An illustration of the immune response to viral infections.

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How the Immune System Protects Animals from Viruses

7 minute read

Dr. Chris Ashworth, DVM contributed to this report.

Part 2 of a 4-part series on viruses in animal production.

Part 1 discussed what viruses are and how they impact animal production.

There are a number of ways that an animal can become infected by a virus. The typical ways are through ingestion and inhalation of air. However, animals can also become infected by coming into direct contact with a surface or an object that has been contaminated or, on a rare occasion, through breeding.

For example, a warts virus can infect the skin of animals anytime that animal comes into direct contact with a surface containing the wart virus. These surfaces could include — but are not limited to — posts and walls in a barn or even a brush used on show animals. Mammillitis and pseudocowpox are examples of viruses that can be spread by milking machines, particularly in dairy heifers.

Once an animal’s immune system recognizes a virus in the body, it starts an inflammatory response to kill the virus. This process can start right away or it can be delayed depending on how long it takes the virus (incubation period) to cause clinical disease.

Let’s take a look at how the immune system works when different viruses infect an animal.

The Immune System in Action

The immune system is comprised of a sophisticated network of cells, proteins and enzymes that are programmed to monitor animal wellness. This network of cells detects and responds to bacteria, viruses, parasites and pollens, as well as toxins from bee stings and tick bites that may invade the body. The cells also respond to stressors, injury or environmental challenges.

When a virus crosses one of the body’s barriers, such as the skin, mucous membrane or blood vessel linings, the immune system will detect the invasion. The immune system then sends signals called cytokines, indicating that help is needed. This initiates the movement of white blood cells toward the site of infection.

The first responders to the site are white blood cells called phagocytes. These cells help protect the body by ingesting harmful foreign particles, bacteria and dead or dying cells. There are two forms of phagocytes:

  • Neutrophils in mammals or heterophiles in birds are small, granular leukocytes that quickly appear at the site of a wound and ingest bacteria.
  • Monocytes are larger leukocytes that appear about three days after infection and scavenge for bacteria, foreign particles and dead cellular material left behind by the neutrophils or heterophiles.

The macrophages and neutrophils appear at the site first and work to en­gulf and destroy the virus. They then display pieces of the viruses on their surface to signal the monocytes to help continue the attack on the invading viruses in a more specific way.

Once a viral infection happens, there’s a race between the immune system and the virus. The virus hides itself in a cell before that cells shows that it is altered and different from other cells. That is the time that a virus has to take advantage and have sufficient offspring to infect new cells.

By the time the immune system starts to recognize those patterns and start the inflammatory process, the virus is already spreading to additional cells.

Macrophages and lymphocytes play an important role in identifying a viral infection that’s ongoing as well as helping the body defend itself against viral infection.

Additionally, within just a few hours of a viral infection, one of the natural defense mechanisms that the body produces is interferon. This is a molecule that the body makes that plays a role in killing some, but not all, viruses.

Immediate and Delayed Disease from Viral Infections

Some viral infections will show signs of clinical disease right away. Such diseases, like rhinovirus, influenza and coronavirus, will cause disease within one to 14 days. Other viruses, like retroviruses, HIV, bovine leukosis virus and Marek’s Disease, won’t show signs of clinical disease until weeks, months or, in some cases, even years after infection.

These retroviruses multiply at a very slow rate, so it can take these viruses a long time to infect enough cells to make an animal sick. With some retroviruses, like equine infectious anemia, the animal is still able to continue producing red blood cells on a daily basis. For example, if a horse loses 10 red blood cells every day, but makes nine, it will take an extended amount of time for the horse to become anemic.

Another example is HIV in humans. Those infected with HIV don’t experience a complete depletion of bone marrow and T helper cells right away. They will lose a small amount each day and it may take a long time before they get to a point where they don’t have enough of them.

How Long Can a Virus Remain in the Body?

Typically, a viral infection will stay in an animal’s body for at least 10 days but could stay for the rest of its life. Herpesvirus, for example, does a great job of tricking the body by making it think it is a part of the body and hiding from the immune system for a long time. This prevents the immune system from initiating an inflammatory response to kill the virus and remove it from the system.

Once a mammal is infected with herpesvirus, they will be infected for life. That doesn’t mean, however, that they will always show signs of clinical disease. There are people who have herpes infections and only have one cold sore in their entire life while still having the virus living somewhere in their body. 

Once a pathogen attaches to cells, the immune system will identify it as “self” or “non-self.” When a body recognizes a pathogen as “non-self” it starts the inflammatory process to kill that pathogen. In some instances, the body allows the immune system to over react and turn against itself. We don’t see this very often in farm animals but do see it in cats and dogs. When this occurs, this is called an auto-immune disease. Some examples of these in humans include lupus, pemphigus, rheumatoid arthritis and many others.

The Role of Vaccines in Virus Management

When it comes to vaccines, it’s important to understand that they do not prevent infection but work to reduce the severity of any resulting diseases. When you get a flu shot, for example, a physician will not tell you that you will not become infected with the flu. They should tell you that if you do catch the flu after receiving a vaccine with the same strain of flu virus, you will not get as sick or you won’t be sick for as long as if you had not had the vaccine. 

Vaccines allow a body to create antibodies in the blood that will attach to a virus in the early hours after infection and hopefully carry it away before it has a chance to cause clinical disease.

Antiviral medications, antiseptics and disinfectants have similar effects in that they reduce the infectivity and a virus’s ability to survive in the environment, but do not completely prevent infection.

The Impact of Nutrition in Protecting Against Viruses

A well-balanced diet is critical to protecting an animal from viral infections.

Early nutrition in young animals is especially important as it is a critical time for the development of the immune system. They need adequate nutrition to develop enough immune cells and to fully develop the immune organs.

Cells that are not deficient in vitamins and minerals, both macro and micro, do a better job at recognizing infections earlier. Additionally, cells with an adequate amount of vitamins, minerals and energy work better and are able to make more antibodies quicker. By making antibodies quicker, animals are better able to respond to and control the infection by preventing viral replication.

The trace minerals selenium, zinc and manganese work as antioxidants and scavenge the free radicals that are produced during the inflammatory process and cause extended damage to cells.

Check back for Part 3 of the 4-part series on viruses in animal production. The next article will discuss how essential trace minerals play a role in protecting an animal against viral infections.