Disease Epizootics Research Articles

Eradication of European rabbits ( Oryctolagus cuniculus) from Cabbage Tree Island, NSW, Australia, to protect the breeding habitat of Gould's petrel ( Pterodroma leucoptera leucoptera)

The endangered Gould's petrel ( Pterodroma leucoptera leucoptera) nests, almost exclusively, in rainforest on Cabbage Tree Island, New South Wales, Australia. Since their introduction in 1906, European rabbits ( Oryctolagus cuniculus) have greatly changed the structure of the rainforest, threatening its existence and that of the petrel. Rabbits were eradicated by sequential epizootics of myxomatosis and rabbit haemorrhagic disease (RHD), and a single application of cereal-based bait (Talon® 20P) containing the anticoagulant brodifacoum. The effectiveness of these mortality agents was evaluated by monitoring 70 telemetered rabbits, half of which were vaccinated against RHD. A natural epizootic of myxomatosis from May to July 1997 reduced the rabbit population from about 250 to 100. RHD, released in August 1997, reduced the population by a further 55%, the rabbits dying within 4–23 days. No survivors tested positive to RHD antibodies. Aerial baiting in September 1997 eradicated all remaining rabbits within 13 days. Intensive monitoring over the next 6 months detected no survivors. Non-target deaths included two buff-banded rails ( Gallirallus philippensis) and one pied currawong ( Strepera graculina).

Central nervous disease and blindness in Nankeen kestrels (Falco cenchroides) due to a novel Leucocytozoon-like infection

The histopathological and electronmicroscopic features of a novel parasitic infection causing seasonal epizootics of central nervous disease in Nankeen kestrels (Falco cenchroides) from Western Australia is described. Thorough necropsy, haematological and transmission electronmicroscopical examinations were performed on thirteen affected kestrels between 1996 and 1999. All had an extensive vasculocentric granulomatous meningoencephalomyelitis, pectenitis and proliferative arteritis with endothelial parasitic cysts measuring 40 to 60 μ m in diameter. Arterioles in the brain, optic papillae, pecten and kidney were most consistently affected. In some birds there was severe haemorrhage from the pecten. Less commonly, parasitic cysts were seen in the arterioles of the lung, liver, heart and intestines. Endothelial schizonts contained many spherical merozoites measuring 1 μ m in diameter that were also present free within the lumen of affected arterioles. Haematological examination in three of seven affected kestrels tested demonstrated rare, basophilic intracytoplasmic inclusions in leucocytes with distorted nuclei consistent with Leucocytozoon-like gametocytes. Day-old chickens inoculated with brain homogenate failed to develop similar lesions.

Bacterial Interactions in Early Life Stages of Marine Cold Water Fish.

> Abstract The intensive rearing of various fish species in aquaculture has revealed intimate relationships between fish and bacteria that eventually may affect establishment of a "normal" mucosal microflora or result in disease epizootics. Interactions between bacteria and mucosal surfaces play important roles both at the egg and larval stages of marine fish. Bacterial adhesion and colonization of the egg surface occur within hours after fertilization. The diverse flora which eventually develops on the egg appears to reflect the bacterial composition and load of the ambient water, but species-specific adhesion at the egg surface may also play a role in development of the egg epiflora. Proteolytic enzymes produced by members of the adherent epiflora may cause serious damage to the developing egg and may also affect further adhesion of the epiflora. Ingestion of bacteria at the yolk sac stage results in establishment of a primary intestinal microflora which seems to persist beyond first feeding. Establishment of a gut microflora is likely to undergo several stages, resulting in an "adult" microflora weeks to months after first feeding. Ingested bacteria may serve as an exogenous supply of nutrients or essential factors at an early life stage. Early exposure to high bacterial densities is probably important for immune tolerance, and thus for the establishment of a protective intestinal microflora. Successful rearing of early life stages of several marine fish species depends on knowledge of the complex interactions among the cultured organisms and the bacterial communities which develop at the mucosal surfaces and in the ambient water and rearing systems. The routine use of antibiotics during rearing of fish larvae is not advisable, since it may increase the risk of promoting antibiotic resistance and adversely affect the indigenous microflora of the larvae. The use of probiotics has proven advantageous in domestic animal production, and the search for effective probiotics may have a great potential in aquaculture of marine organisms. Bacteria with antagonistic effects against fish pathogens have been successfully administered to several fish species, resulting in decreased mortality or increased growth rate.http://link.springer-ny.com/link/service/journals/00248/bibs/38n1p1.html

The Penaeid Shrimp Viruses TSV, IHHNV, WSSV, and YHV

ABSTRACT Viral diseases have emerged during this decade as serious economic impediments to successful shrimp farming. While nearly 20 distinct viruses or groups of viruses are known to infect penaeid shrimp, only four, WSSV, YHV, IHHNV, and TSV, pose a threat to the future of penaeid shrimp culture in the Western Hemisphere. TSV and IHHNV have caused serious disease epizootics throughout the Americas and Hawaii. IHHNV was described nearly 20 years ago when it was found to be responsible for cumulative losses of cultured Penaeus stylirostris that often exceed 90%. The threat of high mortalities posed by IHHNV has historically curtailed interest in the culture of this species in favor of the more IHHNV-resistant P. vannamei. While relatively resistant, IHHNV nonetheless infects P. vannamei and causes runt deformity syndrome (RDS) in which affected shrimp display reduced growth, highly variable sizes, lower production, and sometimes reduced survival. Taura Syndrome in P. vannamei is the virtual “mirror image” of IHHN disease in P. stylirostris. Following its recognition in 1992 as a distinct disease of P. vannamei in Ecuador, Taura Syndrome and its viral agent TSV spread rapidly throughout many of the shrimp farming regions of the Americas. Cumulative mortalities due to TSV in affected juvenile P. vannamei populations have ranged from 40 to 95%. Because P. stylirostris was found to be innately TSV-resistant, genetically selected IHHNV-resistant lines of this species disease are being developed and marketed in the Americas. Some shrimp farmers are using domesticated, selected stocks of P. vannamei that show improved resistance to TSV, while others have continued to use wild stocks that are also showing increased resistance, perhaps through intense natural selection. The viruses of the Yellow Head Disease (YHV) and the White Spot Syndrome (WSSV) were first recognized in 1991-92 in Asia, and by 1996 the two diseases had spread and had caused major pandemics throughout much of the shrimp farming regions of East Asia, Southeast Asia, Indonesia, and India. In late 1995, WSSV was found in North America for the first time in P. setiferus at a shrimp farm in south Texas. WSSV has since been detected in cultured and wild shrimp, crabs, and freshwater crayfish at multiple sites in the eastern and southeastern U.S., as well as being commonly found (along with YHV) in imported frozen commodity shrimp. Because Western Hemisphere penaeids are highly susceptible to WSSV and YHV, the introduction and establishment of either of the viruses poses a significant threat to the shrimp farming industry. The available detection methods for IHHNV, TSV, WSSV, and YHV include traditional methods that employ gross signs, clinical history, histopathology, and bioassay with susceptible shrimp hosts. Molecular and serological methods have also been developed, and specific gene probes, monoclonal antibodies, or PCR methods are readily available for each of the four viruses.

Morbilliviral infections in marine mammals

Epizootics of infectious disease were unknown in cetaceans prior to 1987. However, since then there have been at least three epizootics in dolphins and two in pinniped species. Many of the clinical, pathological and epidemiological features of these events were similar to those of morbilliviral infections in terrestrial mammals. There has been speculation that contaminants may have predisposed marine mammals to these and this is discussed. Morbilliviruses are highly pathogenic viruses and caused epizootics in terrestrial mammals long before the advent of anthropogenic contaminants

Epizootic podoknemidokoptiasis in American robins.

Epizootics of scaly leg disease caused by infection with the submacroscopic mite Knemidokoptes jamaicensis (Acari: Knemidokoptidae) in migratory American robins (Turdus migratorius) from a residential area of Tulsa (Oklahoma, USA) are documented during the winters (December through February) of 1993-94 and 1994-95. Estimates of 60 to > 80% of the birds in several different flights arriving in the area had lesions consistent with knemidokoptic mange. Epizootic occurrence of K. jamaicensis also is confirmed incidentally in American robins from Georgia (USA) in 1995 and 1998 and in Florida (USA) in 1991. These are the first confirmed epizootics of scaly leg attributed to infections with mites specifically identified as K. jamaicensis in North America. Severity of observed lesions in American robins ranged from scaly hyperkeratosis of the feet and legs to extensive proliferative lesions with loss of digits or the entire foot in some birds. Histologically, there was severe diffuse hyperkeratosis of the epidermis which contained numerous mites and multifocal aggregates of degranulating to degenerating eosinophilic heterophils; there was mild to severe superficial dermatitis with aggregates of eosinophilic heterophils and some mononuclear cells. Based on limited data from affected captive birds in Florida, we questioned the efficacy of ivermectin as an effective acaricide for knemidokoptiasis and propose that conditions associated with captivity may exacerbate transmission of this mite among caged birds. While knemidokoptic mange apparently can result in substantial host morbidity and possibly mortality, the ultimate impact of these epizootics on American robin populations presently is unknown.

Molecular characterization of the myxosporean associated with parasitic encephalitis of farmed Atlantic salmon Salmo salar in Ireland.

During seasonal epizootics of neurologic disease and mass mortality in the summers of 1992, 1993 and 1994 on a sea-farm in Ireland, Atlantic salmon Salmo salar smolts suffered from encephalitis associated with infection by a neurotropic parasite. Based on ultrastructural studies, this neurotropic parasite was identified as an intercellular presporogonic multicellular developmental stage of a histozoic myxosporean, possibly a Myxobolus species. In order to generate sequence data for phylogenetic comparisons to substantiate the present morphological identification of this myxosporean in the absence of detectable sporogony, polymerase chain reaction (PCR), Southern blot hybridization, dideoxynucleotide chain-termination DNA sequencing, and in situ hybridization (ISH) were used in concert to characterize segments of the small subunit ribosomal RNA (SSU rRNA) gene. Oligonucleotide primers were created from sequences of the SSU rRNA gene of M. cerebralis and were employed in PCR experiments using DNA extracted from formalin-fixed paraffin-embedded tissue sections of brains from Atlantic salmon smolts in which the myxosporean had been detected by light microscopy. Five segments of the SSU rRNA gene of the myxosporean, ranging in length from 187 to 287 base pairs, were amplified, detected by hybridization with sequence-specific probes, and sequenced. Consensus sequences from these segments were aligned to create a partial sequence of the SSU rRNA gene of the myxosporean. Assessments of sequence identity were made between this partial sequence and sequences of SSU rRNA genes from 7 myxosporeans, including Ceratomyxa shasta, Henneguya doori, M. arcticus, M. cerebralis, M. insidiosus, M. neurobius, and M. squamalis. The partial SSU rRNA gene sequence from the myxosporean had more sequence identity with SSU rRNA gene sequences from neurotropic and myotropic species of Myxobolus than to those from epitheliotropic species of Myxobolus or Henneguya, or the enterotropic species of Ceratomyxa, and was identical to regions of the SSU rRNA gene of M. cerebralis. Digoxigenin-labeled oligonucleotide DNA probes complementary to multiple segments of the SSU rRNA gene of M. cerebralis hybridized with DNA of the parasite in histologic sections of brain in ISH experiments, demonstrating definitively that the segments of genome amplified were from the organisms identified by histology and ultrastructural analysis. Based on sequence data derived entirely from genetic material of extrasporogonic stages, the SSU rDNA sequence identity discovered in this study supports the hypothesis that the myxosporean associated with encephalitis of farmed Atlantic salmon smolts is a neurotropic species of the genus Myxobolus, with sequences identical to those of M. cerebralis.

Protostrongylid nematodes in caribou (Rangifer tarandus caribou) and moose (Alces alces) of Newfoundland

Two species of protostrongylid nematodes with dorsal-spined, first-stage larvae, are present in caribou and moose of Newfoundland. Elaphostrongyius rangiferi Mitskevich, 1958, a parasite introduced from Scandinavia, causes periodic epizootics of a severe neurological disease in caribou. Sick animals exhibiting signs of cerebrospinal elaphostrongylosis (CSE) were particularly noticeable in central Newfoundland each winter between 1981 and 1985. Those collected for examinarion were mostly male calves. The disease again became prominent in caribou on the Avalon Peninsula in the winters of 1996 and 1997; it may have spread to that isolated part of the province as recently as 1990. E. rangiferi was also found in moose but no cases of neurologic disease have been reported in this host. Parelapbostrongylus andersoni Prestwood, 1972, was found in caribou, both in central Newfoundland and on the Avalon Peninsula. Moose may also be infected. Of 1407 terrestrial gastropod intermediate hosts examined, 9 (0.6%) contained infective, third-stage, protostrongylid larvae resembling those of E. rangiferi and P. andersoni which are indistinguishable. The small dark slug, Deroceras laeve, dominated gastropod collections and was the only species infected.

Upper respiratory tract disease and mycoplasmosis in desert tortoises from Nevada.

A population of desert tortoises (Gopherus agassizii) at Yucca Mountain (Nevada, USA) was monitored during four sampling periods using enzyme-linked immunosorbent assays (ELISA) to determine the percentage of individuals that had been exposed to Mycoplasma agassizii, a causative agent of upper respiratory tract disease. Respiratory tract disease has been considered a significant factor in the decline of desert tortoise populations in the Mojave Desert (USA). Few differences between sexes in ELISA values or percentages testing positive were noted. From 15 to 23% of samples per period tested positive for exposure to the mycoplasma. However, we noted few clinical signs of upper respiratory tract disease. This is in contrast to an earlier study which reported a similar proportion of seropositive tortoises as well as a high percentage of tortoises with clinical signs. However, our results are consistent with that study's conclusion that seropositivity for M. agassizii was a poor predictor of the likelihood to exhibit clinical signs of upper respiratory tract disease. Earlier reported epizootics of mycoplasma-associated respiratory disease occurred mainly during times of drought. Our samples were collected during a period of average to above-average rainfall, suggesting that manifestation of clinical signs of the disease may depend upon the physiological condition of tortoises which, in turn, is related to environmental conditions.

Perspectives d’utilisation des biopesticides dans la lutte contre les pucerons

Diseases form an important component of the natural enemy complex of aphids. The most common and obvious of these diseases are entomophthoran fungi such asErynia neoaphidis Remaudiere & Herbert,Entomophthora planchoniana Cornu,Zoophthora radicans (Brefeld) Batko andConidiobolus obscurus (Hall & Dunn) Remaudiere & Keller. The pest status of some aphids such as the pea aphid,Acyrthosiphon pisum (Harris), is considerably reduced by natural epizootics of fungal disease. However, disease may contribute little to practical control as it is mainly effective in high density populations when weather conditions are suitable. Introduction of exotic diseases for classical biological control is only rarely possible since most diseases, like their aphid hosts, are distributed world-wide. One exception was the successful introduction into Australia of a strain ofZ. radicans for control of spotted alfalfa aphid,Therioaphis trifolii (Monell) f.maculata in 1979. Attempts to manipulate entomophthoran fungi have had limited success because of problems with mass production, the fragility of the conidia and the need for suitably moist conditions. Hyphomycete fungi such asVerticillium lecanii (Zimm.) Viegas,Metarhizium anisopliae (Metsch.) Sorokin,Beauveria bassiana (Bals.) Vuill. andPaecilomyces spp. are more suitable for development as mycoinsecticides as they are cheap to mass produce and form stable conidia. “Vertalec™”, a formulation ofV. lecanii, has been sold in small quantities commercially in Britain and parts of Europe for many years and used mainly in glasshouses. Recently promising results have been obtained with use ofM. anisopliae for control of lettuce root aphid,Pemphigus bursarius (L.) in the UK. Laboratory studies on selected isolates ofB. bassiana andPaecilomyces spp. show a promising level of activity. Problems may occur with these species as they can kill aphid predators such as coccinelids. In addition, more research is needed on developing improved formulations which enable control to be achieved under low humidity conditions.

Risk of spread of penaeid shrimp viruses in the Americas by the international movement of live and frozen shrimp.

Within the past decade, viral diseases have emerged as serious economic impediments to successful shrimp farming in many of the shrimp-farming countries of the world. In the western hemisphere, the viral agents of Taura syndrome (TS) and infectious hypodermal and haematopoietic necrosis have caused serious disease epizootics throughout the shrimp-growing regions of the Americas and Hawaii, while in Asia the viral agents of white spot syndrome (WSS) and yellow head (YH) have caused pandemics with catastrophic losses. The international transfer of live shrimp for aquaculture purposes is an obvious mechanism by which the viruses have spread within and between regions in which they have occurred. Shrimp-eating gulls, other seabirds and aquatic insects may also be factors in the spread of shrimp viruses between and within regions. Another potentially important mechanism for the international spread of these pathogens is the trade in frozen commodity shrimp, which may contain viruses exotic to the importing countries. The viral agents of WSS, YH and TS have been found, and demonstrated to be infectious, in frozen shrimp imported into the United States market. Mechanisms identified for the potential transfer of virus in imported frozen products to domestic populations of cultured or wild penaeid shrimp stocks include: the release of untreated liquid or solid wastes from shrimp importing and processing plants directly into coastal waters, improper disposal of solid waste from shrimp importing and processing plants in landfills so that the waste is accessible to gulls and other seabirds, and the use of imported shrimp as bait by sports fishermen.

THE ENTOMOPHAGA GRYLLI (FRESENIUS) BATKO SPECIES COMPLEX: ITS BIOLOGY, ECOLOGY, AND USE FOR BIOLOGICAL CONTROL OF PEST GRASSHOPPERS

AbstractThe biology, ecology, disease etiology, and biological control potential of different members of the Entomophaga grylli species complex are discussed. This complex is represented by several pathotypes that include members that produce both conidia and resting spores within a single season, and members that produce only resting spores. This complex is known as a major pathogen of acridids from most areas of the world where populations of these insects are found, including Africa, Asia, Australia, Europe, North America, and South America. Pathogens from this species complex commonly cause disease epizootics in their host populations and are known to reduce significantly outbreaks of grasshoppers, particularly following periods of rain or high humidity. Specific factors that either limit or enhance disease processes and host mortality are discussed in relation to both epizootiology and biological control programmes. Recent biological control efforts are discussed and the potential of using members of the E. grylli species complex in both augmentation and introduction programmes is considered.

THE POTENTIAL OF BACTERIA FOR THE MICROBIAL CONTROL OF GRASSHOPPERS AND LOCUSTS

AbstractBacteria have been implicated in disease epizootics observed in field populations and laboratory-reared locusts and grasshoppers. Two species [Serratia marcescens Bizio and Pseudomonas aeruginosa (Schroeter) Migula] consistently infect locusts when ingested with food and can spread in laboratory populations. However, research on developing these organisms for microbial control of locusts and grasshoppers begun in the 1950s has not been continued. In recent years strains of Bacillus thuringiensis Berliner have been studied for activity against locusts and grasshoppers. Results of additional trials by the authors are reported. Among 393 B. thuringiensis isolates and 93 preparations of other sporeforming bacteria fed to nymphs of Locusta migratoria (L.) and/or Schistocerca gregaria Forsk., none has shown any pathogenicity to the insects. The recent discovery of novel B. thuringiensis strains active against various diverse pests and the many properties of a sporeforming bacterium that satisfy the requirements for a microbial control agent, and the development of Serratia entomophila as a promising agent for control of grass grubs, provide incentive to continue the search for an orthopteran-active sporeforming bacterium and to re-investigate the potential of non-sporeforming bacterial pathogens as microbial control agents of grasshoppers and locusts.

Viral Isolates Derived from Simian Varicella Epizootics Are Genetically Related but Are Distinct from Other Primate Herpesviruses

Epizootics of a natural varicella-like disease occur in populations of nonhuman primates. Several primate herpesviruses have been isolated from these epizootics, but the relatedness of these isolates to each other is not well-defined. In this study, we demonstrated that the restriction endonuclease (REn) profiles of four epidemiologically distinct isolates were similar, although not identical, indicating that simian varicella epizootics are caused by various strains of simian varicella virus (SVV). The genetic variation among the isolates did not map to a specific region of the SVV genome and REn differences were detected within the SVV DNA long component and the inverted repeat region. Southern blot hybridization demonstrated that SVV is more closely related to varicella-zoster virus than to other primate herpesviruses. The study indicates that the current herpesvirus classification scheme should be changed to include SVV as a single taxonomic group within theVaricellovirusgenus of alphaherpesviruses. In addition, REn profiles of SVV isolates, derived from primary and secondary episodes of simian varicella in the same monkey, were identical, providing evidence for SVV reactivation in a latently infected monkey.

Culicoides (Diptera: Ceratopogonidae) Collected During Epizootics of Hemorrhagic Disease among Captive White-tailed Deer

To help determine specific vectors of epizootic hemorrhagic disease (EHD) and bluetongue (BT) viruses for white-tailed deer, Odocoileus virginianus Zimmermann, in the southeastern United States, Culicoides sp. midges were collected during epizootics of hemorrhagic disease among captive white-tailed deer in Georgia, Mississippi, and North Carolina. Culicoides variipennis (Coquillett), a confirmed vector of EHD and BT viruses, was present in low numbers in light-trap collections made at all sites. Collections from deer made in Georgia and North Carolina yielded only a single specimen of C. variipennis. Other Culicoides species present in far greater numbers during the epizootics included C. lahillei Lutz, C. paraensis (Goeldi), and C. stellifer (Coquillett) C. lahillei warrants particular attention as a potential vector because its readily feeds on white-tailed deer and was by far the predominant species collected from deer during the epizootics.

Epizootiological Study of Bacterial Cold-Water Disease in Pacific Salmon and Further Characterization of the Etiologic Agent,Flexibacter psychrophila

Isolates of Flexibacter psychrophila were obtained from chinook salmon Oncorhynchus tshawytscha and coho salmon O. kisutch that had previously sustained epizootics of coldwater disease. The pathogen was readily isolated from kidney and mucus of convalescent fish. The organisms were relatively inert in most standard microbiological media but were structurally and serologically homogenous by examination of whole cell protein lysates by sodium dodecyl sulfate polyacrylamide gel electrophoresis. In contrast to the homogeneity observed in phenotypic and serologic assays, the isolates studied elaborated varied ribotypes. All isolates produced a single rDNA spacer amplification product of about 240 base pairs.

Seasonal Occurrence and Impact of Natural Enemies of Hypera postica (Coleoptera: Curculionidae) Larvae in Iowa

The entomopathogenic fungus Zoophthora phytonomi (Arthur), (Zygomycetes: Entomophthorales), infected populations of Hypera postica (Gyllenhal), (Coleoptera: Curculionidae), larvae at four sites in central and south-central Iowa in 1990, 1991, and 1992. Six epizootics (>50% host infection) were observed during this 3-yr study; five were associated with above average rainfall. First occurrence of disease after 1 January 1991, occurred following the accumulation of 168–207 degree days above 9°C. From 30 March to 28 May (1991), rainfall averaged between 3.6–7.0 mm per day, promoting epizootics of disease that reduced total larval populations from 15–61% at four locations in Iowa. In 1992, enzootics <50% infection) of Z. phytonomi occurred during late May and early June on declining populations of H. postica larvae at three sites. An epizootic occurred in central Iowa despite below-average rainfall. Parasitization of H. postica larvae by Bathyplectes curculionis Thomson (Hymenoptera: Ichneumonidae) and B. anurus Thomson ranged from 2.2 to 27.6% in 1991 and 1992. B. curculionis parasitization rates at two southern Iowa locations declined during epizootics of Z. phytonomi each year, suggesting a negative effect of Z. phytonomi on this larval parasitoid. B. anurus became the most abundant larval parasitoid at the two locations in southern Iowa after epizootics of Z. phytonomi.

Relationship between pollution and susceptibility to infectious disease in the eastern oyster, Crassostrea virginica

Infectious disease epizootics seriously affect many populations of marine organisms. Pollution has been hypothesized to contribute to some aquatic epizootics, although this link has not been adequately examined. To further evaluate this hypothesis, the effect of a complex mixture of sediment derived pollutants on the susceptibility of the eastern oyster, Crassostrea virginica, to an infectious disease was investigated. The disease studied is caused by the protozoan parasite Perkinsus marinus (Dermo) and results in significant oyster mortalities in the mid-Atlantic region of the United States. Oysters were exposed to 0.15 and 30% dilutions of water soluble fractions, generated from sediments collected from the Elizabeth River, a heavily polluted subestuary of the Chesapeake Bay, USA. Oysters were then challenged with P. marinus meronts. Pollutant exposure enhanced preexisting P. marinus infections and increased the oysters' susceptibility to experimentally induced infection, in a dose-dependent manner.

Evaluating the economic damage threshold for bont tick (Amblyomma hebraeum) control in Zimbabwe.

Controlling ticks and tick-borne diseases by frequent applications of acaricides (e.g., dipping) is costly, and can leave treated livestock vulnerable to epizootics of tick-borne diseases should the system of applying acaricides break down. The concept of only applying acaricides on an infrequent (strategic) basis often relies on the target tick population displaying a seasonal cycle. However, as adult bont tick (Amblyomma hebraeum) infestations in Zimbabwe's lowveld do not have a strictly seasonal pattern of occurrence, it is recommended that tick control only be applied when bont tick infestations are equal to, or greater than, their economic damage threshold. The economic damage threshold is the minimum average weekly standard female tick burden sufficient to cause damage equal in dollar value to the costs of applying tick control. Assuming that each standard female tick represents a 10 gram weight loss, the economic damage threshold (standard female ticks/week) is equivalent to the ratio of the producer price of beef (liveweight equivalent): per head cost of dipping (Eqn (3)). To illustrate the application of the threshold methodology, it was assumed that the producer price of beef was Z$1.63/kg (U.S.$0.33/kg) and that tick control cost Z$0.29/hd/dip (U.S.$0.06/hd/dip). This gave a threshold of 18 standard female ticks/head/week. Using tick counts obtained from 20 Brahman cattle held at Mbizi in southeastern Zimbabwe, it was shown that for the 1988 calendar year there were only 32 weeks when the economic damage threshold was met or exceeded. This is substantially less that the 44 dippings per year that have been, until very recently, legally required in Zimbabwe. Sensitivity analysis showed that a 10% rise in the cost of dipping reduced to 23 (a 28% decrease) the number of weeks when tick burdens exceeded the economic damage threshold. By further assuming that an acaricide application and residual effects will cause a 3-5 week interval before the next application may be required, the number of weeks when the tick burden was equal to or greater than the threshold of 18 standard females/week fell to just 9-12 weeks. Three factors may cause an alteration in the economic damage threshold: i) tick burdens may cause damage to the udders; ii) secondary infestations (e.g., screw-worm) may cause economic damage; and iii) nutritional stress of the cattle may reduce the actual average per tick weight loss.(ABSTRACT TRUNCATED AT 400 WORDS)

Infectious diseases of the Pacific oyster, Crassostrea gigas

The Pacific oyster, Crassostrea gigas, is known for not having been affected by major epizootics of infectious diseases, unlike many other commercially important oysters worldwide. Nonetheless, review of the scientific literature reveals more than ten infectious diseases of this species including those with viral, bacterial, protozoan, and metazoan etiologies. These include diseases of larval, juvenile, and adult oysters. Diseases such as oyster velar virus disease, herpes-like infection, and ligament disease are known because of their importance in intensive husbandry systems of this bivalve. Nocardiosis, Marteilioides infection, haplosporidiosis, Denman Island disease, and others are primarily known from their effect on extensively cultured populations of the Pacific oyster. These diseases are reviewed in terms of their disease manifestations, etilogy, epizootiology and economic importance, prevention, and management and diagnosis.