Grocer's itch and harvester's rash transient but often annoying are the result of exposure to mites that ordinarily infest grain and stored-food products.

The tropical rat mite and a less common but also widely spread house-mouse-infesting mite, Allodermanyssus sanguineus, occasionally cause complaints through infestation of houses invaded by their rat or mouse hosts. The first species has been accused of acting as a vector of murine typhus and of plague, but experimental evidence has been conflicting, and its importance in this regard remains doubtful. But it can transmit rickettsialpox, the most recently discovered of the typhus-like diseases, which has occurred in some mouse-infested premises in New York and Boston. A. sanguineus is the natural transmitter of rickettsialpox in New York suburban settlements. Some of these mites have also been reported as transmitting tularemia in Russia.

Chicken mites and related species occasionally annoy man by their bloodsucking habit but do not remain on him long.

Chiggers belong to another group of mites of the family Trombiculidae. Only the first or larval stage is parasitic on vertebrates and must have a blood meal for further development. Chiggers are so minute that they are seldom seen by man even though the numerous sites of attachment such as the belt line are evident. In eastern and southern areas of the United States where chiggers are most prevalent, they are not known to carry any disease, but their attack produces severe itching, which may result in secondary infections. This type of attack in the southwest Pacific area is known as scrub itch.

Scrub typhus, a serious malady in the Far East, is called tsutsugamushi disease in Japan, where it was first recognized. It is carried by certain species of the chiggers. The disease agent, related to the typhus group, is passed from one generation of mites to the next through the egg.

The chiggers usually parasitize rats and other rodents, but certain species will attack man. The mortality from scrub typhus can be extremely high more than 6o percent of cases in some parts of Japan, or as low as o.6 percent among Americans in an epidemic in the Schouten Islands, where 1,469 cases occurred in 6 months. Even with this low death rate, the situation is serious when incapacitation averages 2 months or more. Chloromycetin and aureomycin, however, are markedly reducing hospitalization and will aid in progress against this dread disease.

More people have an acquaintance with ticks than with mites. Yet many do not know that there is one family, the Argasidae, or soft ticks, which feed on man rather rapidly like bed bugs and do not remain attached.

The notorious relapsing-fever tick of tropical Africa has become almost completely domesticated, and natives sleeping on the floors of their huts pay little attention to it. A number of related species in various parts of the world, including the United States, have since been found to cause human cases of relapsing fever. These are mostly species that rodents bring into living quarters, mountain cabins, native huts, and the like. Several similar forms of relapsing fever are carried by different species of these soft ticks.

Another group of ticks, the hard ticks, of the family Ixodidae, customarily require several days to complete engorgement after attachment. In temperate climates where cold seasons intervene, some species may require 2 years to complete a generation. The Rocky Mountain wood tick is an example. The adults have been known to survive three winters when kept outside. Many ticks have simple eyes with which they can discern the passing shadow of a potential victim. Others have no eyes. Special sense organs enable them to detect animals 25 feet or more away, so that an unwitting camper may attract ticks from a considerable area. Many ticks are thus able to select favorable sites game trails, for instance for seeking their host.

Ticks have numerous progeny and few enemies, are not greatly affected by weather conditions, can feed upon various kinds of animals, and permit the passage of disease organisms from stage to stage as well as from one generation to another through the egg. Instances of secondary infection at the point of tick attack occur often.

"Spring-summer" encephalitis in Soviet Russia and Siberia is the most important of the filterable viruses carried by several Russian species. Another tick-borne virus is the rather mild, nonfatal Colorado tick fever of our Rocky Mountain region.

Tick-borne typhus-like diseases are assuming increasing importance in various parts of the world. These include American or Rocky Mountain spotted fever in various countries of the New World, and a group of usually less severe diseases in Europe, Africa, Asia, and probably Australia, related to boutonneuse fever, which was first recognized in the Mediterranean region. The latter group of diseases includes South African tick-bite fever, Kenya typhus, and Siberian, Indian, and probably Queensland tick typhuses. In the United States, the Rocky Mountain wood tick in the West, the American dog tick in the East and South, and probably the lone star tick in the South are the chief criminals in human infection with spotted fever.

Q fever is a peculiar, recently recognized disease due to a typhus-like agent which is being discovered in many parts of the world. Ticks have been found naturally infected in North America, Australia, Spain, and parts of North Africa, but only in Australia have they shown any importance in relation to human infection.

Tularemia, an important bacterial disease affecting man, occasionally is transmitted by ticks.

Tick paralysis of man and animals is due to a presumed toxin secreted in the saliva of ticks. Though more frequently observed in tick-infested animals, a number of human cases have been reported in the United States, Canada, and Australia. The ascending paralysis is caused by a rapidly engorging female tick attached to the base of the head, where the hair may hide its presence for longer periods. Complete recovery has followed within 24 to 48 hours of removal of the offending tick. Death occurs if the ascending paralysis reaches the respiratory centers of the human or animal victim before the tick has dropped off or has been removed.

The Acarina are more difficult than insects to control because they are more resistant to insecticides. Benzyl benzoate ointment is one of the best materials for the control of scabies. This acaricide in combination with dibutyl phthalate is an excellent clothing impregnant against chiggers and is more durable than many chemicals previously tried. Chiggers can be controlled in infested areas by applying 1 to 2 pounds of chlordane or toxaphene or one-fourth pound of lindane per acre, employing dusts or sprays. Such mixtures are being constantly improved.

Several substances effective against ticks have been discovered. The control of livestock and wild animals upon which ticks feed reduces the number of these pests. Many of the ticks in infested ground areas can be destroyed by spraying with a 5 percent DDT emulsion. Certain ticks congregate near roads and trails and an insecticide should be applied especially on those areas. Clearing of brush and close cutting of grass is a material aid in tick control.

The best way to remove ticks that are attached to persons is to pull them out. Some ticks have long beaks and their removal may require a needle or knife.

F. C. BISHOPP has general direction of the research in the Bureau of Entomology and Plant Quarantine on injurious and beneficial insects. In 1909 he began investigating ticks and various insects as annoyers of man and carriers of disease. In 1926 he was put in charge of the Bureau's division of insects affecting man and animals and in 1941 became assistant chief of the Bureau.

CORNELIUS B. PHILIP is principal medical entomologist and assistant director of the Rocky Mountain Laboratory, United States Public Health Service, Hamilton, Mont., and a member of the National Defense Virus and Rickettsia) Disease Commission. He has studied insect-borne diseases in many Parts of the world, in part as a colonel in the Sanitary Corps of the Army during the Second World War, and is an authority on medical entomology.

Pacific Coast tick.