Bacteria Behind Mystery Epidemic in Iceland's Horses

Researchers from the Animal Health Trust (AHT), in partnership with British and Icelandic research and veterinary institutions, have identified the cause of an epidemic of respiratory disease which infected not only Iceland’s native horse population, but also dogs, cats, and humans.

The respiratory disease epidemic in early 2010 was characterized by coughing and nasal discharge. The disease spread through the population of 77,000 Icelandic horses within weeks, leading to a self-imposed ban on their export and significant economic cost to the country. Initially, due to the speed at which the disease had spread, a viral cause was suspected. Investigations by researchers at the University of Iceland showed that only Streptococcus zooepidemicus, was consistently recovered from coughing horses and rare fatal cases of infection. However, this bacterium was also often found in healthy horses.

Scientists from the AHT and the Wellcome Trust Sanger Institute, in Cambridge, England, were brought in to investigate.

“To identify the culprit, we sequenced the DNA from 257 samples of bacteria from diseased animals and people,” said Simon Harris, BSc, PhD, senior staff scientist at the Wellcome Trust Sanger Institute. “This showed that one specific strain of S. zooepidemicus, called ST209, was the likely culprit, and we also found this strain in a human case of blood poisoning. This study highlights, for the first time, how DNA sequencing can be used to identify endemic strains of bacteria and distinguish them from the cause of an epidemic infection.”

Iceland is free of all major equine infectious diseases thanks to the ban on the importation of horses into the country in place since 1882. Consequently, Icelandic horses are particularly susceptible to any new bacteria or virus that crosses the border, and so strict biosecurity regulations are in place to help protect them.

Sigríður Björnsdóttir, DVM, PhD, of the MAST Icelandic Food and Veterinary Authority, used information from owners and veterinarians to build an epidemiological network. This enabled her to identify an equine rehabilitation center where horses exercised in a water treadmill. The water treadmill is thought to have provided the perfect conditions for transmitting the disease as water was splashed up and ingested. Horses would complete their rehabilitation and return home, while incubating the disease, taking the infection with them.

The ST209 strain of S. zooepidemicus found in Iceland has also been recovered from a coughing horse in Sweden and an abdominal abscess in a Finnish horse trainer.

“There are a couple of theories as to how the strain entered Iceland,” said Andrew Waller, BSc, PhD, head of bacteriology at the AHT. “These bacteria are able to survive outside a horse for a week or so, which means the import of contaminated equipment or clothing is the most likely route by which ST209 entered Iceland. However, this particular strain could have even infected a human who travelled to Iceland, before spreading the strain back to a horse and triggering the epidemic.

“We are delighted to have helped uncover the likely identity of the cause of this epidemic,” he continued. “Our investigation highlights the ability of S. zooepidemicus strains to cause disease in animals and people. We found evidence that even endemic strains of S. zooepidemicus were likely causing cases of respiratory disease in Icelandic horses, illustrating that this group of bacteria causes more clinical problems in horses than was previously thought. We hope that raising awareness of the cause of this epidemic, and the likely involvement of a water treadmill as a key factor in disease transmission, will encourage veterinarians around the world to improve disease control precautions preventing future epidemics.”

The study, “Genomic Dissection of an Icelandic Epidemic of Respiratory Disease in Horses and Associated Zoonotic Cases,” was published in mBio.