Prescriber 2011: 22(5) (5th March)

Lyme disease

First off, bear in mind that it’s rare.  If anyone who’s just had a tick bite asks you whether they should get antibiotics just in case, advise them against it – in one study of 1000 tick bites, no-one got Lyme disease.

Secondly, erythema in the first couple of days after a bite, or resolving within a couple of days of appearing, is more likely to be an allergic reaction.  You wouldn’t expect erythema migrans to show up for at least three days after the bite (usually between 5 and 14 days, but can be anything between 3 and 30).

Erythema migrans will spread to at least 5 cm diameter and develop the typical central clearing and ultimately ‘target’ pattern, although do bear in mind that that only develops over time and an early rash will simply be a round or oval erythematous patch.  It’s common but not invariable to get systemic symptoms – headache, myalgia/arthralgia, and fatigue. It’s also possible to get the systemic symptoms without the rash (good grief, what a heartsink that must be to sort out), or to be asymptomatic during this stage.

If in doubt, serology may help, but it’s wrong a surprising amount of the time – poor sensitivity in the early days as seroconversion takes 2 – 4 weeks, and not 100% sensitivity even later on.  Also note that the initial test is more sensitive but poorly specific, so an initial positive may well be a false positive – await further testing.  Oh, yes  – high-risk patients such as forestry workers may well have preexisting seroprevalence, as it can take years for positive serology to go back to normal (in one study, 19% of high risk patients were found to be seropositive).  What all this effectively means is that, if you have a barn-door obvious clinical rash, you’re probably better off just treating rather than testing. (In cases where late-stage disease is suspected, on the other hand, it is well worth testing as sensitivity is much greater.)

Treatment at this early stage should be 14 days in duration and can be with doxycycline 200 mg daily (either once daily or divided), amoxicillin 500 mg tds, or cefuroxime 500 mg bd.  For children, use 50 mg/kg/day of amoxicillin in three divided doses, or 30 mg/kg/day cefuroxime in two divided doses at a maximum of 500 mg per dose.  Once you reach the stage of neurological symptoms, you need doxycycline or IV ceftriaxone.

Second-stage Lyme disease may overlap chronologically with the first stage.  (Remember also that the first stage may be asymptomatic, so these symptoms may be the first.)  Seroconversion is much more common by this stage but still not invariable, and a negative test does not rule out the disease.  The symptoms are usually neurological (cranial neuritis, painful radiculitis, or meningitis) but it may rarely present as carditis.  The radiculitis can affect more than one site and may be mistaken for shingles, sciatica, or even renal colic.  Bear it in mind as a possible differential diagnosis of any severe dermatomal pain.  Cranial neuritis most commonly affects the facial nerve and may be bilateral, so that’s one differential diagnosis of Bell’s palsy (and how anyone ever makes it, I don’t know).  Meningitis may be mild and afebrile with intermittent symptoms and thus may get entirely missed.  In short, it just isn’t possible to diagnose on clinical grounds.  Discuss suspected cases with an infectious disease physician, or a neurologist.

Late-stage Lyme disease shows up more than six months later, almost invariably in untreated people – it’s very rare for it to show up if the early stages were adequately treated.  By this stage serology is usually positive but you now come up against the problem of false positives.  This, by the way, is why it looked as though Lyme disease might be a factor in diseases such as CFS or fibromyalgia: it’s possible to have positive serology from previous asymptomatic disease.  However, it’s been found that people with MUS are no more likely to have positive Lyme serology than asymptomatic people.

Anyway, the late stage usually affects joints or skin but can very rarely affect the eye or nervous system.  Joint involvement usually presents with intermittent monoarthritis or oligoarthritis of large joints, most commonly the knee, lasting a few weeks at a time and resolving spontaneously before recurring in the same or a different joint.  Unremitting arthritis or small joint polyarthritis aren’t usually due to Lyme disease.  Treatment gives about a 75% chance of resolution.

Skin involvement consists of subtle bluish-red discoloration and oedema on the dorsa of hands or feet or the extensor surfaces of knees or elbows, progressing gradually over several months to atrophy and sometimes fibrosis or nodules.  Treatment usually improves matters significantly, but peripheral nerves may be affected and, if so, they may not fully recover.

Thiazide diuretics

Some points of interest: Diuretic effect may last for about twelve hours, and usually starts within one hour.  Although they are frequently recommended first-line agents, they aren’t the most successful of second-line agents.  It’s sometimes possible to use them in patients with a history of gout as long as they’re taking allopurinol.  And, if hypokalaemia is a concern, co-administration of a potassium-sparing diuretic is preferable to giving potassium supplements.

Respiratory tract infections

In more severe cases, 7 days of treatment may not be enough – you may need 10 days or, in the case of atypicals, even 14 days.

Pertussis is astonishingly common – it may account for up to a fifth of acute severe coughs.  Treat with 14 days of erythromycin, primarily with the aim of elminating carriage (it can reduce duration and severity of disease in the catarrhal phase but is unlikely to do much once patients are in the paroxysmal phase).

Guidelines for management of acute cough in children are here and in adults are here.

And, last but definitely not least – when patients ask me what they can do to have fewer colds, I can now actually give them an answer.  Regular zinc supplements do reduce the frequency of colds, although they can cause nausea and an unpleasant taste in the mouth.

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About Dr Sarah

I'm a GP with a husband and two young children.
This entry was posted in Infectious Diseases, Prescriber. Bookmark the permalink.

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