Vaccination of koalas with a prototype chlamydial vaccine is safe, does not increase the incidence of lymphoma-related disease and maybe associated with increased lifespan in captive koalas
Hernandez-Sanchez, J, Brumm, J, Timms, P & Beagley, KW 2015, Vaccine, vol. 33, no. 1, pp. 4459-4463.
An experimental Chlamydia vaccination previously tested on a population of captive koalas at Lone Pine Koala Sanctuary has been shown not to increase the incidence of neoplasias, and may also increase the lifespan of captive koalas. The median lifespan for koalas treated with the prototype vaccine was 12.5 years; 3.7 years longer than that of unvaccinated koalas. A comparison of vaccinated and unvaccinated koalas revealed that the vaccine did not increase the incidence of lymphoma or leukemia, and the vaccination is therefore considered safe for koalas.
As well as protecting against a target disease, vaccines can also have non-specific effects that may improve the resilience of the immune system to other pathogens. These non-specific effects may be the result of a combination of two factors. The first is heterologous immunity, which occurs when immunity to a previously encountered pathogen alters an immune response to an unrelated pathogen. The second is trained innate immunity, which occurs when the innate immune system is ‘trained’ to react against pathogens additional to that being targeted directly. The non-specific protection afforded by the prototype chlamydial vaccine for koalas in this study improved immunity to several potential bacterial and fungal pathogens and ectoparasites.
Of the multiple threats causing declines in koala population numbers across Australia, mortality from disease is one of the most significant. Chlamydia pecorum is the most prevalent pathogenic chlamydial species that affects koalas and can cause keratoconjunctivitis and blindness, cystitis and urinary soiling or ‘wet bottom’, inflammation, fibrosis, scarring, and ultimately infertility. The development of a vaccine to prevent the risk of infection with this pathogen in wild koala populations could, therefore, deliver great benefits for koala conservation. There is a risk, however, that such a vaccine could increase the incidence of lymphoma or leukemia in koalas. The development of these life-threatening neoplasias is commonly associated with koala retrovirus (KoRV). Recent evidence suggests that KoRV is currently endogenizing; a process by which the virus becomes part of the genetic material of the host species. If KoRV is endogenizing, the risk associated with the chlamydial vaccine is that the activation of the immune system may enhance the process of viral replication and therefore increase the likelihood of neoplastic development.
Given these potential risks, the finding that this experimental chlamydial vaccine was not associated with an increased incidence of lymphoma or neoplasia is promising for its future use in wild koala populations. The capacity for the vaccine to enhance immunity to a broad spectrum of parasites, fungi, and bacteria with the effect of increasing the lifespan of captive koalas indicates the additional health benefits of the vaccine beyond its effectiveness in providing Chlamydia-specific immunity.
Summarised by Joanna Horsfall
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