Ectoparasites of cattle and small...
Ectoparasites of Cattle and Small Ruminants Roberto Cortinas, DVMa,*, Carl J. Jones, PhDb a Department of Veterinary and Biomedical Sciences, University of Minnesota, 205 Veterinary Science, 1971 Commonwealth Avenue, St. Paul, MN 55108, USA b Department of Entomology and Plant Pathology, 207 Ellington Plant Sciences Building, College of Agricultural Sciences and Natural Resources, University of Tennessee, Knoxville, TN 37996-4560, USA Arthropods are among the most numerous creatures on earth, with more than 900,000 species of insects identified . They are successful creatures, occupying nearly all ecosystems and exhibiting a wide variety of behaviors, including parasitizing other multicellular organisms. Though only a few are ectoparasites of humans and animals, the pain, physical damage, reduction in fitness or gain, disease, and economic costs caused by various insects and acarids (ticks and mites) make their control obligatory. In this article, the authors describe the important ectoparasites of livestock and small ruminants in North America. Understanding the life cycles of these ectoparasites is essential when one is making decisions about their control. Because the life cycles are varied and sometimes complex, details of each are given here. Two different kinds of life cycle are examined. In some instances, the adult arthropod will look very different from a developing immature, as observed in the flies. The change from larva to pupa to adult is referred to as complete metamorphosis (from the Greek word for transformation). By contrast, the change that is observed in some groups (for example, ticks) is gradual in that the immature and mature stages differ only slightly in appearance. This change is called gradual or incomplete metamorphosis. Integrated pest management (IPM) requires that we use the safest and least invasive control technology available while keeping pest and vector levels below damage thresholds. As in any other war, it is important that we attack the targets when it will cost us the least and help us gain the * Corresponding author. E-mail address: firstname.lastname@example.org (R. Cortinas). 0749-0720/06/$ - see front matter �� 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.cvfa.2006.06.003 vetfood.theclinics.com Vet Clin Food Anim 22 (2006) 673���693
most. Some propose that IPM for use in veterinary medicine is different from that used in plant communities because of the mobility of the subjects and the resulting limitations on successful control technology. Additionally, the client generally has become habituated to nearly complete control, rather than simply keeping pest numbers below the economic threshold or vector numbers below the transmission threshold. Nevertheless, use of IPM will often reduce both the financial burden on the client and the cost to the environment. Arthropods: phylum Arthropoda Among the common characteristics of arthropods, perhaps the most im- portant is the presence of an exoskeleton with jointed appendages. Briefly, the cuticle of the exoskeleton is impregnated with pigments and sclerotized by means of protein polymerization, creating a tough, protective surface and characteristic color patterns. This protective cuticle with flexible joints de- fends arthropods against desiccation, lending them the ability to survive in nearly every environment. Other characteristics shared by arthropods in- clude a complete digestive tract, an open circulatory system (hemocoel), sep- arate sexes, eyes (either simple or compound), and a body with observable divisions: head, thorax, and abdomen in insects capitulum and idiosoma in ticks and gnathosoma and idiosoma in mites. Although the magic of Hollywood has created some monstrous arthropods for entertainment, the practical weight limit appears to be 22 kg, attained by some lobsters. Flies: order Diptera The Diptera, or ������true flies,������ have one pair of wings, although some inti- mate parasitic species are wingless. Although they all undergo complete metamorphosis, other life-cycle characteristics, including nutritional re- quirements, parity mode, and larval development sites, vary widely among these insects. Mosquitoes, black flies, biting midges, and sandflies Mosquitoes, black flies, biting midges, and sandflies are grouped into the suborder Nematocera. Adults in this group are generally small and delicate, with long, segmented antennae. Females of a few species of this suborder, not including mosquitoes, can complete the first gonotrophic cycle without a blood meal. In general, these ectoparasites may be considered micropreda- tors, because they consume a portion of the host (blood) without killing but do not stay with a single host. As a result, they usually have moderate to broad host specificity. Mosquitoes are the best known of the blood-sucking insects. The imma- ture stages are found in virtually every aquatic habitat. Adults of various 674 CORTINAS & JONES
species are active throughout the day, although individual species may only be active for specific periods, such as dawn and dusk. Depending on the species, eggs may be deposited singly or in rafts in sites as diverse as soil, above the waterline of treeholes, or on the water. Eggs may undergo install- ment hatching or diapause, but once they hatch, the larvae must complete all four larval instars and the pupal stage in water. Both larvae and pupae require air, which they acquire through appendages touching the surface of the water. Surface oils may take advantage of this characteristic to control immatures in small habitats. Only the females take blood meals, but both male and female adults can be found in nectaries. In most cases the transmission of infectious disease agents by mosquitoes requires two bites, one to acquire the agent from an infected host and a second to transmit it to an uninfected host. At this time, the only mosquito-borne disease that is of concern for ruminant hosts exists only in enzootic foci in Africa (Rift Valley fever). Therefore, the pri- mary effect mosquitoes may have on ruminants is irritation, although Steel- man  and others have noted the economic effects of unabated mosquito populations on cattle. Black flies are small dipterans that deposit their eggs in or near running water. Although only 1 to 5 mm in length, these daytime biters can appear in huge swarms, causing severe annoyance and blood loss. Pathogenesis can in- clude toxicosis, dermatitis, allergies, and death (apparently by asphyxiation) in cattle, sheep, and wild ruminants. Black fly fever in humans is a systemic response to black fly salivary antigens, and cattle may have similar symp- toms, including nausea and fever. In sheep, the development of pruritic papules in feeding zones has been observed. At least one species serves as a vector for Onchocerca lienalis in the United States . The immature stages spend their lives in water. They are filter feeders that remove organic material from the water or actively feed on substrate mate- rials. In temperate climates, there is rarely more than one generation of each species per year, although the succession of species through a summer may make a single species appear to be constantly present. Although eggs typically cannot withstand desiccation, they are the stage at which diapause occurs. As in mosquitoes, females require blood meals and have anesthetic components to their salivary secretions. Black fly control is generally best practiced when the flies are in their larval habitat or when they are attacking their hosts. With the increasing purity of flowing water in the United States, larval habitat for the black flies has increased. Consequently, more communities are being tormented by black flies and are asking pest and vector control authorities to provide re- lief. Moderately e���cacious control may be provided to livestock operations using pyrethroid pour-ons and other repellent and toxicant compounds. Culicoides species, or no-see-ums (Ceratopogonidae), are very small flies that deliver no anesthetic when they bite. Culicoides vary from 0.6 to 5 mm in length, depending on the species, and often are so small that they 675 ECTOPARASITES OF CATTLE AND SMALL RUMINANTS