The castor bean tick (Ixodes ricinus), also known as sheep tick or deer tick, is an arachnid in the mite family (so not an insect) which has become familiar to an ever-greater number of people in Northern Europe as the winters have become increasingly mild. It lives by sucking blood from animals and birds (as well as humans) and it is notorious for its ability to spread a number of diseases, with borreliosis/Lyme disease being the best known. This is a disease caused by the Borrelia burgdorferi bacterium, which uses the castor bean tick as its host. It is the most common of the so-called vector-born diseases in Europe, and it is characterised by several stages.
Most people who have been bitten by, or who have seen a tick will have seen an adult castor bean tick with its characteristic small black head and a large bluish-grey abdomen that is filled with blood. If the tick is left undisturbed while it is sucking blood, it will eventually release its grip on the host and bury itself in the ground, where the female will lay its eggs . When these hatch, the small tick larvae, which are about one millimetre in size, find a host and remain attached to suck blood for a few days before falling onto the ground. Over the following months, they then transform into nymphs that then try to attach to a new host. Once the nymph has sucked blood, it begins to mature into an adult tick. Both the larvae and nymphs can suck blood from humans, but they are harder to detect due to their small size and because they quickly fall off, for example if the host scratches themselves. Nevertheless, it is believed that tick nymphs, rather than the adult tick, are responsible for the majority of infections with Borrelia (!).
The Borrelia bacterium can only cross from the tick to animals/humans, and it can take up to a month for symptoms of disease to become noticeable. The Borrelia bacterium was discovered as late as in 1982, while the illness itself, Lyme borreliosis/Lyme disease, was described in the mid-70s when there was a cluster of children who had developed acute joint inflammation (arthritis) in the town of Lyme in Connecticut, USA. Joint pain as a consequence of infection in the joints is one of the early symptoms of the disease.
Lyme disease can be categorised into three stages: early localised, early disseminated and late disseminated.
Early localised disease is characterised by a distinctive rash known as erythema migrans, which is visible as a red ring around the bite itself. Not everyone who is bitten develops this rash, while some people may have them at several places on their body. Fever, reduced general condition and headache are other symptoms that appear in the early phase. If the disease remains untreated, it can progress to what is known as the early disseminated phase, in which numerous organs may be affected by inflammatory changes. Most often affected by far are the nervous system and the heart, though there are considerable variations. Where pains/symptoms last for more than a year, this is referred to as late disseminated disease, which is characterised primarily by symptoms from the nervous system such as fatigue, depression and paralysis, but also joint pain.
This more chronic form has attracted increased attention in recent years, and a lot of research is currently being conducted in this area.
Diagnosis is usually based on a blood test, but in some cases samples of bone marrow or from one of the inflamed joints are required in order to make a diagnosis. Treatment is with antibiotics, though both type and duration are the subject of some discussion. Since the castor bean tick is also a potential carrier of other types of bacteria, a more broad-spectrum antibiotic such as doxycycline or amoxicillin is generally recommended. Treatment should nevertheless be initiated and monitored by a doctor as soon as the diagnosis has been confirmed.