The 2016 outbreak of H7N2 influenza in cats in a New York shelter, with transmission to one veterinarian, got a lot of attention, including a few posts on our blog. H7N2 influenza is known to have been circulating in birds in the US since at least 1994, and two human infections had been identified (a person involved in culling infected birds in 2002, and an immunocompromised person with no bird contact in 2003). However, the strain hadn’t been identified since 2006.

A recent paper in Clinical Infectious Diseases (Lee et al 2017) provides information about the transmission and public health response during this most recent outbreak. Here are some highlights:

  • The index case was a cat that was admitted to the Manhattan, NY shelter 12-Nov-2016. It died Nov 25, and tested positive for H7N2 influenza virus.
  • Widespread (no details, though) infection of cats was subsequently identified in the shelter, with no infections detected in dogs, chickens or rabbits.
  • 385 people adopted a cat from the shelter between Dec 15 and Dec 21. Of those, 188 were contacted and agreed to be interviewed. Three of them had experienced disease potentially consistent with influenza and were tested. All were negative (but they only tested for virus shedding, which doesn’t last long, so I wouldn’t consider that definitive).
  • 265 people worked or volunteered with cats at the shelter. Between Dec 16-18, 165 of those were interviewed and tested. 24 had experienced flu-like illness and influenza virus was detected in one, but it was seasonal flu (H3N2) so not associated with the cats.
  • On Dec 19, a veterinarian who had helped collect throat swabs from healthy cats at the shelter reported flu-like signs. The vet had prolonged exposure to cats during sampling and did not use a mask or other respiratory precautions.  H7N2 influenza infection was diagnosed, treatment with oseltamivir (Tamiflu) was prescribed and the person recovered uneventfully.
  • The viral isolate from that vet was nearly identical to that from the cat. Those viral isolates were similar to the H7N2 strain that was circulating in birds in the early 2000s and those that caused the two earlier human infections.
  • No human-human transmission was identified among the vet’s contacts. The same day the vet got sick (often the peak time of virus shedding) the person traveled by commercial airline. Some contacts on the plane were reached and no illness was detected.

This report highlights a few interesting issues.

  • Infection with this flu virus had not been detected in many years. Where it came from is a great and unanswered question.
  • Infection of the cats was also very notable, since they were not known to be susceptible. This should be a reminder that “not knowing” whether a species is susceptible to a flu strain is not the same as “knowing that they are not” susceptible.
  • It’s surprising that, in the course of a flu outbreak investigation, typical infection control precautions (e.g. wearing a mask) weren’t taken when cats were being sampled.
  • The human infection is likely one of these oddball infections that occurred because of a confluence of factors (e.g. a cat happened to be shedding a lot of virus, the person did things that increased exposure, their immune system didn’t handle exposure like it would most of the time), particularly since no other people were identified as having been infected.

Fortunately, the outbreak in cats ended and only one person was affected. The H7N2 strain didn’t cause serious human disease or spread widely, which is always the concern with zoonotic influenzas. A big concern with flu is if a new strain becomes able to infect another species, and it causes serious disease and spreads widely, it could have devastating effects. The fact that seasonal flu (one that spreads readily in people) and H7N2 were present at the same time in people associated with the shelter shows the potential for flu viruses to meet up and potentially swap RNA.  That’s part of the reason why influenza in animals is tracked carefully in most regions.