FOREIGN PESTS AND VECTORS OF
FOREIGN ANIMAL DISEASES
(Vector-borne Diseases and Arthropod Vectors)
Wilson DD, Bram RA. Foreign pests and vectors of arthropod–borne diseases. In: Foreign Animal Diseases. Richmond, VA: United States Animal Health Association; 1998. Available at: http://www.vet.uga.edu/vpp/gray_book02/fad/fpv.php
Accessed 11 Dec 2006.
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 disease.
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
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 lifecycle 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).
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