Medical Entomology is the study of arthropods, and the diseases they transmit. The role of various species of arthropods as vectors of disease, or as agents directly responsible for human discomfort, is of considerable significance in the study of parasites.
Classification of arthropods
The medically important classes are the following:
1. Class Chilopoda (centipedes): e.g., Scolopendra cingulata
2. Class Diplopoda (millipedes): e.g., Brachyiulus lusitanus
3. Class Arachnida (العَنْكَبِيَّاتٌ): e.g., ticks, mites, spiders, and scorpions.
4. Class Crustacea (القشريات): e.g., Cyclops, crabs, and crayfish.
5. Class Insecta (الْحَشَرَات): e.g., mosquitoes, houseflies, lice, fleas, and bedbugs.
1. Class Chilopoda (centipedes)
Centipedes are often called 100-legged worms and have one pair of legs on each of their body segments. All centipede species are more or less wormlike and have a flattened body with a distinct head that bears a pair of long antennae. Jaws containing poison glands are located on the first body segment immediately behind the head. Depending on the species, centipedes can vary in length from 1 to 12 or more inches when mature.
Centipedes are carnivores and kill their prey by injecting them with venom. Centipedes have maxillipeds with which they attack prey. These are the venomous modified legs of the centipede's first segment and can be used in defence, as well. A centipede bite can kill small prey. Some species of centipedes can be hazardous to humans because of their bite. While a bite to an adult human is usually very painful and may cause severe swelling, chills, fever, and weakness, it is unlikely to be fatal. Bites can be dangerous to small children and those with allergies to bee stings. The venomous bite of larger centipedes can induce anaphylactic shock in such people. Smaller centipedes are generally incapable of piercing human skin. Even nonvenomous centipedes are considered frightening by some humans due to their dozens of legs moving at the same time and their tendency to dart swiftly out of the darkness towards one’s feet.
2. Class Diplopoda (millipedes)
Millipedes are often called 1,000-legged worms or rain worms. They are wormlike, with rounded body segments that each bear two pairs of legs. The head is rounded with short antennae. Species can vary in length from less than 1 to 2 or more inches. They are typically light brown to black. Millipedes can climb walls easily and will often enter homes through foundation cracks above ground level.
Millipedes are primarily detritivores. They feed primarily on decaying organic matter and they may eat the roots and leaves of seedling plants. Millipedes are ecologically esteemed as agents of microbial decomposition and soil nutrient cycles. In the absence of stinging structures, millipedes employ defensive secretions in order to protect themselves from predators. These secretions are produced by their segmental defensive glands. Such irritating secretions can be harmful if it comes in contact with human eyes or mouth.
3. Class Arachnida (العَنْكَبِيَّاتٌ)
Characteristics of arachnids include:
Body regions: Cephalo-thorax and abdomen
Antennae: Absent
Wings: Absent
Legs: Four pairs
1- Ticks (الْقُرَاد):
Ticks are responsible for spreading diseases such as babesiosis, typhus, tick-borne encephalitis and Lyme disease.
2- Mites (الحلم):
Scabies is caused by a small mite Sarcoptes scabiei who burrows into the outer layers of the skin and cause the characteristic itch which can be almost unbearable. The sarcoptic mite does not carry any infectious disease.
House dust mites (HDM, or simply dust mites) are mites found in association with dust in dwellings. They are known for causing an allergy.
3- Spiders (العَنَاكِبُ):
Spiders are predators that feed on insects and other arthropods. Most often they are quite beneficial to our environment and are harmless.
Few species are harmful to humans. Spider bites have been associated with significant pathology and very rare reports of death.
4- Scorpions:
Scorpion sting may cause death.
4. Class Crustacea
Characteristics of crustaceans include:
Body regions: Cephalo-thorax and abdomen
Antennae: Two pairs
Wings: Absent
Legs: Five pairs
Some crustaceans serve as intermediate hosts of human parasites:
A freshwater copepod of the genus Cyclops is the intermediate host of the Guinea worm Dracunculus medinensis.
A freshwater crayfish or crab is the second intermediate host of the lung fluke Paragonimus westgermani.
Freshwater crustaceans of the genera Diaptomus and Cyclops serve as intermediate host of the broad fish tapeworm, Diphyllobothrium latum.
5. Class Insecta (insects)
Characteristics of insects include:
Body regions: Head, thorax, and abdomen
Antennae: One pair
Wings: One or two pairs of wings in most insects (some insects have no wings).
Legs: Three pairs
The most common medically important insects:
1- Mosquitoes:
Female mosquito belongs to the genus Anopheles is the vector of malaria.
Female mosquito belongs to the genus Culex is the vector of filariasis.
Female mosquito belongs to the genus Aedes is the vector of dengue fever and yellow fever.
2- Houseflies (filth flies):
The scientific name of the housefly is Musca domestica.
Houseflies are more than simply being a nuisance; they are carriers of a variety of disease organisms.
3- Lice:
The scientific name of the head louse is Pediculus humanus capitis.
The only significant disease spread by lice is typhus.
4- Fleas:
The scientific name of the human flea Pulex irritans.
Different species of fleas, especially rodent fleas, are able to transmit diseases (endemic typhus and plague).
5- Bedbugs:
The scientific name of the bedbug is Cimex lectularius.
They do not carry any diseases, but their bites can be very uncomfortable.
They tend to bite the face, arms, and legs.
Effects of arthropods on human health
I- Direct effects:
(1) Annoyance: Annoyance comes from disruptive activities of insects, such as flying around or landing on the head, and from feeding, possibly causing a blood loss. Insects usually do not take sufficient blood to cause a medical problem, although anemia. Nevertheless, annoyance is not a trivial effect of insects. Human activities frequently are disrupted by insects, and in some instances, such as when recreational facilities cannot be used because of insects, annoyance can cause substantial economic losses.
(2) Bites: Is the most obvious mechanism for direct injury. Annoying, painful, and can decrease a person’s productivity level. Bites may result in significant lesions, but not because of injected venom (except for spiders). Bite lesions are generally a result of immune reactions to salivary secretions injected during the biting process.
(3) Envenomization: Is the direct injection of venom into the body through a bite or sting. The damaging results of a bite or sting can range from dermatitis, or inflammation of the skin (as seen with flea bites) to actual tissue damage (the result of the bit of the brown recluse spider). Most stings result when social insects, such as bees, and wasps, defensively attack persons coming near or disturbing their nests. Venom is injected on stinging. Thus, the term envenomization is an accurate descriptor.
(4) Entomophobia: This condition is the irrational fear of real or imaginary insects. This may range from unwarranted fears of innocuous insects to sensory hallucinations.
(5) Dermatosis and dermatitis: Dermatosis is a disease of the skin, dermatitis is an inflammation of the skin. Both dermatosis and dermatitis can be caused by arthropod activities.
Many mite species, such as scabies mites and chiggers, produce acute skin irritations. Human scabies, a skin disease caused by infestations of the itch mite (Sarcoptes scabiei) is an important public health problem and periodic outbreaks are common.
Blister beetle dermatitis is a cutaneous condition that occurs after contact with any of several types of beetles. Blister beetles secrete an irritant called cantharidin, a vesicant that can get onto humans if they touch the beetles.
Some millipede species also can cause stains or burns on human skin via defensive body fluids. The liquid is composed of a variety of chemicals, such as hydrogen cyanide, phenol, and organic acids, which can induce a contact dermatitis in humans.
(6) Accidental injury to sensory organs: Insects can cause accidental injury to our sensory organs. Any insect that enters the ear, nose, or eye can cause severe irritation.
(7) Myiasis: A condition in which fly larvae (maggots) invade a human host. Fortunately, myiasis is a rare condition in humans, but it commonly occurs in livestock. Besides the detrimental effects of myiasis itself, many additional complications can arise from myiasis, such as secondary microbial infections, secondary infestations by other insects, and debilitation. Myiasis can be fatal.
(8) Allergic reactions (anaphylaxis): a hypersensitive response to insect proteins. The mechanisms associated with envenomization can also cause exposure to allergens. In fact, human deaths from bee and wasp stings usually are associated with hypersensitive reaction rather than direct effect of a toxin.
Cockroaches seem to be most often involved in allergic responses from ingestion. Allergens are present in cockroach feces, which can be inadvertently ingested in heavily infested areas. Other allergens are present in cockroach saliva and exoskeletons which can be introduced into foodstuffs.
Several insect or mite species (or their body parts) may cause irritation and/or allergic reactions when inhaled and, less commonly, when ingested. Dust mites that are found in many buildings and in bedding materials such as mattresses and pillows.
Allergies to the venom of some biting and stinging arthropods are also prevalent. Numerous arthropods can cause allergic reactions in persons by their stings, including various wasps, bees, ants, scorpions, and even caterpillars. Bites from some arthropods may produce allergic reactions, including anaphylaxis and other systemic effects. However, systemic hypersensitivity reactions to arthropod bites are much less common (almost rare) than those resulting from stings.
II- Indirect effects:
Disease transmission is the primary indirect effect of arthropods on human health.
A- Mechanical disease transmission: disease agents are carried from one host to another by arthropods simply mechanically carried by the body parts (e.g., wings, hairs, feces, vomitus, etc.). In this type of disease transmission, no change takes place in the number, form, or developmental stages of the organism, but simply deposited in the body, food, or drink of the host.
B- Biological disease transmission: the agent will exhibit changes in form and/or number of developmental stages in the arthropod before entry to the host. This includes hereditary (transovarian) and transital transmissions (propagative, cyclodevelopmental and cyclopropagative).
(1) Transovarian transmission:
It is a type of disease transmission, whereas the causative agent is transmitted to the immature stage (usually to the egg) from the adult insects and/or other arthropods which carry disease pathogens. When the infected egg completes its developmental stage; it becomes infective or can transmit the disease to man and other animals. Ticks and sandflies are very good examples of arthropods that exhibit hereditary disease transmission.
(2) Propagative:
In propagative type of disease transmission only the number of pathogens increases, and the developmental stage remain constant. The diseases plague and typhus are good examples of propagative type of disease transmission.
(3) Cyclodevelopmental:
In this type of disease transmission, only the developmental stage (form) of the disease pathogen is changed (small to big, immature to matured stage, etc.), while the number of the pathogenic organism remains constant, e.g., Wuchereria bancrofti
(4) Cyclopropagative:
This type of disease transmission is a combination of both propagative and cyclodevelopmental; whereby the disease pathogen undertakes a change both in number and developmental form (stage), e.g., Plasmodium spp.