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 commerce.
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 thetreatment 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 cooperation.
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 feasible.
Summary
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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