There’s an unpleasant fact about keeping backyard chickens that none of us probably thought about at all when we brought home our first little peeping bundles. Someday they will all die. Death is the elephant in the coop. We don’t like to think about it or talk about it, but we can’t ignore it. It is important, actually, to wonder about what causes the lives of the birds in our backyard flock to end, and there’s nothing gratuitously macabre about finding the answers.

There are people who have made a career out of studying why people die. If you’re searching for the answer to this important and frequently asked question, it isn’t hard to find. I did a quick Google search and learned from the CDC that in 2018 the top five causes of human death, in the US, in order, were heart disease, cancer, unintentional injuries, chronic lower respiratory diseases, and stroke. The WHO has the statistics for the world and lists the top five causes of death, worldwide and in order, to be cardiovascular disease, cancer, respiratory disease, lower respiratory infections, and dementia.

It’s great that somebody is compiling cause-of-death information about people, right? But what’s the cause-of-death story with chickens? Sad fact: Until very recently, nobody knew. Why not? Because prior to the backyard flock movement, it hadn’t ever occurred to anybody to do that research. Chickens, as you already know and I will tactlessly mention anyway, have traditionally become dinner long before the end of their natural lives. And, in general, veterinary science has been more interested in flocks than individual birds. In commercial flocks, if there’s an infectious disease that has the potential to wipe out an entire flock, that’s significant and bears looking into. If one bird is doing poorly, that bird is simply culled—no muss, and sadly, no fuss. Basically, the underlying cause of death of geriatric chickens has been unknown because there haven’t been any geriatric chickens and nobody has cared.

But you and I care, of course. Backyard chicken folk rightfully regard their birds as pets and companion animals. If Henrietta is under the weather, you and I want to know why. Thus, the blossoming of a plethora of books, websites, blogs and podcasts dedicated to Henrietta’s health. Plus, the proliferation of veterinarians who will examine, diagnose, and care for individual sick chickens. And Henrietta is not going to become pot pie even when her egg production decreases. She’ll remain in your loving care until she dies of natural causes. What we backyard chicken folk need to know is what all the various “natural causes” really are. What forms will the angel of death take when it ultimately visits each bird?

At last, there’s a scientific study that has collected the cause of death of a statistically significant number of backyard fowl over a wide-ranging geographical area in order to put together some meaningful information on the cause of death of backyard flocks that are allowed to live out their natural lives.

The Study

The study “Causes of mortality in backyard poultry in eight states in the United States” was a combined effort by researchers in nine academic institutions and research centers and was published in the Journal of Veterinary Diagnostic Investigation in 2019. Over a three-year period, from 2015 through 2017, these researchers collected and performed necropsies on 2,687 birds (The vast majority, 96%, of the birds submitted in this study were chickens but turkeys, ducks, and geese were also included) from eight states (California, Colorado, Georgia, Hawaii, Iowa, Pennsylvania, South Carolina, and Texas) to determine cause of death of each one. What they learned from this study begins to fill in the gaps in our knowledge about why our backyard chickens die, and as more information is collected by similar studies, it could have a definite effect on our chicken husbandry practices and how we interact with our flocks.

The necropsy results were reviewed by veterinarians who placed each cause of death into one of four broad categories and named those categories with descriptive terms that are a bit vet-jargon-y. (Rest assured that I’ll provide definitions later in this article.) The four categories are infectious, noninfectious, neoplastic/lymphoproliferative, or undetermined. Then they broke down each broad category - infectious diseases were subdivided into bacterial, viral, parasitic, or fungal diseases, and they made note of which of ones could potentially spread from poultry to humans. Noninfectious categories included nutritional, developmental, toxic, and traumatic causes of death. Neoplastic/ lymphoproliferative diseases, diseases caused by and characterized by tumors, were categorized by where the tumors appeared in the body and if they were caused by a virus (as is the case in Marek’s disease and the leukosis/sarcoma cluster of diseases).

What They Found

When the results were categorized by primary cause of death, almost half of the birds died from some type of neoplasm. Neoplasms, simply stated, are tumors. In chickens, tumors are often caused by viruses, as in Marek’s Disease. Over a third of the birds in the study died of an infectious disease. Around two in ten died from a noninfectious process, and the rest, about 5%, died from undetermined causes.

The necropsies showed that almost seven in every ten birds had more than one cause contributing to their death. So, when the results were categorized by all causes of death, rather than just the primary one, the numbers shifted a bit. Neoplasms (1131/2687) and bacterial disease (1135/2687) were essentially tied for first place, with both being found in 42% of the birds. Parasites were found in 28% (755/2687) of the birds. A noninfectious malady was discovered in 16% (444/2687). Nutritional, management, or environmental problems were found in 10% (261/2687). And viral disease was seen in 7% (195/2687). Other causes of death included trauma, fungal disease, toxins, and developmental issues. (Note that the percentages add up to more than 100% since some birds had more than one disease.)

Here’s a closer look at the top six causes of death in this study:

1 – Neoplasia

Neoplasia occurred in 42% (1131/2687) of the birds. Neoplasia is the abnormal and excessive growth of tissue. The masses of abnormally growing tissue are called neoplasms. “Neoplasm” is simply veterinary-speak for “tumor.”

Some tumors are caused by viruses, and more than 63% (717/1131) of the birds with tumors in this study had virally-induced tumors. The main culprit for virally-induced tumors? Marek’s disease—accounting for more than 51% (582/1131) of the birds in this category, and responsible for more deaths overall than any other cause. I’m not surprised. Marek’s disease occurs worldwide and is practically everywhere. Complete books have been written about just this one disease. I’ve written a long and somewhat technical post about Marek’s that, in spite of its length and plethora of big words (like plethora), continues to be one of my most popular posts; no doubt because so many people are searching for information. Because they’re dealing with this devastating disease in their flocks. Chickens acquire Marek’s disease by inhaling the virus, which is usually contained in dander shed by infected chickens. Once a chicken is infected, there is no cure. Fortunately, though, there’s a vaccine, which is over 90% effective in preventing symptoms of Marek’s disease from occurring.

Almost 9% (99/1131) of the cases were of the leukosis/sarcoma group of viruses. This related group of viruses cause an overlapping set of diseases that cause the formation of tumors in a variety of locations including the liver, spleen, eyes, bones, and blood. There is no vaccine for this group of viruses.

About 37% (414/1131) of the cases of neoplasia were not virally induced. The most common non-viral cases were ovarian cancers (about 28% or 113/414). Laying an egg almost every single day is hard on hens in a number of ways. Perhaps continuous ovulation, a trait that has been bred into domestic hens that doesn’t occur in their wild progenitors, results in high rates of ovarian cancer.

2 - Bacterial Disease

Bacterial disease was found in over 42% (1135/2687) of the necropsied birds, essentially a tie with neoplasms. The most frequently found organism (34% or 386/1135) was E. coli, a bacterium that lives in the intestines of normal, healthy, happy chickens (as well as normal, healthy, happy humans and all other birds and mammals). It is supposed to live in chicken intestines and it needs to be there for the chicken to be normal, healthy, and happy. There are, unfortunately, strains of E. coli that have acquired “virulence genes” and have become pathogens that can spread from chicken to chicken and make them sick. And sometimes, even regular old garden-variety E. coli can infect chickens that are already under the weather due to some other underlying disease. And when chicken poop gets into a part of a chicken’s body that normally doesn’t contain bacteria, some of the bacteria that inhabit the poop, often E. coli, can cause a severe infection.

Hens frequently suffer from reproductive diseases—30% of the birds in this study suffered from reproductive tract diseases or infections including salpingitis (an infected oviduct—often caused when a hen’s own poop backs up into her oviduct), coelomitis (also called egg yolk peritonitis and usually caused when a hen’s infected oviduct ruptures into her abdominal cavity), and vent prolapse. The most commonly found bacteria in these infections was E. coli.

Mycoplasma was second most commonly found bacteria. Mycoplasma gallisepticum and/or Mycoplasma synoviae were seen in 26% (296/1135) of the birds. Both of these pathogens cause chronic respiratory diseases. Symptoms include swollen and weepy eyes, nasal discharge, gasping, coughing, and sneezing. M. synoviae also causes joint and tendon infections resulting in lameness, swollen hocks, and lethargy. Chickens acquire mycoplasma from other chickens who are sick with it, from infected equipment, or from humans who carry it to their flocks on their clothes or shoes.

6.5% of the bacterial cases (74/1135) were Gallibacterium anatis, an emerging bacterial pathogen that poultry veterinarians are just beginning to understand and characterize. This bacterium can live in the upper respiratory tract and lower reproductive tract of healthy chickens but can also become a pathogen. It has been seen in infections of the oviduct, abdominal cavity, liver heart and blood.

3.5% of the cases (40/1135) were Pasteurella multocida, the bug that causes fowl cholera. Chronic fowl cholera is characterized by lesions on a chicken’s wattles, joints, tendon sheaths, and footpads, but the first sign of an acute infection can be when you walk into the coop in the morning and find a large number of dead birds that all appeared perfectly healthy the day before. A flock is usually exposed to fowl cholera through contact with infected rodents, wild birds, pets, and other animals.

There were small numbers of a whole list of other bacteria including 27 cases of paratyphoid Salmonella enterica, 6 cases of Mycobacterium avium, 4 cases of Listeria monocytogenes, and 5 cases of Campylobacter jejuni. These last four chicken pathogens deserve special mention because they can also cause disease in humans, and we can catch them directly from our chickens—the reason CDC keeps telling us not to kiss our chickens.

3 – Parasites

Parasites were found in 28% (755/2687) birds. The most common parasitic disease was Coccidiosis—which was found in 39% (297/755) of the birds infested with parasites. Coccidiosis is caused by coccidia, tiny single-celled parasites. The coccidia that cause disease in chickens belong to nine different species in the genus Eimeria. Young or unexposed chickens acquire the disease by consuming contaminated feed, water, or soil. The parasites settle in the chickens’ intestines and cause diarrhea, which can become bloody, and in severe cases can cause high mortality in the flock.

The other parasites found were a mixed bag of external and internal parasites including mites, scaly leg mites, various roundworms, tapeworms, and single-celled parasites. Infestation with more than one type of parasite was detected in 28% (210/755) of the birds with parasites.

4 – Generalized Noninfectious Diseases

Systemic diseases and organ failure were grouped together under the “Generalized Noninfectious Diseases” category and accounted for 16.5% (444/2687) of the deaths. The most common disease detected was hemorrhagic liver syndrome (HLS), which was found in 29.5% (131/444) of the birds in this group. Birds with HLS have excess fat deposits throughout their abdomen and in their liver. As a result, their liver becomes large and is susceptible to damage and prone to bleeding. The liver can hemorrhage when excessive pressure is applied, such as when a hen is straining to lay an egg.

A variety of diseases afflicting the heart, lungs, kidneys, reproductive and other organs made up the rest of the conditions in this category.

5 – Nutritional, Management, & Environmental Issues

Another name for this category could be “poor flock care.” The birds in this group suffered from malnutrition, starvation and thirst and accounted for around 10% (261/2687) of the deaths. Almost 95% (247/261) of the birds in this group suffered from both nutrition-related and management-related issues. Nutritional imbalance was found in 25% (64/261) of the birds. Evidence of lack of access to water accounted for the death of 20% (51/261). Starvation was seen in 16% (43/261). Management related issues included exposure to extreme temperatures resulting in heat stroke or frostbite, overcrowding, lighting issues, air-quality problems, and foot issues related to wet litter.

6 – Viral Diseases

Disease-causing viruses were found in 7.3% (195/2687) of the birds. Viral diseases can be devastating and catastrophic to flocks. An epidemic of virulent Newcastle’s Disease afflicted backyard flocks in southern California beginning in early 2018, shortly after the data collection for this study was completed. Since California was one of the states involved in the study, virulent Newcastle Disease would have, without a doubt, been a significant cause of death had the outbreak and the study coincided. The California outbreak was finally declared to be eradicated on June 1, 2020. Over one hundred thousand backyard chickens perished in that two-year period.

The most commonly isolated agent in this study is a virus we’ve been hearing a lot about lately—coronavirus. Only it isn’t that coronavirus. Avian coronavirus, a cousin to the virus causing the current on-going human pandemic causes infectious bronchitis in chickens and can also cause kidney and reproductive system infections. It was found in 41.5% (81/195) of the birds infected with a virus.

Gallid alphaherpesvirus 1, which is a herpesvirus, like the virus that causes Marek’s disease, was found in 20% (39/195) of the virally infected birds. This infectious virus causes infectious laryngotracheitis, a severe inflammation of a chicken’s larynx and trachea.

Avipoxvirus, which causes fowl pox, was found in 20% (40/195) of the infected birds. Avipoxvirus is a member of the viral family Poxviridae, which also includes the human disease smallpox. Like smallpox, fowl pox is characterized by the formation of pus-filled lesions, poxes, on the bird’s skin. It can also cause necrotic membranes to form inside a bird’s mouth and upper respiratory tract. This virus can be spread from bird to bird and flock to flock by contaminated objects, but is primarily spread by mosquitoes.

It is important to take note of the fact that the authors of this study chose to place neoplastic diseases caused by viruses in the “Neoplasms” category. Had they included them with viruses, viral diseases would jump to the top of list and become the leading cause of death.

This study has shown that infectious diseases are the main killer of backyard chickens. We backyard flock keepers often assume that we’re providing a better life for our birds than the life they would endure in a commercial flock, and in so many ways that assumption is absolutely spot on. Yet when it comes to protecting our flocks from infectious diseases, the main killer of our backyard birds, we have failed. In part two of this article, “Preventing the Preventable,” I will examine some changes we could make in our flock management practices - especially the way we control infectious diseases.