A tick that lives for decades and bites without being felt

Most tick-borne disease advice assumes you will find the tick. Ornithodoros
ticks defeat that assumption completely.

**They can live for decades, and once infected with relapsing fever
Borrelia spp., can transmit the bacteria to humans throughout their
lifetime through brief and painless bites that are often not detected.**

They live in rodent nests and burrows rather than on vegetation, so the
exposure is to a place, not a walk.

Where in the United States

The exposure data matches exactly. Among 177 western patients with
documented exposures: cabins in 131 (74%) and camping in 15 (8%). Among
11 central Texas patients, four had documented cave exposure.

The decade

251 cases in 11 states, 2012–2021, a median of 24 a year (15 in 2020 to
41 in 2014), no significant change over the period (p = 0.21).

Median age 39 (range 2–92), 60% male. **55% were hospitalised. No
deaths.** Four states account for more than three-quarters: **California
(33%), Washington (18%), Colorado (14%), Oregon (12%).**

In the western states there is a clear **summer peak: 154 of 217 cases (71%)
during June–September** — cabin season.

Why it is called relapsing fever

The initial illness — **high fever, often with headache, nausea, muscle and
joint pain** — lasts about three days. Untreated, **febrile episodes can
recur every 7–10 days for two or more cycles**, because of **the spirochetes'
ability to repeatedly evade the host's immune system**. Each relapse is the
bacterium changing its surface faster than antibodies can be made against it.

Rare complications: neurological and ocular disease, myocarditis, acute
respiratory distress syndrome. **Infection during pregnancy can result in
pregnancy loss, transplacental transmission and neonatal death.**

Treatment is highly effective — doxycycline, beta-lactams such as
penicillin or ceftriaxone, or azithromycin.

The old test is still the best test

Microscopy is most sensitive during a febrile episode, because fever
coincides with high levels of spirochetes in the blood — so when the blood is
drawn matters as much as what is done to it. Looking down a microscope still
outperforms molecular testing here, which is not the usual direction of travel.

Four reasons the count is too low

The report is unusually direct about its own limits:

1. Underrecognition and underdiagnosis — and some state health departments
   have noted misdiagnosis as Lyme disease, because the antibodies
   cross-react with some Lyme serologic assays. **Hard tick relapsing
   fever** (Borrelia miyamotoi) complicates it further: **most serologic and
   PCR assays do not distinguish the two.**
2. Underreporting — states rely primarily on providers to report
3. Inconsistency — only 7 of the 12 reporting states used a case
   definition at all, and the definitions differed. **There is no nationally
   standardised case definition**
4. It is not reportable everywhere — a small number of cases have
   historically come from Oklahoma, Kansas, Ohio and the US Virgin Islands

And a structural gap: **a large proportion of cases occur in non-resident
visitors** — someone who holidays in a mountain cabin and falls ill at home.
**Cases in returned visitors who live where STRF is neither endemic nor
reportable are more likely to be missed** by clinicians and public health
alike.

Avoid rodent-infested structures and rodent habitats such as caves where
the disease is endemic. It is the only prevention there is, because you will
not feel the bite.

Source: Centers for Disease Control and Prevention, MMWR.