CDC suspects Rocky Mountain spotted fever has a less severe doppelganger
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Researchers say the expansion of lone star tick populations might be spreading a mild form of spotted fever that is being confused with the more serious disease. Pictured here are dorsal views of a female lone star tick, Amblyomma americanum, including an engorged female (right). (Photo by James Gathany and CDC/ASTMH)
When it comes to tick-borne illness, Lyme disease gets a lot of the attention. But another bacterial infection, Rocky Mountain spotted fever, can be just as bad — or worse. Despite the name, it’s found in every state.
At first, it’s just a fever and a headache. Then, there’s a rash. And soon, Rickettsia rickettsii, the bacteria that cause Rocky Mountain spotted fever, can take over.
“It has a case fatality rate in untreated disease that exceeds 20 percent,” said Chris Paddock, a CDC pathologist. “In some countries we see it as high as 40 percent, so it rivals Ebola in some respects in terms of its lethality.”
A surge in the number of cases nationwide over the last 20 years has puzzled researchers because the death toll hasn’t gone up in step.
But new work discussed this week at the American Society of Tropical Medicine and Hygiene meeting in Philadelphia suggests it’s a case of mistaken identity.
“What we think is happening is that more and more cases of these other Rickettsial infections are being recognized, but being misdiagnosed — or misinterpreted is probably a better word — as Rocky Mountain spotted fever,” said Paddock.
The main driver, Paddock suspects, is a rise in the number and spread of the lone star tick, which often carries another spotted fever bacterial species, Rickettsia amblyommii. That species appears to be less dangerous to humans than Rickettsia rickettsii, but because it’s so similar, anyone previously infected by it might test positive for Rocky Mountain in a blood test.
The hotbed for Rocky Mountain spotted fever is in the American south, especially Arkansas. But next in line is Delaware, with the sixth highest infection rate in the country.
Delaware state epidemiologist Paula Eggers isn’t sure why.
“There is some beauty to being small, so it may be reflective of our surveillance system and our ability to conduct the surveillance for the cases,” she said.
She hasn’t noticed an increase in Rocky Mountain cases, although the CDC hypothesis seems reasonable.
The good news, Paddock said, is that even with all the confusion, all types of spotted fevers can be successfully treated with the same antibiotic.
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