Fantastic new research by Niewiadomska and Gifford sheds light how extraordinary complex evolution can be and how human actions can create newly emergent disease.
The human actions involved were all good-meaning and intended as beneficial for the conservation of biodiversity and to combat human disease: captive breeding in zoos and human medicine. Unforeseeable circumstances and accidents have allowed a pathogen to jump hosts between classes of animals (mammals to birds) and circulate in new hosts due to medical procedures – production of vaccines – and livestock propagation in a globalized world.
Whilst the new strain causes no or only mild disease in birds, it shows how unexpect the results of human mitigated evolution can be. It is a model example of the unpredictability of evolutionary trajectories and unintended effects of human actions. We must take care when manipulating natural systems, including the potential effects of modern biotechnology.
A group of viruses – reticuloendotheliosis virus, REV, a retrovirus – is now widespread chickens, turkeys, pheasants, ducks, and geese. Sometimes, REV causes disease. Interestingly, the genetic make-up of REV is not related to other avian retroviruses. Where does it come from and how did it get into birds? Historical evidence (from records of animal keeping in Bronx Zoo to records from medical research), phylogenetic data from viruses, birds and mammals (i.e. information on evolutionary relationships using genetic data) and paleovirological evidence (i.e. viral ‘fossil record’ that is created by the transmission of a virus’ genetic material into the genetic material of a host during infection) support the following scenario as the most likely scenario:
- From a mammal to a pheasant in a zoological garden. Genetic analysis has confirmed that the avian REV is nearest related to retroviruses in mammals. REV are found in two Malagasy mongoose species – ring-tailed and narrow-striped mongoose – which are commonly being exhibited in zoos. Under certain circumstances, viruses can “jump” between species, even as distantly related as between species from different classes such as birds and mammals. A sad reminder is the evolution of the influenza virus in the early 20th century, which has killed 100 million people and more. Unusual overlap of species distribution and near contact can create such “certain circumstances”. Zoological parks – where animals, which do not occur together in the wild, are forced into close proximity – have often been involved in such jumps. Zoos have been involved in many cases in the involuntary transmission of pathogens into new species and into the wild. Importantly, all avian REVs are highly related, meaning that the entire avian REV lineage almost certainly derives from a single founder event. Because the avian REVs are nearest related to mammalian REVs, indicating that the founder came from a mammal. Altogether, it is highly plausible that REVs were transmitted between mongooses and the Borneo firebacked pheasant, Lophura ignite, which was housed in the New York Zoological Park, now Bronx Zoo.
- Creating an experimental model system for malaria research using the Borneo firebacked pheasant at Bronx Zoo. In the 1930, the urgent need for an experimental model system for malaria research arose. Such a model system was found in a Plasmodium parasite isolated from blood of one of the aforementioned Borneo firebacked pheasant. The newly isolated plasmodium was named after it’s host, Plasmodium lophurae, and was isolated only once in 1937.
- Use of the newly found experimental model system for malaria research. P. lophurae was transmissible to young chicken. For experimental vaccine and drug research purposes, the Plasmodium required to be maintained over time and was propagated in poultry, chicken, duck, and turkey. Thus, the pathogen was experimentally – deliberately -adapted to be transmissible in a foreign host species.
- Infection of the experimental, avian-derived model system with mammalian REV. The culture was distributed to many US laboratories. At one point, the culture as contaminated with retroviruses from the (mammalian) REV lineages. The infection occurred either at a later stage during propagation or, more plausible, straight from the beginning, when P. lophurae was isolated from the pheasant (see above).
- Worldwide spread from poultry to wild birds. Today, REV is widespread in poultry and circulates in wild birds.