Chlamydiosis in livestock is caused by a number of species or strains of bacteria in the family Chlamydiaceae. This family has recently been divided into two genera Chlamydia and Chlamydophila, and nine species.29 In this chapter, the words chlamydiosis and chlamydia(e) will be used to refer to diseases and strains from both genera, except when a reference to a specific species is necessary to avoid confusion.

The disease chlamydiosis is characterized by the development of a variety of clinical syndromes depending on the species and strain of Chlamydia or Chlamydophila, the host species, and the affected organ system. A wide range of hosts may be affected by chlamydial diseases, including horses, cows, pigs, sheep and goats. Chlamydial strains are fairly host specific, each primarily infecting a given species or type of animal and causing specific clinical diseases. These diseases may affect the gastrointestinal, respiratory, reproductive and nervous systems, and joints and eyes. Disease severity can vary from life-threatening to asymptomatic, but the usual syndrome is a moderate to mild clinical disease leading to persistent infection. Recurrence of the disease and/or shedding of the organism can occur following stress or physiological changes such as oestrus. What is currently unknown is the extent of synergistic effects these persistent chlamydial infections may have with concurrent infections with other pathogens.

Avian chlamydiosis is the disease caused by Chlamydophila psittaci in birds.4 It has been called psittacosis and ornithosis depending on the type of bird it was infecting or the type of bird from which it was contracted. Psittacosis and ornithosis are now considered to be the same disease, the term avian chlamydiosis being preferred. However, the avian strains have been known to infect humans and other mammals with severe consequences. Transmission of these strains from animal to animal occurs readily.

The disease in birds is not dealt with in this chapter; several reviews on the avian disease and its diagnosis in birds and domestic fowl are available.1, 3, 4, 108

Chlamydiosis was first recognized in 1879 as a disease that could spread from parrots to humans, when a Swiss physician observed respiratory illness in a household with a sick parrot. Avian chlamydiosis gained world prominence during a pandemic in 1929 to 1930 that involved over 12 countries in Europe and North America. The outbreak led to strict regulations on the importation of parrots from South America. In 1930, Leventhal, A.C. Coles and Lillie independently observed for the first time very small basophilic bodies in the tissues of infected birds and humans, and suggested that they were the causative agent (cited in Pienaar et al. 66). In that year, Bedson and Bland (cited in Meyer55) established the aetiological relationship between the basophilic bodies and the disease. It soon became clear that chlamydial infections were not limited to psittacine birds, but that they were widespread in almost all avian species, and that chlamydiae from other birds were transmissible to humans. Chlamydial infection in pigeons was first reported in South Africa in 1940 by J.D.W. Coles after a pigeon fancier who had been experiencing mortalities in his flock sent two affected birds to the Onderstepoort Veterinary Institute.13

Enzootic abortion in ewes was first shown to be caused by a chlamydial infection in 1950 by Stamp and his colleagues. 95 Since then chlamydial abortion has been recognized worldwide. It was first seen in South Africa, in 1972, when a severe epidemic of abortions swept through the sheep-raising areas within a single lambing season, resulting in up to 60 per cent abortions in some flocks.88 In cattle, Schoop and Kauker86 furnished serological and cytological proof that incriminated chlamydia as the cause of abortions in a dairy herd in 1956. In California, Kennedy and co-workers in 196045 described the pathological lesions of aborted calves and, from some of them, Storz et al. in the same year100 isolated the chlamydial agent.

* For recent changes in the nomenclature of these organisms, refer to the introduction to Section 3, Rickettsial and chlamydial diseases

Because of the sporadic occurrence of outbreaks, the disease was named epizootic bovine abortion. The agent is similar to, or the same as, that causing abortion in sheep, in which the disease is called epizootic abortion of ewes.

Thereafter, chlamydiae in ruminant livestock were also found to be involved in enteritis, pneumonia, arthritis, conjunctivitis, and encephalomyelitis. These diseases are usually caused by the Chlamydophila pecorum strains, which are different from the abortion strain, and outbreaks are usually sporadic, primarily affecting young animals. Morbidity may be high, but mortality is usually low.

Aetiology and life cycle

Articles in the historical literature on chlamydia and its taxonomy contain many later-abandoned terms which are, however, still important in order to understand the literature. Chlamydiae were once classified as viruses and grouped with other filterable organisms that required living cells in which to multiply. Some of the names in the literature include the psittacosis-lymphogranulomatrachoma (PLT) group, miyagawanella, bedsonia, prowazekia, rakeia...

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