Foreign Vectors & Diseases




(Vector-borne Diseases and Arthropod Vectors)

In many areas of the world, particularly the tropics, arthropod-borne diseases are

among the major limiting factors to the efficient production of livestock and

poultry. These diseases result in debilitation, lameness, blindness, wasting,

congenital defects, abortions, sterility, and death. Some exotic arthropod-borne

diseases of livestock are zoonotic and affect humans as well as animals.

All of the major groups of pathogenic organisms have representatives that are

transmitted by arthropod vectors and cause disease in domestic livestock or

poultry. For example, over 400 arthropod-borne viruses (arboviruses) have been

recognized, including the etiologic agents of such major livestock diseases as

African swine fever, Akabane disease, bovine ephemeral fever, the equine

encephalitides, bluetongue, and epizootic hemorrhagic fever (16). Rickettsial

agents that are primarily tick-borne cause several extremely important livestock

disease problems, including bovine and ovine anaplasmosis, heartwater, tick-

borne fever, bovine infectious petechial fever, epizootic bovine abortion, Jembrana

disease, and Q fever. Arthropod-borne bacteria cause such well-known diseases as

borreliosis of cattle and horses, spirochetosis of poultry, tularemia, and Lyme


Some of the most devastating of all animals diseases are caused by arthropod-

borne blood protozoa, including babesiosis of cattle, sheep, goats, horses, and

swine; theileriosis, the East Coast fever syndrome, and Mediterranean fever; the

trypanosomiases causing illness in cattle, sheep and goats, camels, pigs, dogs,

and many wild game species; as well as several arthropod-borne protozoa that

cause diseases of birds. Bovine filariasis is a prime example of an exotic helminthic

disease that is arthropod-borne. In fact, over half of all exotic diseases of livestock

and poultry of critical concern to the United States are arthropod-borne.

The most prominent groups of arthropods that transmit etiological agents

pathogenic to livestock are those that are blood-feeding (hematophagous) and are

biologically involved in transmission cycles. Ticks, tsetse flies, mosquitoes, and

biting midges, for example, have leading roles in the biological transmission of

agents causing significant livestock and poultry diseases. Of somewhat lesser

general importance are those hematophagous arthropod groups that mechanically

transmit pathogens. Horse flies, deer flies, stable flies, horn flies, and others have

been incriminated in disease transmission through interrupted feeding.

There are also those arthropod groups in which the many species are not blood

sucking — such as muscoid flies, beetles, or grasshoppers — but which

mechanically transport pathogens or serve as intermediate hosts of helminths. Of

course, examples can also be found for any variety of transmission methods and

cycles within each of the major vector groups.

As a whole, ticks are the most versatile vectors, for they parasitize all vertebrate

groups except fish. The tick-borne diseases that they transmit are among the

most significant animal health deterrents to efficient livestock production. The

methods of pathogen transmission employed by ticks are both mechanical and

biological. In the case of soft ticks belonging to the family Argasidae, the ability of

some individuals to survive for 3 years or more between blood meals permits

them to assume the dual role of vector and reservoir, which is particularly

important in the transmission of African swine fever virus (16).

Mosquitoes are notorious as proven vectors of some of the most devastating

human diseases. There is little need to document the impact on human public

health of malaria, yellow fever, filariasis, and several mosquito-borne diseases of

arboviral etiology. Rift Valley fever and the equine encephalitides are important

livestock diseases transmitted by mosquitoes. Although over 2,500 species of

mosquitoes have been described worldwide in 18 genera and subgenera, those

species of greatest importance as vectors of pathogenic agents are found in the

genera Aedes, Culex, Anopheles, and Mansonia.

Biting midges, particularly species of the genus Culicoides, have been incriminated

in the transmission of viral, protozoal, and filarial agents pathogenic to livestock

and poultry. Owing to their small size and difficulties encountered in colonization,

scientific progress on their role as animal disease vectors has been delayed.

However, considering the fact that biting midges are frequently among those

species of biting flies in greatest abundance that attack livestock, increased

attention should be given to them as animal disease vectors.

Although tsetse flies are limited in their distribution to sub-Saharan Africa, the

importance of the animal trypanosomiases (nagana of cattle) on that continent

ranks tsetse as one of the world's major arthropod-vector groups. The very

complex developmental cycle of the trypanosome within the tsetse vector is

further complicated by several of other factors related to the biology of the vector,

pathogen, and host. Not only are the various species of tsetse flies characterized

by differences in their distribution, biology, and host preferences, but even within

the same species environmental factors (especially humidity, temperature, and

vegetation), densities and composition of mammalian hosts, and vector population

densities affect their epidemiological role.

In addition, there are wide intraspecific variations in both morphology and pathogenicity of trypanosomes. Certain parasite antigens that stimulate production of protective antibodies by the host change before the parasites are completely eliminated; new antibodies are then produced by the host, and the parasites change their antigenic constitution again to

maintain themselves.

The key to the success of arthropod-borne disease transmission lies in the

competence of vector efficiency (6). Whereas one vector species may be

extremely efficient in the transmission of a particular pathogen, a closely related

species may be totally incompetent as a vector. Even within a single vector

species, individuals and populations vary dramatically in their competence to

transmit a particular pathogenic agent. The expression of vector competence

appears to be controlled, in part, by genetic factors involving multiple genes. For

example, although the biting midge species, Culicoides varipennis, is incompetent

to transmit bluetongue virus in the Northeastern United States, populations of the

same species from the Southwest and Western States are extremely efficient

vectors of the virus. Genetic crosses between families of the insect vector species

showed results consistent with the theory that a single genetic locus controls

insect vector competence for infection with the bluetongue virus (12, 15).

Foreign Arthropod Pests and Arthropod-Borne Disease Factors

Although the introduction and establishment of any exotic arthropod pest of

livestock or poultry, or any arthropod-borne disease vector, could have

devastating results to affected industries, certain foreign species are of

considerably greater importance than others. On the basis of potential for

introduction, establishment, and economic impact, three categories of foreign

arthropod pests and arthropod-borne disease vectors have been established

(Appendix 2).

Category A. These species have the highest potential for introduction,

establishment, and economic impact. They consist of five tick species, one

parasitic mite, one blowfly, and one muscoid fly. The southern cattle tick,

Boophilus microplus, is a vector of bovine babesiosis, bovine anaplasmosis, and

benign bovine theileriosis. This tick is found is the hotter, more humid parts of the

West Indies, Mexico, Central America, South American, Africa, Australia, the

Orient, and Micronesia. At one time it was also established in southern Florida, in

several counties in southern Texas, and is found in Puerto Rico and St. Croix, U. S.

Virgin Islands. A closely related species, B. annulatus, the cattle tick, was once the

most important external parasite of cattle in the Southern United States. It is a

principal vector of bovine babesiosis and has also been incriminated in the

transmission of bovine anaplasmosis, benign bovine theileriosis, and spirochetosis

of cattle, sheep, goats, and horses. The cattle fever tick has been eradicated from

the continental United States, but periodic introductions from Mexico continue to

occur. It is also found in western and central Africa, the Mediterranean basin, and

the Near East.

Another exotic tick species of great concern to this hemisphere is the tropical bont

tick, Amblyomma variegatum (Fig. 53). A native of Africa south of the Sahara

Desert, the tropical bont tick was introduced into the Caribbean island of

Guadeloupe around 1830 on cattle imported from Senegal. This tick is a common

vector of Cowdria ruminantium, which is the etiological agent of heartwater that

affects cattle, sheep and goats. The bont tick is also associated with the spread of

dermatophilosis and has been incriminated in the transmission of Nairobi sheep

disease. An international effort is under way to eradicate the tropical bont tick

from the Western Hemisphere. A. hebraeum (Fig. 54), the bont tick, is also of

African origin and is a common vector of heartwater. The exceptionally long

mouthparts enable it to produce deep- seated painful wounds that often become

infected and lead to abscess formation.

The brown ear tick, Rhipicephalus appendiculatus, is widely distributed in the

wetter areas of Africa. Although primarily a cattle tick, there are numerous

secondary host species. Because the most important predilection site of this

species is the inside of the earflap, it is the most important species involved in

transmitting the etiological agent of East Coast fever. Rhipicephalus

appendiculatus has also been incriminated in the transmission of bovine

babesiosis, other pathogens of the East Coast fever syndrome, louping ill, Nairobi

sheep disease, and Kisenly sheep disease.

Another tick species of high vector potential is the European castor bean tick,

Ixodes ricinus. This tick is common throughout most of Europe, including the

British Isles, and is found in North Africa and limited areas of Asia. It has never

been established in North America, although closely related species of the genus

Ixodes do exist in this hemisphere. The European castor bean tick is responsible

for transmitting the causative agents of bovine babesiosis, bovine anaplasmosis,

louping ill, and tick-borne fever of cattle, sheep, and goats. Completion of the life

cycle can require as long as 3 years.

The sheep scab mite, Psoroptes ovis, is recognized as an exotic arthropod pest

having highest potential for introduction because it has been eradicated from the

United States and could easily be reintroduced from other countries of this

hemisphere. Interceptions at port of entry have been made from sheep, goats,

llamas, and alpacas.

Another exotic arthropod pest of highest importance is the New World screwworm,

Cochliomyia hominivorax. This species has been eradicated from the United States

and Mexico through the classic application of the sterile male technique, and the

program continues to approach its goal of eradication throughout Panama.

Screwworms were introduced into Libya from South America and subsequently

eradicated through an international effort utilizing the sterile male technique. Until

a barrier is established in Panama, there is a persistent threat for the

reintroduction of screwworms on infested mammalian hosts from areas that have

not yet been eradicated.

The louse fly, Hippobosca longipennis (Fig. 55), which inflicts a painful bite, is an

ectoparasite of all hairy animals, including livestock, dogs, cats, and wild game.

The louse fly has been introduced into the United States on a shipment of

cheetahs destined for zoological parks and subsequently eradicated from six

states. This species has also been introduced on bat-eared foxes.

The final species in Category A is a licking fly, Musca vitripennis. This species has

been reported as being a tenacious feeder on the facial secretions of cattle, a

mechanical vector of the etiological agent of infectious keratoconjunctivitis, and a

biological vector of bovine filariasis. Adults of this fly have been intercepted on

several occasions in aircraft originating from the Azores, but this species has not

yet become established in North America (13).

Category B. Exotic arthropod pests and arthropod-borne disease vectors in

Category B merit particular concern with respect to introduction, establishment,

and economic impact. So many arthropod species could be assigned to this

category that they are listed by genera rather than by individual species. As

before, the lead is taken by hard ticks of the genera Amblyomma, Dermacentor,

Hyalomma, Ixodes, and Rhipicephalus, followed by soft ticks of the genera Argas

and Ornithodoros. Mosquitoes of the genera Aedes, Anopheles, and Culex are a

continual concern for introduction and establishment, as has recently occurred

with the Asian tiger mosquito, Aedes albopictus. Muscoid flies (Musca) could be

introduced in bedding material of animal importations. The numerous species of

tsetse flies, Glossina spp., are listed within Category B because they are all limited

to the African continent and, in view of their biological cycle and naturally low

reproductive efficiency and population density, are less likely to be a threat to

introduction. However, should a tsetse species become established in a tropical or

semitropical area of this hemisphere, eradication would undoubtedly be a

formidable task.

Category C. Species of foreign arthropod pests and arthropod-borne disease

vectors assigned to Category C are those with some potential for introduction,

establishment, and economic impact. They originate from all areas of the globe

and are too numerous to characterize even at the generic level. Thus, species of

particular concern are found in the families Ceratopogonidae (biting midges),

Simuliidae (black flies), Oestridae (bot flies), Chloropidae (eye gnats),

Sarcophagidae (flesh flies), Ixodidae (hard ticks), Tabanidae (horse flies and deer

flies), Culicidae (mosquitoes), Muscidae (muscoid flies), and Cuterebridae (robust

bot flies).

Examples of Interceptions and Introductions

Historically, some of the most economically important arthropod pests of livestock

found in the United States were introduced from Europe (2). There is evidence to

suggest that the house fly and stable fly were introduced when the first settlers

brought livestock with them from their home countries. The horn fly, a pest of

cattle throughout the United States, was first discovered near Camden, New

Jersey, in 1887. By 1990, it had spread to all states of the United States and all

provinces in Canada. More recently, the face fly, a livestock pest and carrier of

parasites, entered Nova Scotia in 1952 on cargo transported by air from England.

Face flies now infest cattle in all but the southernmost states.

Examples of arthropod vectors that have been intercepted at ports of entry or that

have been detected on premises and subsequently eradicated are numerous and

alarming (3,8,11,17). Records on exotic arthropod pests found on animals and

products have systematically been compiled for over 35 years. Since that time,

over 70 species of exotic ectoparasites, primarily ixodid ticks, have been collected

from a wide variety of both domestic and zoological animals at ports of entry into

the United States. Many of the species intercepted are known vectors of some of

the most economically important livestock diseases in the world, including bovine

babesiosis, heartwater, East Coast fever, corridor disease, Nairobi sheep disease,

louping ill, and tropical disease (Table 1). Other species intercepted, such as the

sheep scab mite, New World screwworm, and louse flies, although not disease

vectors, could become serious pests of our nation's livestock population if they

were to become established in the United States. Most of the exotic pests

intercepted were found on animals while in quarantine at a USDA import center.

Examination and precautionary treatment routinely provided to these animals

ensure that they are free of ectoparasites before being released from quarantine.

When exotic animal pests are found on animal or plant products, baggage, cargo,

etc., at ports of entry other than USDA quarantine stations, treatment of the

infested material is provided to eliminate the pest before further movement into


The greatest threat to the livestock industry comes from those animals that may

enter the United States without being held in quarantine or undergoing a

precautionary treatment before entering. Such animals are those zoological

specimens not regulated by the USDA. Table 2 summarizes those arthropod pests

of livestock that have been introduced into the United States. In some cases,

lengthy and expensive eradication programs had to be conducted to ensure that

these pests did not become established. Specific examples of some of these

introductions are briefly discussed below.

In 1960, the red tick, Rhipicephalus evertsi, was discovered at a wild animal

compound in Florida (3). This was the first time that this tick had been identified

in North America. It was never determined when and how the red tick was

introduced into the United States; however, it was probably brought in on eland or

zebra imported from Africa. The tick was found as a result of an intensive

surveillance campaign by the USDA and the State of Florida during an eradication

program of the southern cattle tick, B. microplus, in Florida. Many of the wild

animals representative of the various species at the compound were inspected to

determine the relative abundance of the red ticks. Systematic application of

pesticide to the entire compound, lasting for 9 months, was implemented and the

tick eradicated.

In 1972, the louse fly, H. longipennis (Fig. 55), was identified in California on

cheetahs that had been imported from Africa in 1970 (7). Subsequent

investigations revealed that the louse fly had also become established at

zoological compounds in Georgia, Texas, and Oregon. Although primarily an

ectoparasite of wild carnivores, there was concern that H. longipennis would

become an endemic pest of pet animals, native wildlife, or livestock. As a result,

treatments began at the various parks in 1972. However, because of the louse

fly's adaptability and the relative ineffectiveness of the pesticides used early in the

treatment program, the eradication effort was not successfully completed until

1975. The louse fly was reintroduced in 1983 when bat-eared foxes imported from

Africa were found infested with this species at a zoological park in North Carolina.

Systematic treatment of the foxes and the area in which they were housed was

conducted and the infestation eliminated.

The New World screwworm, C. hominivorax, was successfully eradicated from the

United States in 1966. Since that time, it has been introduced on five occasions,

twice in 1987, once in 1990, and twice in 1997 (in 1988, screwworm larvae were

collected from 1 of 45 Argentine polo ponies during quarantine at a USDA

quarantine facility; the larvae were removed and both the wound and the

quarantine facility were treated with an appropriate pesticide). The 1987

introductions occurred when screwworm larvae were collected from dogs returning

to the United States from either South or Central America. In both cases, sterile

screwworm flies from Mexico were released around the area where the dogs were

located in the United States. In 1990, screwworm larvae were removed from a

head wound of a paratrooper who had jumped from a plane into Panama, was

injured, and subsequently evacuated to Ft. Sam Houston Military Hospital, San

Antonio, TX. Even though climatic conditions were not conducive for

establishment, surveillance activities were conducted in the area to ensure that

screwworms were not present. The 1997 introductions occurred when dogs

returning from Panama were found with infestations of screwworm larvae. In both

instances, the infestations were discovered early enough to preclude the release of

sterile screwworm flies. However, in both cases, the infested wounds were treated

for screwworms, and all conveyances used to transport the dogs and the premises

where the dogs were housed were cleaned and disinfected.

In 1997, the African tortoise tick, Amblyomma marmoreum, an experimental

vector of heartwater, was discovered on the premises of a reptile breeder in

central Florida (1). Surveillance data indicated that the infestation was restricted

to the one premises. Appropriate actions to eradicate the tick, including treatment

of the infested animals and the premises, are under way.

The recent trend towards placing zoological animals in situations that directly

expose them to susceptible domestic and native wildlife greatly increases the risk

of introducing exotic arthropod pests of livestock. Two introductions of hard ticks

serve to emphasize this risk. The first, in 1984, occurred when the bont tick, A.

hebraeum, a vector of heartwater, was collected from black rhinoceroses imported

into the United States from South Africa (17). Some of the infested rhinoceroses

were placed on a working cattle ranch in south Texas. The rhinoceroses and

premises were systematically treated. After an intensive 6-month surveillance

program, it was determined that this tick had not become established in the

United States. In the second introduction, other vectors of heartwater, including A.

gemma, A. lepidum, and A. variegatum, were introduced into the United States on

ostriches imported from Africa in 1989 (10). Like the black rhinoceroses, some of

the ostriches were placed in ecological settings favorable for the establishment of

exotic ticks, whereas others were placed in situations that directly exposed them

to domestic livestock. Premises with the ostriches were placed under quarantine,

and the ostriches and premises systematically treated with an acaricide to

eliminate the ticks.

Principles of Exclusion and Eradication

Historically, arthropod pests and their associated diseases have migrated with

humanity and their animals. When travel was slow and difficult, and trading in

animals and animal products was limited, pests of livestock moved slowly.

Moreover, many of these pests were excluded from many parts of the world by

natural environmental barriers such as mountains, oceans, deserts, rivers, and

unfavorable climates (9). These barriers served to limit the distribution of both the

pests and their hosts. Today, however, because of the volume and rapidity of

international commerce, these natural barriers are not nearly effective in limiting

the distribution of pests as in the past. As a result, strategies have been

developed to prevent pests from entering the United States on animals, animal

products, or other articles of commerce. Guidelines for eradication of arthropod

pests and their associated diseases have also been formulated.

Effective strategies for exclusion or eradication of livestock pests must be based

upon detailed knowledge of the pest's biology, host preference, and susceptibility

to pesticides. In addition, those factors that limit the pest's distribution and

methodologies for its surveillance must also be known. For exclusion efforts to be

most effective, knowledge of the avenues by which the pests might enter the

United States and become established is also needed. For example, a knowledge

of the host preference(s) of ectoparasites such as ticks, helps alert animal health

officials in determining the potential for introduction, whereas knowledge that

some species of ticks have preferred attachment sites on the host helps focus the

attention of the inspector during an examination of animals for ectoparasites.

International cooperation also plays an important role in the exclusion of many

pests of livestock. For example, in some situations, inspection of certain animals

(including zoo animals) destined for export to the United States and certification

that they are free of ectoparasites are two of the requirements that must be met

prior to export. In other situations, it may be a requirement of the exporting

country to certify that the animals have been treated for ectoparasites within a

specified time prior to export. Cooperation of neighboring countries with mutual

interests can also play a role in the exclusion or eradication of certain livestock

pests. The joint effort by the United States and Mexico in eradicating the New

World screwworm from Mexico and Central America is a recent example of such


Regulating the import of certain animals, particularly domestic livestock, is the

principal means by which livestock pests and their associated diseases are

prevented from entering the United States. Livestock and certain zoological

animals are required to remain in quarantine before entering into commerce in the

United States. During quarantine, which is usually for a 30-day period, the animals

are carefully examined for ectoparasites. The ears, flanks, escutcheon, and other

less accessible areas of the host's body as well as the more obvious sites of

attachment are carefully examined. With horses and other equines, particular

attention is given to the careful examination of the nasal diverticula (false

nostrils). If an ectoparasite is found, the animals are treated with an appropriate

pesticide. An additional treatment is provided if warranted. Animals are not

released from quarantine until they are free of ectoparasites.

When nonregulated animals, particularly zoological specimens, enter the United

States without being held in quarantine or given a precautionary treatment with a

pesticide before entering, the risk of introducing an arthropod pest of livestock is

greatly increased. The risk is minimized for those zoological specimens destined

for well-established and well-run zoos or zoological parks or gardens where

animals are thoroughly examined and treated, if necessary, for ectoparasites.

However, in situations where nonregulated zoological specimens are imported by

private individuals and are subsequently sold or traded to others, many of the

animals end up being exposed to domestic livestock or native wildlife. The

deleteriousness of this practice is exacerbated by the ignorance of the animal

owners who are not aware of the potential danger that these animals present to

our Nation's livestock industry. When an arthropod pest of livestock is identified

from these animals, States cooperate with Federal animal health officials to

eradicate the pest. The first action taken by State animal health authorities is to

quarantine the premises where the animals are located to prevent further spread

of the pest. If the arthropod pest is a known or potential vector of a foreign animal

disease, infested animals are observed for clinical signs of the disease.

Tracebacks, conducted by Federal authorities, are made of other animals that may

have come into contact with the infested animals since their entry into the United

States. In some situations, because of the extensive movements of the infested

animals from the time they enter the United States and the time the pest is found,

tracebacks may become extremely complex and time consuming. If, through the

traceback procedure, other premises are found with infested animals, these too

are quarantined. Surveillance activities are undertaken on the infested premises

and, if appropriate, on adjacent premises as well. Once the extent of infestation is

determined, the infested animals and the premises where they are located are

systematically treated with pesticides known to be effective against the pest on

and off the host. Surveillance activities are continued throughout the quarantine

and treatment procedures to ensure the pest is eradicated.

To date, introductions of exotic arthropod pests of livestock have been relatively

localized or have involved pests whose spread has primarily been related to the

movement of their hosts (e.g., ticks and louse flies). As a result, activities to

eradicate these pests have been relatively inexpensive and of short duration.

However, if broad-area introductions were to be made, or if highly mobile pests

such as mosquitoes or flies were to be introduced into the United States,

eradication could be exceedingly costly and lengthy. In addition, because of

increasing environmental concerns, eradication activities involving the widespread

use of pesticides may not be sociologically acceptable and may therefore not be



Several economically important arthropod pests of livestock in the United States

have been introduced. For the most part, these introductions occurred during the

time when livestock entered the country without restriction. Now, however,

extensive efforts are made to preclude the introduction of exotic arthropod pests

of livestock and poultry and arthropod-borne disease vectors. Regulating the

import of live animals, particularly domestic livestock, is the principal means by

which arthropod pests are prevented from entering the United States. These

animals are required to remain in quarantine until it can be determined that they

are free of pests and disease.

The greatest risk of introducing pests of livestock and poultry comes from the

importation of nonregulated animals — particularly zoological specimens. Such

animals can enter the United States without being held in quarantine to ensure

that they are free of exotic pests and diseases. When an arthropod pest of

livestock or an arthropod-borne disease vector is identified from these animals,

State and Federal animal health officials cooperate to eradicate the pest.

Depending on the circumstances, these eradication efforts may be expensive and

time consuming.


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D. D. Wilson, Ph.D., USDA-APHIS, Emergency Programs Staff, Riverdale, MD

R. A. Bram, Ph.D., USDA-ARS (Retired), Greenbelt,MD


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