As concern grows over the spread of ticks that carry Lyme disease in the U.S., a young mom’s death is raising questions about whether the illness can be misdiagnosed.

Claire Diss — a British woman whose family says her doctors thought she was suffering from ALS (amyotrophic lateral sclerosis) before she was diagnosed with Lyme disease — passed away on February 13. She was 35.

“It is with a broken heart that I write Claire's final update,” her husband announced on Facebook, signing his name with Emilie, the couple’s 3-year-old daughter.

“Wherever you are imagining Claire is right now, she is able to run, jump and show us that wonderful smile. She will be hugging Emilie and dancing around us all, finally able to move her body again.”

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Lyme disease is caused by a bacterium carried and transmitted by deer ticks. People often don’t know they’ve been bitten, but most develop a telltale red rash.

Most cases can be treated successfully with a few weeks of antibiotics, but if left untreated, the infection can spread to joints, the heart, and the nervous system, the CDC notes. About 30,000 cases of Lyme disease are reported each year in the U.S., but the agency says the actual number of people diagnosed could be 10 times that figure.

Most U.S. cases are concentrated in the Northeast and upper Midwest, but there’s concern about what’s next. The disease has been spreading across the country over the past several decades, the Entomological Society of America notes. In Michigan, the number of human cases has increased five-fold in some parts of the state, a new study found. The authors weren’t sure what’s behind the spread of the ticks and the disease they carry.

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Another recent study confirmed deer ticks infected with the bacterium that causes Lyme are present in nine national parks in the East, including Acadia National Park, Gettysburg National Military Park and Fire Island National Seashore.

“If you have very bad Lyme disease, you can be severely disabled in the acute and near-acute infection,” Dr. Harold Weinberg, clinical professor of neurology at NYU Langone Medical Center, told TODAY.

It can lead to facial paralysis, brain infections and spinal cord infections, he noted. In about 1 percent of cases, Lyme disease bacteria enter heart tissue in a potentially-deadly condition known as Lyme carditis.

However, it would be very unlikely for Lyme disease to be mistaken for ALS, said Weinberg, who was not involved in the case of Claire Diss but commented in general.

Diss’ family did not respond to requests for comment, but she left behind a chronicle of her illness online.

She had visible tick bites and a “blotchy red rash around them” when she was admitted to the hospital with meningitis in August 2014, she wrote on Facebook. Doctors did not seem to pay much attention — “I guess they are not educated on what to look for,” she wrote.

Diss, who lived in Chelmsford, England, was later diagnosed with ALS and told she could expect to live only two to five more years, according to her fundraising page. But when some of her symptoms were inconsistent with ALS, a friend suggested she get tested for Lyme disease. The first result came back negative, but a subsequent test confirmed she had Lyme.

She traveled to the U.S. for treatment, where a doctor ordered “aggressive IV antibiotic therapy,” according to her fundraising page. “He doesn't believe this is ALS, but instead complex Lyme neuroborreliosis,” — Lyme disease affecting the nervous system — she wrote.

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Diss complained that she struggled with her balance and walking, and had deteriorating strength in her arms and hands. She described her health issues as a “living horror story.”

She returned to the UK last fall, but said the side effects of the treatment left her unwell and bed ridden. She had become very ill and died after contracting a chest infection and then pneumonia, her husband wrote.

Most people should not be worried about doctors misdiagnosing ALS and Lyme, Weinberg said. ALS is a pure muscle and motor disease — it involves strength, while Lyme disease can affect strength but also always involves sensation, so patients get numbness or tingling that would not be seen in ALS, he noted.

“So only by the most atypical presentations of Lyme disease… could it possibly be mistaken for ALS,” Weinberg said. “It would stretch the imagination to see how the two could present in the same way.”

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