Ticks & Tick-Borne Diseases

What are Ticks?

Tick-borne Diseases

 
Ticks are excellent vectors for disease transmission. They are second only to mosquitoes as vectors of human disease, both infectious and toxic. They can transmit a wide variety of pathogens, including bacteria, protozoans and viruses to humans and animals.
 
As our native ticks do not attach to humans, ticks would not be of any concern to human health in New Zealand if it was not for the introduced species of the cattle tick (Haemaphysalis longicornis) that is known to transmit theileriosis, babesiosis, tick-borne encephalites and several other diseases.
Every year there are interceptions at the border of other tick species (Heath and Hardwick, 2011), which can carry diseases of greatest concern to human health including Lyme disease, ehrlichiosis, babesiosis, rocky mountain spotted fever, tularemia, tick-borne encephalitis and tick-borne relapsing fever, as well as major animal diseases including babesiosis and anaplasmosis. Therefore personal protection and tick control is very important as is biosecurity and border control.
  Lyme disease is caused by Borrelia burgdorferi, a bacterium carried by the blacklegged tick or deer tick (Ixodes scapularis), the western black legged tick I. pacificus and the sheep tick I. ricinus in Europe and by I. persulcatus in Asia. These ticks are usually found feeding on cattle, sheep, horses, dogs and cats.
Lyme disease is endemic to North America and Eurasia. Symptoms include fever, headaches, fatigue and a distinctive “bullseye” rash on the skin (Erythema migrans). Infection can spread into the heart, joints and nervous system if it remains untreated.
Lyme disease can usually be confirmed through a blood test to detect the presence of antibodies designed to fight the disease. However, it takes six to eight weeks for the antibodies to show up, and so a blood test done soon after contracting the disease may be negative (falsely indicating absence of Lyme disease). Even after the disease has progressed and antibodies are present, the tests may sometimes still be negative when the disease is present (a "false negative" result). If there are signs of early symptoms, especially the telltale rash, immediate treatment is usually advised. As there is no certain cure for Lyme disease the blood test will continue to be positive for life, however it can be effectively treated with antibiotics. The earlier the disease is treated the better the prognosis for complete recovery. Successful treatment of the disease will not prevent contracting getting Lyme disease again. A Lyme disease vaccine is now available.
Rocky Mountain Spotted Fever (RMSF)
Rocky Mountain spotted fever (RMSF), like all rickettsial infections, is classified as a zoonosis. Zoonoses are diseases of animals that can be transmitted to humans. RMSF is caused by Rickettsia rickettsii, a bacteria that is transmitted to humans by the American dog tick (Dermacentor variabilis) and the Rocky Mountain wood tick (D. andersoni) in the United States, and Amblyomma cajennense in South America. Despite its name, RMSF is found in many areas outside of the Rockies, occurring throughout North, Central and South America.
These ticks are vectors and primary reservoirs for this bacterial pathogen. Mice, deer, grounding-feeding birds, wild rodents, and dogs are also reservoirs. Risk factors for contracting the disease are those who are frequently exposed to dogs and who live near wooded areas or regions with tall grass.
Symptoms of RMSF include sudden onset of fever, headache, and muscle pain, followed by the development of a rash. It can be difficult to diagnose in the early stages of the disease and can prove fatal if it is not treated. As the name implies, the illness presents with a very distinctive rash that indeed looks like spots.
The rickettsia are introduced into humans after an infected tick feeds for more than 6 hours. The tick bite is painless and frequently goes unnoticed. Tick-borne relapsing fever (TBRF) is caused by several species of spiral-shaped bacteria (Borrelia spp.) that are transmitted to humans through the bite of infected soft ticks of the Ornithodoros genus. Most cases occur in the summer months in mountainous areas of the Western United States. TBRF is a disease characterized by relapsing or recurring episodes of fever, often accompanied by headache, muscle and joint aches and nausea.
  Ehrlichiosis is a tick borne disease caused by several species of bacteria in the genus Ehrlichia, which are pathogens that cause disease in humans, dogs, cattle, sheep, goats, and horses. Currently, three species of Ehrlichia in the United States and one in Japan are known to cause disease in humans. There are three distinct ehrlichioses in the United States. The first is caused by the bacteria Ehrlichia chaffeensis, is transmitted by the lone star tick (Amblyomma americanum) which occurs in South eastern and southern central parts. Human granulocytic ehrlichiosis (HGE) represents the second recognized ehrlichial infection of humans in the United States. The name for the species that causes HGE has not been formally proposed, but is carried by the black legged tick (Ixodes scapularis) and the western blacklegged tick (Ixodes pacificus) in the United States.
The third and most recently discovered ehrlichiosis is caused by Ehrlichia ewingii which has so far been limited to a few patients in Missouri, Oklahoma, and Tennessee. The full extent of the geographic range of this species, its vectors, and its role in human disease is currently under investigation.
Symptoms of Ehrlichiosis include headaches, myalgia, rigors and vomiting.
Sennetsu fever, caused by Ehrlichia sennetsu, in Japan is characterized by fever and swollen lymph nodes. This disease is very rare outside the Far East and Southeast Asia, and most cases have been reported from western Japan.
Canine ehrlichiosis is caused by Ehrlichia canis which is transmitted by the brown dog tick (Rhipicephalus sanguineus). Symptoms of canine ehrlichiosis include lameness and fever.

 

Babesiosis is an uncommon malaria-like parasitic disease caused by piroplasms, protozoan parasites of the genus Babesia. Babesia microti uses the same tick vector (Ixodes scapularis) as Lyme disease and HGE, and frequently occurs in conjunction with them. Babesiosis in humans is a rare potentially fatal disease, but is a common infection in animals. People can be infected with both babesiosis and Lyme disease at the same time.
Babesiosis occurs in the north east of the United States, especially the offshore islands of New York and Massachusetts. Cases have also been reported in Wisconsin, California, Georgia, and in some European countries.
Babesiosis causes a disease very similar to Malaria. Infection with Babesia parasites can be asymptomatic or cause a mild non-specific illness, and therefore many cases go unnoticed. In mild cases, people may experience mild fevers and anaemia. In more severe cases, fevers go up to 105°F / 40°C with shaking chills, and anaemia (haemolytic anaemia) can become severe. Organ failure may follow including adult respiratory distress syndrome.
In animals, there are a number of Babesia species which cause disease. Babesia canis rossi causes canine babesiosis and is vectored by the brown dog tick (Rhipicephalus sanguineus). This tick will feed on a wide variety of mammals, but dogs are the preferred host in the United States and appear to be required to develop large infestations. Canine babesiosis symptoms include fever, anorexia and anaemia.
Out of at least six Babesia species that have a considerable impact on livestock health and productivity, two species, Babesia bovis and Babesia bigemina have the greatest affect. Both of these piroplasms cause bovine babesiosis (tick-fever or cattle-fever) in cattle, economically the most important tick-borne disease of cattle worldwide.
Babesia bigemina is transmitted by Boophilus ticks while Babesia bovis is vectored by ticks of the genera Boophilus, Rhipicephalus, and Ixodes. Particularly severe forms of this disease can include a severe haemolytic anaemia.
B. bigemina is distributed wherever Boophilus ticks are encountered, which includes North and South America, Southern Europe, Africa, Asia and Australia. No vaccine against babesiosis is available.
  Tick-borne encephalitis (TBE) or tick-borne meningoencephalitis (FSME) is a tick-borne viral infection of the central nervous system affecting humans as well as most other mammals. Caused by a member of the genus Flavivirus, the tick-borne encephalitis virus (TBEV) which has two subtypes; the European subtype, vectored by the sheep tick (Ixodes ricinus) and the Far Eastern subtype (Russian spring-summer encephalitis virus (RSSEV)), vectored by the taiga tick (I. persulcatus).
The ticks act as both the vector and reservoir for the TBEV. The main hosts are small rodents, with humans being accidental hosts. Large animals are feeding hosts for the ticks, but do not play a role in maintenance of the virus. The virus can chronically infect ticks and is transmitted both transtadially (from larva to nymph to adult ticks) and transovarially (from adult female tick through eggs).
The virus can infect the brain (encephalitis), the membrane that surrounds the brain and spinal cord (meningitis) or both (meningoencephalitis). The disease is incurable once manifested, but infection can be prevented by vaccination, and the virus can be inactivated, halting disease progression. In humans the disease can be fatal. Person-to-person transmission has not been reported although vertical transmission from an infected mother to foetus has occurred.
TBE is an important infectious disease in many parts of Europe, the former Soviet Union, and Asia, corresponding to the distribution of the ixodid tick reservoir. Tularemia (also known as "rabbit fever" and "deer fly fever") is a disease that was first recognized as a plague-like disease of rodents in 1911 in Tulare, California. It is caused by a highly infectious bacterium (Francisella tularensis) that is widespread "in nature”, occurring in a variety of wild animals, in water, and even in soil. The bacterium is not dependent on arthropod transmission, but can be transmitted by the lone star tick (Amblyomma americanum) and also from deer flies. This disease has a worldwide distribution, but exists primarily in the northern hemisphere, including Asia and North America. Most cases occur in the south central United States.
General symptoms include fever, headache, chills, nausea, and dry cough. An ulcerated lesion at the site of the bacteria inoculation (e.g. a tick bite) occurs in about 80% of patients. There is no vaccine for the general public, but one is available for people in high-risk occupations.  

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