Viral incubation: why do bugs hide before they strike?

Different viruses have different incubation periods so some make you ill sooner than others. MIKI Yoshihito/Flickr, CC BY-SA

Health workers returning to their home countries after working in Ebola-affected areas are being asked to place themselves in quarantine for 21 days. This period is important because it’s the maximum time the virus needs to incubate. Other viruses have different incubation periods.

The term “incubation period” describes the time between a virus entering a person’s body (infection) and the first appearance of disease symptoms. But why do viruses have incubation periods? And why do these vary from virus to virus and person to person?

Viral incubation periods are affected by the rate at which a virus replicates once it has infected someone, the host’s susceptibility and their immune response to the virus, the viral dose that infected the host and the infection route.

What Ebola does

For the Ebola virus, the incubation period depends on all of these factors. Direct inoculation into the skin through accidental needle-stick injuries or needle sharing, for instance, leads to a shorter incubation period than transmission of body fluids into a mucous membrane, such as through the eye, mouth or nose.

The Ebola virus can infect any cell in the body that has a particular cholesterol transporter. But as it enters the body through a mucous membrane or a break in the skin, the virus needs to infect immune cells that can carry it to other sites around the body.

The most important cells it initially infects are immune cells, which then carry the virus to the lining of blood vessels, liver and spleen.

Once these immune cells have been infected, the virus is able both to replicate within the cell and to use that cell as a vehicle to travel to other sites in the body. When it has reached sufficient numbers within the cell, the virus ruptures or buds out to infect neighbouring cells. This cycle continues until large numbers of cells have been destroyed and the organ starts to fail.

While this is happening, the body starts to fight against the virus by producing chemical mediators to control its replication and stimulate the wider immune system. The chemicals include chemokines and cytokines which, while helpful, can also harm the body.

The Ebola virus causes massive numbers of these molecules to be produced, leading to systemic inflammation in the host. And because a type of immune cell has been infected, the body cannot trigger an adaptive immune response, which targets a specific virus.

Instead, the white blood cells, which usually target and kill invading viruses, die due to the inflammatory response being produced. This is how the Ebola virus cripples the body’s immune response and renders the person helpless to fight infection.

The release of cytokines and chemokines by cells that sense the virus and the damage to blood vessels and liver cells caused by these inflammatory chemicals cause the characteristic symptoms of Ebola virus disease such as rashes, vomiting, diarrhoea and bleeding.

Only when it sufficiently stimulates the production of the chemical mediators and infects blood vessel cells or liver cells in high enough numbers will Ebola symptoms manifest.

The infection of multiple cells in the body and subsequent replication of the virus takes time. While the virus remains at low levels, it can neither be detected nor transmitted. This is why this time is known as the virus’ incubation period.

Other viruses

In comparison, the influenza virus is transmitted through the air and enters the body through the nose or mouth. The virus rapidly infects the cells in the respiratory tract and replicates in them, soon producing symptoms.

As it doesn’t need to travel elsewhere in the body, this reduces its incubation period to a mere one or two days. It would be longer if the influenza virus had to infect the brain or some other organ far from its point of entry.

At the other end of the spectrum, the rabies virus has an incubation period of between 30 and 100 days. In rare circumstances, it has been in the order of years. The virus usually enters through the site of a wound and travels along the peripheral nerves to the brain where it causes inflammation of the brain tissue.

Rabies-infected bites to the face are associated with a shorter incubation period than bites on the extremities. Effectively, the further the site of primary infection is from the host response or tissue damage that causes the symptoms, the longer a virus’ incubation period.

Whether or not an infected person is contagious during this incubation period depends on the virus.

With Ebola virus, the amount of virus in bodily fluids during its incubation period is so small that molecular tests cannot detect it. There’s virtually no chance of transmitting it to others, particularly as the route of transmission requires symptoms.

But viruses that are present in significant quantities during their incubation periods may be shed; both rotaviruses, the main cause of infant diarrhoea, and polio viruses are known to be shed prior to the onset of symptoms.

The Ebola virus’ lack of contagiousness during the incubation period makes the isolation of potentially infected, but currently asymptomatic, people unnecessary. Twice-daily monitoring for fever, which is the first sign of the virus being at significant enough levels to be transmitted, is all that is required.