Chicken anemia virus (CAV) has a world-wide distribution. Infection of young chickens causes anemia, decreased weight gain, transient immunosuppression, and increased mortality. Infection of chickens older than 3 or 4 weeks of age usually does not cause clinical signs, but can cause immunosuppression resulting in secondary infections or can result in economic losses even in the absence of evidence of any disease. There is no specific treatment, and control of disease is through vaccination or natural exposure of breeders. Infection can be diagnosed by reduced hematocrit and detection of CAV nucleic acids in blood lymphocytes by PCR, or postmortem by thymic atrophy, lymphoblast depletion of the thymic cortex, pale bone marrow, and detection of CAV DNA or antigens in tissues.
Etiology and Pathogenesis of Chicken Anemia Virus Infection
Chicken anemia virus (CAV), a 25 nm, nonenveloped, icosahedral virus with a very small (2.3 kb), single-stranded, negative sense, circular DNA genome, is the only recognized member of the Gyrovirus genus of the Anelloviridae family. It was previously classified as a Circovirus, but important differences in genome organization led to its reclassification into the new Anellovirus family. The genome codes for three viral proteins (VPs). VP1, the only structural protein, is the capsid protein, but VP2 may be needed as a scaffold protein to allow proper folding of VP1. VP2 also has serine/threonine and tyrosine protein phosphatase activities, and mutations affecting these activities result in attenuation of virus replication in vivo. VP3, or apoptin, is a nonstructural protein that induces apoptosis in infected cells and has other functions in viral replication.
When day-old susceptible chicks are inoculated IM with CAV, viremia occurs within 24 hours. Virus can be recovered from most organs and rectal contents as long as 35 days after inoculation. The principal sites of CAV replication are hemocytoblasts in the bone marrow, precursor T cells in the cortex of the thymus, and dividing CD4 and CD8 T cells in the spleen. Replication in and destruction of the hemocytoblasts leads to anemia, whereas replication in and destruction of the T cells causes immunosuppression. Neutralizing antibodies are detectable 21 days after infection, and clinical, hematologic, and pathologic parameters return to normal ~35 days after infection.
Chicken anemia virus infection has adverse effects on proliferative responses of spleen lymphocytes and on the production of interleukin-2 and interferons by splenocytes. Infection can cause a marked decrease in generation of antigen-specific cytotoxic T cells and T-helper cells directed against other pathogens. In addition to T-cell defects, macrophage functions such as Fc-receptor expression, phagocytosis, and antimicrobial activity may be impaired. Because of decreased T-helper cells, antibody production after vaccination against other pathogens can be reduced.
Subclinical, horizontally acquired infection with CAV in broiler progeny of seropositive parent flocks may be associated with impaired economic performance. In addition, subclinical infection of chickens older than 4 weeks of age, after maternal antibody has waned, can cause immunosuppression resulting in secondary infections and can result in economic losses even in the absence of evidence of any disease. Syndromes involving CAV infection along with other pathogens include hemorrhagic aplastic anemia syndrome, gangrenous dermatitis, inclusion body hepatitis (IBH), and IBH/hydropericardium syndrome.
Epidemiology and Transmission of Chicken Anemia Virus Infection
Horizontal transmission of chicken anemia virus infection is by the fecal-oral route, perhaps by the respiratory route, and through infected feather follicle epithelium. Contaminated litter is a common source of introduction. Vertical transmission occurs when seronegative hens become infected and continues until adequate levels of neutralizing antibodies develop in the hens. Chicks hatched from these eggs are viremic, and CAV can rapidly spread horizontally from these chicks to susceptible, maternal antibody–negative hatchmates. Roosters shedding CAV in semen are another source of vertical transmission. Vaccination of seronegative flocks before the onset of egg production is recommended to prevent vertical transmission.
Maternal antibody–negative chicks are susceptible to infection and disease until 1–2 weeks old. In contrast, maternal antibody-positive chicks are protected from disease and probably from infection. Age resistance to clinical disease, but not infection, begins at ~1 week of age and is closely associated with the ability of chickens to generate antibody to the virus after infection. The age resistance can be overcome by coinfection with viruses causing immunosuppression such as infectious bursal disease virus, Marek's disease virus, and reticuloendotheliosis virus.
Many SPF flocks develop antibodies to CAV during or after onset of sexual development. Spread of infection by CAV-contaminated embryo- or cell-culture-derived vaccines is possible.
Clinical Signs of Chicken Anemia Virus Infection
Signs of illness or adverse effects on egg production from chicken anemia virus infection do not occur when seronegative adult chickens become infected. However, vertical transmission or infection of maternal antibody–negative chicks before 1 week of age can cause clinical disease 12–17 days after hatching or infection.
anemia (PCV ≤27)
watery and slowly clotting blood
reduced weight gain
high mortality rates resulting from secondary complicating infections (generally 10%–20%, but as much as 60%)
anemia, leukopenia, and pancytopenia seen often on blood smears
Organs are pale; the thymus is generally atrophied, and the bursa of Fabricius may be small. Bone marrow is pale or yellow. Hemorrhages may be present in or under the skin and in muscle and other organs. Lesions associated with secondary infections may also be present.
Lymphoid cell populations are depleted in primary and secondary lymphoid organs. Most notable is severe lymphocytic depletion of the thymic cortex. Granulocytic and erythrocytic compartments in the bone marrow are atrophic or hypoplastic.
Diagnosis of Chicken Anemia Virus Infection
A tentative diagnosis of chicken anemia virus infection is based on history, signs, and gross and histopathologic lesions. Confirmation requires detection of virus or viral DNA in the thymus or bone marrow. PCR and quantitative PCR techniques are commonly used to demonstrate the presence of CAV DNA. Highly conserved portions of the CAV genome enable design of PCR assays that reliably detect most CAVs. Alternatively, viral antigens in the thymus can be detected by immunohistochemistry or immunofluorescence. Viral isolation is generally not used in the course of diagnosis because it is slow and expensive. In addition, it is difficult because of the limited number of cell lines supporting CAV replication. Only limited chicken lymphoblastoid cell lines are available, some of which can become resistant to CAV infection with cell passage. Furthermore, these cell lines grow in suspension, so the cytopathic effect of CAV infection is difficult to recognize. To isolate CAV, chloroform-treated extracts of tissues are inoculated in MDCC-MSB1 or MDCC-147 cultures (both are lymphoblastoid cell lines derived from Marek's disease tumors) or into maternal antibody–negative day-old chicks. The presence of CAV DNA or proteins in the culture or chicken tissues must then be verified by PCR or antibody assays (e, immunohistochemistry or immunofluorescence). Commercial ELISA kits are available to detect serum antibodies to CAV and can be used to identify breeder flocks that are seronegative before egg production and to monitor the efficacy of vaccination.
Treatment, Control, and Prevention of Chicken Anemia Virus Infection
There is no specific treatment for chicken anemia virus infection. Secondary bacterial infections may be treated with antibiotics. One strategy to control CAV infection is vaccination of breeder flocks with commercially available live vaccines before the start of egg production. Because only a single serotype of CAV has been identified, the vaccine does not need to be matched to regional variants. Vaccine administration is by injection or by addition to the drinking water, depending on the type of vaccine available in individual countries. Many operations rely instead on natural exposure of breeders to CAV before the onset of egg production, with serological monitoring to confirm that seroconversion has occurred. In some areas, transfer of litter to noncontaminated premises and the addition of crude homogenates of tissues from affected chickens to the drinking water have been used to ensure infection and seroconversion of parent flocks before they begin to lay eggs, thereby diminishing the risk of egg transmission. However, these alternatives to vaccination are extremely risky because they do not ensure uniform seroconversion and are thus not acceptable. Variability in seroconversion can lead to vertical transmission and infection of progeny with suboptimal maternal antibody levels via horizontal transmission. In addition to vaccination of breeders, a vaccine is approved in the USA for vaccination of broilers as young as 7 days old; administration is by addition to the drinking water. Because of the synergism between CAV and other immunosuppressive viruses, control of the latter is also important, especially to prevent disease in chickens infected after the decay of maternal antibodies. Eradication of CAV from premises is not a feasible strategy for control because of its extreme resistance to chemical disinfectants and heat.
Zoonotic Risk of Chicken Anemia Virus Infection
Chicken anemia virus poses no known zoonotic risk to humans, nor does it infect other mammals. CAV DNA has been detected by PCR in chicken meat intended for human consumption and in human stool samples, but there is no evidence that CAV replicates in humans or is associated with human disease. CAV has also been detected in or isolated from feces of stray mammals and ferrets fed chicken meat, but it is unlikely that the virus replicates in these hosts.
CAV infection causes anemia and transient immunosuppression in vertically infected chicks or CAV maternal antibody-negative chicks infected before 2–3 weeks of age.
Subclinical infection of chickens older than 3–4 weeks of age can still cause transient immunosuppression, resulting in secondary infections and economic losses, even in the absence of disease.
CAV is ubiquitous throughout the world in poultry operations; eradication of the virus from premises is not possible due to its extreme resistance to chemical disinfectants and heat.
The most commonly used and definitive methods of diagnosis of CAV infection are detection of CAV DNA in thymus or liver, for example by PCR, and detection of CAV antigens in thymus, for example by immunohistochemistry or immunofluorescence. Virus isolation is difficult and tedious and is not commonly used for diagnosis.
CAV infection is controlled by vaccination of breeders or natural exposure of breeders to this ubiquitous virus before the start of egg production. Infection of breeders during egg production leads to vertical infection and disease in progeny.
Only one serotype of CAV has been identified.
Monitoring of CAV antibody levels in breeders, using commercially available ELISA kits, is important to ensure adequate maternal antibody levels for protection of progeny.
CAV poses no known zoonotic risk.