Understanding The Chain Of Infection

When I’m not busy ‘chickening’ I work as a Health Promotion Educator for a non-profit organization dealing with HIV, Hepatitis c, harm reduction and substance use issues, My job, in part, is to provide workshops for a wide-range of audiences about the origins, transmission, prevention and treatment of blood-borne pathogens, like HIV and Hepatitis.

I thought I would take some of that basic knowledge and apply it to disease transmission in chickens. I’d had birds for almost a decade before I started blogging. My desire to educate myself and other chicken keepers has pushed me to learn more about avian anatomy, physiology and the pathogens that can potential cause illness. I’ve learned a lot in the last year.

In order to understand disease transmission in chickens it helps to have some basic understanding of what can potentially infect them, how they enter the body and what they do once there.

Pathogens – bacteria, viruses, fungi, protozoa – are living organisms and, as such, are continually evolving to find ways to increase their survival. The ways in which pathogens and parasites are transmitted are complex and diverse, varying among strains and host populations.

I think lots of us who spend time in online chicken groups have some basic understanding of the common pathogens affecting chickens, how they might be transmitted, symptoms and possible outcomes. In this article, I want to provide a theoretical framework to explain the necessary components of transmission and the language used to describe them.

Do you all know what ‘fomite’ means? I didn’t, and it is a critical piece for understanding how chickens are infected in their environment. Stay tuned and you’ll know what it is in a minute.

I’ll try to break down the discussion in layman’s terms so it’s user-friendly and you’re able to apply it to improve the health of your flock.

Components of Transmission

• Pathogen: Bacteria, viruses, fungi, parasites, protozoa
• Reservoir: Contaminated surfaces/equipment; water; soil; other animals/insects (e.g. wild birds, mosquitoes, earth worms, rodents)
• Portal of exit: aerosols (saliva/mucous); dander; poop; feathers
• Mode of transmission: Direct or indirect contact; ingestion; inhalation; sexual
• Point of Entry: respiratory tract; mucous membranes; broken skin; vent
• Vulnerable host: young, older and sick birds; chickens with pre-exisiting health issues or compromised immune systems

Modes of Transmission:

• Aerosol: Pathogens contained in aerosol droplets are passed from one bird to another, through coughing or sneezing. Pathogens don’t survive for extended periods of time within the droplets and close proximity of infected and susceptible animals is required for transmission (i.e. on roost bars and in the coop)
• Direct contact: A susceptible bird becomes exposed through physical contact when germs from an infected animal or the environment enters open wounds, mucous membranes, or the skin through saliva, dander or feathers. Some diseases can spread between animals of different species.
• Oral: Consumption of pathogens in contaminated feed, water or carried on objects in the environment (e.g. equipment, feeders, water troughs).
• Fomite: A contaminated inanimate object that transmits disease between birds. It involves a secondary route of transmission (direct contact or oral) for the pathogen to enter the host (e.g. contaminated shovels, clothing, bowls/buckets). A vehicle, trailer, or people can spread pathogens through contaminated tires, wheel wells, undercarriage, clothing, or shoes/boots by spreading organic material from one farm to another.
• Vector-borne: An insect or rodent (e.g. flies, fleas, ticks, mosquitoes, lice, rats and mice) acquires a pathogen and transmits it either by carrying it to, or inserting it into, a potential host

Horizontal vs. Vertical Transmission

Horizontal transmission refers to disease transmission between members of a flock, regardless of age, although young birds are often most vulnerable. Vertical transmission occurs when pathogens are able to spread between a hen and her chicks via the egg before it was laid (i.e. Salmonella, Mycoplasma, Newcastle Disease and anaemia).

Think of the components above as part of a larger equation, each one a link in a larger chain: pathogen + reservoir + portal of exit + mode of transmission + point of entry + host = potential disease transmission. The things that we can do that disrupt that equation can also break the cycle of transmission.

Breaking the Cycle of Transmission

• Start with healthy stock from a reliable breeder
• Provide proper feed, fresh water and plenty of space to ensure your birds stay healthy and are resistant to potential pathogens (i.e. sick birds are more vulnerable to secondary infections)
• Understand the pecking order and the potential stress of integrating new birds into a flock
• Vaccinate, if possible (e.g. Marek’s, Coccidiosis, Fowl Pox, Salmonella)
• Practice good biosecurity: clean, disinfect and sterilize equipment, especially anything that has been exposed to birds outside your flock
• Quarantine new birds
• Discourage pests (i.e. rodents) which may be vectors for disease
• Clean and disinfect waterers on a regular basis
• Wash your hands often, and every time after you handle your birds
• Wear coop-specific footwear and clothing
• Isolate and treat sick birds
• Invest in a necropsy for euthanized birds, or those that have died unexpectedly

Feature Image Credit: Mind Your Mind