The Lyme disease tick life cycle is approximately two years long although some ticks may take three years. Humans are rarely hosts to tick larvae and most tick bites are from nymphal ticks in their second year of life. Some tick types however are more reticent to bite humans as nymphs and are only really problematic during the adult stages of their lives. Understanding the behavior of the ticks endemic to a local area aids in developing appropriate Lyme disease prevention strategies and public health warnings especially as ticks in the various stages of their lives are active at different times of the year.
In the first year of a tick’s life they are laid as eggs by adult ticks in the Spring before hatching as larvae in the Summer. These larvae feed on birds, mice, and other small animals which tend to be close to the ground during the fall. The animals that these larvae feed on may provide a host reservoir of Borrelia bacteria which causes the tick larvae themselves to become infected (this is why Lyme disease is described as a zoonosis). Ticks tend to hide in deep vegetation whilst digesting their meal at each stage before climbing to the tips of grasses as adult ticks to await a passing animal. Larvae usually hatch from a batch of around 2000 eggs and begin searching for food after just a few days. Tick larvae have six legs and are just about visible with the naked eye. These larvae also climb up nearby vegetation and clamp onto the skin of any passing animal. Two or three days of feeding will see the larvae swell by about ten to twenty times their initial weight. These engorged larvae will then drop off back into the dense vegetation to digest their meal. After obtaining a blood meal from these smaller animal hosts the ticks have sufficient energy to move into the next stage of their development and they will be mostly inactive during the Winter of their first year.
When Ticks Bite Humans
In the Spring and Summer of the second year of a tick’s life the larvae change into nymphs which also tend to feed on smaller animals and birds. The six-legged tick larvae moults to an eight-legged nymph that is around 1.5-2mm in size. The nymphs require around four to five days of feeding on a squirrel or mouse and continue to feed from late Spring to early Summer until they have sufficient energy stored to continue their transformation. In feeding on one or more animals those ticks that were infected as larvae will likely transmit the Borrelia bacteria to other host animals and those that were initially uninfected may now become infected. Humans may also be bitten by nymphal ticks and their minuscule nature can make them extremely difficult to detect, thereby increasing the likelihood of them remaining in place long enough for Lyme disease transmission to occur.
In the Fall of this same year the nymphs, providing they have successfully fed, will turn into adult ticks around 4mm in length when unfed. During Fall and into the early Spring of the ticks’ third year of life these adult ticks will start to feed on larger mammals, including deer, sheep, and humans. The ticks will also mate on their animal hosts to create eggs for the next generation of Lyme disease ticks. The female ticks tend to take their meal over a period of seven days, during which time they will also likely mate with a male adult tick, and then drop off to hide in the undergrowth until it is time to lay their eggs. Male ticks tend to stay for longer on their animal hosts, taking smaller blood meals sporadically and mating with those female ticks present. Both male and female adult ticks can swell to the size of a small bean or grape and commonly consume around 5ml of blood. The following Spring will see these adult female ticks laying their eggs on the ground for the cycle to begin again.
Avoiding Ticks – Know Your Enemy
The tick life cycle is commonly separated into three stages, each a year long, but some ticks may only live a year and some up to six years. Feeding on the blood of their animal hosts for only a short time during each period, the ticks require sufficient energy to move onto the next stage of their life cycle. Where feeding is interrupted, say if the tick is brushed off by an animal, the tick will attempt to find another host to complete its meal, thereby increasing the spread of disease. It is in this way that a human may be infected by a tick brought into the house on a dog’s coat for example, or, indeed, vice versa where the dog is infected by a tick brushed off the clothes or skin of their human companions.
Ixodes ricinus in Europe tends to feed on animals between March and October with Spring and early Summer the most active periods during which adult ticks are seeking hosts. A second, smaller peak of activity is sometimes seen in late Summer and Fall. Where the vegetation provides sufficient shade and humidity the ticks can survive for weeks without feeding. Lyme disease outbreaks tend to occur during the summer months due to both the activity of the ticks and the increased presence of humans in the outdoors during warmer weather. The heavy layers of clothing worn in winter also provide an extra deterrent to tick bites during all stages of the Lyme disease tick life cycle.
Continue Reading –> Lyme Disease Prevention – Reducing Tick Exposure