The Invasive Longhorned Tick and Its Potential Health Effects
An Animal of Concern
The longhorned tick feeds on the blood of both birds and mammals, including humans. In some parts of the world, it carries bacteria and viruses that can infect people and cause disease. In these areas it’s classified as a public health threat. The tick is native to East Asia but is also found in other areas, including Australia and New Zealand. It has recently been discovered in the continental United States and is spreading. It hasn't yet been shown to transmit human diseases in the U.S. but is being monitored carefully.
The scientific name of the longhorned tick is Haemaphysalis longicornis. It’s also known as the bush or cattle tick. To the unaided eye, the tick is very small and can be mistaken for a speck of dirt when it hasn’t eaten human or animal blood recently. It’s red brown in colour and has darker markings. When the animal is engorged with blood its appearance changes dramatically. The swollen tick is grey and its dark markings often appear to be yellow, as shown in the photo above. A magnified view of the unfed tick can be seen in the opening screen of the video below.
Ticks aren't insects. They belong to the phylum Arthropoda and the class Arachnida. Spiders also belong to this class. Hard ticks belong to the family Ixodidae (pronounced iks-sod-i-dee) within the class Arachnida.
Ticks are small animals with an oval body. They have four pairs of jointed legs, like spiders. The ends of the legs are hooked. Hard ticks (including the longhorned tick) have a plate-like structure on their back called a scutum. The female's scutum is shorter than the male's.
The capitulum or gnathosoma (feeding structure) of the animals can be seen in the "head" region of the body. Many North American ticks have a long and narrow capitulum, but the longhorned tick has a short and wide one. On each side of the capitulum is a sensory structure called a pedipalp. In between the two pedipalps are the chelicerae, which cut through the skin of the host, and the hypostome, a long, narrow, and often barbed structure that acts like a harpoon.
The hypostome is used to anchor the tick to its food source. This is an important function because ticks need to stay anchored to their host for a long time in order to take up enough blood through their mouthparts. Some ticks secrete a substance that acts as a cement to hold the hypostome in place. The structure is sometimes grooved to facilitate the flow of liquid.
Other Features of the Animals
Some ticks have eyes, which are located on the sides of the scutum. At least some species have a structure called the Haller's organ in the outermost segment of each foreleg. This organ seems to detect a wide variety of stimuli, including specific chemicals, humidity, and heat.
On each side of the body behind the last leg is an opening called the spiracle, which leads to the respiratory system. (The mouth isn't used in respiration.) The anus is located towards the rear of the animal on its undersurface. In the middle of the undersurface is the opening of the reproductive system.
The common name of Ixodes ricinus is the castor bean tick. The photo above gives a close-up view of typical hard tick features. The capitulum, legs, and a spiracle can be seen. The dark plate behind the capitulum is the female's scutum. The smaller scutum of the female compared to that of the male allows her body to expand when she swallows blood. Adult male ticks don't appear to feed on blood, but they may absorb some body fluids from a host.
The Longhorned Tick
If it hasn't eaten recently, the adult longhorned tick is only 3 to 4 mm long. Like other ticks, when it's swollen with blood its appearance changes so much that it looks like a different animal. It's often said to resemble a raisin in appearance if not in colour. It may look like a swollen sac, or it may be swollen and also have indentations as raisins do. Apart from its appendages, it's grey or grey-green in colour. During this stage the tick is about the size of a pea.
Like other ticks, the longhorned species finds a host by a behaviour called questing. The tick climbs to the top of a tall plant and positions itself with its head facing downwards. It then holds its front legs outwards ready to latch onto a passing animal. The tick may wave its legs around as it quests. Ticks don't jump, so they have to quest very near the area where animals or humans travel. They can detect the presence of a host in a variety of ways. Certain chemicals, such as exhaled carbon dioxide, body heat, and vibrations are clues that tell a tick that a host is nearby.
Life Cycle of Haemaphysalis longicornis
The life cycle of the longhorned tick contains four stages: egg, larva, nymph, and adult. The tick exists in male and female forms and reproduces sexually. After fertilization, the female generally lays 800 to 2000 eggs in the soil in mid-summer. Female ticks die after laying eggs. Males often die after fertilization.
Each egg hatches into a larva, which is very small and has only six legs. Like the adult, it feeds on blood. After several days, it molts and turns into a larger nymph. The nymph has eight legs and is about 2 mm long. It's the overwintering stage in the life cycle. The longhorned tick is known for its ability to survive cold winters. In the spring, the larva eats blood and then molts to become an adult.
The longhorned tick also has the ability to reproduce by parthenogenesis, which is the process in which an unfertilized egg produces an embryo. The existence of parthenogenesis means that the female tick has the ability to clone itself and doesn't need to take the time to find a male in order to reproduce.
According to an article written by University of Virginia scientists (referenced below), while Asian populations of the tick exist as males and females, the American population is thought to be entirely female and to reproduce by pathenogenesis.
Longhorned Ticks in the United States
The longhorned tick has occasionally been found on animals arriving in the United States at ports, but until recently it wasn't believed to have become established in the country. It was discovered in the U.S. on August 1st, 2017 but wasn't identified until November 9th. The discovery was made in New Jersey and involved a woman and her pet sheep.
The woman discovered a heavy infestation of ticks on her pet and took some of them to the authorities to be identified. The authorities found that the woman had numerous ticks on her clothing. They later discovered that her property also contained lots of longhorned ticks. Larvae, nymphs, and adults were discovered on the sheep.
The sheep was the only domesticated animal on the property and hadn't left the area for years, so the arrival of the ticks was puzzling. The infested area was later cleared of ticks, but the investigators say that some of the animals may have been missed.
Since the original discovery in New Jersey, the tick has been found in other parts of the state as well as in Virginia, West Virginia, and Arkansas. It appears to be spreading.
The tick lives in tall grass when it isn't feeding on blood from a host. It settles near the surface of the soil where moisture prevents it from drying out. It feeds on the blood of farm animals, wildlife, pets, and humans. At the moment, it seems to be infesting animals instead of humans.
Potential Health Problems
It's unknown whether longhorned ticks are harmful to humans in the United States. There is currently no evidence that they are. The fact that they transmit disease in some parts of the world is a concern, however. Investigators think that the ticks are currently a greater threat to animals than humans, but this situation could change. The situation is being monitored closely with respect to its potential effects on agriculture and on human beings.
In some parts of the world, the longhorned tick transmits the following pathogens to humans:
- Anaplasma phagocytophilum, a bacterium that causes a disease called human granulocytic anaplasmosis and is present in other ticks in the United States
- Rickettsia, another bacterial genus that is already transmitted by some U.S. ticks
- Species of Borrelia bacteria (One species of this bacterium causes Lyme disease in the U.S.)
In some areas the longhorned tick also spreads the Powassan virus, which causes encephalitis, or inflammation of the brain. Like the bacteria mentioned above, the virus is already present in some ticks in the United States.
In addition, the tick is known to transmit the SFTS virus, which isn't present in other U.S. ticks. The name of the virus is an abbreviation that stands for severe fever with thrombocytopenia syndrome. Thrombocytopenia is a disorder in which the number of thrombocytes (or platelets) in the blood is abnormally low. Platelets play a vital role in the blood clotting process. The disease has a ten to thirty percent mortality rate.
A tick-borne disease is passed from one organism to another by the animal. Pathogens enter the tick when it withdraws blood from one host and then contaminate another host's bloodstream as the tick feeds.
Avoiding and Removing Ticks
According to experts, there is no need to panic about the longhorned tick with respect to its effects on humans. It is recommended that people take precautions in tick infested areas, however, especially since these will help to prevent the transmission of diseases by other ticks.
- Avoid travelling through areas with tall grass or dense plant growth. When following a trail through these areas, stick to the centre of the trail so that you don't brush against plants. The same rules apply to pets travelling through the area.
- Tuck the legs of pants into socks so that skin isn't exposed. Also tuck in shirts and wear long sleeves.
- Wear light-coloured clothing so that ticks are easier to see.
- Perform a tick check of human bodies and pets after travelling in a potentially infested area. (Ticks don't transmit disease immediately after attaching to a human. The time varies.)
- Remember to check hidden areas of the body where ticks like to hide, such as under the armpits, in the groin, and on the scalp.
- Consider treating skin, clothing, pets, and farm animals with a tick repellent. (This step requires careful research into the safety of the repellent.)
- If a tick is discovered, remove it with tweezers. Grab hold of the animal close to the skin. Try to pull it out slowly and in a straight line. Twisting the tick may cause body parts to break off and stay in the wound. Wash the wound once the tick has been removed.
Ticks are known to transmit a variety of unpleasant diseases in North America. It's therefore important to take steps to avoid them, whether or not the longhorned tick is dangerous. Hopefully this invader won't hurt us, but in case it does, we need to be proactive.
Hard and soft ticks and tick-borne disease from the University of Missouri Extension
Exotic tick fact sheet from the State of New Jersey Department of Agriculture
Facts about longhorned ticks from Virginia State University Extension
Self-cloning tick in New Jersey from The Conversation
Asian ticks found on a New Jersey sheep from NPR (National Public Radio)
Detection of the tick in Arkansas from the NPS (National Park Service)
© 2018 Linda Crampton