Infectious Diseases and Immunology of the Koala

The koala, a native Australian marsupial, is experiencing population declines in the wild due to a number of natural and anthropogenic factors. Infectious pathogens of conservation concern include a gammaretrovirus known as koala retrovirus and the obligate, intracellular bacterium Chlamydia pecorum.

A recent review article, published in Developmental and Comparative Immunology, provides an overview of the current literature focusing on infectious diseases and immunology of this unique animal.

Koala Immunology

Despite their divergence from placental mammals more than 148 million years ago, marsupial species (including the koala) have a complex immune system. The koala possesses an intact innate immune system and also produces adaptive humoral and cell-mediated immune responses to infectious pathogens, including Chlamydia.

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Newborn koalas, or “joeys,” rely on innate and passive forms of immunity, including ingestion of immune cells and immunoglobulins in milk. Although immune tissue development has not been studied explicitly in the koala, research in other marsupials indicates that these tissues mature while the joey is still in the mother’s pouch. The thymus, which is the site of T-cell maturation, develops completely by 8 months of age; however, it begins to involute after sexual maturity at 2 to 3 years of age, thereby increasing susceptibility to disease in adult animals.

Secondary lymphoid tissues, including the spleen and lymph nodes, are relatively small in the koala but still initiate an adaptive immune response. The mucosa-associated lymphoid tissues are intact on the body’s mucosal surfaces, including the lung and eye; however, poor development of the gut-associated lymphoid tract may be due to the germicidal effects of the koala’s main diet of eucalyptus.

The Relationship Between Koala Retrovirus and Chlamydia Infection

Scientists believe koala retrovirus was first introduced into the koala population via an intermediate host. Prevalence in native Australian populations is currently estimated between 18% and 100%, depending on locale. The virus may be either exogenous (horizontally transmitted) or endogenous (vertically transmitted) with viral sequences inserted into the koala genome. Similar to other retroviruses, including HIV and FeLV, koala retrovirus appears to cause immunosuppression in the host and likely increases susceptibility to opportunistic diseases, including C pecorum and neoplasia. In support of this belief, one study found that Chlamydia-infected koalas had elevated koala retrovirus RNA concentrations in the blood.

C pecorum infection in the koala targets the mucosal tissues of the reproductive tract and eye. Infection may be asymptomatic or, alternatively, may have severe, persistent effects, including infertility and blindness. Immunologic studies show that infection initiates a strong humoral response with increased IgG and IgA concentrations against Chlamydia antigens, a finding that may allow for the future development of a koala-specific vaccine against Chlamydia.

Saving the Koala

The Australian Koala Foundation estimates current wild koala populations to be fewer than 100,000, and the species faces continued threats including infectious disease. The review article’s authors hope that increased understanding of koala immunity will enable vaccine development against important pathogens, namely C pecorum, to ultimately facilitate conservation of this vulnerable species.

Dr. Stilwell received her DVM from Auburn University, followed by a MS in Fisheries and Aquatic Sciences and a PhD in Veterinary Medical Sciences from the University of Florida. She provides freelance medical writing and aquatic veterinary consulting services through her business, Seastar Communications and Consulting.