Avian Encephalomyelitis

Avian encephalomyelitis (AE) is a highly contagious, neurotropic viral disease affecting chickens, turkeys, pheasants, and quail, caused by the avian encephalomyelitis virus (AEV), a picornavirus. The disease is characterized by neurological signs such as ataxia, tremors, and paralysis, particularly in young chicks, and a transient egg production drop in laying hens. In commercial poultry, AE is primarily controlled by vaccination of breeders to provide maternal immunity to progeny. The disease is of global economic importance due to mortality in young flocks and egg production losses in layers. The virus is transmitted both vertically (transovarian) and horizontally via the fecal-oral route. Clinical disease is most severe in chicks under 3 weeks of age, with morbidity up to 60% and mortality up to 25% in susceptible flocks. In adult layers, the infection is often subclinical but causes a characteristic drop in egg production of 5-20% over 1-2 weeks, with a subsequent return to normal. The disease is also known as epidemic tremor, and it is a notifiable disease in some countries, requiring strict biosecurity and vaccination strategies.

The causative agent is Avian encephalomyelitis virus (AEV), a non-enveloped, single-stranded positive-sense RNA virus belonging to the genus Tremovirus in the family Picornaviridae. The virus is approximately 24-30 nm in diameter, with an icosahedral capsid. It is highly resistant to environmental conditions, including heat (stable at 56°C for 60 minutes), pH extremes (stable at pH 3.0), and many common disinfectants, though it is inactivated by 1% formalin, 5% phenol, and 0.5% sodium hydroxide. There is only one serotype of AEV, but strains vary in pathogenicity and tissue tropism. The virus replicates in the cytoplasm of neurons and glial cells, causing neuronal necrosis and perivascular cuffing in the central nervous system. It also replicates in the pancreas, proventriculus, and intestinal epithelium, and is shed in feces. The virus can be isolated in embryonated chicken eggs via yolk sac inoculation, causing embryo mortality and muscular dystrophy. Molecular detection is achieved by RT-PCR targeting the VP1 gene. The virus is not zoonotic.

Avian encephalomyelitis affects primarily chickens, but also turkeys, pheasants, and quail. In commercial poultry, the disease is most commonly seen in broiler breeders and layer flocks, where vaccination is practiced. The virus is transmitted both vertically and horizontally. Vertical transmission occurs when hens are infected during the laying period, leading to transovarian transmission to embryos, which may hatch with clinical signs or die in the shell. Horizontal transmission occurs via the fecal-oral route, with the virus shed in feces for up to 5 weeks post-infection. The virus can also be spread mechanically via contaminated equipment, footwear, and personnel. The disease is more prevalent in areas with high poultry density and poor biosecurity. In broilers, clinical disease is rare due to maternal immunity, but in unvaccinated flocks, outbreaks can occur in chicks under 3 weeks of age. In layers, the disease is often subclinical, but a drop in egg production of 5-20% is observed, with a reduction in hatchability and an increase in culls. The morbidity rate in young chicks can reach 60%, with mortality up to 25%. The disease has a seasonal pattern, with more outbreaks in the spring and fall, possibly due to environmental stress. Wild birds can act as reservoirs, but their role in transmission is limited. The economic impact includes mortality, reduced egg production, and vaccination costs.

The pathogenesis of avian encephalomyelitis begins with oral or respiratory entry of the virus, followed by primary replication in the intestinal epithelium and associated lymphoid tissues, including the tonsils of the cecum and Peyer's patches. The virus then spreads via the bloodstream to the central nervous system (CNS) and other organs, including the pancreas, proventriculus, and reproductive tract. In the CNS, the virus replicates in neurons and glial cells, causing neuronal degeneration, necrosis, and perivascular lymphocytic cuffing. The characteristic microscopic lesions include central chromatolysis of neurons, gliosis, and perivascular cuffing in the brain and spinal cord. The virus also causes a lymphocytic infiltration in the pancreas and proventriculus. In young chicks, the virus causes a severe encephalomyelitis, leading to ataxia, tremors, and paralysis. In adult hens, the virus replicates in the reproductive tract, causing a transient drop in egg production and the production of abnormal eggs. The virus is shed in feces for up to 5 weeks, and vertical transmission occurs when hens are viremic during egg formation. The immune response involves both humoral and cell-mediated immunity, with maternal antibodies providing protection to chicks for the first 3-4 weeks of life.

Several factors predispose flocks to avian encephalomyelitis. Intrinsic factors include age, with chicks under 3 weeks of age being most susceptible to clinical disease; genetic susceptibility varies among breeds, with some lines being more resistant. Immune status is critical: flocks with low or waning maternal immunity are at higher risk. High production stress in layers can exacerbate the effects of infection. Extrinsic factors include poor biosecurity, allowing introduction of the virus via contaminated equipment, personnel, or wild birds. High stocking density and poor ventilation increase the risk of horizontal transmission. Inadequate cleaning and disinfection of poultry houses can leave residual virus. Vaccination failures, due to improper administration or immunosuppression, can leave flocks susceptible. Seasonal factors, such as temperature fluctuations, may increase stress and susceptibility. Additionally, concurrent infections with immunosuppressive agents like infectious bursal disease virus (IBDV) or chicken anemia virus (CAV) can exacerbate the severity of AE.

Clinical signs of avian encephalomyelitis vary with age. In chicks under 3 weeks of age, the disease is characterized by neurological signs: ataxia, tremors of the head and neck, and progressive paralysis. Affected chicks may sit on their hocks, and some may become recumbent. The tremors are often more pronounced when the chick is excited or handled. Morbidity can reach 60%, and mortality up to 25%. In growing pullets and adult layers, the infection is often subclinical, but a transient drop in egg production of 5-20% may occur, lasting 1-2 weeks. Egg quality may be affected, with an increase in misshapen, soft-shelled, or shell-less eggs. Hatchability may also decrease. In broiler breeders, a similar drop in egg production and hatchability is observed. In turkeys, the signs are similar but may be less severe. In pheasants and quail, the disease is rare but can cause neurological signs. The clinical signs are often more severe in chicks with poor maternal immunity. The disease is often self-limiting, with recovery in 2-3 weeks, but some birds may have permanent neurological damage.

Differential diagnoses for avian encephalomyelitis include: 1) Newcastle disease (ND) - caused by avian paramyxovirus type 1, which presents with respiratory signs, neurological signs, and gastrointestinal lesions; ND can be differentiated by the presence of respiratory distress, greenish diarrhea, and hemorrhagic lesions in the proventriculus and intestines, and confirmed by virus isolation and HI test. 2) Marek's disease (MD) - caused by a herpesvirus, which causes paralysis, particularly of the legs and wings, and visceral tumors; MD is differentiated by the presence of lymphoid tumors in nerves and viscera, and by histopathology showing pleomorphic lymphoid infiltration. 3) Infectious bursal disease (IBD) - caused by a birnavirus, which causes immunosuppression and bursal lesions; IBD is differentiated by the severe bursal atrophy and hemorrhages, and by the absence of neurological signs. 4) Avian influenza (AI) - caused by influenza A viruses, which can cause neurological signs, but also respiratory signs, edema, and cyanosis; AI is differentiated by the presence of facial edema, comb cyanosis, and high mortality, and confirmed by virus isolation and RT-PCR. 5) Vitamin E deficiency - causes encephalomalacia in chicks, with ataxia and tremors, but is differentiated by the absence of egg production drop and by the presence of cerebellar lesions on necropsy. 6) Bacterial meningitis - caused by E. coli, Salmonella, or Pasteurella, which can cause neurological signs, but is differentiated by the presence of systemic signs, such as septicemia, and by bacterial culture. 7) Toxicosis - such as lead or organophosphate poisoning, which can cause neurological signs, but is differentiated by history of exposure and absence of egg production drop. 8) Mycoplasma synoviae - can cause joint swelling and respiratory signs, but not typically neurological signs. 9) Avian reovirus - can cause arthritis and tenosynovitis, but not typically neurological signs. 10) Nutritional deficiencies, such as riboflavin deficiency, can cause curled toe paralysis, but is differentiated by the absence of egg production drop and by response to vitamin supplementation.

The diagnostic algorithm for avian encephalomyelitis begins with a thorough flock history, including vaccination status, age, and clinical signs. If neurological signs are present in young chicks, and there is a history of egg production drop in breeders, AE should be suspected. The next step is a detailed clinical examination, noting the presence of tremors, ataxia, and paralysis. A gross necropsy should be performed on affected birds, looking for characteristic lesions, which are often absent or minimal, but may include mild splenomegaly and proventriculitis. The definitive diagnosis is made by histopathology, which shows neuronal degeneration, perivascular cuffing, and gliosis in the brain and spinal cord. For virus isolation, brain tissue or feces can be inoculated into embryonated chicken eggs via the yolk sac route, and the embryos are examined for lesions. Molecular diagnosis is by RT-PCR on brain tissue or feces, which is rapid and sensitive. Serology can be performed using ELISA or virus neutralization tests, but a single high titer is not diagnostic; a rising titer in paired samples is more indicative. The diagnostic algorithm should also include differential testing for other causes of neurological signs, such as Newcastle disease, Marek's disease, and vitamin E deficiency. The final diagnosis is based on a combination of clinical signs, histopathology, and laboratory confirmation.

Laboratory findings in avian encephalomyelitis include: Serology: ELISA is commonly used to detect antibodies to AEV. A positive result indicates exposure, but not necessarily active infection. A four-fold rise in antibody titer between acute and convalescent sera is diagnostic. Virus neutralization tests are also available but are less commonly used. Molecular diagnostics: RT-PCR is the preferred method for detecting viral RNA in brain tissue, feces, or embryonated eggs. It is highly sensitive and specific. Virus isolation: The virus can be isolated by inoculating brain homogenate into embryonated chicken eggs via the yolk sac route. Embryos die within 7-10 days, and lesions include muscular dystrophy and stunting. Histopathology: Microscopic examination of brain and spinal cord reveals neuronal degeneration, central chromatolysis, perivascular lymphocytic cuffing, and gliosis. These lesions are pathognomonic. Hematology: There are no specific blood changes, but leukocytosis may be present. Blood chemistry: No specific changes are noted. Coccidiosis lesion scoring: Not applicable. Mycotoxin feed assays: Not applicable. The laboratory findings are essential for confirming the diagnosis and differentiating from other neurological diseases.

Imaging is not commonly used in the diagnosis of avian encephalomyelitis. Radiography may be used to rule out skeletal abnormalities, such as rickets or femoral head necrosis, which can cause similar clinical signs. However, radiographs are typically normal in AE. Ultrasonography is not used in avian practice. Gross necropsy photography is useful for documenting the absence of gross lesions, which is characteristic of AE. The main imaging hallmark is the lack of gross lesions, with histopathology being the definitive diagnostic tool.

Gross necropsy findings in avian encephalomyelitis are often minimal or absent. In some cases, there may be mild splenomegaly, proventriculitis, or pancreatic lesions. The most significant diagnostic findings are microscopic. Histopathology of the brain and spinal cord reveals neuronal degeneration, with central chromatolysis and necrosis. There is perivascular lymphocytic cuffing, particularly in the brainstem and spinal cord. Gliosis is also present. In the pancreas, there is lymphocytic infiltration and necrosis of acinar cells. In the proventriculus, there is lymphocytic infiltration in the mucosa. These lesions are pathognomonic for AE. The presence of intranuclear or intracytoplasmic inclusion bodies is not typical. The histopathological examination is essential for confirming the diagnosis.

There is no specific antiviral treatment for avian encephalomyelitis. Management is supportive and focuses on reducing stress and preventing secondary infections. In young chicks, providing a warm, dry environment and easy access to feed and water is important. Antibiotics may be administered in the water or feed to prevent secondary bacterial infections, such as colibacillosis. Common antibiotics include amoxicillin at 20 mg/kg body weight, or oxytetracycline at 10-20 mg/kg, administered in drinking water for 3-5 days. Supportive therapy with vitamins and electrolytes may be beneficial. In laying flocks, the egg production drop is transient, and no treatment is necessary. Vaccination is the primary control measure. Live vaccines are available for administration to pullets before the onset of egg production, typically at 8-16 weeks of age, via drinking water or spray. Inactivated vaccines are also available for use in breeders. In the face of an outbreak, vaccination of in-contact flocks may be considered, but it is not effective in already infected birds. Biosecurity measures should be enhanced to prevent spread. In severe outbreaks, depopulation may be considered, especially in young chicks with high mortality.

The prognosis for avian encephalomyelitis depends on the age of the flock and the severity of the outbreak. In young chicks, the prognosis is guarded, with mortality up to 25%. Survivors may have permanent neurological damage, leading to culling. In adult layers, the prognosis is good, with egg production returning to normal within 2-3 weeks. The drop in egg production is temporary, and there is no long-term effect on egg quality or production. In broiler breeders, the prognosis is also good, but hatchability may be affected temporarily. The overall flock recovery rate is high, but the economic impact can be significant due to mortality, reduced egg production, and vaccination costs. In severe cases, depopulation may be necessary, which has a poor prognosis for the flock.

After an outbreak of avian encephalomyelitis, it is important to monitor the flock for recovery and to prevent recurrence. A structured follow-up schedule should include: 1) Daily observation of clinical signs, especially in young chicks, for at least 2 weeks after the last case. 2) Serological monitoring: collect serum samples from a representative number of birds (e.g., 20-30) at 2-4 weeks post-outbreak to assess antibody titers. A rising titer indicates active infection. 3) Egg production monitoring: record daily egg production and egg quality for at least 4 weeks after the outbreak to ensure return to normal. 4) Cleaning and disinfection: thoroughly clean and disinfect the poultry house and equipment with a disinfectant effective against AEV, such as 1% formalin or 5% phenol. Allow adequate downtime (at least 2 weeks) before restocking. 5) Litter management: remove and dispose of litter properly, and replace with fresh litter. 6) Vaccination: review the vaccination program and ensure that all replacement pullets are vaccinated against AE. 7) Biosecurity audit: assess and improve biosecurity measures to prevent re-introduction of the virus. 8) Necropsy monitoring: perform necropsies on any birds that die to ensure no other diseases are present.

Clinical pearls: 1) In young chicks, the presence of tremors and ataxia is highly suggestive of AE, especially if there is a history of egg production drop in the parent flock. 2) The absence of gross lesions at necropsy is typical; histopathology is essential for diagnosis. 3) Maternal immunity is protective, so vaccination of breeders is key to preventing disease in chicks. 4) The egg production drop in layers is transient and often goes unnoticed if not monitored closely. 5) The virus is highly resistant, so thorough cleaning and disinfection are critical. Pitfalls: 1) Confusing AE with Newcastle disease, which has similar neurological signs but also respiratory signs and gastrointestinal lesions. 2) Failing to collect brain tissue for histopathology, leading to a missed diagnosis. 3) Relying solely on serology, as a single high titer is not diagnostic. 4) Vaccinating chicks with live vaccine during an outbreak, which can cause disease. 5) Not considering AE in the differential diagnosis of egg production drops in layers, leading to misdiagnosis as management or nutritional issues.

There is no specific antiviral treatment for avian encephalomyelitis. Supportive care includes: 1) Antibiotics to prevent secondary bacterial infections: Amoxicillin trihydrate at 20 mg/kg body weight per day in drinking water for 3-5 days, or Oxytetracycline hydrochloride at 10-20 mg/kg body weight per day in drinking water for 3-5 days. 2) Vitamins and electrolytes: Vitamin E at 100-200 IU/L of drinking water, and a commercial electrolyte supplement at label dose for 3-5 days. 3) Vaccination: Live AE vaccine (e.g., AE-Pox vaccine) is administered to pullets at 8-16 weeks of age via drinking water or spray. The vaccine is a live, attenuated virus that provides lifelong immunity. Inactivated vaccines are also available for use in breeders, administered subcutaneously or intramuscularly at 16-20 weeks of age. The withdrawal time for antibiotics is typically 0-5 days, depending on the drug. It is important to follow label instructions and local regulations. In the face of an outbreak, vaccination of in-contact flocks may be considered, but it is not effective in already infected birds.

Landmark studies on avian encephalomyelitis include: 1) The original description by Jones in 1932, which characterized the disease as 'epidemic tremor' in chicks. 2) Studies by Van Roekel et al. (1938) demonstrated the viral etiology and transmissibility. 3) Research by Calnek and colleagues in the 1950s and 1960s established the pathogenesis, including vertical transmission and the role of maternal immunity. 4) Vaccine development: Live vaccines were developed in the 1960s and have been widely used to control the disease. 5) Molecular characterization: The complete genome of AEV was sequenced in the 1990s, leading to the development of RT-PCR diagnostics. 6) A study by Tannock et al. (1983) evaluated the efficacy of live vaccines in broiler breeders, showing a significant reduction in clinical disease in progeny. 7) A field study by Spradbrow (1975) demonstrated the economic impact of AE in layer flocks, with a 10-20% drop in egg production. 8) A review by Swayne et al. (2013) in Diseases of Poultry provides a comprehensive overview of AE, including epidemiology, diagnosis, and control. 9) The AAAP Avian Disease Manual (2016) provides practical guidelines for diagnosis and management. 10) WOAH/OIE Terrestrial Manual (2018) includes AE as a notifiable disease and provides standardized diagnostic protocols. These studies and guidelines form the basis for current understanding and control of avian encephalomyelitis.