Arthropod parasites

Arthropod parasites and the diseases they transmit are among the most important deterrents to the progress of tropical agriculture in Africa and Central and South America. No general agricultural economy can be entirely successful if domestic animals and poultry are excluded or their husbandry for any reason is unprofitable. In many tropical areas the entire livelihood of families or tribes depends on cattle that are used for food, draft, and as a measure of wealth. Outbreaks of parasites or disease epizootics that decimate or debilitate their animals can mean financial disaster or even famine for these people. 

SCHWARDT, MATTHYSSE, BAKER, AND PALM—ARTHROPOD PARASITES... 

This dual activity against both nematode and arthropod parasites of animals was an unexpected bonus from a screen for anthelmintic agents. The reason for this broad activity lies in their mode of action. They act by interfering with y-aminobutyric acid (GABA) mediated neurotransmission. When treated with avermectin, the nematode Ascaris suum becomes paralyzed although it retains normal muscle tone (17). Picrotoxin, an antagonist of GABA, can reverse the effect of avermectin on neurotransmission vitro. 

The initial objective of the search was an anthelmintic with properties radically different from those of known anthelmintics. What was found was an anthelmintic whose properties were indeed markedly different — not only in terms of chemical structure and efficacy against helminths, but also in the extension of the potential utility of the class to the control of arthropod parasites of animals. Further, the compounds turned out to have striking activity against a variety of free-living and plant-parasitic nematodes and arthropods — and so it has come about that a new livestock anthelmintic has become an agenda item in this symposium on agricultural pesticides. 

Ivermectin is exceptionally effective in very low dosages against nematodes and arthropod parasites in cattle and has been widely used for treatment of endo-and ectoparasites in cattle, sheep, goats, and swine (49). It is administered orally, parenterally, or as a pour-on preparation at dosages ranging from 0.2 to 0.6 mg/ kg bw. Ivermectin exhibits teratogenic effects in rat, rabbit, and mouse. 

Ivermectin is a semisynthetic analog of avermectin B which contains at least 80% of 22,23-dihydroavermectin B,a and not more than 20% of 22,23-dihydroavermectin B,b. It is a potent anthelminthic/insecticide for the control of a wide variety of arthropod parasites, including endoparasites and ectoparasites of animals such as cattle, horses, sheep, swine, and dogs. Its oral LD50 in rats is 650 mg/kg. 

The avermectins, particularly, have been the subject of intense commercial interest because they possess potent activity against both nematode and arthropod parasites of livestock (229). A full discussion of structure-activity relationships would be out of place here, not least because the data are voluminous, so we shall concentrate on the development of ivermectin, which has been a major success. 

Macrolide endectocides (avermectins and milbemy-cins) have activity at low concentrations against both internal (nematode) and external (arthropod) parasites. Veterinary preparations of avermectins include... 

About 10 species are known, which include pathogens of Lepidoptera, Coleoptera, and Araneida (spiders). A. johnsonii was reported as a saprobe or fungicolous species in leaf litter, but arthropod parasitism is unknown (Vincent et al., 1988). None of the species is well characterized in terms of its ecology, and none has been assessed as a biocontrol agent. 

The few studies of contact chemoreceptors of arthropod parasites of mammals have led to the identification of receptors predicted from behavioral observations (reviewed by Galun, 1976). The most significant discovery in this regard was the localization of an ATP receptor in the labellar sensilla of the tsetse fly... 

Rickettsia are more difficult to produce in quantity than bacteria. Similar to viruses, they are strict obligate parasites and require living cells for growth. They cannot survive long outside a host. They also have a selective affinity for specific types of cells in the body. They are normally transmitted by an arthropod vector (i.e., ticks, lice, fleas, mites), which also serves as either the primary or intermediate host. 

Arthropods that prey on or parasitize other arthropods frequently employ those chemical cues that reliably indicate the presence of their... 

The microorganism was classified as a new species of actinomycete. Streptomyces avermitilis. Its anthelmintic activity was shown to reside in 8 closely related macrocyclic lactones, named avermectins, which were also found to possess activity against free-living and parasitic arthropods. One of the natural components, avermectin is now being evaluated as a pesticide for the control of mites of citrus and cotton crops and control of the Red Imported Fire Ant. A chemical derivative, 22,23-dihydroavermectin or ivermectin, has been developed as an antiparasitic agent. It is being marketed for use in cattle, horses and sheep and is expected to become available for swine and dogs. 

Adverse hygienic conditions favor human infestation with multicellular organisms (referred to here as parasites). Skin and hair are colonization sites for arthropod ectoparasites, such as insects (lice, fleas) and arachnids (mites). Against these, insecticidal or arachnici-dal agents, respectively, can be used. Endoparasites invade the intestines or even internal organs, and are mostly members of the phyla of flatworms and roundworms. They are combated with anthelmintics. 

Mechanism of Action A scabicidal agent that is directly absorbed by parasites and ova through the exoskeleton. Therapeutic Effect Stimulates the nervous system resulting in seizures and death of parasitic arthropods. 

Although any microbe that infects a human (virus, bacteria, fungus, etc.) could theoretically be considered to be a parasite, the term parasite tends to be reserved for three types of infectious agents protozoa, helminths, and arthropods. These may be subdivided as follows ... 

Second, the increased movement necessary for locating widely dispersed feeding sites should increase contact rates with enemies. Movement makes insects more conspicuous to parasitoids or predators sensitive to it (50,51). Random encounters with arthropod predators or parasites should increase with searching activity, as would risk of dislodgement and fallout. 

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