

Altan-Bonnet: As single free virus particles that moved independently of one another. Noroviruses were first observed and identified by Al Kapikian in 1972 using electron microscopes. They were seen as free naked viral particles disseminated throughout stool samples. But processing samples for electron microscopy involves a lot of harsh invasive treatments that disrupts membranes. This is probably why the membrane packets carrying the noroviruses were not observed.

Also, until our studies, viruses were always assumed to spread as independent infectious agents. So folks had preconceived notions of what to look for: individual virus particles. When we revisited some of Al Kapian’s old electron micrographs we were able to find vesicles containing norovirus even in those harsh preparations.

RG: How do they actually spread?

Altan-Bonnet: Using gentler, less invasive light microscopy techniques and by developing tools that isolate these vesicles from stool, we were able to show that much of the rotavirus and norovirus in stool samples of humans and animals exists inside vesicles. These vesicles are membrane bound packets containing multiple rotavirus or norovirus particles.

What we also showed is that these membrane-cloaked virus clusters are quite stable in stool and as they pass through the GI tract of the next host. This was quite surprising.


“We think the membrane cloak around the clustered viruses protects the viruses from being seen by the immune system.”


RG: What difference does that make for the viruses?

Altan-Bonnet: Pound for pound, viruses clustered inside vesicles are much more virulent than equal numbers of free viruses. We think this is due to several factors. For one, by remaining intact as they pass through the GI tract, the packets are able to deliver a high dose of virus simultaneously into the intestinal cells. We also think the membrane cloak around the clustered viruses protects the viruses from being seen by the immune system of the gut.

Also, RNA viruses such as rotavirus and norovirus are known to carry many mutations within their genetic material (RNA). Some of these mutations can be quite deleterious for the virus, preventing it from replicating in another cell. It’s possible that when they are delivered in multiples into a cell from a vesicle, these viruses interact in a way that lessens the effects of the mutations carried by individual virus particles. Of course, these kinds of interactions could also lead to much more rapid emergence of traits that cause resistance to drugs and vaccines.

In contrast, free virus particles become diluted and even inactivated by enzymes and antibodies as they are transiting through the GI tract. When a few of them do eventually reach the intestinal cells, they are too few in number to infect the cells efficiently and fight off the immune system.

RG: So people get sicker because the viruses spread in clusters?

Altan-Bonnet: Based on our transmission studies with animals, we think that they get sicker faster and stay sicker for longer. The vesicles also appear to be highly stable under a variety of harsh environmental conditions, so people may come in contact with them over a longer period of time and may still get infected as the vesicles carry multiple viruses for delivery into that person.

RG: Are any other types of viruses transmitted in clusters?

Altan-Bonnet: We also have found that poliovirus, Coxsackievirus (causes foot and mouth disease in children), and rhinovirus (a cause of the common cold) are also transmitted as clusters inside vesicles. As we have seen with our current work, the list will likely grow as this form of transmission is highly advantageous for the viruses.

RG: How and when do the viruses form these clusters?

Altan-Bonnet: The viruses are clustered and packaged within membrane-bound vesicles inside the cells where they are replicating. The organelle sources for these membranes can be different for different viruses.


“When I look back, it seems so obvious that viruses would want to be transmitted together.”


RG: What happens to a virus cluster once it’s in a new host’s body?

Altan-Bonnet: The membrane cloak keeps the clusters together all the while they transit through the GI tract. Once they reach the upper intestines, the vesicles are taken up by cells and the viruses are transferred en masse into the cell that took up the vesicle. So that cell suddenly becomes infected by multiple viruses simultaneously. These multiple viruses within a cell then start to band together and rapidly replicate, compensating for each other’s deficiencies, to churn out viral progeny which are then released in vesicles back into the gut as well as into the blood stream. The ones released into the gut make their way into stool and eventually to another susceptible host via the oral-fecal route.

RG: Could knowing these viruses are transmitted in clusters contribute to the development of better antivirals?

Altan-Bonnet: Yes, in particular to the development of antivirals that target clustering by disrupting the membrane cloaks, by preventing the formation of the membrane cloaks within cells, and by inhibiting virus- virus interactions that help sequester these clusters into the membrane cloaks.

RG: Are there other applications?

Altan-Bonnet: Yes, anywhere where you actually want to enhance the infection efficiency. For example, when a virus is being used as an oncolytic agent, replicating in tumor cells and eventually killing them. Being clustered inside vesicles may boost the efficiency of this process.

RG: What’s next for this research looking at this transition mechanism?

Altan-Bonnet: This is an entirely novel mode of viral transmission that enhances virulence and had never been observed before. When I look back, it seems so obvious that viruses would want to be transmitted together to boost their infectivity. While our work is largely focused on the vesicles transporting multiple viruses clustered together, there will likely be other yet-to-be-discovered mechanisms by which viruses transmit themselves in clusters and reap some of the benefits of this mode of transmission.


Featured image courtesy of NIH.