Emergencies/Illness Health Issues

Microbial Pathogens In Chickens

How Bacteria, Viruses & Protozoa Work

I’ve spent many hours recently painting my house and listening to our public radio programs while I worked. I enjoyed it all (the radio, not the painting), but especially liked the medical and science shows. One had a Q & A period in which a child asked how a virus worked to attack its host given that it doesn’t have a nucleus. I’m a bit of a science and nature nerd so I listened intently for the answer. I realized that lots of chicken keepers probably don’t understand the difference between various microbial pathogens and how they actually make our birds sick.

So here is a quick overview about what bacteria, viruses and protozoa are and how they work to infect their hosts; in our case, chickens. I’ve dealt with fungal infections here

Structure, Size & Shape

Bacteria are single-celled organisms that can live in soil, water and inside the gut of avian and mammal hosts. They have a simple internal structure that lacks a nucleus, but contains DNA.

They generally have two protective coverings: an outer cell wall and an inner cell membrane. Certain bacteria, like the mycoplasmas, don’t have any cell wall. Some bacteria may even have a third, outer protective layer called the capsule. Whip-like extensions often cover the surfaces of bacteria — long flagella or short pili — that help them move around and attach to a host.

Bacteria are classified based on cell wall, shape or genetics. There are three basic bacterial shapes: round (cocci), cylindrical (bacilli) or spiral (spirilla), which are often reflected in their names. The largest bacterial cells are visible with the naked eye.

Viruses are composed of genetic material (DNA or RNA) that is surrounded by a protective coating made of protein (capsid) and sometimes a spiked envelope obtained from the cell membrane of a previously infected host. There is some debate in the scientific community as to whether viruses are living or non-living organisms. They do contain genetic material, but rely solely on a host for replication.

Viruses typically have spherical, rod or helical shaped capsids and are much smaller than bacteria. They are the smallest of all the microbes and about 1,000 times smaller than bacteria. Five hundred million cold viruses could fit on to the head of a pin and one sneeze could release 20,000 droplets into the environment. Touching or breathing in those droplets is how airborne viruses spread to the next host.

Protozoa come in many different shapes and sizes and live in a wide variety of moist habitats including fresh water and the soil.  They are more complex than bacteria and have structures that perform the function of a mouth, digestive tract and anus. They are able to move using cilia (hair-like structures), flagella (whip-like threads) or temporary protrusions (pseudopodia).

Larger than bacteria and viruses, protozoa come in a variety of shapes including oval, spherical and elongated. Some, like amoebas, can even change their shape.


Both bacteria and protozoa commonly reproduce asexually by a process known as binary fission in which a single cell replicates and divides into two identical separate organisms. Two other forms of asexual reproduction used by protozoa are called budding and schizogony.

Unlike bacteria and protozoa, viruses can only replicate with the aid of a host cell. They typically enter a body through the respiratory system or openings in the skin. When they find the target host cells they attach on to them like Lego. The virus then hijacks the receptors and enters the cell by being engulfed or burrowing through the membrane. Once the viral components are assembled and the newly formed viruses mature, they break open the cell and move on to infect other cells.

Unlike bacteria, which attack cells from the outside, viruses actually move into, live in and make copies of themselves inside cells. In reproducing themselves they damage the host’s cells.

How Do They Cause Illness?

I think most of us like to think of our bodies as pristine and free of ‘cooties’ and once we’re exposed to them we get sick. That’s far from what actually happens: trillions of microbes live on all surfaces of the body: skin, gut and mucous membranes. Mammals and birds – including both people and our chickens – are host to more bacteria than their own cells.

So what are these microscopic organisms doing? Lots of them are actually helpful: they synthesize vitamins, break down food into absorbable nutrients and stimulate the immune system. Think of them as tiny campers living in complex communities inside the body. Most of the time they are doing their own thing which we’re largely unaware of.

Humans typically produce a new generation every two or three decades, while bacteria can accomplish that every 20 to 30 minutes, and viruses are even faster. That rapid reproductive cycle means they can amass huge numbers with great variety in their communities.

Infection doesn’t automatically lead to disease. It’s only when microbes begin to multiply and damage host cells that symptoms of an illness appear. Disease usually only happens in a small proportion of an infected population. Many hosts may carry the microbes and be totally asymptomatic.

The immune system is triggered into action in response to infection. White blood cells, antibodies, and other mechanisms go to work to get rid of foreign invaders. Many of the symptoms during an infection —fever, lethargy, pain—result from the activities of the immune system trying to eliminate an infection.

Pathogenic microbes challenge the immune system. Viruses cause illness by killing cells or disrupting cell function. Hosts often respond with fever (heat inactivates many viruses), the secretion of a chemical called interferon (which blocks viruses from reproducing) or by activating the immune system’s antibodies and other cells to target the invader.

Many bacteria cause illness in the same way, but they can also multiply so rapidly they crowd out host tissues and disrupt their normal function. Sometimes they kill cells and tissues, make toxins that can paralyze or destroy cells’ metabolic machinery or cause a massive toxic immune reaction.

Protozoa that infect chickens often target the respiratory system and digestive tract causing destruction of internal tissue and robbing birds of nutrients.

What Illnesses Do They Cause?

The majority of bacteria are harmless, but some can cause disease in chickens: Botulism, Campylobacter, E.coli, Infectious Cholera, Infectious Coryza, Mycoplasma, Pasturella, Salmonella, Staphylococcus, Streptococcus and Tuberculosis.

Chicken viruses include: Avian Influenza, Encephalomyelitis, Fowl Pox, Infectious Bronchitis, Infectious Bursal Disease, Lymphoid Leukosis, Marek’s Disease and Newcastle Disease.

Most protozoa do not cause disease, but some are parasitic and can make chickens sick, including: Coccidia, Cryptosporidia, Toxoplasma and Trichomonas gallae.

There are vaccines for some chicken illnesses caused by bacteria, viruses and protozoa.

A Note About Antibiotics

Antibiotics are used to treat bacteria, which work by weakening the cell walls allowing the immune system to kill the invaders or by disrupting their ability to function.

Unfortunately they are ineffective on viruses, which are protected by a protein coat. Viruses are much smaller than bacteria and don’t have the same machinery which is targeted by antibiotics.

Treatment for viral infections usually involves medications that treat the symptoms of, and not the virus itself. Keeping our birds’ immune systems strong is paramount because that’s what can fight off viruses. Do not use antibiotics for viral infections.

When using antibiotics for bacterial infections it’s critical to use the one that will be most effective based on the pathogen. Misuse or overuse can lead to antibiotic resistance. Ask your veterinarian for the most appropriate medications.

The take away is that both we, and our chickens, are co-existing with billions of microbes that, for the most part, do us no harm. On the occasions when they can cause disease the greatest defense is a robust immune system, which is developed by natural exposure to small numbers of microbes.

You don’t need to panic every time you see a worm in your chicken’s poop or you hear a sneeze. A healthy chicken isn’t one that is pristine and free of all microbes, but one that can manage the complex relationships with the various tiny campers in its body so they strike a balance. Events that tip the balance in favour of the pathogens is what we are trying to avoid.

Prevention is the key and that’s achieved by keeping your birds healthy:

  • Start off with healthy stock (feed store chicks and shipped birds are often stressed and weak)
  • Practice good biosecurity
  • Quarantine new birds
  • Regularly clean your coop and sanitize chicken equipment
  • Manage stress and overcrowding
  • Understand flock dynamics
  • Feed a balanced diet
  • Remove sick birds from the flock for treatment (or euthanasia)
  • Give the appropriate treatment only once you’ve got a diagnosis. Experimenting with a multitude of options can make your vulnerable bird worse.

Credits: Live Science, Microbiology Society, NCBI, ThoughtCo. Featured photo credit: AgFunder News (microscopic image of bacteria)

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