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Arthropodborne Diseases

 
  1. Historically, arthropodborne diseases have caused more casualties than combat injuries. Arthropodborne diseases alone were responsible for the loss of 15,576,000 man-days among US Armed Forces during World War II.
    1. Today, harmful arthropods represent one of the greatest environmental hazard to soldiers in the field. The chain of infection for arthropodborne diseases involves a pathogenic organism in an infected person or animal (the reservoir), an arthropod to transmit the disease (vector), and a susceptible person (the host).
    2. The significance of vector efficiency in disease transmission from reservoir to host is related to many factors. Some of the factors are species-related such as vector reproductive capacity, physiology, morphology, and genetics.
    3. Other factors that affect the vector�s ability to transmit disease are physical and related to environmental conditions, such as temperature, moisture, rainfall, pH, weather, geographical and topographical location, photoperiod, and wind.
    4. Soldiers in a field environment must break the chain of infection for arthropodborne disease or arthropod injury by limiting arthropod pest exposures.
  2. Arthropods (insects, ticks, mites, spiders, scorpions, and the like) make up over 75 percent of all animal species. Less than 1 percent of the 750,000 species of arthropods are potentially dangerous to humans. However, their impact is significant due to their high numbers and the negative results of their activities. The impact is direct injury and disease transmission to man and other animals; damage to crops; infestation of stored products; and destruction of wooden structures. Still, many species are beneficial as pollinators, predators of other pests, scavengers of waste, manufacturers of food, and a part of the natural balance of nature. However, the economic damage and medical disorders caused by a few arthropods make some pest management practices necessary to control the problem pests. Protection of the soldier from arthropods and arthropodborne diseases is essential to mission accomplishment.

Direct Arthropod Affects on Human Health

In addition to disease transmission, arthropods can cause direct injuries to man. Bites, stings, and allergic reactions are three major categories of injuries caused by arthropods. Arthropods also affect man by annoying and disturbing him. The sound of a single mosquito buzzing around your head while your are trying to sleep is annoying. Standing guard with gnats buzzing around your face can be disturbing. Also, finding cockroaches or other insects or parts of insects in your food is disturbing. The problems of arthropod injury and the exaggerated fear of arthropods can even result in psychiatric problems.

  1. Biting Arthropods. Arthropods bite to feed, probe (taste), or defend themselves. Most penetrations of human skin are made by mouthparts that are developed for ingesting blood, tissue, and tissue fluids of animals or plants. These bites usually result in the arthropod injecting salivary fluids or regurgitating its digestive tract products into the man or animal. Some biting arthropods can also produce skin injuries. Each individuals reaction to arthropod bites can be very different. Biting arthropods are grouped according to the duration of host contact as (short-term) or prolonged (long-term).
    1. Short-term host contact. Most arthropods that bite man have only short-term host contact. Bloodsucking arthropods are frequently winged or highly mobile. This accounts for their ability to quickly attack and escape capture or detection. Some arthropods hide in structures close to the host and only feed when the host is nearby. Others that bite may not have intended to attack, but did so in defense or by mistake. Arthropods can bite in several stages of their development; that is, adult, larvae, or nymph stages. The mouthparts are generally classified into chewing or sucking types. Chewing mouthparts are generally not used for skin penetration. Usually, injuries of this type are not reported, but secondary infections may occur due to bacterial contamination. Sucking mouthparts are structured for skin penetration.
      1. Bloodsucking (hematophagous) arthropods. Blood, normally from warmblooded animals (including man), is used both for life support and growth and/or egg development. The mouthparts of sucking arthropods vary greatly in structure from arthropod-to-arthropod. For example, adults of the order Diptera (two-winged insects) have the most diverse mouthparts. Only the females of the mosquitoes, black flies, biting midges, horseflies, and deer flies are bloodsuckers, while both males and females of tsetse flies and stable flies are bloodsuckers. The mouthparts are different within each of these families of Diptera, but the goal of a blood meal is the same. Other examples of arthropods that are short-term are fleas, true bugs (conenose bugs and bedbugs), and soft ticks.
      2. Nonbloodsucking (nonhematophagous) arthropods. Some plant-feeding arthropods and some arthropod predators have piercing/sucking mouthparts, which are capable of penetrating the human skin. Bites from these arthropods can be as painful as bloodsuckers, if not more so. Bites from these arthropods are usually an act of defense.
    2. Long-term host contact. Some biting arthropods require a considerable time on the host to complete a normal life cycle. Since a continuous food supply is available on one host, the search for another host is reduced. Most of these arthropods are categorized as parasites. They are classified as either ectoparasites or endoparasites. Ectoparasites (those living outside the host body) may be flat (fleas) or thin (lice) which allows them to travel easily through a hairy environment. Their feet are specialized for holding on to hair. The mouthparts of ticks and mites are designed to anchor their bodies to the host. Endoparasites (those living inside the host body) are usually soft-bodied (fly larvae, mites) without legs or with very short legs; their bodies usually have specially arranged spines or hairs.
  2. Stinging Arthropods. Some arthropods affect man by injecting venom (insect toxins) through stingers, fangs, modified front legs, or spines. An arthropod�s injection of poison is in defense or to kill prey. Usually, man is envenomized by arthropods in defense of themselves and their nest or eggs.
    1. Venoms from bites. Spiders and centipedes are arthropods in the category that uses mouthparts for envenomization. Most spiders use venom to kill prey. Man is not part of a spider�s diet. The fangs of many spiders cannot penetrate the human skin. Some species have venom that is more poisonous than other venomous animals, including snakes. Fortunately, most spiders are not aggressive, but will defend themselves and their eggs and/or webs. Antivenom has been developed for the venom of some species of spiders, but they may not be immediately available for use. Some of the more toxic spiders are night hunters, and by day, they hide in clothing and boots that were left on the floor or in tents on the ground. Less common are the bites of centipedes that are also night hunters. In the tropics, some species of centipedes reach 25 centimeters in length; a bite by such a large specimen could be serious.
    2. Venoms from stings. The number of soldiers seeking medical assistance because of arthropod bites is far fewer than those seeking aid because of bee, wasp, hornet, or ant stings. Stings from these arthropods are frequently the result of defensive action. A single sting to an allergic person can be fatal. Even to a person who is not strongly allergic, medical complications due to swelling can occur with stings to the face, neck, or throat. Stings usually occur during daylight hours. However, night maneuvers can result in individuals running into paper wasp nests and being stung by wasps trying to protect their nests; or an individual may stand on an anthill and receive numerous stings from the ants trying to protect their nest. Therefore, individuals who are highly sensitive or have severe reactions to stings should carry an emergency first aid kit for stings as prescribed by a physician. Obviously, high-risk persons should use extreme caution in tropical areas. Another venomous arthropod with a potent sting is the scorpion. Scorpions are active at night. During the day scorpions are usually well-hidden from the light; they hide under rocks or in piles of debris outside and in clothing, cabinets, boxes, and footwear indoors. Stings often are the result of individuals walking barefoot or in stocking feet, or because they use their hand to move the object where the scorpion is hiding. Also, stings occur when individuals put on clothing or footgear without first shaking the item to make sure that it is free of unwanted arthropods. Some scorpion venoms are very painful or they may be deadly, while others are not. The absence of initial pain is not always an indication of no problem. The lack of or the nonavailability of antivenom and the possible medical problems associated with the use of antivenoms, even if they were on hand, are additional reasons for avoiding scorpion stings.
  3. Allergy. Allergic reactions are caused by both the bites and stings of arthropods. Additionally, arthropod parts (live or dead) and their body fluids can cause allergic reactions. Allergic reactions are extremely variable in different people ranging from very mild to severe reactions. Highly sensitive persons should be prepared to deal with their problems in case they are bitten, stung, or exposed to other arthropod allergens.

Arthropodborne Diseases

Diseases transmitted to man by arthropods are some of the most serious known to man. Uncontrolled, these illnesses can cripple or destroy military forces. The effect of these diseases on man can range from a very mild illness to death. For examples of arthropodborne diseases and their vectors see Table 2-1. House flies and other flying insects that are attracted to human wastes or other organic material can spread disease organisms to food and water. The disease organisms or parasites of humans are carried from diseased humans or animals (reservoirs) by arthropods (vectors) to other humans or animals (hosts). By employing individual PMM, soldiers can stop arthropodborne diseases from being a factor in their lives and in their units mission accomplishment. The most common arthropodborne diseases that affect combat troops are discussed below.

   

DISEASE VECTOR
  MALARIA   MOSQUITO
  CHAGAS' DISEASE   KISSING BUG (REDUVIID)
  LEISHMANIASIS   SAND FLY (PHLEBOTOMINE)
  YELLOW FEVER   MOSQUITO
  DENGUE FEVER   MOSQUITO
  ENCEPHALITIS   MOSQUITO
  SANOFLY FEVER OR PHLEBOTOMUS FEVER   SAND FLY (PHLEBOTOMINE)
  TYPHUS FEVER (EPIDEMIC)   BODY LOUSE
  TYPHUS FEVER (MURINE)   FLEA
  SCRUB TYPHUS   LARVAL MITE (CHIGGER)
  BUBONIC PLAGUE   FLEA
  DYSENTERY   FILTH FLIES (PARTICULARLY THE HOUSE FLY)
  TYPHOID FEVER   FLIES AND COCKROACHES (BY FOOD CONTAMINATION)
  SPOTTED FEVER   TICK
  FILARIASIS (ELEPHANTIASIS)   MOSQUITO
  ONCHOCERCIASIS   BLACK FLY
  1. Malaria. Malaria is a serious disease occurring most commonly in tropical and semitropical regions. It is caused by a microscopic parasite carried by the Anopheles mosquito. This parasite destroys blood cells and causes chills, fever, weakness, and anemia. If untreated, malaria can cause death.
  2. Yellow Fever. Yellow fever is a viral disease transmitted by the Aedes mosquito. It occurs in tropical Africa, Central America, and tropical South America. Symptoms are fever, headache, backache, jaundice, and internal bleeding. If untreated, yellow fever can result in death.
  3. Dengue Fever. Dengue viruses of multiple types are now endemic throughout most tropical areas of the world and are highly endemic in Southeast Asia, the Philippines, West Africa, and northern Australia. Like yellow fever, it is transmitted by the Aedes mosquito. Symptoms are fever (lasting about 5 days), intense headaches, skin rash, and muscle pain which can be severe; for this reason, another name for dengue fever is �breakbone� fever. The disease seldom results in death, but the recovery time is usually long and the victim may be fatigued and depressed.