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The model, reported in JAMA Network Open, could help clinicians, particularly those in primary care settings, to more rationally prescribe PEP to people concerned about a potential exposure to the rabies virus (RABV). In the United States, rabies PEP often is given without a comprehensive assessment that considers regional factors as well as species, nature of an attack, and the health and vaccination status of the animal.
Providers err on the side of caution, as rabies infection has a fatality rate near 100%. When exposures are low-risk, however, patients can rack up substantial out-of-pocket expenses or experience unnecessary adverse effects from the series of shots. Those can include injection site reactions, hypersensitivity reactions, and neurological complications.
The authors write that an estimated 55,000 people per year in the United States were treated for potential exposure to RABV in 2017 and 2018, at an estimated cost of more than $3,800 per person treated.
Researchers calculate risk threshold
The researchers, led by Kelly Charniga, PhD, MPH, an infectious disease epidemiologist with the U.S. Centers for Disease Control and Prevention in Atlanta, calculated positivity rates using more than 900,000 animal samples tested for RABV between 2011 and 2020. Other parameters were estimated from surveillance data and the literature and probabilities were estimated using Bayes’ rule.
A convenience sample of state public health officials in all states (excluding Hawaii) plus Washington and Puerto Rico was used to help determine a risk threshold for recommending PEP. Respondents were asked whether they would recommend PEP given 24 standardized exposure scenarios while accounting for local rabies epidemiology.
Their model establishes a risk threshold of 0.0004 for PEP administration, which represents the probability that an animal would test positive for RABV given that a person was exposed, and the probability that a person would die from rabies after exposure to a suspect rabid animal and no PEP. PEP should not be recommended with any value lower than that cutoff.
Alfred DeMaria, DPH, a consultant to the Massachusetts Department of Public Health in Boston, who was not involved with the study, said the work will be particularly helpful for primary care physicians, giving them confidence to not recommend PEP when infection is statistically highly unlikely and thereby to reduce unnecessary and costly measures.
“Concern about rabies is often based on a very unlikely scenario,” Dr. DeMaria said. He gave the example of people coming into primary care worried that they might have been exposed after comforting their dog who had been bitten in a fight with a wild animal.
“Has that ever happened in the history of the human species? Not that we know of,” he said.
Many people also think dogs and other domestic animals are a likely source of rabies, which is not the case in the United States, Dr. DeMaria said.
“In most cases, it is exposure to a raccoon, a skunk, or a bat,” he said. “Most calls are for potential bat exposure, especially in the summer when young bats are flying around and are not very savvy about avoiding humans.”
The authors note the difference between the animals likely to bite and the species that carry RABV: “The most common mammals involved in bite events in the U.S. are dogs, cats, and small rodents. These species, when healthy and provoked into biting, represent some of the lowest risk exposures evaluated in this model.”
The canine rabies variant virus was eliminated in the United States in 2004.
The study authors note that their model should not be used in other countries because “most rabies deaths globally are caused by domestic dogs.”