The parasite stealing the Lyme light
Infection-spreading ticks are on the increase, which has serious implications for man and game
Ticks are on the increase; John and Matt Holland look at the implications
When it comes to dangerous wildlife, and compared to many other countries around the world, the British countryside can be regarded as reasonably safe. However, there is one little beast lurking in the undergrowth that can cause a debilitating disease and it’s on the increase. The creature in question is the ixodid tick. It has the potential to transmit Lyme disease, a bacterial infection that, left untreated in humans, can affect the heart, joints and nervous system.
Lyme disease can be caused by three species of bacteria, Borrelia burgdorferi,
Borrelia garinii and Borrelia afzelii, which are carried by the ticks and passed on to their host during blood feeding. In the UK, Borrelia is usually transmitted by the deer or sheep tick, Ixodes ricinus. It can be difficult to diagnose Lyme disease, not only because of the variety of symptoms but because ticks can transmit other bacteria, viruses, protozoa and nematodes that cause similar symptoms and can be present in the same tick.
Unfortunately, incidences of Lyme disease are on the increase. From 1997 to 2000, there were 0.38 cases for every 100,000 people per year being reported in England and Wales; by 2011, this was up to 1.73 (Public Health England [PHE], 2013). Many cases may, however, go unreported and PHE estimates that there could be almost three times as many cases each year (NHS Choices, Lyme disease). In Scotland, the figure is much higher, being around 5.53 cases per 100,000 but reached 56 cases per 100,000 in 2009/10 (Slack et al, 2011).
THE BIOLOGY OF TICKS
There are more than 20 species of tick in the UK, many of which are specialist parasites of wildlife. The sheep and deer tick, Ixodes ricinus, is most abundant in southern England,
particularly the New Forest and the southern counties from Devon to Kent, and also the Lake District and Scottish Highlands. Ixodes
ricinus feeds on a range of animals, from small mammals to deer, birds (including gamebirds) and also domesticated animals such as dogs, horses, sheep and cattle. This species of tick is most commonly found in woods, heaths and moors but it can occur in parks or gardens. They tend to be in long grass, bracken or other undergrowth that provides the high humidity essential for their survival. They can be particularly abundant in areas where there are many deer because they are hosts for the adult stage.
Ticks have complex lifecycles that last about two to three years. Starting from an egg laid on soil, they develop into larvae, nymphs and finally adults. Typically, they will only feed once during each of the final three life stages.
They increase from larva the size of a speck of soot to adults that are approximately 2mm long, reaching up to 10mm when fully gorged. The larvae typically feed on small mammals or birds, from which they can acquire the infective bacteria. Having fed, they then drop to the ground, moult into nymphs and seek out (known as questing) another small vertebrate host, passing on Borrelia in the process. Once they have fed, the nymphs again drop to the ground and moult into adults and again quest for a new host. In the case of the sheep tick, these are usually larger mammals such as sheep or deer and again the bacteria may be passed. Dogs are also a common host. Ticks will mate at this stage so the large mammals are important for survival. Once gorged, the adults drop to the ground to lay their eggs.
The adult stage is usually found on dogs whereas on humans it is the minute larval and slightly larger nymphal stage, which is why they can go unnoticed.
Ticks can be affected by environmental conditions because they spend most of their time in the vegetation, only questing when the temperature and humidity are right. If it’s too hot or too dry they will die, so wetter autumns and springs allow them to be active for longer.
The relationships between ticks, their mammalian hosts and Borrelia are complex. The Borrelia multiplies within the smaller vertebrates such as rodents and birds (known as reservoir hosts) and not larger ones such
Tick bites can be prevented by keeping legs covered, wearing long-sleeved shirts and using insect repellent
as deer, sheep, dogs and humans. However, Borrelia, once within humans and dogs, can start to attack the central nervous system causing the symptoms of Lyme disease. On a more positive note, the proportion of ticks carrying Borrelia is typically very low in the UK (0% to 10%) but beware in the hotspots where it can be high; it is also on the increase in some urban areas.
While still rare, the risk of contracting Lyme disease is on the increase in the UK. Once infected, there are three stages of the disease. The first occurs between three and 36 days and usually results in a single circular red rash that does not itch but spreads outwards, giving the appearance of a bullseye. Mild, flu-like symptoms can also occur. The second stage, which not all cases progress to, occurs over weeks or months. Symptoms can include joint pains, neurological problems, inflammation around the brain, heart problems and rashes. Stage three is persistent Lyme disease, which has multiple symptoms affecting the joints, skin, nerves, heart and brain. In the latter it can cause disruption of balance, memory and personality. Chronic fatigue is also described.
Diagnosis in stage one is from the rash or flu-like illness, especially if associated with a tick bite, and fortunately antibiotics are usually an effective treatment with no persistent effects. However, symptoms can vary between individuals and for other species of Borrelia a rash may not always occur. Stages two and three are more difficult to diagnose but blood tests may help.
Further information is available online. However, if you believe you have been infected then you should consult with your doctor and let them know you have spent time where ticks occur. Tick bites can be prevented by keeping legs covered and wearing long-sleeved shirts, using insect repellent on exposed skin and not lying down in long vegetation. All parts of the body and head, especially the armpits, waist and groin, should be checked for ticks immediately after visiting areas where they occur, and any attached ticks should be removed quickly and correctly.
in game wildlife
Although greater movement and abundance of deer aids the spread and increases the profusion of ticks, their higher numbers also help to dilute the infection rates. Another uncertainty is whether the increase in the number of released pheasants is exacerbating the spread of Lyme disease. Pheasants are a reservoir host for Borrelia so have the potential to maintain or spread the disease. Those working with game should therefore be especially vigilant for tick bites and take precautions to prevent being bitten.
Besides transmitting diseases, heavy tick infestation would appear to be harmful, particularly to chicks where it can cause mortality. This is most likely to occur in areas where tick burdens are high and with ground-nesting birds, which is why red grouse are vulnerable.
In rough, hill-grazing areas another serious disease, louping ill, can also be transmitted by the sheep tick. This usually poses a threat to red grouse but can cause high mortalities (60%) in newly introduced sheep flocks; even acclimatised flocks can suffer 5% to 10% mortality (Sargison, 2008).
There is still much to learn about ticks, their hosts and disease transmission. In addition, how the management of wildlife and its movement around the countryside influences the risk of ticks carrying Lyme and other pathogens needs further investigation. You can help with this by sending ticks to the tick surveillance scheme run by PHE. Meanwhile, it is best to be vigilant and take precautions to avoid tick bites, especially if, unfortunately, you are one of those people they seem to find more attractive.
Above: one of the sneaky places ticks hide on dogs Below: a circular rash is a symptom of Lyme disease
Top: Ixodes ricinus, the sheep and deer tick Left: you are most at risk in woods, heath and moor
Greater movement and abundance of deer has aided the spread and increase of ticks