Lyme Disease Transmission
Lyme disease transmission occurs through the bite of an infected tick, with the blacklegged tick (Ixoxes scapularis) the main vector in the northeastern and midwestern United States and the western blacklegged tick (Ixodes pacificus) responsible for the spread of Lyme disease on the Pacific coast. Ixodes ricinus and Ixodes persulcatus are the main vectors across Europe, Russia, and Asia. Ticks that spread Lyme disease bacteria can bite any part of the human body but tend to attach to areas which are difficult to see, hard to reach, and where they are unlikely to be disturbed as they feed. Checking the scalp, armpits, groin, and even between toes is important following outdoor activities in a Lyme endemic area. To transmit the Borrelia bacteria responsible for Lyme disease the ticks must be attached to the host for 36-48 hours although some infection may occur more quickly (between 24 and 36 hours) if the bacteria are already present in the ticks’ saliva rather than in their midgut.
Adult and Nymph Tick-Bites
Immature ticks, or nymphs, are most likely to cause infection after biting a human as these are very small (less than 2mm) and, therefore, very difficult to spot and remove in time. Nymphs tend to feed during the spring and summer which is the reason for most cases of Lyme disease diagnosis occurring at these times, or in later summer as symptoms progress. The transmission of Lyme disease from nymph bites is less common where Ixodes persulcatus is the predominant tick type, with the adult ticks much more likely to spread the disease. Adult ticks are usually easier to discover and remove promptly as they are larger. They also tend to be more active in the cooler months making it more likely that clothing will act as a barrier to the skin. Assessing the risk of Lyme disease in an area involves finding out if the area maintains a large tick population and whether or not these ticks are infected with Borrelia. The rate of infected ticks varies quite considerably in different locations with, for example, only around 5% of ticks in Maryland thought infected with Borrelia burgdorferi compared to Lyme-endemic areas in the northeastern US with 50% or so of ticks infected. Tick infection rates in the Pacific coastal states is thought to be around 2-4% although tick-testing is not as prevalent there so specific figures may not be representative.
Lyme Disease Bacteria in Tick Saliva
Once a tick bites and starts to take a blood meal from its host the Borrelia bacteria in the tick’s gut will detach from the gut wall and begin making their way up into the tick’s saliva. Tick saliva contains certain proteins which can enable the spirochaetes of Lyme disease to be transmitted to the host without prompting an acute immune system reaction. The increased presence of bacteria in the saliva of the Ixodes persulcatus tick however increases the likelihood of Lyme disease transmission in a shorter space of time. Borrelia afzelii, the bacteria that causes the majority of Lyme disease cases in Europe is also transmitted faster than Borrelia burgdorferi which is more commonly found in the North American tick population. Preventing tick bites and removing ticks promptly if bitten is, therefore, even more important in Europe than in the US where the transmission time-frame may be a little longer. Lyme disease transmission is still considered rare however, with only around 1% of recognized tick bites thought to result in Lyme disease.
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