Ticks That Affect Domestic Animals and Poultry

ALLEN McINTOSH AND WILLIAM C. McDUFFIE.

TICKS are among our commonest parasites.

They are blood feeders and transmit disease-producing organisms to man and domestic animals. In man, several tickborne diseases are known in the United States, including Rocky Mountain spotted fever, tularemia, relapsing fever, Colorado tick fever, and tick paralysis. In domestic animals, babesiosis, anaplasmosis, tularemia, and avian spirochetosis are tickborne.

An appraisal in 1954 of the estimated annual losses by ticks to cattle was nearly 14 million dollars. The estimate was made after the eradication of tick fever in the United States; previously cattle fever caused estimated losses of 40 million dollars each year.

Not all ticks are known to transmit disease-producing organisms. They may cause injury by secondary infections of the wounds produced during feeding, by injection of toxic substances, and by extraction of blood.

Death may occur in both domestic and wild animals when the ticks are numerous. Sir Arnold Theiler, in the Union of South Africa, in 1909 reported a case in which a horse died from acute anemia, caused by the blue ticks, Boophilus decoloratus. Fourteen pounds of engorged ticks were collected from the horse in 3 days and that was only about half the ticks present.

In range animals and certain wild mammals, especially moose and elk, death may result from heavy infestations of the winter tick, Dermacentor albipicus. They are particularly bad in late winter and early spring when feed is short.

Ticks are parasitic arthropods of the class Arachnids, order Acarina, super-family Ixodoidea. There are two families, Argasidae and Ixodidae. They are usually larger than mites, to which they are related. Ticks range in body length from 1.2 millimeters (unengorged females of Ixodes soricis) to 28 millimeters (engorged females of Amblyomma varium, the largest tick known).

Ticks occur on toads, lizards, snakes, turtles, birds, and mammals. Most of the species are not host specific, but are found on a large variety of animals. In number of valid species, ticks are less numerous than the insects or the mites. The total number of ticks probably does not exceed 450 species but that does not mean that ticks are not abundant.

Some species are noted for their great fecundity. C. B. Phillip, of the Rocky Mountain Laboratory at Hamilton, Mont., has recorded more than 18,000 eggs laid in one batch by a single specimen of one of our common ticks, Amblyomma maculatum. Some species (argasids) lay only a few eggs but are able to survive long periods of fasting.

Ticks must have a meal of blood if they are to produce eggs, and some can wait 9 or 10 years for it.

Ticks occur in all parts of the world, except in the extreme frigid zones.

Some species are restricted in their distribution by lack of suitable hosts; others, by climatic requirements. Two common species, the brown dog tick and the fowl tick, have become established in most of the tropical and temperate areas of the world.

Some species in their development may require three hosts. Some may require only one host. The three-host ticks drop from the animal to molt after each stage has become fully engorged; each metamorphosis, from larvae to nymphs and from nymphs to adults, takes place off the host. The engorged larvae and nymphs of the one-host ticks do not drop but remain attached to the host; they spend their quiescent periods, or metamorphoses, on the same animal.

THE LIFE HISTORY of a tick involves four stages egg, larva or "seed tick," nymph, and adult.

Eggs are not deposited until the engorged tick has left the host. The preoviposition (pre-egglaying) period in many instances is 5 to 14 days perhaps shorter in warm weather and a month or more in cool weather. Some ticks (the fowl tick, for example) lay a few hundred eggs, return to the host for another blood meal, and then lay more eggs; they repeat the process for several feedings. Most ticks, however, lay but a single batch of eggs and die when oviposition is completed. Oviposition usually lasts 1 to 2 weeks if the weather is warm.

The incubation period for the eggs of several of the common ticks is about 10 days to 3 weeks, but some may take 3 months.

The six-legged larva, or "seed tick," must find a suitable host when it hatches if the life cycle is to be continued. The period of larval engorgement may be 2 days for some species and 2 to 3 weeks for others. Thereafter they remain attached, if they are one-host species, or drop off, if they are of the three-host kind. After a quiescent period, which may last 10 days to several weeks, larval metamorphosis is completed.

The eight-legged nymph finds a new host, or at least a new place of attachment, if it is of the one-host variety. The engorgement period for nymphs is like that of the larvae, but the quiescent period is longer. The nymphs may vary in size. The smaller ones become males and the larger ones become females.

After the nymphs have engorged, a few days to several months may elapse before they transform into adults. The final molt having been completed, the adults search for new hosts or (in the case of the species that molt on the host) the female reattaches and the male goes searching for a mate. Most species mate on the host.

Many individuals in each of the developmental stages larvae, nymphs, and adults die without finding suitable hosts. Many of the species, however, can survive long periods of fasting. Larvae of the brown dog tick, an ixodid, has been known to live 253 days; the nymphs, 2 to 6 months; and adults, up to 596 days. This ability to survive without a blood meal makes the pest difficult to control.

Some argasids, or leather ticks, may have even longer periods of fasting. Gorden E. Davis, of the Rocky Mountain Laboratory, kept a specimen of Ornithodoros turicata for 9 years without a blood meal and for 3.5 years more after one feeding.

TAXONOMICALLY, ticks are divided into two family groups:

1. The scutate, or shield ticks, family Ixodidae. They are characterized by a dorsal scutum, which covers the anterior part of the body in the female and immature stages and completely covers the body in the adult males.

2. The nonscutate ticks, of the family Argasidae, which lack a scutum and have a leathery integument.

SIXTEEN SPECIES OF ARGASIDS are known in the United States. Four, Ornithodoros talaje, O. turicata, O. hermsi, and O. parkeri, are known vectors of relapsing fever spirochetes.

The fowl tick, Argas persicus, is a vector of avian spirochetosis (rare in the United States) and also is believed to be the cause of fowl paralysis. It causes considerable losses. Heavy attacks may cause death. The hosts include most species of domestic fowls, particularly chickens, and several species of wild birds, including some game species, quails, turkeys, and doves. The fowl tick is most prevalent in Florida and the Southwestern States. And it occurs also in Mexico, Central America, South America, and many other countries.

The spinose ear tick, Ornithodoros (Otobius) megnini, is troublesome to livestock. It is not known to be a vector of any disease-producing organism, although the infectious agent of Q-fever has been reported from it. The immature stages are found deep in the ears of the host. Cattle, sheep, goats, and horses are the common hosts. It also attacks mules, asses, hogs, cats, dogs, and a number of wild mammals. Most records of this tick have come from animals in the Southwestern States and Mexico. It is found also in Central America and South America and in parts of South Africa.

The spinose ear tick is unique in that the adults never feed. The nymphs engorge until they reach adult size, drop to the ground, and seek a place to molt. After molting, the adults mate and the females lay their eggs.

Great losses are attributed to this tick. The annual loss in the United States for sheep alone has been estimated at 1.3 million dollars.

The pajaroello, tick, Ornithodoros coriaceus, has been taken from cattle and deer. It attacks man, particularly cattlemen, readily. Its venomous "bite" is painful. It is not known to be a disease vector. It occurs in California and the Pacific coast region of Mexico.

Two additional species of argasids, Ornithodoros turicata and O. talaje, occasionally attack domestic animals. The former attacks pigs, cattle, and horses. The latter attacks dogs, cats, and chickens. Both species are vectors of spirochetes that cause relapsing fever. Both occur in Florida and several Southwestern States and Mexico.

THE FAMILY Ixodidae is represented in the United States by 50 species comprising five genera: Ixodes, Dermacentor, Ambylomma, Haemaphysalis, and Rhipicephalus. Two additional species belonging to a sixth genus, Boophilus, B. annulatus and B. microplus, once were present in this country.

Many species of the genus Ixodes occur in America, but only three are common on domestic animals. The black-legged tick, Ixodes scapularis, and the California black-legged tick, Ixodes pacificus, are common on cattle and horses in the Southeastern States and on the Pacific coast, respectively.

Ixodes cookei occurs on cattle, sheep, dogs, cats, and people in the Eastern States. Wild mammals are its chief hosts.

The Ixodes are not known to be disease vectors, but I. Pacificus is a suspected vector of Pasteurella tularensis.

Experiments have indicated that I. scapularis may transmit anaplasmosis. Ixodes have long mouth parts, which often are broken off in the tissue of the host when the ticks are removed.

Nine species of the genus Dermacentor occur in the United States. Six are common parasites of domestic animals. Three of the species attack man. Two or more species are present in most States. In this group of ticks we find the vectors of the dreaded Rocky Mountain spotted fever, tularemia, and Colorado tick fever and the cause of tick paralysis.

Three species of the genus Dermacentor D. albipictus, D. nigrolineatus, D. nitens are one-host ticks. The immature stages remain attached to the host and molt on it. The others leave the host to molt, and the second host may be quite different from the first one.