An Introduction to Medical Entomology
For educational purposes. Quote cited references only.
Ticks differ from insects by having eight legs and other Acarina by having one pair of stigmatal plates located behind the 4th leg pair. They are also much larger with a tough integument. Of only two families the Ixodidae or "Hard Ticks" possess a dorsal shield or scutum and s specialized structure, the capitulum. that is situated at the anterior margin and can be seen from the dorsal surface, while in the Argasidae, or "Soft Ticks" there is no dorsal shield and the capitulum is located on the underside and barely visible from a dorsal view (Matheson 1950). The possession of a capitulum distinguishes the animal. Important differences between "Hard" and "Soft" ticks were summarized by Service (2008) [See: Hard vs. Soft Ticks].
Ixodoideas a comparatively small Superfamily with only about 425 species identified as of 2017. However, they often occur in very large numbers. They are considered of the utmost importance as ectoparasites and disease vectors. They usually are found on mammals and reptiles, but birds and amphibia are also hosts. and the blood and lymph of their hosts is consumed. Life cycles differ among the species with some requiring only one host while others will move to additional hosts. During feeding they may swell up from only 2 mm to nearly 25 mm.
Their distribution is worldwide, especially in tropical areas. Although humans are usually only bothered by two species, Ornithodoros moubata and O. rudis, others may also attack. They serve as intermediate hosts of many important diseases of humans and animals. Just the bites alone can result in severe reactions. Matheson (1950) grouped the effects of ticks and disease into three categories (1) Bites and their effects, (2) Paralysis termed "Tick Paralysis," and (3) Vectors of pathogenic organisms.
All ticks are external parasites of mammals, birds, reptiles and some amphibia. The life cycle has four stages: egg, larva, nymph and adult. All species lay their eggs on the ground or in the environs of their hosts. A hexapod larva hatches from the egg and is very active seeking its host. After feeding the larva drops off and molts on the ground or remains on the host and molts. The nymph has eight legs and a tracheal system. Following another feeding the nymph leaves its host again and molts, or it may remain on the host. The adult stage is similar to that of the nymph save that it has developed genitalia. The Argasidae have several nymphal stages, but the Ixodidae have only a single nymphal stage. Adults do not molt but feed and mate on their hosts or on the ground. Ixodid males die soon after mating and females die after laying eggs. Argasid adults of both sexes live longer, sometimes even several years. There is nevertheless considerable variation in development between both families. A few species are known to reproduce by parthenogenesis.
Being parasites the Ixodidae have developed a simplified body where the main regions from head to abdomen are contiguous. A scutum or shield is located on the dorsum whose size and appearance is diagnostic. Females have a smaller shield than males. The eyes occur near the margin of the anterior portion of the scutum. There are four pairs of legs in adults, but only three in larvae (See Dermacentor andersoni).
A Capitulum near the anterior portion of the body simulates a true head and is useful in the identification of species. The basal portion is the Basis Capituli, which is made u of a broad ring that is constricted to form the neck, which leads to the anterior opening of the body. Extending beyond the ring are the mouthparts that serve for piercing and sucking blood. Palpi arise from the lateroventral margin of the capituli. The first segment is typically short followed by second and third longer ones, and a fourth that is found in a depression on the third segment. The fourth segment regularly has a row of hairs, which are believed to be sensory.
The Hypostome in the shape of an arrow emanates from the median ventral surface of the basis capituli and extends forward underneath the mouth opening. Important and complex cutting organs above the mouth are the Chelicerae, which lacerate host tissue (See Capitulum).
A shield or "scutum" occurs on the top of the body with variable sizes that aid in species identification (See Dermacentor andersoni). The genital opening is situated between the 1st and 2nd or 3rd pair of legs.
There are many genera in this family and the majority is cosmopolitan feeding on mammals, reptiles, amphibians and birds.
IMPORTANT IXODID GENERA
Boophilus annulatus (Say) is the cattle tick of North America into Mexico. It attacks only one host and lays its eggs on the ground. The ticks attach to hosts from the grass on which they reside. It is important as a vector of Piroplasma bigemina or Texas Fever.
Some of the most important North American ticks are found in this genus, and up-to-date information on problem species may be sought online. Following are several of the more common species that may be found:
Dermacentor variabilis (Say), the wood- or dog tick, is widely distributed in North America. It has three hosts, with adults preferring large mammals. The larvae attack mice. It is an important vector of Rocky Mountain Spotted Fever virus and Tularemia.
Dermacentor andersoni Stiles bears the name "Rocky Mountain spotted-fever tick." It is also distributed widely in Western North America. It is a 3-host tick with a complicated life cycle involving different rodents over two years.
Dermacentor occidentalis Marx ranges along the Pacific coast from California to Oregon where it is active during the entire year. It is a 3-host tick that attacks larger domestic mammals as well as humans. A number of rodent hosts are attacked during its life cycle. On humans the bite of this tick can be painful and it can transmit tularemia and possibly Rocky Mountain spotted fever.
Dermacentor parumapertus Neum. of Western North America attacks rabbits primarily. Although it does not frequently encounter humans it is important as a reservoir for the Rocky Mountain spotted fever virus.
Dermacentor albipictus (Pack.) is known as the Elk Tick. Its appearance differs from all other ticks in the genus, and it attacks large game and domestic animals in winter months. Humans are rarely affected by this species that is distributed all over North America.
The taxonomy of this genus continues to be revised, but as of 2017 there were about 55 species known. Some important species are noted as follows:
Ixodes ricinus (L.) is a cosmopolitan tick whose hosts include larger mammals and humans. It is a 3-host tick and a vector of the Louping Illness in sheep and humans. It also vectors Piroplasmosis (Babesia bovis) of cattle in Europe. The life cycle covers a whole year.
Ixodes pacificus Cooley & Kohls ranges from Canada to Mexico west of the Cascade Mountains. It is a 3-host tick and may be important in disease transmission.
Ixodes cookei Pack. is common in eastern North America attacking small mammals, cattle and humans.
The genus contains a large number of species that are common in tropical and subtropical regions of South America and Africa, with some species found in Eastern North America. They are very difficult to identify. Some important species are noted as follows:
Amblyomma americanum (L.), the Lone Star Tick occurs widely in Eastern North America. A 3-host tick that breeds all year long, the larvae and nymphs have a wide host range that includes birds, mammals and humans. The bite causes considerable pain followed by persisting soreness. It has vectored Rocky Mountain spotted fever in the Central United States, and may also spread "Q" Fever, Bullis Fever and Tularemia.
Amblyomma cajennense (Fab.) ranges from South Texas through Central America and eastern South America. It is a 3-host tick where all stages attack domestic and wild mammals and humans. This tick in South America vectors Brazilian Spotted Fever and Tobia Fever.
Amblyomma maculatum Koch attacks livestock and small wild birds and mammals from Eastern North America south through South America. The inflammation resulting from its bite can stimulate attack by screwworms and the death of domestic animals.
Amblyomma hebraeum Koch, the "Bont Tick" ranges throughout southern Africa. A 3-host tick where all stages attack humans, domestic and wild animals. This tick is a vector of "Tick Bite Fever" in humans and "Heart Water " of cattle.
Most species in the Rhipicephalus genus occur in Africa, with only one cosmopolitan species in North America.
Rhipicephalus sanguineus (Latr.), the "Brown Dog Tick" occurs in most of the tropical and temperate world regions. It is a 3-host tick with all stages developing on dogs and sometimes on humans as well. The Rocky Mountain spotted fever virus has been recovered from this species in North America. It is also a vector of canine piroplasmosis (Babesia canis), and may be involved as vector of South African Tick-Bite Fever..
There are only a few species of these small ticks involved in the transmission of disease.
Haemaphysalis leachi (Aud.) has a wide distribution from Africa through Australasia. It vectors "Canine Piroplasmosis" (Babesia canis) and "Tick Bite Fever:" in southern Africa.
The family differs from Ixodidae primarily by lacking a dorsal shield or scutum, and the capitulum is on the ventral surface (See "Examples."). While the underside of the tick resembles the Ixodidae, the top or dorsal surface is very different, and sclerotization is minimal. Also, the palpi resemble the legs of a spider instead of palps.
Argas persicus (Oken), the common fowl tick seeks out domestic fowl, but humans are attacked when in close contact with the preferred hosts. This species is cosmopolitan and occurs primarily in structures where poultry is housed.
Argas mianensis attacks on humans in Iran causes Mianch Fever.
Argas vespertilionis (Latr.) attacks bats in northern Europe, and in Africa, India and Australia.
Argas reflexus (Fabr.) attacks pigeons in Europe and northern South America, but in North America it is rare and does not attack pigeons.
Argus brumpti Neum. is a large species (ca. 20 mm.) from East Africa.
Genus ANTRICOLA: (data being sought).
The genus has a number of species that are important in transmitting human diseases. Some of the most important are the following:
Ornithodoros moubata (Murray) prefers humans in all its developmental stages. It also attacks an array of domestic animals. It occurs in the dry parts of Africa from Lake Chad to the Red Sea and south to southern Africa and Madagascar.
Ornithodoros savignyi (Aud.) occurs in Africa and east to Arabia and India. It is a vector of relapsing fever.
Ornithodoros hermsi Wheeler is a small tick that occurs at higher elevations in the western United Stages where it attacks small rodents. Adults are long-lived and are vectors of relapsing fever.
Ornithodoros turicata (Duges) is a large tick that is abundant in the southwestern and southern United States and sections of central Mexico. It attacks an array of domestic and wild animals and humans. It is an important vector of relapsing fever.
Ornithodoros purkeri Cooley of the western United States. It attacks small rodents and humans. It may be capable of transmitting spotted fevers.
Ornithodoros talaje (Guerin-Men.) is widespread from the southern United States south to Argentina. It attacks an array of mammals, birds and reptiles and can transmit relapsing fever to humans.
Ornithodoros rudus Karsch) attacks humans primarily in Central and South America, and is a vector of relapsing fever.
Ornithodoros coriaceus Koch is a large tick attacking large mammals and gives a nasty bite to humans. It is found from California south into Mexico.
Otobius megnini (Duges) resides in the ears of horses, cattle and other domestic animals. It is widespread in North America south through Central and South America. It is also invaded in southern Africa.
Otobius lagophilus whose principal host is rabbits occurs in the northwestern North America.
The bite of ticks can produce serious illness, and the loss of blood in domestic and wild animals can result in weakness or death. The bites of some tick species can result in wounds that are slow to heal and which may become infected or attract flies that can cause myiasis. Great care should be taken when removing an embedded tick as crushing may cause infections. Various techniques include applying heat to the tick's body or covering it with adhesive tape. An antiseptic should be applied to all wounds
Tick bites can result in "Tick Paralysis" especially in young children and domestic animals. A muscular weakness precedes paralysis that can quickly progress to a loss of leg movement and a spreading to other parts of the body. Removing the tick is essential to avoid respiratory paralysis and death.
As vectors of diseases caused by viruses and pathogens the role that ticks play has been well known by more than a century. Matheson's (1950) detailed reports of some of the important diseases are worth noting.
Table 1. Tick Species That Inflict Harmful Bites
RELAPSING FEVER.-- A large number of relapsing fevers, caused by Spirochaeta spp. have been recognized. These fevers are characterized by repeated attacks that last from 3-5 days. Durations vary from 5-10 days. Causative agents are species of Spirochaeta present in blood, cerebrospinal fluid and other fluids of the body. During the incubation periods they may not be obvious from the blood stream although experiments have shown their presence. Ticks are the vectors of various species of Spirochaeta even though other arthropods are known to play a role sometimes. The presence of the spirochetes in the blood stream during the entire infection period is very significant, especially when prophylactic measures are deployed. Spirochaeta recurrentis (Lebert) was the first species observed to infest the blood of humans in 1868 by Obermeier, and was described and named by Lebert in 1874. Ross (1924) demonstrated that a peculiar fever of West Africa was caused by a spirochete (S. duttoni), and that the spirochete was transmitted to humans by the tick Ornithodoros moubata (Murray). Then Todd (1914 & 1919) demonstrated that O. moubata was the vector of the spirochete. The newly hatched offspring of infected ticks transmitted the disease. Ever since it has been shown that infection in the tick can pass through the eggs even to the third generation. Today many species of spirochetes have been found in the blood of humans and animals. There are believed to be more than 15 species or strains occurring in humans alone.
Relapsing fevers are now found throughout the world, and most are passed across generations through the tick eggs. The way transmission of spirochetes occurs varies for different species. The ticks obtain the spirochetes while feeding on animal blood that is infected. In the tick the spirochetes multiply by transverse fission. They then invade the tissues and body cavity of the tick. After an infected tick bites a new host, the spirochetes gain entrance either through the coxal fluid glands, which eject their secretion of by way of the bite (Davis 1945).
ROCKY MOUNTAIN SPOTTED FEVER.-- Ever since settlement certain areas of the North American west sustained outbreaks of a very fatal disease among the people living there. The disease was first recognized around 1890. A high fever starts the infection followed by arthritic and muscular pains and skin rashes that begin on the ankles, wrists and forehead, but later may spread over the entire body. The disease can run a rapid course, which may end in death after 6-12 days. If the fever drops and the person lives two weeks recovery is typically quick. Two strains of the disease exist, a mild and a virulent type. These appear to be present in most of the geographic regions in which it occurs. Mortality rates vary from 80 percent for the virulent strain to about 4-6 percent for the mild strain. The disease bets its name from its area of origin in the Rocky Mountains of North America. It is not contagious but highly infectious and transmitted solely by ticks. Warren (1946) suggested that ticks carried the disease and Ricketts (1906) showed that the disease is mainly an infection of rodents. Large mammals, except humans, are not susceptible. The tick, Dermacentor andersoni, was found to be the vector for humans. Wolbach (1916) described and named the parasite Dermacentroxenus rickettsi. For years the disease was known only from a small area in the Rocky Mountains, but Rumreich et al. (1931) showed it to be present in eastern North America as well. The dog tick, Dermacentor variabilis, was found also to vector the disease in the east. Today we know that the disease is very widespread throughout North America and down to South America, with Amblyomma cajennense being the principal vector in the Southern Hemisphere.
There are many different species of ticks now known to vector the disease over its range. Of course, transmission can only occur from infected ticks. The incubation in humans after infection varies from 2-12 days. Vaccines have been developed for the disease, which can reduce or eliminate symptoms entirely.
TULAREMIA.-- This is a kind of plague in rodents caused by the bacterium Pasturella tularensis. It was first discovered in California rats by McCormack (1921). It was later isolated from squirrels and described by McCormack (1921). Francis (1919, 1920, 1921) showed that "Deer Fly Fever" in humans and the disease of rodents were identical, being caused by the same organism. It was later named "Tularemia."
Tularemia is very infectious to humans being transmitted by several arthropods through their bites or crushed bodies, or by the feces and body fluids of rodents. It is spread throughout North America, Europe, North Africa and Japan. There are many natural reservoid hosts. Burroughs et al. (1945) listed 44 bird and mammal hosts from different parts of the world. Infection of humans occurs through contact with reservoir hosts, especially rabbits. The bacterium is very infectious being able to penetrate human skin. Handling or being around infected animals can result in infection. Some arthropods are important as natural reservoirs and also in transmission to humans. Francis (1921) first demonstrated that the deer fly, Chrysops discalis can vector the disease.
AUSTRALIAN "Q" FEVER.-- This fever was found to occur among the meat handlers in Queensland. The causative organism was found to be Rickettsia burneti by Burnet & Freeman (1937). A related fever "American Nine-Mile Fever" was found in Montana in 1938 with the infectious agent, Rickettsia diaporica, being isolated from the tick Dermacentor andersoni (Davis et al. 1939-1943; Cox 1940). Later it was found that both incitants are identical organisms.
The bandicoot rats and other bush animals are reservoirs in Australia with the tick Haemaphysalis humerosa probably being the main vector. Dermacentor andersoni, Dermacentor occidentalis and Amblyomma americanum are vectors in North America. Infection of this disease is also very common by inhalation around infected animals and meat.
Q Fever was a serious problem among troops during World War II in Europe with different strains being involved.
COLORADO TICK FEVER.-- Occurring in the Rocky Mountains of North America, the disease is associated with the bite of Dermacentor andersoni. A rash does not occur and the fever is a remittent type with rare fatality.
BULLIS FEVER.-- The fever was reported this disease from Texas in 1943 and was isolated from troops during World War II. The vector was reported as Amblyomma americanum because of its frequency around infected individuals (Matheson 1950).
TICK TYPHUS.-- This disease has been found from different parts of the world. The tick Dermacentor nuttalli Olenev was reported as a vector in Russia, with rodents being reservoirs. It seems to be spread in India, East Africa and the Americas, but detection is not always certain.
BOUTONNEUSE FEVER.-- First reported from Tunisia by Conor & Bruch (Matheson 1950) the disease is known to be widespread in Europe and Ethiopia. The causative agent is Rickettsia conori with the tick vector Rhipicephalus sanguineus. Rodents and domestic pets serve as reservoirs for the disease.
SOUTH AFRICAN TICK BITE FEVER.-- Closely related to spotted fevers caused by rickettsia, this disease was called Rickettsia rickettsi conori or a strain of R. r. pijperi in southern Africa. The reservoirs are dogs and the vectors the dog ticks, Haemaphysalis leachi, Amblyomma hebraeum and possibly Rhipicephalus sanguineus. However, only the larval stage was believed to transmit the pathogen. It is usually associated with tick bites followed by a sore and lymphadenitis. Transovarial transmission is also possible by the tick.
RUSSIAN TICK BORNE ENCEPHALITIS.-- This is one of the very few encephalitis of humans that is transmitted by ticks. The distribution is primarily Russia all the way to the Far East where there is virgin forest. The tick Ixodes persulcatus Schulze is a principal vector, although Dermacentor silvarum, Haemaphysalis concinna and H. japonica are suspected.. Transmission to humans that spend time in forests of the distribution area. Infection occurs in spring and summer, with the first from the overwintering ticks and the second from the young that hatch from eggs deposited in springtime.
SAINT LOUIS ENCEPHALITIS.-- This virus is transmitted by the Chicken Mite, Dermanyssus gallinae, and chickens serve as reservoirs for the disease. The mosquito, Culex pipiens, and a number of other mosquito species that attack humans acquire the virus from chickens and thereby become infectious for other animals including humans.
ANIMAL DISEASES TRANSMITTED.-- Many diseases of domestic and wild animals are transmitted by ticks, including Anaplasmosis, East Coast Fever, Fowl Spirochetosis, Heartwater of Sheep & Cattle. Louping Ill Disease. Nairobi Sheep Disease. Piroplasmosis, Red Water Fever, Texas Fever, etc. As these hosts may serve as disease reservoirs, close contact with the infected animals may pose a hazard for humans as well (Please check the Internet for more information on animal diseases).
Avoiding tick habitats and the diseases they carry is about the best way to avoid bites and infection. Traditionally various dips and sprays have been used for domestic animals. There are also vaccinations available for some diseases, and consulting a physician is advised for the latest treatments.
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