Darwin Chronicles

June 2006

Zoonosis reversal: Animals contracting MRSA from humans?

ATLANTA — Microbiologists from the Ryan Veterinary Hospital at the University of Pennsylvania have isolated methicillin-resistant Staphylococcus aureus (MRSA) from companion animals, Shelley C. Rankin, PhD, said at the 2006 International Conference on Emerging Diseases.

Since 2002, the clinical microbiology laboratory has collected 77 MRSA isolates from 56 animal cases. “Is MRSA of companion animals a zoonosis, or is it a reverse zoonosis?” she asked. Zoonotic transfer from the companion animal reservoir back into humans has been documented, she said.

Although Rankin said the answer would take more investigation, she speculated that humans are infecting their animals. “We must remember that S. aureus is not part of the commensal flora of dogs. So, when we find it there, and we find it in association with an infection process, then the chances are that there has been some mechanism by which those dogs and cats and rabbits acquired that infection: probably from a human,” said Rankin, who is an assistant professor at the Ryan Veterinary Hospital in Philadelphia.

“In July 2002, we identified our first confirmed MRSA. By the end of 2002, we had another four cases. So, we thought, that’s not too bad really. From July 2002 until the point we put this data together, we’ve identified 77 isolates from 56 individual cases.

“Those are mostly dogs and cats, but we’ve had three parrots and one rabbit,” Rankin said.

She presented characterization data for 39 of the isolates at the conference. Eleven of the isolates were SCCmec types II and three were SCCmec IV, which indicated that most of the strains were health care-associated. “That is not at all what we were expecting. We assumed we were going to see community-acquired MRSA,” she said.

All of the isolates were susceptible to vancomycin, trimethoprim-sulfamethoxazole, gentamicin, and chloramphenicol; almost all were resistant to erythromycin and the fluoroquinolones, she added. In addition, they found that six isolates were resistant to linezolid, which is not a drug used in animal health, she said.

“Linezolid is not a drug that is used in veterinary medicine. I guess there are some vets who would think about using it, but certainly in our hospital, it is not part of the formulary,” Rankin said.

“When we started looking at this data, we assumed that if MRSA was present in companion animals, we were going to see community-acquired strains, rather that health care–associated strains, so then the big question became: Is there a problem with our own facility?” she asked.

They did a retrospective case review and found that six cases were “almost certainly” infected at the hospital, but they were sporadic, not clustered by date or location. Three were isolated from animals in the ICU, but the small outbreak was quickly contained. Another 12 cases might have been related to the hospital, but it was hard to confirm this.

For example, they treated a dog “who presented to the emergency service with a laceration on his right carpus, but he had been to the local vet to have that wound dressed and then he was referred to VHUP, and the MRSA was isolated three days post-admission to our hospital, there are so many questions here. How was the laceration caused? Was the local vet implicated or was it something that happened at our own hospital?” she said.

However, of 38 cases, 21 definitely had no hospital-associated risk factors, according to Rankin.

“So what are the implications for public health? MRSA is not prevalent in companion animals, but it can cause infection,” she said. “Our dermatologist, Daniel Morris, DVM, has just completed a study that shows that 0.1% of cats and dogs admitted to our hospital have an MRSA.”

No one knows the implication for human or animal health at the moment, she said, but she does think that this is a case of humans transmitting infections to animals. “The high prevalence of SCCmec type II MRSA strains in this study suggests direct transfer from humans to animals. All we have to do is confirm it,” she said.

This may be difficult to do, however, because veterinarians are not permitted to ask people about their own medical conditions; they can only treat animals.

—Marie Rosenthal

NJ cluster of botulism reported after man serves salted fish at party

ATLANTA — It was almost unbelievable, but it happened: A New Jersey man got one of the world’s rarest diseases – twice. About 15 people a year get foodborne botulism in the United States. That’s one in 20 million, according to a recent presentation at the recent International Conference on Emerging Infectious Diseases.

The man presented to a New Jersey emergency department (ED) on Independence Day, July 4, stating that he thought he had botulism. How did he know?

“In 1992, this patient had been the index case of a botulism outbreak in New Jersey from a salted fish that he and three relatives had eaten,” said Jeremy Sobel, MD, of the CDC.

In the new case, which occurred in July 2005, the man, patient A, tasted a home-salted fish in his home on July 3, experienced symptom onset on July 4 at 2 p.m., and presented at the ED two hours later. A party was held later at the patient’s home, where the salted fish was served. A 49-year-old woman (patient B) and a 54-year-old woman (patient C), who ate the fish, presented at a different ER, suffering from gastrointestinal (GI) symptoms, and were diagnosed with gastroenteritis.

“The 16-year-old daughter of patient C brought the patients to the hospital. She made no complaints, she was sitting in the ER waiting area, when she was told by the ER doctor that her mother might have botulism,” Sobel said.

The ED staff related the following conversation, according to Sobel: “She says, ‘OK, I’m going to call my dad.’ So, she takes out her cell phone, she looks at it. She holds it near; she holds it far, and she says, ‘hey do you think you can dial for me? I’m having trouble seeing the numbers.’ The alert ER staff appropriately concluded that she might be exhibiting the early neurologic symptoms of botulism.”

The girl reported no previous eye problems. She admitted being at the party and eating fish, becoming patient D, according to Sobel.

Meanwhile, a 66-year-old woman, in yet another ED became patient E, when she was admitted with nausea, vomiting and diarrhea. She was also diagnosed with gastroenteritis.

“All patients experienced symptom onset within less than 24 hours of exposure, and two patients within less than four hours,” Sobel said. Both of the patients with neurological symptoms received antitoxin within less than 12 hours of presentation.

Botulinum toxin type E was identified in fish remnants collected from the trash at patient A’s home, but there was not enough to perform quantitative assays, Sobel said.

This was considered conclusive proof that salted fish containing botulinum toxin E was the cause of the outbreak. GI symptoms predominated, but botulinum toxin could not be confirmed. Two of five of the patients had neurological findings consistent with mild botulism.

Failure to detect toxin in early serum suggests sub-threshold toxin levels, and possibly, there were low levels of toxin in the fish, according to Sobel.

“How did the fish become contaminated with toxin? Fresh, ungutted whitefish, was placed with salt in a Ziploc bag that was then sealed and left on the kitchen countertop for 30 days,” he said.

This “recipe” dangerously recreates the anaerobic conditions that favor the growth to botulinum-toxin producing bacteria found in fish intestines.

The investigation was limited by the incomplete history obtained from patients, and the number of institutions involved. Patients with mild GI symptoms were discharged early with no follow-up, Sobel said.

– Marie Rosenthal