Vectors of Disease

Disease vectors are animal carriers that transfer infective agents from one host to another. They usually are arthropods (insects, arachnids, crustaceans) but may be other animals. They can be classified into two types-biological and mechanical. Biological vectors are animals in whose bodies the infecting organism develops or multiplies before it can infect the recipient animal. Mechanical vectors are animals that transmit infective organisms from one host to another but, in themselves, are not essential to the life cycle of the parasite.

Mosquitoes that transmit malaria and yellow fever are biological vectors. The black horsefly, which transmits anthrax, and many insects that transmit plant diseases are mechanical vectors. Higher animals, and man himself, sometimes act as vectors. Swine are host to trichine, which produces trichinosis in man when he eats inadequately cooked, infected pork. Dogs, cats, skunks, foxes, and some other animals transmit rabies.

Pests

The meaning of the term pest as used here is restricted to certain animals (excluding microorganisms) that interfere with the health of other organisms. These pests live on or within the animals, or they are associated with them in other injurious ways. Pests are known as parasites when they obtain their food from living host cells. The presence of a large number of parasites on the surface of the host's body, producing only mechanical effects, is known as infestation. Invasion of the tissues of the body of the host by parasites, producing injury followed by host reaction, is known as infection. Living organisms that consume or destroy food, clothing, and forest products also are characterized as pests.

Although many insects are beneficial to agriculture, great losses are caused by plantfeeding insects and by insect-borne plant diseases. Other serious losses result from the destruction of stored food, clothing, and forest products by such pests as rats and moths. Some pests affecting the animal kingdom are mites, ticks, spiders, scorpions, chiggers, lice, bedbugs, and flies. Some pests affecting plants are the Japanese beetle, snails, corn earworm, boll weevil, and the elm leaf beetle. Other pests that take their toll of man's products and have potential value as BW agents are rats, mice, groundhogs, starlings, and crows.

EFFECTS OF BIOLOGICAL AGENTS

Biological agents may be selected to produce various strategic or tactical goals. These goals range from brief but crippling diseases to widespread serious illnesses with many deaths. The effects of biological agents vary widely, depending upon the agent or agents selected.

The mere presence of a disease-producing organism on or in the body of a host does not guarantee infection or illness. In fact, pathogenic organisms are frequently present and cause no harm in the human body for long periods of time. The factors that determine whether infection will result from contact between a pathogen and a host are not completely understood, but the following are known to be important: l The general state of health of the

individual

. The immunity of the particular individual to the particular organism

. The number of organisms to which the individual is exposed

. The ability of the organisms to cause disease

Remember that the effects of pathogens are always delayed. There is always an incubation

period between the time organisms enter the body and the time that symptoms of disease are observed. This period may vary from several hours to several weeks.

To some extent, the effects of pathogens are determined by the route that the infecting organisms use to enter the body. Many organisms require a specific portal of entry to produce infection or disease. Other organisms can cause disease when they enter by any route. The usual ways in which pathogenic organisms enter the body are by inhalation, by swallowing, by direct contact, and by injection. Injection includes insect and animal bites.

DISSEMINATION OF BW AGENTS

Biological agents may be spread in various ways. They may be used as fillings in bombs, shells, or aerial or surface spray tanks. Biological agents may be released from munitions such as aerosols. The aerosols are cloudlike formations of solid or liquid particles in which the biological agents are held suspended.

DETECTION OF BW AGENTS

There are no simple and rapid methods to detect biological agents such as those used to detect chemical agents and nuclear radiation.

The positive detection and identification of a pathogen can be obtained only by taking samples of the organizms, growing a culture of the organisms under laboratory conditions, and then subjecting the culture to a variety of biochemical and biological tests. Obviously, the final identification of pathogens is a problem for medical personnel.