Chicken anaemia, chicken infectious anaemia and blue wing disease all describe an immunosuppressive syndrome in poultry caused by chicken anaemia virus (CAV), also known as chicken infectious anaemia virus (CIAV). The virus was first isolated in J...
CAV is the smallest DNA virus that belongs to the Gyrovirus genus, which has been reassigned to the Anelloviridae family (Rosario et al., 2017). The genome contains a single-stranded negative circular DNA, approximately 2.3 kb in size (Gelderblom et al., 1989; Adams et al., 2016).
CAV has been isolated in a significant number of countries and is considered to have a worldwide distribution including commercial poultry and in specific pathogen free (SPF) flocks (Fadly et al., 1994; O’Rourke et al., 1994). CAV causes anaemia in young chicks and immunosuppression in young and adult chickens; CAV may be transmitted vertically from parent to the chick causing a clinically apparent immunosuppressive disease (McNulty, 1991; McNulty et al., 1991) and horizontally from chicken to chicken resulting in an inapparent infection if transmission occurs after a few weeks of life.
Vertical transmission results in acute disease with clinical signs at 10-14 days of age with mortality peaks of 5-10% usual, but up to 60% mortality has been documented (Yuasa et al., 1979; Taniguchi et al., 1982; McNulty et al., 1991). Disease signs include weakness, depression, anorexia and stunting, as well as gangrenous dermatitis, atrophy of thymus and bone marrow leading to severe anaemia and immunosuppression. Anaemia may be seen on non-feathered areas such as the combs and wattles, and skin lesions are also characteristic, especially on the wings (Yuasa et al., 1979; Engström et al., 1988; McNulty, 1991). The disease appears to inhibit blood cell production, probably by affecting stem cell development, hence anaemia, with lowered white cell and platelet counts. Lymphoid tissues are also affected.
Vertical transmission appears to be of particular importance in intensive operations if breeding birds become infected. The breeders transmit CAV to the chicks and the developing chick develops signs within 2 weeks of hatching. Disease in the resultant chicks is severe. Vertical spread can be controlled by vaccination of the breeding hens with common vaccination regimes being controlled exposure to the wild-type virus. Such vaccination protocols control vertical transmission but increase the probability of horizontal transmission.
Horizontal transmission occurring after a few weeks of age does not result in overt signs but studies on CAV-antibody positive and CAV-antibody negative flocks have indicated that the economic impact is still quite substantial but inapparent.
CAV is a pathogen of domestic fowl; however, antibodies to CAV have also been found in quail but not duck or crow (Farkas et al., 1999). CAV has also been isolated from humans (faeces, blood and skin) and from faeces of dogs and cats (Zhang et al., 2012; 2014; Li et al., 2017). More recently, a novel CAV, designated as pigeon-CIAV-1906, has been isolated from two diseased pigeons in China (Shao et al., 2021), suggesting that pigeon can be a host for CAV. The isolation of pigeon-CIAV-1906 and its molecular characterization provide evidence for cross-species transmission of CAV from chicken to pigeon, but experimental infection of pigeons with pigeon-CIAV-1906 is needed to understand the pathogenesis and transmission of CAV in pigeons. It is unlikely that CAV infects other avian species. In the laboratory, CAV can only be grown in a limited number of transformed chicken cell lines (Yuasa, 1983; McNulty, 1991; Coombes and Crawford, 1998) and has not been successfully grown in other cell lines, suggesting that CAV is species-specific. At this stage, there is no evidence of infection of humans or other animal species and hence no health implications for humans or other species.
Similarly, there is no evidence of insect vectors, other animal carriers or environmental reservoirs of the virus. Fomites (i.e. inanimate objects), dust (due to the stability of the virus) and worker movements may play a role in transmission.
There do not appear to be any predisposing host factors to infection other than lack of maternal antibody to CAV. Non-immune poultry become infected and poultry that have previously been exposed to CAV (i.e. possessing CAV antibodies) do not become infected. However, the age of the bird has a marked effect on the development of clinical signs. With vertical transmission, the hatched chicks develop signs from about 10 to 14 days of age with mortality rates up to 60%. Birds infected horizontally after about 14 days old do not exhibit clinical signs but production may be affected.
Antibodies to CAV and the virus itself have been isolated in a significant number of countries/states/regions. Results have generally been reported in the scientific literature and hence will not represent the totality of CAV infections worldwide. In fact, CAV is now recognized as being of worldwide distribution and is usually found when looked for. It is relatively easy to detect antibody to CAV in birds but CAV isolation is needed for the definitive diagnosis. Growth of CAV in cell culture presents many difficulties hence many reports of CAV are based on antibody evidence. CAV has been found in commercial domestic fowl, in SPF flocks and in village chickens (Fadly et al., 1994; O’Rourke et al., 1994).
The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.
The characteristic histopathological findings in CAV-infected chicks include a change from a red colour to a yellow colour to a white colour in the bone marrow and intense atrophy of the lymphoid organs, including the thymus, bursa of Fabricius, and to a lesser extent, the spleen, with a depletion of lymphocytes, followed by hyperplasia of reticular cells. This results in a severely immunocompromised state that increases the chicks' susceptibility to secondary bacterial infections and ultimately shortens life expectancy (Taniguchi et al., 1982). Haemorrhages throughout the skeletal muscle and subcutaneous tissue are also a common finding (Yuasa et al., 1979; Taniguchi et al., 1982; 1983; Engström et al., 1988). These characteristic features are usually observed in the second week of infection (McNulty, 1991).
CAV infection is characterized by severe aplasia of the bone marrow with haematopoietic cells reduced severely or replaced almost completely by adipose tissue, giving the bone marrow a watery texture and its characteristic yellow colour (Taniguchi et al., 1982). The changes occurring in the bone marrow cause a severe anaemia.
Typical clinical signs include weakness, depression, anorexia, stunting, gangrenous dermatitis and atrophy of thymus and bone marrow combined with obvious anaemia characterized by pale comb and wattles, eyelids and legs and a pale carcass. Skin lesions, commonly on the wings, are also observed.
A pale watery bone marrow, watery blood with a decreased haematocrit (red and white cells and platelets are reduced) is evident. Haematocrit values of 27% (normal range 32-37.5%) and below are commonly used to identify infected chicks (Yuasa and Imai, 1986) with values as low as 10% being recorded (Taniguchi et al., 1982).
From 16 days post-infection, recovering birds will exhibit juvenile forms of erythrocytes, thrombocytes and granulocytes in the blood, characterized by an increase of 30% or more of immature blood cells (Taniguchi et al., 1983).
Virus isolation in cell culture or in embryonating eggs is considered the gold standard procedure for CAV detection. The virus can be isolated using yolk sac route in embryonated chicken eggs, in MDCC-MSB1 cells or in SPF chicks, but these procedures are tedious, time-consuming and expensive (Yuasa et al., 1979; McNulty et al., 1989).
Numerous laboratory techniques for the detection of CAV infection have been reported, including immunofluorescence, immune-peroxidase staining in situ hybridization and ELISA (Hoop and Reece,1991; McNeilly et al., 1991; Sander et al.,1997; Toro et al., 2006), but these techniques are time-consuming and costly, hence limiting their use in routine practice.
Laboratory detection of CAV is most readily achieved by the detection of CAV antibodies or antigens using paired sera sampling, using commercially available ELISA tests (Todd et al., 1999) and polymerase chain reaction (PCR) based tests (Schat and Santen, 2008). This approach has been used not only to detect CAV in experimental work and in the field (Cardona et al., 2000; Hailemariam et al., 2008), but also to test for CAV contamination in poultry vaccines (Amer et al., 2011; Hermann et al., 2012). Different PCR techniques, such as nested and conventional PCR, strain-specific real-time PCR for quantitation and real-time quantitative PCR, have shown high sensitivity and specificity and provide a relatively inexpensive and fast alternatives for testing on a large scale (Imai et al., 1998; Cardona et al., 2000; Markowski-Grimsrud et al., 2002; Santen et al., 2004; Amer et al., 2011). PCR can detect CAV DNA in a number of tissues, including the caecal tonsils, thymus, Harderian gland, spleen, reproductive tissues and in infected cell lines (Cardona et al., 2000; Markowski-Gimsrud et al., 2002; Santen et al., 2004; Joiner et al., 2005).
For monitoring of CAV in specific pathogen free (SPF) flocks, the use of a quantitative PCR (qPCR) is recommended because qPCR analysis of samples is much faster compared with the standard PCR (Vagnozzi et al., 2018).
Typical signs observed at the onset of clinical disease (10-14 days post-hatching) include weakness, depression, anorexia and stunting, gangrenous dermatitis, atrophy of thymus and bone marrow. Anaemia is noticeable on the non-feathered areas such as the comb and wattles, eyelids and legs and the carcass appear quite pale.
Clinically infected birds fail to thrive and may be runted/stunted. CAV infection reduces the effectiveness of the immune system and birds may become adventitiously infected with secondary bacterial pathogens. Mortality peaks in the third week of life.
There is no specific treatment for infected birds but culling of birds is appropriate.
Horizontally infected birds do not exhibit any signs and the only evidence of infection may be a drop in production parameters; however, very careful analysis is required to demonstrate such a decrease.
CAV can be transmitted vertically from the parent to the chick (McNulty, 1991; McNulty et al., 1991) and horizontally from chicken to chicken, resulting in clinical and subclinical infections, respectively. Vertical infection occurs when breeder flocks with no previous exposure to CAV become infected as they come into egg production. After hatching, the virus causes a disease that is acute at onset with clinical signs appearing at 10-14 days of age. Mortality peaks within the third week of life, usually from 5% to 10% but spikes up to 60% have been recorded (Yuasa et al., 1979; Taniguchi et al., 1982; McNulty et al., 1991). Chicks hatching from eggs over a three- to six-week period are affected. After this time, the breeder flocks develop sufficient CAV antibody to stop transmitting the virus through the egg (McNulty, 1991), instead maternal antibody to CAV is transmitted and chicks are protected during the first critical weeks of life. Thus, vaccination of breeders provides good protection from clinical CAV infection.
Horizontal infections occur in older chickens lacking maternal antibody to CAV. Virus is excreted by a small number of vertically infected hatch mates and acquired via the faecal-oral route or may be introduced via an external source such as on workers' boots or clothing. CAV is particularly hardy and survives in poultry houses for a considerable period of time. Clinical signs do not develop but the growth and health of the birds may be affected (McNulty et al., 1991).
Seasonal variations in the incidence of subclinical/horizontal CAV infection have been reported in Denmark, possibly due to better cleanout between flocks and hence virus inactivation (Jørgensen et al., 1994; 1995a; 1995b).
The economic impact of CAV infection is twofold, related to the mode of infection. Vertical transmission results in clinical signs, reduced production with mortality of 5-10%, with spikes up to 60%. Vaccination of breeding birds is cost effective but specific dollar figures regarding losses and costs of vaccination are not available. Such figures could be calculated for individual commercial operations based on mortality figures of 5-10%. It should be remembered that CAV infection also interferes with the effectiveness of vaccination programs for other avian pathogens with concomitant increases in other diseases, e.g. IBDV. These factors would also need to be considered when calculating the economic importance and impact of CAV infection.
Horizontal transmission does not lead to clinical signs or death but production is significantly affected. Horizontal infections are hidden as the worldwide distribution of CAV and the non-evident nature of horizontal infections makes analysis difficult. Vaccination of all birds to control horizontal transmission is a significant impost but in situations where clinical CAV exists, it may be a very useful control mechanism, i.e. complete eradication of CAV from flocks may have substantial economic benefits.
Economic Impact of CAV on Commercial Broiler Production in the UK
CAV appears to be restricted to chickens and there is no current evidence of CAV being able to infect or replicate in any other species. As such, CAV is not considered a zoonosis and there are no direct food safety implications.
Vertical CAV infection leads to immunosuppression of the infected chick, reducing the effectiveness of vaccination programs and increasing the likelihood of a pool of adventitious infections in the flock. As such, clinical CAV infection may reduce the overall health of a flock and increase the chances of other diseases in the flock that may have human and food safety implications.
CAV is a particularly resistant virus and difficult to eradicate from the housing environment. CAV remains active after heating at 70°C for 1 h and is resistant to many commonly used disinfectants, including lipid solvents, such as ether or chloroform, and pH 3 (Gowthaman, 2019). Implementation of stringent biosecurity measures and keeping the other immunosuppressive viral diseases under control by vaccination are essential for controlling the infection. The use of rigorous cleaning out methods combined with hypochlorite, iodoform or formalin is recommended. Virus is excreted in faeces so scrupulous attention to clothing and footwear is required by personnel and control of dusts may be important.
National and International Control Policy
Quarantine may be an effective approach but is beset with difficulties due to the resistance of CAV to inactivation and its persistence in the environment.
Immunization and Vaccines
The only viable veterinary measure is vaccination either through exposure of breeders to the wild-type virus (Controlled Exposure) or with an attenuated vaccine. Vaccination of broilers would be dependent on economic factors and the demonstration of production losses associated with vertical transmission.
Vaccination strategies are based on the vaccination of breeders to prevent vertical transmission and horizontal transmission of CAV to chicks (Schat and Santen, 2008). The breeders should be vaccinated between 8 and 16 weeks of age with a live vaccine; MDA titre of >8 log2 is required to limit the vertical transmission (Gowthaman, 2019).
Controlled exposure of breeders has the advantage of being cheap but may increase the degree of horizontal CAV infection with concomitant and hidden reduction in production of older birds.
Live attenuated CAV vaccines are commercially available for vaccination of breeders from a young age e.g. 6 weeks, and up to 6 weeks prior to lay. Vaccination later than this could result in inadequate antibody levels in the breeders, putting progeny at risk. The live vaccines should be administered according to each manufacturer’s instructions, as recommended routes vary.
