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40 Seiten, Note: A
TABLE OF CONTENTS
LIST OF TABLES
LIST OF ACRONYMS
3. GENERAL ANALYSIS
The Host, Habitat and Geographical Range, and Disease Relationships of Venomous Arthropods
The Host, Habitat and Geographical Range, and Disease Relationships of Parasitic arthropods
The Host, Habitat and Geographical Range, and Disease Relationships of Arthropod-Borne Parasites
The vector range of other arthropod borne pathogens and diseases
6. GENERAL RECOMMENDATIONS
Table 1: The Host, Habitat and Geographical Range, and Disease Relationships of Venomous Arthropods
Table II: The Host, Habitat and Geographical Range, and Disease Relationships of Parasitic arthropods
Table 3: The Host, Habitat and Geographical Range, and Disease Relationships of Arthropod-Borne Parasites
Table IV: The vector range of other arthropod borne pathogens and diseases
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Venomous arthropods are those that release a poisonous substance (venom) when disturbed. They release the venom in their defense against intruders. Examples of venomous arthropods are scorpions, wasps, some caterpillars and bees. The word “parasite” is derived from two Greek words, “para”, meaning “beside”, and “sitos” meaning “food”. Therefore, a parasite literally means an organism that is beside another organism for purposes of obtaining food. Adam, et al. (1979) defined a parasite as an organism which depends for part of its life or for its entire life on another organism, called the host, from which it obtains food and shelter. According to Smyth (1996), hosts are normally of a different species from their parasites. Parasitism is a kind of adaptation for survival, and in any case, a true parasite should not kill its host, lest it will kill its source of survival and/or kill itself too. Although many parasitic organisms are harmless to the host, others are pathogenic; they cause disease in their hosts, leading to morbidity and death of the host. The parasitic mode of life must have been a survival mechanism developed by certain organisms since the beginning of life on earth, about 4 billion years ago. It must have been impossible for these organisms to survive on their own. It is known that environment can change; presenting different conditions each time and organisms struggle to cope and survive with the change. We are aware that the earth has gone through periods of ice, fire, meteorite strikes, volcanic eruptions, global dust veils, acid rain, and continental upheavals. When such changes occur in the environment, organisms naturally take refuge in safer habitats, hosts or geographical locations. Better still, others change behaviour or transit through an evolutionary process into a new organism in order to fit within the new conditions. So, changes in the environment present with both challenges and opportunities. Organisms which were once free living can become parasitic and those which were once non-pathogenic can become pathogenic. Those which do not have survival mechanisms to cope with the change will disappear from the scene. For example, 99% of the life forms that ever appeared on earth met with extinction already (WWH, 2012). The remaining 1% today consists of an estimated 5-30 million species of flora and fauna. Of these, approximately 1.7 million have been named, and more than half are insects. An insect is that animal as defined and described by Davies (1988), Anonymous (2012a) and Wikipedia (2012a). It is estimated that insects make up 75% (1.3 million) of the known animal kingdom. Insects evolved all through from the time they appeared about 400 million years ago (Devonian period) to occupy every possible land habitat on earth, including freshwaters, except in the depths of saline waters of the sea. Because insects occupy almost every conceivable terrestrial niche, they interact with humans and other organisms, including micro-organisms and domestic animals in countless ways, with varying degree of impact. One case of association of insects with other organisms is parasitism. Such information about insects is obtained through the discipline of Entomology, a study done by scientists called entomologists. The definition of Entomology has been reviewed in Cambridge International Dictionary (1995) and Wikipedia (2012b). Entomological studies have revealed that insects do not exist as stand alone organisms; they co-exist with other closely related organisms. In biological classification and taxonomy, insects (class Insecta) belong to yet a larger group (phylum) of animals called Arthropoda. Arthropoda means ‘jointed feet’ and includes all animals with segmented legs, segmented bodies and exo-skeleton. Therefore, arthropods include millipedes, centipedes, horseshoe crabs, crayfish, crabs, shrimps, lobsters, sowbugs, pillbugs, scorpions and relatives, spiders, mites, ticks, insects, and many other related organisms. Although entomology is synonymous with the scientific definition of a true insect, it evolved primarily to concentrate on both insects and arachnids. However, some other arthropod classes like Diplopoda, Chilopoda are still being considered by entomologists. Even a few non-arthropod groups like the phylum Mollusca (snails and slugs) and phylum Nematoda (nematodes) are sometimes referred to entomologists (Anonymous, 2012a). Therefore, entomology still encompasses arthropods other than insects as well. Owing to their generally small size, numerical superiority, ability to inhabit every ecological niche on land and freshwaters, and behaviour and activities; arthropods, as a whole, have over the time developed varying associations with organisms, including parasitism, as observed with the insect subgroup earlier. Entomological studies have also revealed that arthropods such as ticks and tumbu fly are parasites in their own right. Others like mosquitoes, tsetse flies carry parasites that cause disease in humans and animals. Some of them like ticks and lice are both parasites and vectors of other parasites and pathogens. Further still, others like ticks are all through being venomous, parasitic and vectors of disease causing parasites. These discoveries all started when Josiah Nott, a mobile, Alabama, physician, proposed (1848) that the causative agents of malaria and yellow fever were transmitted by mosquitoes. In 1881, Carlos Finlay, a Cuban physician, postulated that mosquitoes transmitted the yellow fever agent, setting the stage for Major Walter Reed and associates to verify his claim. However, the foundation for modern medical and veterinary entomology was said to be laid by Louis Pasteur, a French microbiologist who formulated the theory of microbial causation of disease (germ theory), based on his work with the silkworm, Bombyx mori in 1887. This was followed by Theobold Smith who in 1889 discovered the causative agent of Texas cattle fever and, working with F. I. Kilbowen, showed in 1893 that the cattle tick, Boophilus annulatus, was the vector. In 1897, Ronald Ross demonstrated the occurrence of the malaria parasite in mosquitoes that fed on a human patient, whose blood contained the parasite, thus leading to the elucidation of the epidemiology of malaria. Their work paved the way for the present day research, knowledge and practices of arthropod vector borne diseases and management. Many arthropod vector borne diseases have now been known to occur in different parts of the world, including the human African trypanosomiasis, sometimes called Sleeping Sickness, reported in 37 sub-Saharan African countries (Biryomumaisho, 2007a; and Science Daily, 2012). It is caused by a protozoan of genus Trypanosoma, transmitted by tsetse flies (Glossina spp.), mainly while taking a blood meal from a human host victim. Tsetse flies also transmit a form of trypanosomiasis to domestic animals called animal trypanosomiasis or nagana. Malaria has been confirmed to be a disease transmitted by female Anopheles mosquito to humans, also while taking a blood meal. It is caused by a protozoan of genus Plasmodium. Malaria is a killer disease, but difficult to control, yet it continues to be a major public health problem in most countries of the tropical world, with majority of the deaths being reported from Africa (WHO, 1995). Another arthropod vector borne disease is East Coast Fever (ECF), caused by a protozoan organism, Theileria parva. It is transmitted by ticks, Rhipicephalus appendiculatus, to cattle, mainly in eastern, central and southern Africa. The disease presents with a number of early clinical signs, including reduced milk yield (Anonymous, 2008) with the consequent loss of income to the farmer. Complicated form of ECF is even terminal. Although the emphasis here appears to be on the actual disease parasites they transmit, the biting and blood sucking behaviour of some of the arthropods in itself is being parasitic, as mentioned earlier, should not be taken for granted. From the explanations above, what comes to the mind is that certain arthropods can be very disturbing to human and livestock health and well being. For a long time their management has been based on the theory that they have certain specific hosts, habitats and geographical distribution patterns; knowledge obtained through research. However, with the realities of global warming and climate change on ground, it is widely feared that the parasites will expand their hosts, habitats and geographical locations; thus shifting and enlarging the incidence and distribution of diseases. There will be emergence of new diseases and re-emergence of diseases previously suppressed or eradicated. This calls for taking an account of currently known arthropod-borne parasites and their host, habitat and geographical range. This will help to trace global warming and climate change effects on the parasites and to better understand and manage the dynamics involved in case they take on new hosts, habitats and geographical locations. This paper therefore reviews the host, habitat and geographical range; and disease relationships of venomous and parasitic arthropods, and arthropod-borne parasites.
Many people have experienced the discomfort and pain caused by venomous arthropods. Examples of venomous arthropods are ants, wasps, spiders, fleas, ticks, scorpions. Their venom can cause more than mere pain. Harris (2013) reported that more human deaths in the United States are attributed to venomous arthropods than any other group of venomous animals, including snakes. Although deaths caused by arthropod bites or stings represent only a tiny fraction of the millions of victims, a higher number of about 25,000 per year in the United States have severe reactions. Arthropods often live in close proximity to people and can be abundant, resulting in a high contact rate with people. Arthropods cause three types of envenomization (exposure to venom): piercing/biting, vesicating/urticating, and stinging. Piercing/biting arthropods inject a toxin through their mouth parts, and they include the chiggers, fleas, ticks, spiders, centipedes, wheel bug; millipedes are not venomous, but some of the large tropical species are said to release poisonous or pungent fluids from cutaneous glands when they are irritated. Vesicating/urticating arthropods release toxins on contact through venomous hairs (urticating) or small body openings (vesicating), and they are represented by blister beetles, certain caterpillars with stinging hairs or spines; e.g. Io moth caterpillar, puss caterpillar, and saddleback caterpillar. Stinging arthropods inject a toxin through a stinger located on the posterior end of the abdomen and the group includes paper wasps, hornets, yellowjackets, cicada killers, fire ants, and scorpions. The biology, ecology and systematics of these arthropods have been reviewed by Davies (1988), and Harris (2013). Their host, habitat and geographical range is highlighted in Table 1, but it is important to note that most of them do not have specific host organisms. Fortunately, they are conspicuous organisms, and in a likely event of change of their ecological habitat to escape harsh conditions brought about by global warming and climate change, they could still be noticed in their new ecological habitats, and be avoided or controlled and their venom treated as in Harris (2013). Therefore, more emphasis will be put on the parasitic arthropods and arthropod-borne parasites which hang on other organisms as their hosts for survival. It is important to map out their host, habitat and geographical range. Should the currently known parasitic arthropods and arthropod-borne parasites take on new hosts, habitats and geographical locations, the situation could be more complex and difficult to manage! So, a deeper understanding of them will help to better envisage what would happen if they expand their hosts, habitats and geographical locations. Currently, the widely known malaria causing parasites, plasmodia, belong to phylum Apicomplexa, class Sporozoea, subclass Coccidia, suborder Haemosporina, family Plasmodiidae, and genus Plasmodium (Smyth, 1996); while the vector Anopheles mosquito belongs to class Insecta, order Diptera, family Culicidae, and genus Anopheles (Davies, 1988). There are 4 main malarial parasites of man: Plasmodium vivax, P. malariae, P. falciparum, and P. ovale; but the most virulent being P. falciparum (Smyth, 1996). As said earlier, malaria is one of the greatest killer diseases, ranking with cancer and heart disease. Despite continuous research and control programmes in many countries, the situation has shown little improvement, and malaria continues to be a major public health problem in most countries of the tropical world. WHO (1995) reported that of the total world population of about 5.4 people by then, 2200 million were exposed to malaria infection in some 90 countries or areas. It is estimated that there may be 300-500 million clinical cases each year, with countries in tropical Africa accounting for more than 90% of these. Malaria is also said to be the cause of an estimated 1.4-2.6 million deaths worldwide every year, with more than 90% in Africa alone. It is one of the most important causes of mortality and morbidity among infants and young children, and infection during pregnancy contributes, primarily in primiparae, to maternal mortality, as well as to neonatal mortality and low birth weight. The burden put by this disease on household income and human labour force in the affected communities is enormous. Global warming is further compounding the situation. For example, in certain areas like in Kabale District in Southwestern Uganda which used to have cold weather, unfavorable for mosquitoes, are now warming up which has increased incidents of malaria more than previously reported (Environmental Alert, 2010). The need to review and understand the host, habitat and geographical range of the parasite is therefore paramount. Another disease being transmitted by mosquitoes is filariasis, caused by filariae. Although there is no universally agreed classification of nematodes yet, filariae are, for convenience, put in phylum Nematoda, subclass Secernentea (Phasmidea), order Spiururida, suborder Spirurina, and superfamily Filarioidea. The biology of filariae is quite substantially covered by Smyth (1996). A number of the species of filariae are known to parasitize man. In many of the human cases, infections give rise to revolting fleshy deformities which are collectively called elephantiasis. This condition results from inflammation of the walls of the lymphatics and the consequent hyperplasia and partly from mechanical blockage by the worms. Many species of filaria exhibit periodicity. Of the common species of filariae of man, 3 of them responsible for most of the cases of human filariasis are Wuchereria bancrofti, Brugia malayi and Onchocerca volvulus. Wuchereria bancrofti, vectored by members of Culex (C.) pipiens “complex” in urban areas and species of Anopheles, Aedes and more rarely Mansonia.; causes bancroftian filariasis, resulting in elephantiasis in man. Brugia malayi, resembling W. bancrofti, causes Malayan filariasis in man. Approximately 750 million people are at risk of lymphatic filariasis, mainly caused by W. bancrofti. Nearly 80 million people are infected and some 30 million of them experience the chronic disease. Of those with chronic infection, more than 1 million suffer from overt elephantiasis, the most disfiguring form of the disease (WHO, 1995). Documentation of the host, habitat and geographical range of these filarial parasites will help in designing better control strategies for filariasis. Dengue and dengue haemorrhagic fever are the most important arboviral diseases, transmitted again by mosquitoes. The main vector in urban areas is Aedes aegypti and in suburban and rural areas is Ae. albopictus. Vector Ae. albopictus is spreading in the world and giving cause for concern. Epidemics of dengue and dengue haemorrhagic fever threaten nearly two-fifths of the world’s population, in 100 countries, accounting for millions of cases of disease and thousands of deaths each year. Dengue has recently caused extensive epidemics in non-immune populations in Africa, the Americas, Asia, the Pacific islands and certain countries of WHO’s Eastern Mediterranean Region. 37 countries have experienced outbreaks of dengue and dengue haemorrhagic fever, and in many countries, outbreaks of dengue and dengue haemorrhagic fever are the leading cause of hospitalization of young children (WHO, 1995). Highlighting the knowledge of the vector range for the pathogen is crucial. Japanese encephalitis is another viral disease transmitted by mosquito, Culex tritaenirhynchus. It has been reported in some countries in Asia and the Pacific islands. Yellow fever is yet another viral disease transmitted by mosquitoes. The number of cases of yellow fever reported to WHO from Africa in the mid-1980s was 5104 but decreased to 2561 cases in 1991. Understanding the vector range of Japanese encephalitis, yellow fever and other mosquito viruses is very important for developing better control strategies in future. Current control options for malaria and other mosquito-borne diseases have been reviewed by WHO (1995) and CDC (2012a). Ticks are one of the parasitic arthropods in their own right while at the same time transmitting disease causing parasites to humans and animals. Description of parasitic arthropods will be detailed in the following section. Tick transmitted diseases are collectively called Tick-borne Diseases (TBDs). Ticks belong to phylum Arthropoda, class Arachnida, order Acarina, suborder Ixodoidea, and 3 families of Argasidae, Ixodidae, and Nuttallielidae. Ticks obtain blood meal from vertebrate or invertebrate host. This has led to transmission of various disease agents from infected animals and humans to uninfected animals and humans. Nuttallielidae has only one species whose economic importance is yet to be known. There are about 170 species of soft ticks (Argasidae) and 700 species of hard ticks (Ixodidae), majority of which are found in Africa. Several of these are known to transmit a wide range of micro-organisms; viruses, rickettsia, bacteria, spirochetes, and also protozoa and nematoda, and are reported to surpass all other arthropods in the number and variety of diseases they transmit to animals and man (Okello-Onen, et al., 1999). The biology, ecology and taxonomy of ticks have been reviewed by FAO (1984a), Okello-Onen, et al. (1999) and Anonymous (2008). Ticks of economic importance to livestock in Africa belong to the family Ixodidae. Only 9 species (from 4 genera) out of a total of over 650 species (from 13 genera) are known to be vectors of economically important diseases. These are Rhipicephalus (R. appendiculatus), Boophilus (B. decolaratus, B. microplus, and B. annulatus), Amblyomma (A. variegatum, A. hebraeum) and Hyalomma (H. anatolicum, H. detritum, and H. dromedarii). They transmit a variety of TBDs that can be categorized into protozoan diseases (Theileriosis and Babesiosis), rickettsial diseases (Anaplasmosis and Cowdriosis) and tick-associated dermatophilosis, etc. Although management of TBDs has been reviewed in FAO (1984b) and Anonymous (2008), there is still need to review the host, habitat and geographical range of ticks and tick-borne parasites. Flagellates of the genus Leishmania are parasites of man and other mammals, including dogs. Leishmania spp. belong to the phylum Sarcomastigophora, subphylum Mastigophora (Flagellata), class Zoomastigophorea, order Kinetoplastida, suborder Trypanosomatina, family Trypanosomatidae, and genus Leishmania. They cause diseases collectively known as leishmaniases, which can be grouped into three: cutaneous leishmaniasis, mucocutaneous leishmaniasis (espundia), and visceral leishmaniasis (Kala-azar or black disease or “dum-dum fever” or “ponos”). These are serious debilitating and disfiguring diseases which occur in African, Asian, American, and Mediterranean region countries. All forms of leishmaniasis of man are transmitted by the bite of female sandflies of the subfamily Phlebotominae, which contains about 600 species and subspecies; some 70 of these are proven or suspected vectors of leishmaniasis (Smyth, 1996). Tsetse flies belong to class Insecta, order Diptera, and family Glossinidae. They inhabit sub-Saharan Africa. The biology, ecology, systematics, and distribution of tsetse have been extensively studied and documented by Smart et al. (1943), Jordan (1961), Ford (1968), Kangwagye (1968), Kangwagye (1988a), Nash (1969), Locke and Smith (1980), Service (1980), Turner (1980), FAO (1982a), FAO (1982b), Young (1982), Turner (1987), Dransfield (1988), Yu et al. (1996), Kalyebi (1998), Vreysen and Khamis (1999), and Mugasa (2007a). 31 species and subspecies of tsetse flies are known to exist in Africa. As already stated in the introduction section, tsetse fly is known to transmit trypanosomes parasitic to both man and livestock, human African trypanosomiasis (sleeping sickness) and animal trypanosomiasis (nagana) respectively. Smyth (1996) grouped trypanosomes in the phylum Sarcomastigophora, subphylum Mastigophora (Flagellata), class Zoomastigophorea, order Kinetoplastida, suborder Trypanosomatina, family Trypanosomatidae, and genus Trypanosoma. Between 50,000 and 70,000 people are infected with human African trypanosomiasis (sleeping sickness) and about 60 million are at risk of human African trypanosomiasis (sleeping sickness) infection, as reported in Science Daily (2012). Although trypanosomiasis is known to occur in 37 sub- Saharan African countries, it is important to get a renewed picture of the host, habitat and geographical range of the various Trypanosoma spp. to guide planners and implementers of human and livestock health projects and programmes. Past and present control options for tsetse and tsetse-borne diseases have been reviewed by Jordan (1986), Kangwagye (1988b), FAO (1982c), FAO (1992), FAO (1993), Biryomumaisho (2007b), Biryomumaisho (2007c), Mugasa (2007b), Waiswa (2007a), Waiswa (2007b), Waiswa (2007c), Waiswa (2007d) and CDC (2012). Chagas’ disease is caused by Trypanosoma cruzi, transmitted by brightly coloured triatomid bugs belonging to the family Reduviidae, and subfamily Triatominae, all stages of which (larva, nymph and imago) are susceptible to infection (Smyth, 1996). Trypanosoma cruzi is passed to the victim through defecation by the vector after a blood meal. Chagas’ disease occurs throughout South and Central America; hence it is sometimes referred to as South American Trypanosomiasis. It is essentially a zoonosis, affecting both humans and other wild mammals. The main vectors are known. However, the focus has been on the human pathology, and by 1985 over 24 million people were infected, or at least serologically positive for T. cruzi (Smyth, 1996). Do we precisely know the host, habitat and geographical range for T. cruzi ? Onchocerca volvulus causes a disease called onchocerciasis in man. Onchocerciasis is one of the world’s distressing diseases of helminth origin, often resulting in blindness, generally referred to as “river blindness” because its main vector, black flies of genus Simulium occupy riverine habitats. The population at risk of onchocerciasis infection in the world is 85,583,780 while that in Africa, including Sudan is 80,750,000. The population infected with onchocerciasis in the world is 17,757,700 while that in Africa, including Sudan is 17,640,500. Current options for onchocerciasis and vector control have been reviewed by WHO (1987). Other species of Onchocerca have been detected in domestic animals. Fleas belong to class Insecta, order Siphonaptera; and a number of families, subfamilies, genera and 2,400 species are so far known. One important example is the “tropical rat flea”, Xenopsylla cheopis which transmits bubonic plaque bacillus to man. Dracunculoid worms belong to phylum Nematoda, subclass Secernentea (Phasmidea), order Spiururida, suborder Camallanina. They are parasitic in vertebrates, and vectored by a copepod host. One key example, Dracunculus medinensis, sometimes called guinea worm, is infective to man. There are over 140 million people at risk of Dracunculus medinensis infection. The annual incidence in Africa is estimated at 3.32 million (Smyth, 1996). It is has been known to be vectored by Cyclops sp. Human infection is brought about by accidentally taking infected copepods in drinking water. The disease caused by this parasite in humans is called dracontiasis or dracunculiasis, and some control options have been reviewed by Smyth (1996). Mansonella perstans, M. streptocera, and M. azzardi vectored by midges, Culicoides spp. infect man, among other mammals. They cause diseases collectively called mansonellosis in different parts of the world. There are many other parasites (Tables 3) pathogens (Table IV) vectored by arthropods. In general, arthropods of major medical and veterinary importance have been identified among mosquitoes (Culicidae), tsetse flies (Glossinidae), horse flies and deer flies, moth flies, black flies, muscid flies, skin bots, grubs, louse flies, lice, mites, ticks, kissing bugs, bedbugs, Lepidoptera and Hymenoptera, fleas, spiders and scorpions.
Major arthropod groups that have so far been found to be parasitic in their own right include lice, Pediculus humanus and Phithrus pubis (Anoplura: Pediculidae); jigger flea, Tunga penetrans (Siphonaptera); mites, Sarcoptes scabei; chiggers (Trombiculidae); bot fly (berne), Dermatobia hominis (Diptera), tumbu fly, Cordylobia anthropophaga (Diptera), and ticks (Acarina) as in Hamilton (2013) and Ghaffar and Hunt (2013) . The kind of parasitism exhibited by the bot fly and the tumbu fly is different from the rest in the list. Their larvae infect and bury themselves in tissues of man, domestic and wild animals; a condition called myiasis. It is an obligatory step in the life cycle of some flies and incidental for others, a phenomenon typical of dipterous larvae. Other species exhibiting myiasis are found in genera Cochliomyia (Screw worm fly), Calliphora, Oestrus, Sarcophaga, and Gastrophilus. Myiasis may be cutaneous, arterial, intestinal or urinary, in normal tissue or in pre-existing wounds, some of which may result from other infections. Larvae can burrow through necrotic or healthy tissue using their mandibular hooks aided by proteolytic enzymes. They can cause mechanical damage and the affected area may be the site of a secondary infection. Control and treatment for pediculosis (infection by Pediculus humanus), phithriasis (infection by Phithrus pubis), acariasis (infection by mites), myiasis (infection by dipterous larvae), and tick infection have been reviewed by FAO (1982c), and Ghaffar and Hunt (2013). Arthropods outlined in Table 3 are vectors of parasites. The parasites have been categorized into protozoans, filarial nematodes, trematodes and cestodes. Of the 85 arthropod-borne parasites reported, more than three quarters (66, 78%) are protozoans, 12 filarial nematodes (14%), 2 trematodes (2%), and 5 cestodes (6%). Again of the 85 parasites reported, 70 (82%) of them have been reported to be pathogenic, causing disease; 55 (65%) protozoans, 8 (9%) filarial nematodes, 2 (2%) trematodes, and 5 (6%) cestodes; leaving only 18% as a group of non-pathogenic and those whose pathogenicity has not been established. The later group is represented by 11 protozoans (13%), 4 filarial nematodes (5%), no trematodes (0%), and no cestodes (0%). All the trematodes and cestodes reported are pathogenic. 22 protozoans (26%) are zoonotic; causing diseases to both man and animals, including 5 filarial nematodes (6%), 1 trematode (1%), and 5 cestodes (6%). All cestodes have been observed to be zoonotic. From the above analysis, it is apparent that protozoans are responsible for most of the arthropod-borne diseases in man and livestock, as earlier observed by Adam, et al (1979). One protozoan parasite, Dirofilaria immitis, a flariid parasite is best known to occur in dogs. Its distribution is now worldwide, although it was long considered to be limited to tropics and subtropics. Some 60 species of mosquitoes are recorded as vectors of this species, and since many of these attack man, it is not surprising to find that man is occasionally infected, with some 100 cases so far reported in literature (Smyth, 1996). Diseases caused by arthropod-borne protozoan parasites include Chagas’ disease, a zoonosis, caused by a protozoan, Trypanosoma cruzi, a form of trypanosomiasis and transmitted by triatomid bugs to humans mainly in the Americas; African trypanosomiasis, a zoonotic disease (sleeping sickness in man and nagana in cattle), caused by other Trypanosoma spp. and vectored by tsetse fly (Glossina spp.). Many Trypanosoma spp. are still non-pathogenic and vectored by other organisms like triatomid bugs, tabanid flies, fleas and ked. Histomoniasis in birds is another protozoan infection caused by Histomonas meleagridis and vectored by nematode worm, Heterakis gallinarum; leishmaniasis, a zoonosis, caused by Leishmania spp. mainly in man and transmitted by Sandflies, Phlebotomus spp. and Lutzomyia sp.; malaria, a zoonotic disease, affecting mainly humans, caused by Plasmodium spp. and transmitted by female Anopheles mosquitoes. However, the avian form of malaria is transmitted by mosquitoes of culicinae subfamily. Avian malaria can also be caused by other Plasmodium -related protozoans, Haemoproteus columbae, Leucocytozoon simondi, and Leucocytozoon (Akiba) caulleryi transmitted by hippoboscid flies, black flies, Simulium spp. and midges, Culicoides spp. respectively. The malaria caused by Leucocytozoon simondi in turkey is sometimes called Leucocytozoonosis. Babesiosis, a zoonotic disease, affecting mainly domestic animals and rarely man, is also a protozoan disease caused by various species of Babesia and transmitted by various tick species (Acarina). The vectors of some Babesia spp. found in pigs and cats are still unknown. Theileria spp. cause diseases collectively called theilerioses, including East Coast Fever (ECF), Mediterranean Coast Fever (MCF), Malignant Theileriosis (Ovine Theileriosis), Tropical Theileriosis, and Corridor Diseases, in domestic animals, all also transmitted by various tick species. Protozoans belonging to genus Anaplasma cause disease called anaplasmosis in domestic animals; the parasites are transmitted by various tick species. Some Anaplasma -related protozoans of genus Eperythrozoon and Haemobartonella are relatively non-pathogenic except some low degree of pathogenicity exhibited by Eperythrozoon suis in pigs. Filarial nematodes, ranking second only to protozoans, are among the major arthropod-borne parasites responsible for a number of diseases in domestic animals and man; including onchocerciasis (river blindness) caused by Onchocerca volvulus in man and transmitted by black flies of Simulium species. Other arthropod vector borne filarial diseases are Bancroftian filariasis (elephantiasis) in man caused by Wuchereria bancrofti and vectored by mosquitoes; dirofilariasis in dogs caused by filarial nematode, Dirofilaria immitis and transmitted also by mosquitoes; Loaiasis (eye worm disease) in man caused by Loa loa and transmitted by tabanid flies, Chrysops silacea and Chrysops diminiata; and dracunculosis in man caused by a filarial nematode or guinea worm, Dracunculus medinensis and vectored by copepod, Cyclops sp. Cestodes and trematodes are also important parasites of man and domestic animals. Cestode (tapeworm), Diphyllobothrium latum vectored by fresh water copepods, Cyclops sp. is known to cause diphyllobothriasis in man, pigs and dogs. Cestode (tape worm), Diphylidium caninum are parasites in dogs, cats, foxes, and man, causing human dipylidiasis mainly in children. The disease is vectored by fleas; dog flea, Ctenocephalides canis; cat flea, C. felis; human flea, Pulex irritans; and dog louse, Trichodectes canis. Cestodes (tape worms), Diphyllobothrium and Spirometra species both cause sparganosis (sparganum infection) in man, cats, dogs, vectored by fresh water copepods, Cyclops sp.; cestode (tape worm), Hymenolepsis diminuta causes rat tape worm infection in rats, other rodents, other mammals, including man, vectored by fleas, Xenopsylla cheopis and other various rodent fleas, Ceratophyllus fasciatus, Ctenocephalides canis, etc, and beetles, Tenebrio molitor (larva), Tribolium confusum (larva). Cestode (tape worm or dwarf tape worm), Hymenolepsis nana, vectored by flour beetle, is known to cause hymenolepiasis disease in rodents and man. Paragonimus westermanni, a trematode, causes paragonimiasis in man, cats and dogs. It is vectored by crustacean crabs and crayfish. Another group of organisms of great importance is the genus Schistosoma. Species of Schistosoma belong to phylum Platyhelminths, class Trematoda, class Digenea, and family Schistosomatidae. The biology of helminthes in general and schistosomes in particular is reviewed by Smyth (1996). Five species of Schistosoma are known to be pathogenic parasites of man. Of these, the chief ones are Schistosoma mansoni, S. haematobium, and S. japonicum, with S. mekongi and S. intercalatum having limited distribution. Other species, S. matheei and S. bovis, are occasionally parasites of man and S. incognitum may also prove to be infective to humans. The disease caused by schistosomes called schistosomiasis (bilharzias) is the most important disease of helminth origin and causes untold misery in some 75 countries. Some 200 million people are probably infected and 500-600 million more are exposed to infection (Smyth, 1996). However, schistosomiasis is not transmitted by an arthropod vector, but rather by a molluscan host, snails, of genus Biomphalaria. Human infection with schistosomiasis is brought about by bathing or wading in infected waters. Control of schistosomiasis depends basically on the control of the snail vectors, a problem which presents an extremely complex ecological situation. A summary of the above diseases appear in Table 3. Pathogens other than parasites that cause diseases in animals and man and are also vectored by arthropods have been listed separately in Table IV. They have been categorized into bacterial/rickettsial and viral pathogens. The diseases they cause include, among others, Rocky Mountain Spotted Fever, Lime disease, tularemia, human anthrax, Scrub Typhus (Tsutsugamushi disease) , Colorado Tick Fever (CTF), yellow fever, Nairobi sheep disease, St. Louis Encephalitis, and dengue fever.
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