Diseases of backyard poultry (Proceedings)


As stated in the accompanying lecture, Introduction to Backyard Poultry, bio security is an essential part of any poultry health management program.

Infectious diseases

As stated in the accompanying lecture, “Introduction to Backyard Poultry,” biosecurity is an essential part of any poultry health management program.  Steps to prevent exposure to infectious agents should be taken for every backyard flock, including minimizing visitor contact, maintaining a “closed” flock or quarantining new birds for at least two to four weeks, isolating sick birds, and providing periodic preventative veterinary services.  Many poultry diseases are zoonotic, such as salmonellosis and campylobacteriosis.  Many zoonotic and other diseases are reportable to state regulatory agencies.  The poultry practitioner must be keenly aware of the public health concerns and concerns to the commercial poultry industry with regards to zoonotic and reportable diseases.

Selected viral diseases

Marek's disease (MD, or MDV)

Marek's disease (MD, or MDV) is one of the more common diseases encountered in backyard chicken medicine.  The causative agent of Marek's disease is an alphaherpesvirus with numerous strains of varying pathogenicity.  MDV is highly contagious and is transmitted horizontally by direct contact or indirectly through the air or by fomites (eg. feathers, dander, litter).  An infected bird sheds virus indefinitely.  Virus can remain infectious in the environment for many years.  The disease only affects chickens.  Morbidity is generally high and mortality is low.  Clinical signs are most often seen in 10 to 20 week old chicks, but birds 3 to 4 weeks of age to 3 years of age can also be affected.  Clinical signs include unilateral paresis or paralysis (can progress to bilateral involvement) typically between 6 and 12 weeks of age (often associated with unilateral swelling of the affected ischiatic nerve at necropsy) or lymphoma, often associated with extensive visceral or dermatologic neoplastic involvement.  If mononuclear infiltrates are present within the iris, the eye may change color from yellow to a pale tan or grey.  MD must be differentiated from lymphoid leukosis (LL), a retroviral disease with similar clinical and pathologic findings.  Serology (eg. ELISA) and PCR are available for diagnosis.  No treatment exists for MD, and as infected birds are persistent shedders, euthanasia should be advised for confirmed cases.  Vaccination is standard for commercial birds, either within the egg or at one day of age.  Vaccination is over 90% effective in preventing disease.  Purchase of vaccinated birds for the backyard flock is advised.  On-site vaccination is impractical and not advised.  A 1:10 dilution of bleach inactivates MDV. 

Lymphoid leukosis (LL)

Avian leukosis virus (ALV), a retrovirus, is associated with the leukosis/sarcoma group of diseases in chickens.  Lymphoid leukosis (LL) is the most common manifestation of this group of diseases, but numerous other neoplastic diseases are associated with ALV in chickens.  ALV is transmitted both horizontally and vertically.  LL is typically seen in birds 14 to 40 weeks of age but can be seen later in life.  Clinical signs are generally non-specific and include inappetance, weight loss, weakness, and diarrhea.  Premortem diagnosis can be made by serology (eg. ELISA) and virus isolation.  Necropsy findings include grey to white tumors on viscera including the liver.  ALV induces numerous neoplasms such as erythroid, lymphoid, and myeloid leukosis as well as fibrosarcomas, hemangiomas, and nephroblastomas.  Clinical and pathologic findings of LL and MD can be difficult to differentiate.  However, clinical signs of LL rarely develop before 14 weeks of age, which helps to differentiate it from MD.  As with MD, no treatment exists for LL.  Currently no vaccine is available.  The best preventative measure is to test all birds and cull all positive breeding stock.

Avian influenza (AI)

It is unlikely for avian influenza (AI), also known as “fowl plague,” to be seen in the typical backyard flock.  However, sporadic outbreaks do occur, and the backyard poultry veterinarian should be familiar with this serious and reportable disease.  In fact, highly pathogenic avian influenza (HPAI) H5N8 was reported in a small (about 100 birds) backyard poultry flock of guinea fowl and chickens in Winston, Oregon in December, 2014.  The causative agent is a type A influenza virus, an orthomyxovirus. Transmission is through aerosol spread, ingestion, or through fomites.  Clinical forms of AI range widely from low pathogenic (LPAI) to highly pathogenic (HPAI) and the signs vary widely from mild clinical illness including inappetance, diarrhea, decreased egg production, and mild respiratory disease for LPAI, to severe respiratory, gastrointestinal and neurologic signs and significant mortality with HPAI.  Rapid diagnostic methods such as agar gel immunodiffusion (AGID) are generally preferred over classic diagnostic methods such as virus isolation. rtPCR is also available.  Further classification is generally performed by the National Veterinary Services Laboratory (NVSL) in Ames, Iowa. No specific treatment exists for AI in birds.  Depopulation is generally performed for outbreaks of disease.  Vaccination is generally not practiced in the United States and is tightly controlled by governmental agencies.


Viscerotropic velogenic (exotic) Newcastle disease (vvND; END)

As with avian influenza, exotic Newcastle disease (END) is unlikely to be seen in the backyard poultry flock, but the veterinarian should be familiar with this reportable and zoonotic disease.  California has experienced several outbreaks of viscerotropic velogenic Newcastle disease (vvND) in fighting cocks and small household flocks in the last several decades.  Unlike in other parts of the world, vvND is not endemic, and so is also known as exotic Newcastle disease (END).  The causative agent is a paramyxovirus, which is generally spread through respiratory secretions, ingestion, or by fomites. Newcastle disease types are referred to as lentogenic, mesogenic, or velogenic.  The mildest form (lentogenic) is associated with mild respiratory signs and mild generalized illness.  The mesogenic form is also associated with respiratory disease and is also associated with a drop in egg production with occasional neurologic signs but low mortality.  For the most severe form (vvND), neurologic signs (eg. torticollis, paresis, paralysis) may be seen along with more severe respiratory signs.  Mortality rates for vvND are high.  Diagnostic testing for this reportable disease (eg. rtPCR) is usually performed by laboratories in the National Animal Health Laboratory Network (NAHLN), although further classification is generally performed at the NVSL.  Vaccination is considered unnecessary for backyard flocks unless there is a known exposure in the area. Exposed and infected flocks are generally depopulated.

Selected bacterial diseases


There are numerous mycoplasmas capable of infecting poultry species and not all cause disease. Infections with Mycoplasma gallisepticum (MG) are typically silent in chickens, although coinfection with Escherichia coli can result in Chronic Respiratory Disease (CRD) in chickens.  MG causes infectious sinusitis in turkeys. MG is a reportable disease in many states.  Transmission is transovarial or through aerosol or fomites.  Culture is the gold standard for diagnosis, although PCR and serology can be used.  Although tylosin and tetracyclines can be effective at diminishing clinical signs, no antibiotic completely clears infection.  Prevention is preferable, through purchasing replacement birds only from MG free sources and through quarantine.

Infectious coryza

Infectious coryza is common in the southern United States but rare in the Midwest.  It is caused by Avibacterium (Hemophilus) paragallinarum, a Gram negative rod.  Chickens, pheasants, and guinea fowl are susceptible.  Transmission is through aerosol, ingestion, or fomites.  Clinical signs include unexpected mortality or severe respiratory signs such as oculonasal discharge, facial edema, or swollen infraorbital sinuses.  Diagnosis is generally through culture on appropriate media.  Although treatment may be attempted with antibiotics such as sulfonamides, tetracycline, or erythromycin, depopulation followed by disinfection and restocking are often performed since recovered birds are lifetime carriers.  A vaccine is available but seldom used for backyard flocks.

Pullorum and other Salmonella-associated diseases

Infection with Salmonella pullorum is associated with white diarrhea and high mortality in young birds, whereas adults are generally asymptomatic carriers.  The disease is primarily egg-transmitted.  Infected chicks that do not die can produce infected eggs at sexual maturity.  Septic arthritis has been seen in some birds.  Cecal plugs observed at necropsy are characteristic of the disease.  Diagnostic options include culture and the whole blood plate test.  Affected flocks are generally culled under supervision of state regulatory agencies.  Other Salmonella species can infect poultry and are often associated with gastrointestinal signs and signs of generalized illness, such as S. gallinarum (fowl typhoid), and paratyphoid Salmonella (paratyphoid).  Pullorum and fowl typhoid have been nearly fully eradicated in the United States and are reportable diseases in many states. 

Selected parasitic diseases


Coccidiosis is common in backyard chickens and is caused by infection with the protozoal parasite Eimeria spp.  Eimeria spp. are species-specific.  There are numerous species of Eimeria (at least 9 in chickens), with cecal and intestinal forms.  Most outbreaks involve infection with two or more species. Infection with Eimeria tenella (a cecal parasite) is associated with hematochezia and a high morbidity and mortality.  Clinical signs include pale combs and wattles, lethargy, and shivering.  Diagnosis is made through fecal examination and by histopathology.  To control coccidiosis in a flock, coccidiocidal drugs or coccidiostats should be used on a semi-annual rotational basis.  Control measures include using deep litter and salting the coop floor.  Coccidiostats such as amprolium and sulfamethazine should be added to the feed for chicks less than 16 weeks of age. 


Infection with Ascaridia galli causes a common roundworm infection in chickens and turkeys.  The nematode resides in the upper small intestines.  Heavier worm burdens are more likely in birds with concurrent illness and a high environmental load.  Birds less than 12 weeks of age are most susceptible.  Signs include generalized illness, decreased egg production, diarrhea, and occasionally intestinal obstruction.  Diagnosis is through fecal examination.  Piperazine is the drug of choice, although it is somewhat weak and not intended for use in layers.  Fenbendazole can be used off-label.  Using litter at least 4 to 6” deep and frequent cleanings reduce exposure.


Selected non-infectious diseases


Rickets is a nutritional disease that primarily affects young, growing birds.   It is associated with inadequate dietary calcium, an inverse Ca:P ratio, inadequate dietary vitamin D3, or excessive dietary protein.  Nutritional deficiencies can be caused by concurrent illnesses such as enteritis and nephritis.  Clinical signs include lameness, gait abnormalities, ataxia, and lethargy.  At necropsy, metaphyseal flaring (thickened growth plate) of long bones, rotational limb deformities, pathologic fractures, and perosis (slipped tendon) are often seen. Beak deformities and soft beaks are also seen.  The disease is reversible if caught early.  Hobbles, boots, and splints can be used for early, mild limb deformities.  Surgical correction may be necessary for more severe deformities, but the prognosis is guarded at best for complete return to normal function.


Trauma is very common in backyard poultry, either through predator attacks (eg. dog, raccoon, coyote, owl), persecution (attacks from conspecifics), and vehicular injuries. With dog attacks, there are often bite marks or scratches on the back and sides with associated feather loss.  Birds are frequently presented in hypovolemic shock from blood loss and dehydration.  Treatment is aimed at restoring perfusion with fluid therapy and through antibiotic and analgesic treatments and through the repair of damaged soft tissues and bone.  Wound therapy techniques described for other avian species can be used in poultry species.

Reproductive disorders

Reproductive disorders such as oviductal impaction, egg binding, dystocia, and yolk peritonitis are very common in female poultry seen in clinical practice. It is believed that the reproductive tract weakens with advancing age and allows excessive bacteria to enter the oviduct passively or through retrograde peristalsis.  E. coli is often incriminated with reproductive tract infections, although other bacteria such as Salmonella spp. have been isolated.  If fluid is present in the intestinal peritoneal cavity, or if excessive egg products have accumulated in the oviduct or peritoneal cavity, there can be a sagged appearance or a base-wide stance and enlargement of the belly with or without a palpable mass.  Affected birds may be lethargic or inappetant and may be septic.  Diagnosis can be made through digital cloacal exam, survey radiographs, ultrasound, and abdominocentesis.  Excessive fluid can be drained and birds can be managed with appropriate antibiotics and analgesics.  However, exploratory surgery and removal of retained egg products with or without salpingohysterectomy or partial oophorectomy may need to be considered for more advanced cases.  Other reproductive diseases commonly seen in hens include ovarian and oviductal carcinoma and cystic persistent right oviduct. Cystic persistent right oviduct can be treated surgically.

Suggested reading

Beam A, Garber L, Sakugawa J, et al. Salmonella awareness and related management practices in US urban backyard chicken flocks. Prev Vet Med 2013;110(3):481–488.

Behravesh CB, Brinson D, Hopkins BA, et al. Backyard poultry flocks and salmonellosis: A recurring, yet preventable public health challenge. Clin Infect Dis 2014;58:1432–1438.

Garber L, Hill G, Rodriguez J, et al. Non-commercial poultry industries: Surveys of backyard and gamefowl breeder flocks in the United States. Prev Vet Med 2007;80(2):120-128.

Greenacre CB, Morishita TY (eds). Backyard Poultry Medicine and Surgery: A Guide for Veterinary Practitioners.  Wiley Blackwell, Ames, IA, 2015.

Greenacre CB.  Backyard poultry: A crash course so you are prepared.  Proc Assoc Avian Vet 2013:3–12.

Grunkemeyer VL. Zoonoses, public health, and the backyard poultry flock. Vet Clin No Amer Exotic Anim Pract 2011;14(3):477–490.

Madsen JM, Zimmermann NG, Timmons J, et al. Prevalence and differentiation of diseases in Maryland backyard flocks. Avian Dis 2013;57(3):587–594.

Mete A, Giannitti F, Barr B, et al. Causes of mortality in backyard chickens in northern California: 2007-2011. Avian Dis 2013;57(2):311–315.

Pollock SL, Stephen C, Skuridina N, et al. Raising chickens in city backyards: The public health role. J Commun Health 2012;37(3):734–742.

Smith EI, Reif JS, Hill AE, et al. Epidemiologic characterization of Colorado backyard bird flocks. Avian Dis 2012;56(2):263–271.

Whitehead ML, Roberts V. Backyard poultry: Legislation, zoonoses and disease prevention. J Small Anim Pract 2014;55(10):487–496.

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