Avian Chlamydiosis

Avian chlamydiosis (AC) is caused by the bacterium Chlamydophila psittaci. AC occurring in humans and all birds was originally called psittacosis, but later the term ornithosis was introduced to identify the disease contracted from or occurring in domestic and wildfowl, while the name of the disease contracted from or occurring in psittacine birds remained psittacosis. These diseases are similar when contracted by humans. The genus Chlamydia was divided into two, Chlamydia and Chlamydophila. A proposal to re-combine them into the single genus Chlamydia is under consideration but has not been adopted for this chapter. The avian strains of Chlamydophila psittaci include at least fifteen genotypes, some of which correlate with the avian species from which they are usually isolated. Chlamydiosis as it occurs naturally in mammalian species and not contracted from avian species is caused by different species of the organism. Depending on the virulence of the chlamydial strain and the avian host defence, chlamydiae cause pericarditis, conjunctivitis, sinusitis, airsacculitis, pneumonia, lateral nasal adenitis, peritonitis, hepatitis, and splenitis. Generalised infections result in fever, anorexia, lethargy, diarrhoea, and occasionally shock and death. Special laboratory handling (biosafety level 3) is recommended because avian chlamydial strains can cause serious illness and possibly death in humans. While the disease in psittacine birds is best known, the infection in ducks and turkeys is of particular concern as transmission to humans is common during handling and slaughter of the birds. The diagnosis of AC requires the isolation and identification of the organism, the demonstration of chlamydiae in tissues, or the demonstration of a four-fold increase in specific humoral antibody, as well as typical clinical signs. Identification of the agent: Isolation of chlamydiae requires the inoculation of embryonated eggs or cell cultures and testing for chlamydiae by cytochemical stains or immunohistochemical methods. The direct inoculation of samples into cell cultures is preferable as they are as sensitive for the isolation of most avian strains of chlamydiae as are chicken embryos. The cell cultures are then stained by immunofluorescence or by other appropriate stains at appropriate times to demonstrate the presence of inclusions. Histochemical staining of impression smears from the liver, heart, and spleen are commonly made. The technique gives a rapid diagnosis, but requires some experience. Enzyme-linked immunosorbent assays (ELISAs) developed for detecting Chlamydophila trachomatis antigen in humans have been used for diagnosing chlamydiae in birds. Many of the earlier tests were developed using monoclonal or polyclonal antisera against lipopolysaccharide epitopes, some of which were shared with other Gram-negative bacteria. Their use when screening individual birds is questionable, as they lack sensitivity and specificity. Molecular tools (conventional and real-time polymerase chain reaction, restriction length polymorphism, DNA microarray or sequencing) and immunohistochemical staining of histological sections are now widely used in diagnostic laboratories. All of them are rapid and do not require the live agent. The current PCR tests target the ompA gene or the ribosomal RNA genes (16S or 23S). Validated and standardised protocols of both species-specific and family-specific assays are available. Nested and real-time PCRs can be as sensitive as isolation. There has been an increase in the use of immunohistochemical staining of histological sections because of the recent development and availability of automated staining equipment. Serological tests: The standard serological test for chlamydial antibodies is the complement fixation (CF) test. The modified direct CF test can be used with most sera. The antigen is a groupreactive lipopolysaccharide antigen present in all strains. The occurrence of high CF titres in the majority of individuals in a flock with clinical signs is presumptive evidence of active infection. The demonstration of a four-fold increase in titre in an individual bird is considered to be diagnostic of a current infection. Other serological tests, such as the ELISA, latex agglutination, elementary body agglutination, micro-immunofluorescence, and the agar gel immunodiffusion tests can be used. These tests are of value in specific cases and may replace the CF test; however, comparisons of reliability and reproducibility are not yet available. Requirements for vaccines: There are no commercial vaccines available for chlamydiosis control in poultry. Antibiotics are the only current means of control. Chlamydophila psittaci is susceptible to a number of antibiotics. The drug of choice varies from country to country.