What Are Viruses and How Do They Impact Animal Production?

Dr. Marco Rebollo, DVM also contributed to this post.

Part 1 of a 4-part series on viruses in animal production.

Viruses are having a major impact on animal production all around the world. Viruses can affect just about any tissue in the body, including the intestines, lungs, liver, kidneys, uterus, ovaries, the spinal cord, the brain and even the skin.

When viruses infect animals, it can affect growth, performance, reproduction and even lead to death. Controlling harmful viruses, like influenza, infectious bronchitis virus, BRSV and others is key to producing healthy, high-performing animals. First, we need to understand what viruses are and how they work to affect animals.

A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism and lacks the ability to survive or reproduce for a long period of time outside of a host. Viruses are present in plants, animals, insects and the environment all around the world and can cause problems in the normal function and structure of animals and their bodies.

The control of viruses, using vaccines, enhanced biosecurity and antiviral medications, among other strategies, as well as adequate zinc nutrition has helped extend animal life expectancies, improve productivity, performance and reproduction.

How Do Animal Viruses Work?

Viruses are made of either a single or double strand of nucleic acids and are classified as either RNA or DNA with protections in the form of either a capsule or an envelope. Viruses can enter the body through ingestion, inhalation of air, across the skin, or through the reproductive tract at the time of breeding.

Once an animal becomes infected with a virus, the virus can inject its genetic material into the healthy cells of an animal. It is then able to make the cells work to replicate the virus into many viruses, which can then be shed and further spread to other animals or, in some rare cases, to the humans caring for them.

While viruses have been recognized for many years as an etiological agent of disease, there are some viruses that do not cause diseases right away. They will live dormant in the cells of an animal until a stress event occurs. This could be transportation stress, heat stress, cold stress or stress from high stocking densities. Stress could cause a virus to come back out of dormancy and start to produce clinical disease.

While viruses can sometimes lead to death, most viral infections don’t directly lead to the demise of an animal. Instead, they initiate a depression in the immune system, which then allows bacterial pathogens to come into the animal and lead to death from a bacterial infection. This is very common in feedlot cattle.

Where Do Coronaviruses Fit In?

Basically, all species have a coronavirus — respiratory or enteric — that can infect them. Coronaviruses are quite common in animals. For example, cattle have a bovine coronavirus that causes respiratory disease. In poultry, infectious bronchitis virus (IBV) is classified as a coronavirus. Porcine epidemic diarrhea (PED) is a coronavirus that can impact young pigs. Then our companion animals — cats and dogs — can be infected with feline infectious peritonitis and certain respiratory conditions. Those coronaviruses affecting farm animals are different from those affecting humans (SARS-CoV2, SARS-CoV and MERS-CoV). It is highly unlikely that they will infect or cause disease in humans or that those from humans will infect farm animals.

Challenges with Animal Viruses

The big challenge is that viruses are ultramicroscopic, which means that they are under 300 nanometers in size. This means that viruses can easily become aerosolized. They can float in the air and spread very easily, especially inside enclosed spaces. So, the higher the stocking density of animals, the easier a virus can spread between animals.

For example, an outbreak of foot-and-mouth disease, which can affect many cloven hoof animal species, between Britain and France is thought to have spread by the virus being carried on the air across the English Channel.

While a virus can travel on its own through the air, there are other ways that a virus can travel by surviving in a host’s body. For example, Encephalitis viruses (western and eastern) in horses can be carried, maintained and spread by mosquitos and birds. Additionally, bats are known to carry and spread viruses, such as rabies, influenza and coronaviruses without showing clinical symptoms.

Another challenge that animal producers face when it comes to viruses is the ability of the viruses to mutate. As animals build an immunity to a certain virus, the virus can mutate and change. This means that the virus can become more infectious or less infectious, more virulent or less virulent or can start infecting other species, including humans in some cases.  This is always a big fear with avian and swine influenza viruses.

When a virus starts adapting to a new species, it can become very pathogenic. So, in the time it jumps to different individuals of the new species, it starts to adapt. It takes time before that virus establishes a friendly relationship with the new host. That’s why we see massive outbreaks and are going through this current pandemic, which is a virus that is adapting.

Virus Management in Animals

Around the world we have built animal production systems that are conducive to increasing the spread of viral infections, largely due to high stocking densities. As a result, animal producers can reduce the spread of viruses by reducing stocking density. Many poultry systems are already doing this as the industry transitions to antibiotic-free production practices, which naturally has resulted in placing fewer birds in each house.

Other ways to reduce the spread of viruses is using antiviral medications and vaccines. Many viruses, including influenza, Newcastle disease and Marek’s disease, have effective vaccines readily available for animal producers. Antiviral medications, however, are oftentimes cost prohibitive while vaccines take time, research and technology to produce. Therefore, the best way to prevent a virus from infecting your animals is to enhance your physical biosecurity to keep a virus as far away from animals as possible.

Enhancing your physical biosecurity involves:

• Keeping your houses and feed yards separated
• Disinfecting vehicles that are coming in or out of your animal production system and/or not moving equipment from one farm to another, especially if birds and hogs are different ages
• Make workers take showers and change clothes before entering and after leaving each complex
• Wear protective masks, hair covers and gloves when working in a house and wash your hands every time when moving from poultry house to poultry house
• Delivery trucks should be disinfected, and drivers should stay in the truck or be disinfected as well
• Allow downtimes long enough – more than 10 days – to allow viruses to die and allow enough time for cleaning and disinfecting the facility

Quarantining animals prior to placing them in with the larger herd is an excellent way to decrease the chance of introducing a disease into a group of animals. In cattle, when producers bring new animals into their systems, the animals should be quarantined for 21 days. In cattle, for example, a BVD virus can live in beef or dairy cattle for up to 18 days in animals that are not persistently infected.

In addition to biosecurity, it is important to make sure your animals have good immune systems. Ensure the animals are eating adequate amounts of protein and energy and have good body condition. It is also important for the animals to have an excellent mineral and vitamin status to improve white blood cell function. Finally, animals need to have access to clean of water to prevent the ingestion of more bacterial and viral pathogens into their intestinal tract.

Check back for Part 2 of the 4-part series on viruses in animal production. The next article will explain how the immune system reacts to viral infections in animals. Then, Part 3 will dive into the role essential trace minerals play in protecting animals against viruses.

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