Lymphoid Leukosis

Lymphoid Leukosis is a neoplastic disease of chickens caused by avian leukosis virus, a retrovirus that induces the formation of lymphoid tumors primarily in the liver, spleen, and bursa of Fabricius. This economically significant disease occurs worldwide and represents one of the most important tumor-inducing diseases in commercial poultry, causing significant losses through mortality, condemnation of carcasses at processing, and reduced production performance. The disease typically affects chickens older than sixteen weeks of age, with peak incidence occurring in adult birds, distinguishing it from similar tumor diseases such as Marek's disease which tends to affect younger birds. All breeds and types of chickens are susceptible, though some genetic lines show varying degrees of resistance to tumor development.

The causative agent of Lymphoid Leukosis is Avian Leukosis Virus, a member of the Alpharetrovirus genus within the family Retroviridae. Several subgroups of ALV exist, designated A through J, with subgroups A and B being the most commonly associated with classical lymphoid leukosis in chickens. The virus can be transmitted both horizontally through direct contact with infected birds or contaminated material and vertically from infected hens to their offspring through the egg. Congenital infection through vertical transmission is particularly significant because chicks infected in this way become immunologically tolerant to the virus and shed large amounts of virus throughout their lives, serving as efficient sources of infection for flockmates.

The impact of Lymphoid Leukosis on poultry health and economics extends beyond the direct mortality caused by tumor development. Infected birds that do not develop tumors often show subclinical effects including reduced growth rates, decreased egg production, and impaired feed conversion efficiency. The immunosuppressive effects of ALV infection can increase susceptibility to other infectious diseases and reduce the effectiveness of vaccinations. At processing, birds with tumors are condemned, representing a direct economic loss. The presence of ALV infection in breeding flocks can lead to widespread dissemination of the virus to commercial progeny through vertical transmission, amplifying the economic impact across the industry.

There is no treatment available for Lymphoid Leukosis, and once tumors develop, the disease is invariably fatal. Prevention through eradication of the virus from breeding flocks represents the only effective control strategy, achieved through testing and removal of virus-shedding birds to break the cycle of vertical transmission. Understanding the epidemiology and transmission patterns of ALV is essential for implementing effective control programs. Working closely with an avian veterinarian and participating in coordinated industry eradication programs provides the best approach to eliminating this costly disease from poultry operations.

The primary cause of Lymphoid Leukosis is infection with Avian Leukosis Virus, a member of the Alpharetrovirus genus belonging to the family Retroviridae. This enveloped RNA virus contains reverse transcriptase, which allows it to convert its RNA genome into DNA that integrates into the host cell genome, establishing permanent infection. Multiple subgroups of ALV are recognized based on envelope glycoprotein characteristics and receptor specificity, with subgroups A, B, C, D, E, and J being the most relevant to poultry. Subgroups A and B are classically associated with lymphoid leukosis, while subgroup J primarily causes myeloid leukosis. Subgroup E viruses are endogenous, meaning their genetic material is integrated into the chicken genome and inherited genetically, though these are generally non-pathogenic.

Genetic factors influence susceptibility to Avian Leukosis Virus infection and tumor development in complex ways. Some chicken lines have been bred for resistance to specific ALV subgroups through selection for specific receptor variants that do not allow virus entry. The presence of endogenous ALV sequences in the chicken genome can provide some degree of interference immunity against related exogenous viruses. However, recombination between endogenous and exogenous viruses can generate new pathogenic variants. Certain genetic lines show increased susceptibility to tumor development even when infected with the same virus strain, suggesting that host genetic factors influence the oncogenic process beyond simple susceptibility to infection.

Transmission of Avian Leukosis Virus occurs through both vertical and horizontal routes, with vertical transmission being particularly important for the epidemiology and control of the disease. Vertical transmission occurs when infected hens shed virus into developing eggs, infecting the embryo before or during egg formation. Chicks infected congenitally develop immunological tolerance to the virus and become persistently viremic, shedding large quantities of virus throughout their lives. Horizontal transmission occurs through direct contact with infected birds or exposure to contaminated feces, feathers, and other materials containing virus. Close confinement of birds in commercial poultry housing facilitates horizontal spread. Mechanical transmission through contaminated needles, equipment, or personnel may also occur.

Several risk factors increase the likelihood and impact of Lymphoid Leukosis in poultry flocks. Presence of ALV infection in breeding flocks leads to vertical transmission and widespread dissemination of the virus to commercial progeny. Mixed-age production systems where infected older birds contact susceptible younger birds facilitate ongoing horizontal transmission. Lack of testing and monitoring programs allows infected birds to remain in flocks and continue transmitting virus. Stress from poor management, concurrent infections, or environmental factors may increase the likelihood of tumor development in infected birds. The long incubation period means that tumors may not appear until birds are in production, when losses have the greatest economic impact.

The mechanism by which Avian Leukosis Virus causes lymphoid tumors involves a multi-step process of oncogenesis. After infection, ALV integrates its proviral DNA into the host cell genome near cellular proto-oncogenes. In lymphoid leukosis, the most common site of integration is near the c-myc gene, a cellular gene involved in regulating cell growth and proliferation. Insertional activation of c-myc leads to uncontrolled expression of this growth-promoting gene, driving abnormal proliferation of B-lymphocytes. This process takes months to years, explaining the typical onset of tumors in adult birds. The tumors that develop are monoclonal, arising from a single transformed cell that has undergone the critical insertional mutation, and consist of proliferating B-lymphocytes that infiltrate and enlarge the affected organs.

The early warning signs of Lymphoid Leukosis are often subtle and easily overlooked because the disease develops slowly over many months. Infected birds may show gradual, progressive decline in condition before any specific signs become apparent. Slight paleness of the comb and wattles may be noticed as anemia develops from tumor infiltration of the liver and spleen. A gradual decrease in egg production in laying hens may precede more obvious clinical signs by weeks or months. Birds may seem slightly less active than their flockmates but continue to eat and behave relatively normally. Because birds instinctively hide signs of illness, these early changes can be extremely difficult to detect in a large flock setting.

As Lymphoid Leukosis progresses, common symptoms become more apparent and reflect the expanding tumors within the body cavity. Affected birds develop progressive weakness and lethargy as tumor burden increases. Abdominal enlargement may become visible as the liver and spleen enlarge massively with tumor infiltration. The comb and wattles become increasingly pale as anemia worsens. Weight loss occurs despite continued eating, as metabolic demands of the growing tumors outpace nutritional intake. Birds may develop a characteristic hunched posture and reluctance to move. Diarrhea with pasty vent feathers may develop as gastrointestinal function is compromised. Egg production ceases completely in laying hens with advanced disease.

Behavioral changes associated with Lymphoid Leukosis reflect the progressive systemic illness caused by tumor growth. Affected birds become increasingly withdrawn and spend more time sitting or lying down. Normal activities such as foraging, dust bathing, and social interaction decrease as the bird conserves energy. Appetite may remain relatively normal initially but decreases as the disease advances. Affected birds often separate themselves from the flock and seek quiet, protected areas. Vocalization decreases, and birds become less responsive to environmental stimuli. In laying flocks, affected hens may continue to enter nest boxes but produce progressively smaller, abnormal eggs or cease laying entirely.

Physical signs visible on examination of birds with Lymphoid Leukosis provide important diagnostic information. The abdomen feels enlarged and firm on palpation due to massive hepatomegaly and splenomegaly from tumor infiltration. The liver may be palpable extending beyond its normal boundaries in the abdomen. Mucous membranes and the comb and wattles appear pale due to anemia. Birds often appear thin despite the abdominal enlargement, having lost muscle mass while tumors have grown. Feathers may appear dull and unkempt due to decreased preening activity. On postmortem examination, the liver and spleen are dramatically enlarged with diffuse or nodular gray-white tumor tissue, and the bursa of Fabricius may also be enlarged and infiltrated with tumor.

The progression of symptoms in Lymphoid Leukosis typically follows a gradual but relentless course over weeks to months once clinical signs appear. Initial vague signs of poor condition progress to obvious illness as tumor burden increases. The rate of progression varies among individual birds depending on the extent and location of tumor development. Some birds may die suddenly when massive hepatomegaly leads to liver rupture and internal bleeding. Others slowly deteriorate over many weeks before dying or being found in a moribund state. Mortality in affected flocks tends to be ongoing and cumulative rather than occurring as a sudden outbreak, with losses accumulating throughout the production cycle.

Certain symptoms warrant immediate veterinary attention to confirm the diagnosis and implement appropriate control measures. Finding multiple birds with enlarged abdomens and pale combs in a flock over sixteen weeks of age should prompt investigation for Lymphoid Leukosis. Sudden death in birds that appeared relatively normal, potentially from liver rupture, warrants postmortem examination. Ongoing, gradual mortality in a laying flock with no obvious acute cause should trigger diagnostic workup. Any condemnations at processing due to liver or visceral tumors should be reported to the flock veterinarian for investigation. Early diagnosis allows implementation of control measures to reduce losses and prevent spread to other flocks.

The initial examination for suspected Lymphoid Leukosis involves gathering comprehensive flock history and performing clinical evaluation of affected birds. An avian veterinarian will inquire about the age of affected birds, the timeline and pattern of losses, whether losses have been gradual or sudden, and the production history of the flock. Physical examination focuses on abdominal palpation to assess liver and spleen size, evaluation of mucous membrane color for evidence of anemia, and assessment of overall body condition. The age distribution of affected birds is particularly important, as lymphoid leukosis typically affects birds over sixteen weeks old, helping differentiate it from Marek's disease which more commonly affects younger birds. History of ALV status in the source flock or breeding stock is also relevant.

Diagnostic testing for Lymphoid Leukosis utilizes postmortem examination, histopathology, and laboratory detection of the virus. Gross postmortem examination reveals characteristic enlargement of the liver and spleen with diffuse or nodular gray-white tumor tissue replacing normal organ parenchyma. The bursa of Fabricius may be enlarged and infiltrated with tumor, a finding that helps distinguish lymphoid leukosis from Marek's disease in which the bursa is typically atrophied. Histopathological examination shows proliferation of neoplastic B-lymphocytes infiltrating affected organs. ELISA testing can detect ALV group-specific antigen in serum, cloacal swabs, or egg albumin, identifying virus-shedding birds. PCR testing can detect and type the specific subgroup of ALV involved. Virus isolation in cell culture can confirm the presence of infectious virus.

Differential diagnosis is essential because several tumor diseases of chickens produce similar clinical signs and gross lesions. Marek's disease, caused by a herpesvirus, is the most important differential and produces lymphoid tumors primarily in peripheral nerves, skin, and visceral organs, typically in birds younger than sixteen weeks. Reticuloendotheliosis causes similar tumors but with different histological characteristics. Myeloid leukosis, often caused by ALV subgroup J, produces tumors of myeloid cells rather than lymphoid cells. The avian veterinarian considers the age of affected birds, specific gross and microscopic pathology findings, and laboratory test results to differentiate lymphoid leukosis from these other tumor conditions.

Confirmation of Lymphoid Leukosis diagnosis requires integration of clinical findings, pathology, and laboratory testing. The presence of B-cell lymphomas primarily in the liver, spleen, and bursa of Fabricius in birds over sixteen weeks of age is highly suggestive. Histological confirmation that the tumor cells are B-lymphocytes, often using immunohistochemistry to identify cell surface markers, supports the diagnosis. Detection of ALV antigen or genetic material in tissues or secretions confirms viral involvement. Identification of clonal integration of proviral DNA near the c-myc oncogene provides definitive evidence of ALV-induced lymphoid leukosis. The combination of compatible clinical presentation, characteristic pathology, and positive laboratory testing establishes the diagnosis with confidence.

There is no effective treatment for Lymphoid Leukosis, and birds that develop clinical disease with tumor formation will inevitably die. Emergency care focuses on humane management of affected individuals rather than attempted cure. Birds showing advanced clinical signs with severe abdominal enlargement, weakness, and pallor should be humanely euthanized to prevent further suffering. There are no antiviral medications effective against retroviruses in poultry, and chemotherapeutic agents used to treat lymphoid malignancies in mammals are not practical for use in poultry. The focus of management must therefore be on preventing new infections rather than treating affected individuals.

Medical management options for Lymphoid Leukosis are essentially nonexistent because the disease involves irreversible neoplastic transformation of cells. Once proviral integration and oncogene activation have occurred, the transformed cells will continue to proliferate regardless of any interventions. Supportive care to maintain comfort in mildly affected birds may be attempted in pet or valuable individual birds, but this only delays the inevitable outcome and does not alter disease progression. Ensuring adequate nutrition, hydration, and a comfortable environment may provide some palliation, but affected birds will continue to decline. Most affected birds should be humanely euthanized when the diagnosis is established to prevent suffering.

Surgical intervention is not applicable to Lymphoid Leukosis because the tumors are disseminated throughout multiple internal organs rather than localized masses that could be removed. The liver, spleen, and bursa of Fabricius are diffusely infiltrated with tumor cells, making surgical excision impossible. Even if individual tumor nodules could be removed, the underlying viral infection and neoplastic transformation of cells throughout the lymphoid system would continue. There is no surgical approach that can address the fundamental disease process of ALV-induced oncogenesis.

Supportive care for birds with Lymphoid Leukosis serves only to maintain comfort temporarily and cannot alter disease outcome. Providing easily accessible food and water reduces the energy expenditure required for affected birds to meet basic needs. Maintaining comfortable environmental temperatures reduces metabolic stress. Isolating affected birds from flock aggression protects them from injury and harassment. However, these measures provide only temporary palliation, and affected birds will continue to deteriorate. The ethical approach for most affected birds is humane euthanasia once the diagnosis is confirmed rather than prolonged supportive care that merely extends suffering.

Alternative and complementary treatments have no role in managing Lymphoid Leukosis because there is no intervention capable of reversing the neoplastic process or eliminating the viral infection. Immune-stimulating supplements cannot restore normal function to transformed tumor cells. Herbal or nutritional interventions cannot address the integrated proviral DNA or activated oncogenes driving tumor growth. Bird owners should be counseled that no alternative therapy can cure or meaningfully treat this disease, and pursuing unproven treatments only delays appropriate humane euthanasia.

Management decisions for flocks affected by Lymphoid Leukosis focus on minimizing losses through culling affected birds and preventing spread rather than treating individuals. Individual birds with confirmed or suspected lymphoid leukosis should be removed from the flock to reduce virus shedding. Flock-level interventions focus on identifying and removing virus-shedding birds to reduce horizontal transmission. Long-term control requires eliminating vertical transmission by testing and removing infected breeders. Consultation with an avian veterinarian and, for commercial operations, industry organizations helps develop comprehensive eradication strategies. The goal is preventing future cases through breaking transmission cycles rather than treating currently affected birds.

There is no recovery from clinical Lymphoid Leukosis with tumor development because the neoplastic transformation of cells is irreversible. Birds that develop tumors will die from the disease regardless of any interventions. The concept of recovery does not apply to this condition in the same way it might for acute infectious diseases. Instead, management focuses on preventing new infections and minimizing the number of birds that progress to clinical disease. Birds infected with ALV that have not yet developed tumors remain infected for life but may live out their normal lifespan if tumors do not develop, though they continue to pose a transmission risk to susceptible flockmates.

Post-diagnosis management in flocks where Lymphoid Leukosis has been identified focuses on damage control and prevention of further spread. Affected birds should be humanely euthanized rather than allowed to continue suffering. Virus-shedding birds identified through testing should be removed from the flock to reduce transmission pressure. Enhanced biosecurity measures should be implemented to prevent spread to other flocks on the premises. Careful evaluation of breeding stock sources should be conducted to prevent introduction of ALV-infected replacement birds. Comprehensive testing programs should be established to monitor flock status and identify infected individuals for removal.

Prognosis for individual birds with Lymphoid Leukosis that have developed tumors is uniformly fatal, with death occurring within weeks to months of clinical disease onset. Prognosis for infected birds that have not yet developed tumors is variable, as only a proportion of infected birds will progress to tumor formation. The risk of tumor development depends on factors including the infecting virus strain, the age at infection, host genetic factors, and environmental stressors. Congenitally infected tolerant birds have the highest risk of tumor development due to their persistent high-level viremia. Birds infected horizontally as adults may have lower tumor incidence if they mount effective immune responses.

The long-term outlook for flocks affected by Lymphoid Leukosis depends on successful implementation of control and eradication measures. Flocks with endemic ALV infection will continue to experience ongoing losses until the virus is eliminated. Eradication requires comprehensive testing of breeding stock and elimination of virus-shedding birds to break vertical transmission. For commercial operations sourcing birds from breeding companies, ensuring that suppliers maintain ALV-free status is critical. Complete elimination of ALV from a production system can be achieved but requires sustained commitment to testing and removal programs. Once eradication is achieved, strict biosecurity must be maintained to prevent reintroduction of the virus.

Environmental prevention of Lymphoid Leukosis involves biosecurity measures designed to prevent virus introduction and limit horizontal transmission within flocks. Thorough cleaning and disinfection of facilities between flocks helps eliminate residual virus in the environment. ALV is an enveloped virus and is readily inactivated by most common disinfectants. Controlling access to poultry facilities and requiring appropriate sanitation measures for personnel and equipment reduces mechanical transmission risk. Avoiding contact between birds of different ages and sources reduces opportunities for horizontal transmission. Proper needle hygiene during vaccinations and other injections prevents iatrogenic transmission of virus.

Eradication of ALV from breeding flocks is the cornerstone of Lymphoid Leukosis prevention and represents the only truly effective control strategy. Testing breeding birds for ALV shedding using ELISA detection of viral antigen in cloacal swabs or egg albumin identifies virus-positive individuals for removal. Repeated testing and culling progressively reduce the proportion of infected birds until eradication is achieved. Because vertically transmitted virus creates tolerant, persistently viremic birds that shed high levels of virus, breaking this cycle of vertical transmission is essential. Many primary breeding companies have achieved ALV-free status through sustained testing and eradication programs, providing clean breeding stock to the commercial industry.

Dietary and general health management to support robust immune function may reduce the likelihood of tumor development in infected birds, though this cannot prevent infection or eliminate the virus. Balanced nutrition meeting all requirements supports optimal immune function. Minimizing stress from environmental factors, social competition, and management practices may reduce factors that promote tumor development. Avoiding immunosuppressive agents and concurrent infections helps maintain immune surveillance that may limit tumor growth. However, these measures are secondary to preventing infection in the first place and cannot substitute for ALV eradication.

Health maintenance programs for Lymphoid Leukosis prevention focus on maintaining ALV-free status through ongoing surveillance and biosecurity. There is no vaccine available for preventing ALV infection or lymphoid leukosis development. Regular testing of flocks, particularly breeding stock, monitors for virus introduction and identifies any shedding birds for removal. Sourcing replacement birds only from certified ALV-free suppliers prevents introduction of infected birds. Maintaining detailed records of bird sources, test results, and any disease events facilitates epidemiological investigation if problems arise. Working with an avian veterinarian to design and implement appropriate surveillance programs ensures that monitoring efforts are effective.

Early detection and rapid response are essential for successful control when ALV is identified in a flock. Prompt investigation of any unexplained mortality or tumor condemnations at processing allows early diagnosis. Immediate testing of flockmates of affected birds identifies other infected individuals for removal. Quick implementation of enhanced biosecurity prevents spread to other susceptible flocks. Communication with bird suppliers and industry organizations facilitates tracing of infection sources and warnings to other potentially affected operations. The economic benefits of rapid response in limiting losses and preventing further spread far outweigh the costs of surveillance and testing programs.

Daily management of flocks with Lymphoid Leukosis focuses on surveillance for new cases and maintaining flock health while implementing control measures. Regular observation of birds for early signs of illness such as pallor, abdominal enlargement, or decreased activity allows prompt identification of affected individuals for removal. Monitoring production parameters including egg production, feed consumption, and mortality rates helps detect problems early. Maintaining optimal nutrition and environmental conditions supports overall flock health despite the presence of infection. Ensuring adequate biosecurity prevents spread to other flocks while eradication efforts proceed.

The home environment for flocks undergoing ALV eradication programs requires careful attention to sanitation and biosecurity. Facilities should be maintained in clean condition with regular removal of contaminated litter and waste. Thorough cleaning and disinfection between flocks eliminates residual virus in the environment. Footwear changes and hand washing should be required for all personnel entering poultry areas. Equipment should not be shared between different flocks without proper sanitation. Wild bird access should be minimized to prevent potential mechanical transmission of virus.

Maintaining quality of life for birds in flocks affected by Lymphoid Leukosis involves standard good management practices while acknowledging that some birds will inevitably develop fatal disease. Providing high-quality nutrition, clean water, and comfortable housing supports overall flock welfare. Environmental enrichment appropriate for the production system helps maintain normal behavior patterns. Prompt removal and humane euthanasia of clinically affected birds prevents prolonged suffering. Avoiding overstocking and maintaining good air quality reduce stress that might promote disease progression.

Ongoing monitoring and veterinary involvement are essential components of Lymphoid Leukosis management programs. Regular testing of flock samples allows tracking of ALV prevalence and detection of any new infections. Postmortem examination of birds that die or are culled confirms diagnoses and monitors disease patterns. Periodic consultation with an avian veterinarian ensures that surveillance and control programs remain appropriate and effective. Detailed record keeping of test results, mortality, and production data provides information needed to evaluate program success and make management decisions.

Caregiver support for poultry keepers dealing with Lymphoid Leukosis encompasses understanding the disease, managing expectations, and accessing appropriate resources. Accepting that affected birds cannot be saved and that the goal is eradication rather than treatment helps caregivers make appropriate decisions. Understanding that successful eradication requires sustained effort over time helps maintain commitment to testing and culling programs. Connecting with industry organizations, extension services, and veterinary resources provides technical support and guidance. For commercial operations, working with integrators or breeding companies helps coordinate eradication efforts across the production chain.

Chickens are essentially the only species affected by classical Lymphoid Leukosis caused by ALV subgroups A and B, with all types and breeds of domestic chickens being susceptible. Layer chickens, including commercial white and brown egg layers and backyard laying breeds, are commonly affected because they live long enough for tumors to develop. Broiler chickens are less commonly diagnosed because their short production lifespan means most are processed before tumors would become apparent, though breeding stock may be affected. Heavy breed chickens and heritage breeds are equally susceptible to light commercial layers. While genetic selection has produced some lines with increased resistance to ALV infection or reduced tumor development, no breed is completely resistant.

Other avian species show varying susceptibility to different members of the avian leukosis virus group. Turkeys, pheasants, partridges, and quail can be infected with related avian leukosis/sarcoma viruses adapted to those species, though these are distinct from the ALV subgroups causing classical lymphoid leukosis in chickens. Waterfowl are not considered susceptible to chicken ALV. Some laboratory studies have shown experimental infection of various bird species, but natural disease from chicken ALV strains is primarily limited to chickens. Cross-species transmission is generally not a significant concern for lymphoid leukosis control programs focused on chickens.

Screening recommendations for Lymphoid Leukosis focus on identifying and eliminating virus-shedding birds from breeding populations. Primary breeder flocks should undergo regular testing for ALV antigen using ELISA on cloacal swabs and/or egg albumin samples. Testing should be conducted multiple times to ensure detection of all shedders, as virus excretion can be intermittent. Commercial laying and breeder flocks should be sourced only from suppliers documenting ALV-free status. Any flocks experiencing unexplained mortality with tumors should be investigated and tested for ALV. Industry eradication programs typically involve coordinated testing across multiple levels of the breeding pyramid to ensure comprehensive control.

Several tumor and infectious conditions commonly occur in association with Lymphoid Leukosis or share risk factors and epidemiological features. Infection with multiple ALV subgroups can occur simultaneously, potentially leading to different tumor types in the same flock or even the same bird. Myeloid leukosis, caused by ALV subgroup J, produces myeloid cell tumors rather than lymphoid tumors and can occur alongside lymphoid leukosis. Infection with immunosuppressive viruses including Infectious Bursal Disease Virus and Chicken Anemia Virus can increase the severity of ALV infection and accelerate tumor development. Concurrent bacterial infections may be more severe in immunosuppressed ALV-infected birds.

Marek's disease is the most important condition requiring differentiation from Lymphoid Leukosis because both produce lymphoid tumors but have different causes, epidemiology, and control measures. Marek's disease typically affects younger birds, usually under sixteen weeks of age, while lymphoid leukosis primarily affects adults over sixteen weeks. Marek's disease tumors are T-cell lymphomas and commonly involve peripheral nerves, skin, and muscle, while lymphoid leukosis produces B-cell tumors primarily in the liver, spleen, and bursa. In Marek's disease, the bursa is typically atrophied, while in lymphoid leukosis it may be enlarged and tumor-infiltrated. Vaccination is highly effective for Marek's disease but not available for lymphoid leukosis. Accurate differentiation through histopathology and appropriate laboratory testing is essential for implementing correct control measures.

Complications of Lymphoid Leukosis include secondary effects of tumor growth and ALV infection beyond the primary neoplastic disease. Immunosuppression from ALV infection increases susceptibility to other infectious diseases and may reduce vaccine effectiveness. Anemia from tumor infiltration of hematopoietic organs causes weakness and reduced productivity. Liver failure from massive tumor replacement of hepatic tissue leads to metabolic derangement and death. Sudden death from liver rupture and internal hemorrhage can occur when enlarged tumor-infiltrated livers are traumatized. Condemnation of carcasses at processing due to tumor presence represents a direct economic loss even when birds do not die prior to slaughter.