Vectors & Diseases
FOREIGN PESTS AND VECTORS OF ARTHROPOD-BORNE DISEASES
FOREIGN ANIMAL DISEASES
FOREIGN PESTS AND VECTORS OF ARTHROPOD-BORNE 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
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
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
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
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
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
GUIDE TO THE LITERATURE
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Imported into the United States. In Proc. 88th Ann. Mtg. U.S. Anim. Hlth. Assoc.,
D. D. Wilson, Ph.D., USDA-APHIS, Emergency Programs Staff, Riverdale, MD
R. A. Bram, Ph.D., USDA-ARS (Retired), Greenbelt,MD
PLEASE SEE FULL ARTICLE TO VIEW TABLES, CHARTS & ADDITIONAL IMPORTANT INFORMATION.
Full article located here- http://www.phsource.us/PH/PALM/PDA/FOREIGN%20PESTS%20AND%20VECTORS%20OF%20ARTHROPOD-BORNE%20DISEASES.pdf