GENERAL INTRODUCTION: RICKETTSIAL, CHLAMYDIAL AND HAEMOTROPIC MYCOPLASMAL DISEASES

Introduction

The taxonomic diversity of the order Chlamydiales has been significantly expanded over the last 20 years resulting in the addition of thirteen families of genetically related organisms collectively referred to as “Chlamydia-related bacteria”.^{46, 105, 157} In this chapter, we will focus on the family Chlamydiaceae containing the single genus Chlamydia and 12 currently accepted species (C. abortus, C. avium, C. caviae, C. felis, C. gallinacea, C. ibidis, C. muridarum, C. pecorum, C. pneumoniae, C. psittaci, C. suis and C. trachomatis). All of these species are able to infect animals (main hosts are non-human mammals and birds) and some also have the ability to infect humans, with C. trachomatis being the only exception as solely a human pathogen. Chlamydiosis in non-avian livestock is caused by three species, C. abortus, C. pecorum and C. suis, while in birds infection is caused by C. psittaci, C. avium, C. ibidis and C. gallinacea, all of which are the focus of this chapter. The terms chlamydiosis and chlamydia(e) will be used here to refer generally to the diseases and members of the Chlamydiaceae family, except when a reference to specific diseases and species is necessary to avoid confusion.

Chlamydiosis is characterized by the development of a variety of clinical syndromes depending on the species and strain of Chlamydia, the host species, and the affected organ system. A wide range of hosts may be affected by chlamydial diseases, including horses, cattle, pigs, sheep, goats, birds and poultry. Chlamydial strains can be fairly host specific, primarily infecting a given species or type of animal and causing specific clinical diseases, but recent evidence has demonstrated that many chlamydial species are not restricted to one particular host and host species jumping may occur.¹⁵⁶ These diseases may affect the gastrointestinal, respiratory, reproductive and nervous systems, as well as the joints and eyes. The severity of disease can vary from subclinical to life-threatening, but the usual presentation is a mild to moderate 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 Chlamydia 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. Chlamydia psittaci is distributed worldwide and has been reported in nine domestic fowl species and at least 460 free-living bird species frequently kept as pets in 30 different orders.⁶⁶ In birds, infections caused by C. psittaci can be subclinical or result in respiratory, enteric and ocular signs and it is the most important veterinary chlamydial agent in terms of economic and zoonotic impacts. Recently, avian chlamydial species have been extended to include the novel species C. avium, C. gallinacea, and C. ibidis.^{120, 165} Chlamydia avium and C. gallinacea have been reported in pigeons, psittacine birds and mallards (C. avium) and domestic poultry, guinea fowl, turkeys and ducks (C. gallinacea) in Europe, Asia, China, Argentina and more recently in North America, while C. ibidis has been isolated from feral Sacred Ibis. Assessment of their pathogenicity, potential hosts, transmission routes and zoonotic potential warrants further research. More information on avian chlamydiosis due to C. psittaci is available in several reviews.^{121, 161}

Chlamydiosis was first recognized in 1879 by Ritter as a disease that could spread from parrots to humans, when a Swiss physician observed respiratory illness in a household with a sick parrot.¹⁰⁸ 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, Bedson and colleagues, Coles, Levinthal, 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.^{10, 24, 76, 77} In the same year, Bedson and colleagues also established the aetiological relationship between the basophilic bodies and the disease.^{8, 9} 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...