Bug behind farmer’s flesh-eating infection was probably caught from his horses

A farmer required repeated surgeries after a bug he probably caught from his horses developed into a flesh-eating infection that destroyed his thigh muscle.

Details of the case in the 73-year-old Norwegian farmer have been described in a case report in the journal BMC Infectious Diseases.

The bacteria responsible for the infection was identified as Streptococcus equi subsp. zooepidemicus.

The bacterium is able to colonize the upper airways of horses and can produce a diverse range of problems, including respiratory tract infections, mastitis and meningitis.

S. zooepidemicus is primarily an animal pathogen and has been isolated from other mammals, including cows, rabbits, and pigs. It occasionally produces severe infections in animals, including the exceedingly rare necrotizing myositis − the most severe form of necrotizing soft tissue infections.

S. zooepidemicus rarely causes human infection, probably having been transmitted by direct contact with infected or colonized animals, or the consumption of unpasteurized milk products.

Reported human infections by the bacterium include cellulitis, pericarditis, toxic shock syndrome, endovascular infections, pneumonia, septicaemia, meningitis, arthritis and spondylodiscitis.

Bård Reiakvam Kittang and his colleagues said that the previously healthy farmer fell ill following close contact while feeding two symptom-free Shetland ponies in his stable.

He was admitted to his local hospital with acute pain in his left groin. He rapidly developed erythema − that’s superficial reddening of the skin − on his left thigh, then sepsis with multiple organ failure.

Blood cultures were undertaken and antibiotic therapy launched. CT scans of his left thigh and pelvis showed possible necrotizing myositis and he was rapidly transferred to a major hospital.

The clinical course was severe and complicated, the doctors reported, with his medical team battling to rein in the infection.

The study team said it was, to their knowledge, the first reported necrotizing soft tissue infection in a human caused by S. zooepidemicus.

The man required repetitive surgical excision of necrotic muscle, treatment with vasopressors, mechanical ventilation and continuous blood filtration, along with hyperbaric oxygen therapy.

The first surgery revealed extensive muscle necrosis, requiring excision of one of his thigh muscles. Repeated surgical excision of necrotic tissue followed.

The patient was discharged from hospital after 30 days.

The cause of the infection was S. equi, later assigned subspecies identification as S. equi subsp. zooepidemicus.

Further testing identified the strain as a novel type (ST 364), closely related to types previously identified in horses and cattle. It expressed a virulence similar to that of Streptococcus pyogenes.

The case, they said, illustrated the potential of S. equi subsp. zooepidemicus to jump between species, and showed the importance of early clinical recognition, rapid and radical surgical therapy, appropriate antibiotics and adequate supportive measures when necrotizing soft tissue infection was suspected.

The study team said it was likely that early surgery and meticulous surgical follow-up was behind the therapeutic success in the case.

“Prompt and radical surgery along with appropriate antibiotics were the treatment cornerstones in this case, but it is conceivable that hyperbaric oxygen therapy might have contributed to the relatively rapid improvement of the infection.”

Unfortunately, nasopharyngeal swabs were not obtained from the healthy ponies in the present case, they said.

“The patient developed sores and abrasions on his fingers prior to the infection, was in direct contact with the ponies while feeding them, and had no direct contact with other animals before and around the time of infection.

“Hence, a direct transmission of ST 364 from pony to human is suspected and also supported the close genetic relationship between ST 364 and other S. zooepidemicus sequence types from horses …”

They said their molecular data did not allow for firm conclusions on the virulence properties of ST 364. They called for a more thorough genetic and proteomic dissection of this particular strain.

“Although our patient did not have any obvious susceptibility for severe streptococcal disease, the clinical outcome was probably a result of a complex interplay between host factors and bacterial virulence.

“Taken together, we speculate that this case story matches the available microbiological, molecular and clinical data on S. zooepidemicus quite well.

“We suspect that a S. zooepidemicus-strain, perhaps not fit to produce clinically significant nasopharyngeal infection in the ponies, but potentially armed with virulence properties homologous to those of S. pyogenes, was transmitted to a human without known predisposition to infection, causing a severe S. pyogenes-like clinical picture.”

The case-report team comprised Bård Reiakvam Kittang, Veronika Kuchařová Pettersen, Oddvar Oppegaard, Dag Harald Skutlaberg, Håvard Dale, Harald G. Wiker and Steinar Skrede. They are variously affiliated with the University of Bergen, Haukeland University Hospital, and Haraldsplass Deaconess Hospital.

Zoonotic necrotizing myositis caused by Streptococcus equi subsp. zooepidemicus in a farmer
Bård Reiakvam Kittang, Veronika Kuchařová Pettersen, Oddvar Oppegaard, Dag Harald Skutlaberg, Håvard Dale, Harald G. Wiker and Steinar Skrede
BMC Infectious Diseases 2017 17:147 DOI: 10.1186/s12879-017-2262-7

The case report, published under a Creative Commons License, can be read here

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