Infection with spirochaetal bacteria of the genus Borrelia through the bite of an infected tick is considered the principle cause of Lyme disease which is contracted by over 20,000 people in the US each year according to The Centers for Disease Control and Prevention (CDC). Lyme disease is transmitted by the ‘hard ticks’ of the Ixodes genus and infection is thought to occur after the tick has been in place for around 24hrs as the Lyme disease bacteria migrate from a tick’s mid-gut to the saliva. This means that a tick usually has little opportunity to spread infection as those bitten will often have brushed off the tick either consciously or accidentally in the first few hours after being bitten. The increased presence of bacteria in the saliva of the Ixodes persulcatus tick however is likely responsible for infections in cases where a tick is removed after only a few hours as opposed to days. Borrelia afzelii, the bacteria considered culpable in many European cases of Lyme disease, is also transmitted faster than Borrelia burgdorferi which is more commonly found in the North American tick population.
Lyme Disease Transmission
Only around 1% of recognized tick bites are thought to result in Lyme disease infection although the incubation period of 3-30 days prior to the bacteria spreading though the skin into lymph nodes and the bloodstream may make it difficult to connect a bite to subsequent symptoms. Furthermore, the bite is easy to ignore as the ticks’ saliva contains substances which inhibit the pain, itchiness, or irritation of a bite and, therefore, allow the tick to continue feeding from the host for longer whilst spreading the infection. Tick saliva also contains substances which control the body’s immune response and likely allow the bacterial spirochaetes to proliferate upon entering the body (Machácková, et al, 2006). For a long time, tick bites were the only proven way of contracting Lyme disease, although in recent months there has been new evidence that Lyme disease could be a sexually transmitted disease. There remains no conclusive evidence in support of Lyme disease transmission during pregnancy via the placenta to the foetus (Woodrum, et al, 1999). The risk of developing the disease through blood transfusion also appears to be low and most scientists regard an infected individual as a ‘dead end’ host at extremely low risk of transmitting the infection to another human (Gerber, et al, 1994).
Seasonal Lyme Disease Infection and the Species Involve
Most new cases of Lyme disease are diagnosed during the summer or fall months following infection in May or June. This is due to the infection being spread whilst ticks are in their nymphal stage rather than as larvae, although this varies according to geographical region. The adults, rather than the nymphs, of the species Ixodes persulcatus, for example, are thought to be largely responsible for transmitting the infection to humans (Kurtenbach , et al, 1998). Borrelia burgdorferi was the Borrelia species first identified in the 1980s as responsible for Lyme disease but this species has now been reclassified as just one of several strains of the bacteria which have come to be known as the group Borrelia burgdorferi sensu lato. This group includes B. burgdorferi sensu stricto, B. afzelii, and B. garinii amongst others. B. afzelii and B. garinii are largely responsible for cases of Lyme disease in Europe whereas the burgdorferi species is considered the causative factor in most cases of Lyme disease in North America. There are at least eleven Borrelia species, five of which are strongly associated with Lyme disease and two more of which are thought to be accountable in a small number of cases (B. bissettii, and B. valaisiana). Those with a strong association to Lyme Borreliosis include B. burgdorferi sensu stricto, B. garinii, B. bavariensis, B. afzelii, and B. spielmanii.
The Nature of the Borrelia Bacteria
The Borrelia bacteria are Gram-negative, spirochaetal bacteria which have both an inner and outer membrane. This affects both their detection in the bloodstream and their eradication as they appear to be able to express different surface proteins in order to mimic a patient’s own cells so as to go undetected by the immune system. This ability to mimic the body’s own cells is also implicated in the possible continuing autoimmune reaction that some Lyme disease patients display and which may be erroneously diagnosed as chronic Lyme disease. There is also considerable genetic variation between the Borrelia bacteria which can make testing imprecise and inaccurate and often requires a two-fold approach to identifying the presence of infection and the nature of the bacteria. Lyme disease is classed as a zoonosis as it is an infectious disease transmitted from another vertebrate animal, in this case by a vector (the tick). Not all infected animals act as a ‘reservoir’ for infection however, with some testing positive for antibodies to the bacteria or evidence of the presence of bacterial DNA but not leading to infection of ticks or larvae feeding from that animal.
Animal Hosts of Lyme Disease Bacteria
The types of tick that carry the bacteria vary between countries, favoring different host animals such as sheep and deer, along with a variety of rodents, birds, cattle, and even feral cats. Lizards also play host to some ticks and were largely thought for many years to have a zooprophylactic action on the Borrelia bacteria, actually cleansing the ticks of the infection. The evidence now supports their role as hosts of ticks infected with at least one kind of Borellia species B. lusitaniae, which has implications for environmental-engineering strategies for disease control (Dsouli et al., 2006). Such strategies are based on the idea that infected tick populations and, therefore Lyme disease incidence in humans, could be controlled by engineering the animal environment. Experiments in Monhegan, Maine, and Mumford Cove, Connecticut appear to have succeeded to some degree in reducing the incidence of Lyme disease by reducing the numbers of those animals (such as deer) which act as hosts of infectious ticks.
Engineering the natural environment is impractical in most areas however, and the focus in Lyme disease prevention has remained on developing awareness of the importance of checking for ticks and promptly, and safely removing those found. Some strategies for medicating wildlife such as deer have been developed such as the 4-poster deer treatment bait station which uses pesticide-impregnated rollers to transfer tickicide to the ears of the deer where most of the tick population resides, this is then transferred to other body parts during grooming and appears effective at reducing the tick population in the areas used although long-term effects on the deer and other wildlife are unknown.
The tick responsible for carrying the Lyme disease bacteria in Europe is known as the sheep tick or castor tick and is of the species Ixodes ricinus. Ixodes scapularis is the predominant carrier of the disease in eastern North America and is referred to as the deer tick or black-legged tick. The disease-carrying ticks are not thought to be endemic in the central states of North America or in the Southeastern US as far as Texas as the tick population here is made up largely of the lone star tick (Amblyomma americanum). The Amblyomma americanum is thought responsible however for a condition with similarities to Lyme disease, but milder, known as southern tick-associated rash illness. Lyme disease also occurs in China and is attributed to the presence of Ixodes persulcatus, known as the taiga tick.
Species-Specific Symptoms of Lyme Disease
The type of tick and the type of Borrelia strain present in an area has implications for the development and treatment of the disease. This is because each strain appears to give rise to specific features of Lyme disease at different stages. The B. burgdorferi senso stricto is most associated with arthritis, for example, whereas B. garinii is associated with neurological symptoms and B. afzelii with acrodermatitis chronica atrophicans (a chronic skin condition). The occurrence of the characteristic erythema migrans ‘bullseye’ Lyme disease rash also varies with a much more rapid appearance following infection with B. afzelli than with B. garinii. The latter is more likely to cause systemic symptoms than the localized rash and may, therefore, lead to an increased risk of misdiagnosis in those infected with that particular bacterial strain (Bennet, et al, 2006), making it doubly important to consider a differential diagnosis for Lyme disease.
Remaining Vigilant Against Deer Ticks
Knowledge of the specific bacterial strains, their favored ticks, and the ticks’ favored animal hosts has progressed rapidly in recent years leading to a vast information network for researchers into Lyme disease. Increasing knowledge of the tick life-cycle and periods of activity or inactivity is also important as this provides an opportunity to give good quality advice on Lyme disease prevention and act as an early warning system for those residing, or holidaying, in high risk areas. General advice is to increase vigilance when engaging in high-risk activities such as gardening, hunting, or hiking, especially when in long grass. Dogs pose little risk of acting as tick hosts unless they are outdoor pets, whereas feral cats can present a risk; checking pets for ticks following outdoor activities is good practice however. The tiny, poppy seed-sized, ticks are also very difficult to spot which is why wearing light clothes can help locate them early. Making sure to check yourself and your children for ticks after returning indoors from outdoor pursuits is also important and it helps to shower thoroughly following outdoor activity in an area where such ticks are present.
Where a tick is observed it is important to remove it carefully with tweezers so as not to detach the head. Keep the tick in a jar and watch for any signs of infection. If Lyme disease is suspected then take the stored tick to the appointment with the doctor as it may prove useful in identifying the type of infection through a tick test. As always, prevention is better than cure, and it is sensible to wear lightly colored clothing including long-sleeved shirts and pants, with pants tucked into boots/socks when walking in long grass. Check frequently for ticks, including on the scalp as they can quickly move up the body, and use specific tick repellent where necessary.
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