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Psittacosis in the United States, 1979

Morris E. Potter, D.V.M.

Arnold K. Kaufmann, D.V.M.

Brian D. Plikaytis, M.S. Bacterial Zoonoses Activity Division of Bacterial Diseases Center for Infectious Diseases


Psittacosis, a zoonotic disease caused by Chlamydia psittaci, is typically transmitted from infected avian species to humans by inhalation of infectious aerosols. Both symptomatic and asymptomatic infected birds shed organisms in their droppings and other body secretions. Psittacosis was first described as a human disease in Europe in 1879, but was rarely reported in the United States before a pandemic that occurred in 1929-1930. This U.S outbreak was traced to parrots imported for the 1929 Christmas trade and resulted in the first ban on the importation and interstate shipment of psittacine birds in the United States. Since that time, regulations pertaining to the importation of pet caged birds have been modified repeatedly to conform to 1) newer knowledge about psittacosis, 2) the advent of efficacious antimicrobial therapy, 3) the threat of exotic Newcastle disease to the domestic poultry industry, and 4) the popularity of pet-bird ownership. Currently, birds are imported through quarantine facilities owned or supervised by the U.S. Department of Agriculture (USDA).


This surveillance summary is based on individual case reports submitted by state and territorial health departments. As with other diseases, the true incidence of psittacosis is higher than the reported incidence. Many cases are so mild that they go undiagnosed, since the signs do not point specifically to psittacosis and the disease may be self-limited. Psittacosis is a disease of low incidence that tends to occur sporadically. For this reason, it may not be considered in the differential diagnosis of a patient's illness, particularly when the patient's exposure history is not known. Underreporting of confirmed cases also occurs, and psittacosis is not a reportable disease in all states.

The clinical, laboratory, and epidemiologic information on the case reports varies widely in its completeness. Thus, statements about reported frequencies should not be interpreted literally. For example, the statement that 75% of the patients were reported to have fever does not necessarily mean that 25% of the patients had afebrile illnesses.

A confirmedt case is defined as a clinical specimen positive for C. psittacit or clinical illness characterized by any combination of fever, chills, lower or upper respiratory disease, myalgia, headache, photophobia, and splenomegaly, plus a fourfold or greater change in psittacosis complement-fixing (CF) antibody titer to at least 32 between two serum specimens obtained 2 or more weeks apart and studied at the same laboratory. A presumptivet case is defined by compatible illness and either a CF titer of 4!532 on a single serum specimen or a stable titer of 4!532 in two or more specimens obtained after onset of symptoms. A suspectedt case is reported to CDC by a state health department as a case of psittacosis but did not meet the criteria for a confirmed or presumptive case; cases for which insufficient data have been reported are included as suspected cases.


In 1979, the most recent year for which data have been analyzed, 25 states reported 116 cases of human psittacosis to CDC, compared with 33 states that reported 160 cases in 1978. This decrease halted the 5-year upward trend in cases associated with sources of infection other than poultry processing (Figure 1). Forty-seven of these cases were confirmed, 45 were presumptive, and 24 were suspected.

As in the past (1t), most cases of psittacosis were reported in the western states, with California accounting for 27 (23%) of the 116 cases (Figure 2). States in the Pacific Region reported 52 (45%), and states in the Pacific and Mountain regions together reported 63 (54%) of the total. No distinct seasonality was observed for the 112 cases with known month of onset.

A slight predominance of females was associated with cases of psittacosis in 1979 (52% vs 48%). Although patients ranged in age from 5 to 89 years, 76% (88 of 116) were in the 20- to 59-year age group.

Fever (75%), cough (56%), and pneumonia (53%) were the most commonly reported clinical findings for psittacosis cases in 1979. Other signs and symptoms reported by more than 25% of the patients included headache, chills, weakness and fatigue, and myalgia. One patient with a fourfold titer rise to chlamydial group antigens died. This patient, an 89-year-old female with congestive heart failure, had purchased a lovebird in mid-October. Two weeks later the bird died, and a week after that the patient became ill with cough, fever, headache, and chills. While she was hospitalized for treatment of the respiratory illness, the patient's cardiovascular status deteriorated, and she developed an intestinal intussusception and died; psittacosis was considered to be a contributing cause of death.

In 1979, most of the cases occurred in pet-bird owners or bird fanciers (Table 1); pet caged birds were the reported source for 86 (74%) of the 116 cases of human psittacosis. The category labeled "other psittacine species" consists largely of imported birds and was the most frequently implicated source of human infection (31%), followed by "mixed and unspecified psittacine birds" (12%). It is of interest to note that the direction of change in the numbers of birds imported through USDA-approved quarantine facilities (Figure

  1. paralleled changes in the number of reported cases of psittacosis in humans for the years 1974-1979. Interpretation of this observation must await analysis of data for later years. There were no reported cases of psittacosis in poultry processors in 1979.


In addition to causing disease in bird owners and persons inadvertently exposed to pet birds, psittacosis is an occupational health hazard for individuals employed in the importation, breeding, transportation, marketing, and care of pet birds. Also, chlamydial infections in turkey flocks cause sporadic epidemics in poultry-processing-plant workers.

Chlamydiae are unique bacteria by virtue of their obligate intracellularity and their complex method of replication within the host-cell's cytoplasm. The disease process begins with inhalation of an infectious elementary body that is not metabolically active but is capable of surviving in an extracellular environment. After it attaches to a respiratory epithelial cell, the elementary body is actively phagocytized by the host cell and then reorganizes into the larger initial (reticulate) body, which is the metabolically active replicative form of the organism. Following repeated binary fission, the developmental cycle is completed by reorganization of the initial bodies into infectious elementary bodies (2,3t).

Host defense against chlamydiae may involve both cell-mediated and T-cell-dependent humoral antibody responses. However, chronic or latent infections are the rule in many host species, including birds, after primary infection. Even in species that resist reinfection (e.g., the mouse with murine pneumonia caused by C. psittacit), chronic infections occur (4,5t). Humans exhibit no prolonged immunity, and relapses or reinfections have been reported even for individuals with high CF antibody titers (6t). Persistence of Chlamydiat may reflect depression of T and B lymphocyte function by the organism and its metabolites.

The genus Chlamydiat is divided into two species, C. trachomatist and C. psittaci, that are differentiated by a few relatively simple biochemical tests (7t). This simple subdivision may be confusing because each species comprises biologically distinct members differing in host-range and associated disease states. In addition to psittacosis, organisms classified as C. psittacit cause a variety of animal diseases such as feline pneumonitis; polyarthritis in lambs, calves, and pigs; abortions in cattle and sheep; and sporadic bovine encephalomyelitis. These last diseases have little or no known human health significance.


  1. CDC. Psittacosis surveillance report. Annual Summary 1978. Issued June 1979, Atlanta.

  2. Schachter J, Caldwell HD. Chlamydiae. In: Starr MP, Ingraham JL, Balows A, eds. Ann Rev Microbiol. Palo Alto, California: Annual Reviews Inc, 1980;34:285-309.

  3. Moulder JW. The relation of the psittacosis group (Chlamydiae) to bacteria and viruses. Ann Rev Microbiol 1966;20:107-30.

  4. Williams DM, Schachter J, Grubbs B, Sumaya CV. The role of antibody in host defense against the agent of mouse pneumonitis. J Infect Dis 1982;145:200-5.

  5. Lammert JK, Wyrick PB. Modulation of the host immune response as a result of Chlamydia psittaci infection. Infect Immun 1982;35:537-45.

  6. Potter ME, Kaufmann AF. Psittacosis in humans in the United States, 1975-1977. J Infect Dis 1979;140:131-4.

  7. Schachter J, Dawson CR. Human chlamydial infections. Littleton, Mass: PSG Publishing Company, Inc. 1978.

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