K7EE2A Eastern equine encephalitis virus, computer illustration. This is an RNA (ribonucleic acid) virus from the Alphavirus genus. It is an arbovirus (arthropod-borne virus) and is transmitted to humans, birds and horses by mosquito bite. Symptoms in humans range from mild flu-like symptoms to inflammation of the brain, coma, and death.
We find out what’s new in the veterinary world as species-hopping infections come under the spotlight
Going viral
In the current climate of Covid-19 infection, there has been increased awareness of the unusual property of some infections suddenly to skip between species. C19 itself is known to be a virus that came originally from a wild animal, probably a bat, and then jumped species, acquiring the ability to infect humans.
Over generations, most pathogens develop to live in a kind of balance with their host. After all, the virus or the bacterium would not survive and persist if it killed every infected host animal. This host-pathogen balance means that the majority of infections are specific to one host, having evolved together for a very long time. Just occasionally, however, an infection that has been present in one species for ages hops to another.
Molluscum contagiosum (MC) is a human skin infection caused by a virus in the pox-virus family. Most common in children under 10, the infection causes inflamed spots or nodules and can spread widely on the body, limbs and face. It almost always clears up after a few weeks and treatment is rarely necessary.
Research has revealed that an MC virus has been isolated from a horse in Tanzania, East Africa, and that genetic sequencing of the virus shows close similarity with human MC. Whether or not this horse acquired the infection from a person is not clear, but it illustrates that such viruses can affect species other than their primary host.
Virologists recently discovered a cluster of horses in Brazil infected with vaccinia, the virus used in human smallpox vaccination. It seems that the vaccinia virus spread from vaccinated humans and is now circulating in a limited area from horse to horse.
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Psittacosis, a bacterial infection of birds, has a worldwide distribution and is caused by Chlamydia psittaci.
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Human doctors are well aware of psittacosis because it is easily passed from pet birds to people, causing a serious disease with clinical signs that include fever, pneumonia and profound weakness. In Europe, wild birds such as feral pigeons are often infected with a less virulent form of the disease than that carried by wild parrots elsewhere in the world.
Vets in Australia were puzzled recently by two cases of unexplained late abortion in mares on a stud in Victoria. The usual causes were ruled out and foetal tissues were examined in the laboratory for evidence of unusual pathogens. Surprisingly, C. psittaci DNA was discovered in both cases.
A month later, on another stud 75km away, a weak foal was born which died after 24 hours. Tissues from this foal also revealed C. psittaci DNA. The lab scientists were able to sequence the bacterial DNA from these foals; when they compared the strain with other known strains of psittacosis, they discovered that it was exactly the same as that harboured by the wild parrots of Victoria.
These cases confirm what we have come to realise through the C19 epidemic: occasionally, and inexplicably, bacterial and viral pathogens can jump from one species to another. Thankfully, scientific work has so far enabled us to spot and to combat such occurrences.
Further reading
MC: Journal of General Virology 2020, DOI 10.1099/jgv.2020.001357
Vaccinia: Frontiers in Microbiology 2018, DOI 10.338/fmicb.2018.00402
Psittacosis: Australian Veterinary Journal 98, 11 570-573
Our equine veterinary expert has more than 30 years experience in practice. She is an FEI vet and has attended the past three Olympic and two World Equestrian Games in her role. She has a particular interest in anaesthesia, pre-purchase examinations, ophthalmology and dermatology amongst other clinical concerns. She has been Horse & Hound’s official veterinary advisor since 1991. Karen is part of the Bell Equine team.
