Amplification Hosts Research Articles

Simulation studies of St. Louis encephalitis virus in south Florida.

Two simulation models were used to investigate the epidemiology of St. Louis encephalitis virus (SLEv) in south Florida, one including sentinel hosts (chickens) and amplification hosts (wild birds), while the other one included age structure in the amplification host population. The overall population size of the vector, Culex nigripalpus, was a major factor in the likelihood of epizootics for both models, but the seasonal dynamics of the vector alone did not explain variation in transmission. Interactions between seasonal factors in the mosquito and reproduction in the wild amplification avian hosts appeared to be important in the likelihood of epizootics. Biased feeding between sentinel and amplification hosts affected the time course of virus prevalence and may have implications for the interpretation of sentinel data. The time of virus introduction strongly affected the timing of outbreaks but did not affect the likelihood of epizootics. In most cases, the outbreak occurred immediately after virus introduction; however, in some cases the outbreak was delayed until the mosquito population increased. This has implications for the timing of control strategies directed against mosquito populations.

Virulence and viremia characteristics of 1992 epizootic subtype IC Venezuelan equine encephalitis viruses and closely related enzootic subtype ID strains.

Following a 19-year hiatus, Venezuelan equine encephalitis (VEE) reemerged in western Venezuela in December 1992. This outbreak is important in understanding VEE emergence because phylogenetic studies imply that sympatric, enzootic, subtype ID VEE viruses mutated to generate the epizootic/epidemic. Although the 1992-1993 strains belong to subtype IC, a serotype implicated in extensive outbreaks during the 1960s and in 1995, relatively small numbers of human and equine cases occurred in 1992-1993. We, therefore, evaluated the pathogenicity of these Venezuelan enzootic ID and epizootic IC viruses to determine 1) if they exhibit phenotypes like those described previously for more distantly related enzootic and epizootic strains, and 2) if the 1992-1993 outbreak was limited by the inability of these IC viruses to exploit equines as amplification hosts. All strains were virulent in mice and guinea pigs, but were benign for cotton rats, natural hosts of enzootic viruses. However, only the IC strains produced equine disease, with mean peak viremias of 10(5) suckling mouse 50% lethal doses per mL serum, and some titers exceeding 10(7). These viremias approximate those observed previously with VEE strains isolated during more extensive epizootics, suggesting that efficient equine amplification did not limit the scope and duration of the 1992-1993 outbreak. Enzootic ID virus infection protected all horses from challenge with epizootic strain P676, supporting the hypothesis that epizootics bypass regions of enzootic transmission due to natural immunization of equines by enzootic VEE viruses.

Mycobacterium bovis infection in wildlife in New Zealand

Bovine tuberculosis (Tb) is the most important disease of livestock in New Zealand, and it puts at risk the nation's trade in dairy, beef and venison products. Elimination of the disease from livestock is based on a herd test and slaughter programme and carcass inspection at abbatoirs. However, this programme has not been as successful as expected, because the disease also occurs in wild or feral animals acting as vectors of the disease to livestock. Brushtail possums are the major wildlife vector and self-sustaining maintenance host of Tb, and play a role analogous to that of the badger in Great Britain. In contrast, some deer species and ferrets may act as vectors of the disease, but their role in transmitting Tb to livestock is unclear. Hedgehogs, pigs, cats, sheep and goats are now considered to be amplifier hosts, and spread the disease to other species only when inspected or their carcasses scavenged. In the absence of infected possum populations, these species do not appear to be capable of maintaining the infection in their own populations and are not thought to be involved in the maintenance of Tb in livestock. Tuberculosis has also been recorded from stoats, hares, and a rabbit, but the level of infection recorded in their populations indicates these species are unlikely to spread the disease to other animals and hence are not involved in the transmission of Tb to livestock.

Frequency of Saint Louis encephalitis virus in humans from Florida, USA: 1990-1999.

Four general frequencies of human St. Louis encephalitis (SLE) virus (epidemic, focal, sporadic, and no transmission) occurred in Florida between 1990 and 1999. An epidemic with 226 clinical cases and 11 deaths was reported from 28 Florida counties between July 1990 and January 1991. During the autumn of 1993, a focal outbreak was reported from Lee (5 cases) and Collier (3) Counties in southwest Florida. During the autumn of 1997, sporadic transmission to nine humans was reported from five Florida counties (Brevard [1 case], Polk [3], Charlotte [1], Lee [2], and Palm Beach [2]). Human infection with SLE virus depends on a number of variables that drive virus transmission. These include vector, virus, and avian host abundance, and meteorological events, especially rainfall. We monitored the abundance and serological status of wild avian amplification hosts, virus isolations from Culex nigripalpus Theobald females, and SLE virus transmission to sentinel chickens during 1990, 1993, and 1997. The epidemic of 1990 was characterized by conditions that produced an unusual abundance of vector mosquitoes and avian amplification hosts early in the year. We propose that epidemics may result when a specific combination of biotic and abiotic conditions favor SLE virus minimum field infection rates that approach 1:1,000 in Cx. nigripalpus vectors.

Development of PbF 2–GaF 3–InF 3–ZnF 2–YF 3–LaF 3 glass for use as a 1.3 μm Pr 3+-doped fiber amplifier host

The PbF 2–GaF 3–InF 3–ZnF 2–YF 3–LaF 3 glass system is developed for potential use as an efficient Pr 3+-doped 1.3 μm fiber amplifier (PDFA) host. It is shown that 38PbF 2–25GaF 3–13InF 3–16ZnF 2–4YF 3–4LaF 3 (mol%) glass has a glass transition temperature of 252±1°C, a crystallization temperature of 350±1°C, a refractive index ( n D) of 1.597±0.001 and an infrared transmission window up to 8 μm. We confirm that its thermal stability against crystallization is similar to that of stable ZrF 4-based fluoride glasses. An optical fiber with a minimum transmission loss of 43 dB/km at 3.36 μm is successfully fabricated using this glass.

Fabrication of low-loss and high-Δn single-mode fluoride fiber for 1.3 μm praseodymium-doped fiber amplifiers

The crystallization properties of ZrF4-based fluoride glasses containing PbF2 are clarified and single-mode fibers for an optical amplifier operating at 1.3 μm are fabricated. The crystallization tendency increases as the PbF2 content of the glasses increases and the optimum PbF2 content ranges from 11 to 12.5 mol % in terms of glass stability and refractive indices. The 4%-Δn single-mode fiber with a transmission loss of 44 dB/km at 1.2 μm was prepared as the amplifier host fiber. ZrF4-based glass with a PbF2 content of 11 mol % was used as a core glass and HfF4-based glass was used as a cladding glass.

Evaluation and measurement of the resonant group-velocity dispersion in erbium-doped fiber lasers

Erbium doping introduces a resonant contribution to the group-velocity dispersion of the amplifier host fiber. A method for evaluating the erbium-induced group-velocity dispersion is proposed. This dispersion method requires the knowledge of the cold-cavity loss along with the gain and absorption spectrum of the active medium. An experimental measurement of group-velocity dispersion in a tunable actively mode-locked erbium-doped fiber laser confirms the validity of the method.

Viremia in three orders of birds (Anseriformes, Galliformes and Passeriformes) inoculated with Ockelbo virus.

One-hundred six birds of 14 species were inoculated with approximately 10(2.7) plaque-forming units of Ockelbo virus and bled daily for 5 days to determine viremia levels. Virus was detected in birds of all 14 species tested (four Anseriformes, one Galliformes and nine Passeriformes). The onset of viremia occurred earlier and viral titers were higher in very young anseriforms and galliforms than in older birds. Adult passeriforms had Ockelbo viremias of higher titer and longer duration than did adult anseriforms. Viremia titers in adult birds of all three orders tested were sufficient to induce high transmission rates in enzootic mosquito vectors, and viremias in passeriforms could induce high transmission rates in bridging vectors as well. Passeriforms of the genera Turdus and Fringilla could serve as amplification hosts for Ockelbo virus based on the presently demonstrated viremia of high titer and long duration in these birds, and the previously demonstrated high prevalence of Ockelbo virus neutralizing antibodies in free-ranging individuals and great population size compared to birds of other taxa. Bird species of all three orders tested, however, could function as incidental hosts of the virus.

Antibodies to Ockelbo virus in three orders of birds (Anseriformes, Galliformes and Passeriformes) in Sweden.

Sera from 324 birds collected in an Ockelbo virus disease endemic area in central Sweden were examined for the presence of specific antibodies to Ockelbo virus by a plaque reduction neutralization test. Birds examined belonged to the orders Anseriformes (n = 207), Galliformes (n = 66) and Passeriformes (n = 51). Ockelbo virus neutralizing antibodies were detected in 26 (8%) of the specimens, including species from each of the three orders tested. Specific antibodies found in caged birds and in 6- to 10-week-old birds suggested local transmission. The highest antibody prevalence (27%, 14/51) was observed in the Passeriformes in which 5 of 9 species tested contained antibodies. The high antibody prevalence in passeriforms and the very large population of this group in relation to other avian groups in Sweden gives them a high potential as amplification hosts for Ockelbo virus.

California serogroup virus infections in Wisconsin domestic animals.

A serologic survey and experimental virus transmission studies were done to assess the role of domestic animals as amplifier hosts of La Crosse (LACV) and Jamestown Canyon (JCV) viruses. Serum from 319 cows, 88 dogs, 122 equines, 47 swine, 10 goats, and 4 cats were tested for neutralizing antibody to LACV, JCV, trivittatus (TVTV), and snowshoe hare (SSHV) viruses. Antibody prevalences of LACV, TVTV, and SSHV were less than 10% in all species. Antibody to JCV was detected in all species except cats. Prevalence ranged from 10% in goats and swine to 29% in dogs. No age-associated trends in JCV prevalence were noted. Two of 6 adult dogs, and 2 of 4 pigs inoculated with 6.3-6.5 log10 suckling mouse intracerebral 50% lethal doses (SMICLD50) of LACV developed viremias ranging of less than 1.0-2.9 log10 SMICLD50/ml 1-3 days after inoculation. Of 4 puppies inoculated with LACV, 3 developed fatal infections. Viremias were not detected in 4 cows, 4 ponies, 7 cats, or 6 sheep. Two cats fed LACV infected suckling mice shed virus from the oropharynx for 1 day each. All animals except 1 cow, 1 cat, and 1 sheep had greater than or equal to 4-fold rise in antibody titers. Five additional dogs fed upon by LACV-infected Aedes triseriatus mosquitoes did not develop viremias or antibody and uninfected Ae. triseriatus engorging on the dogs 1-5 days after feeding by infected mosquitoes failed to become infected. Five ponies, 6 calves, 2 ewes, 6 dogs, and 5 piglets were inoculated with 3.6-7.3 log10 SMICLD50 of JCV. None developed detectable viremias, although greater than or equal to 4-fold rises in antibody titers developed in 60% of the ponies, 17% of the calves, 50% of the dogs, and 1 of 2 ewes. None of the pigs developed corresponding rises in antibody titers. We conclude that juvenile and adult animals of the species tested are not efficient amplifier hosts of LACV or JCV, but may be useful sentinels of local virus transmission.

Prevalence of Giardia sp. in a beaver colony and the resulting environmental contamination.

The prevalence of Giardia sp. in a beaver (Castor canadensis) colony in Colorado was determined by the collection and analysis of fecal samples over a period of 14 mo. Environmental contamination was monitored through the use and analysis of water filter samples. Beaver shed cysts of Giardia sp. in their feces throughout the year with temporal variations in the prevalence, and became infected as kits and remained infected as juveniles and adults. Beaver served as amplification hosts for Giardia sp. and contaminated surface waters downstream from their dams in late spring and early fall. In slow moving waters the cysts of Giardia sp. settled rapidly. Muskrats (Ondatra zibethicus) were the only other species of wildlife shedding cysts of Giardia sp. on the study area.

Leptospirosis in red foxes in Ontario.

The role of the red fox (Vulpes vulpes) in the epizootiology of leptospirosis in southwestern Ontario was investigated in 1973-1974. Leptospira interrogans serovar pomona (kennewicki by DNA analysis) was isolated from the kidneys of three of eight foxes tested. Severe hemorrhagic nephritis and interstitial nephritis were common to these foxes and to five others out of nine foxes examined. Autumnalis antibodies were detected at titers 10(-2) to 10(-5) in 12 of 100 fox sera. Pomona antibodies occurred in 6% of the sera, always accompanied by autumnalis antibodies, and at titers never exceeding the autumnalis titers. Cultural, serological, and pathological findings together indicated that the red fox could have been acting as an amplifier host, but not as a maintenance host, for pomona.

Arbovirus infections in Sarawak: the role of the domestic pig

The possible role of pigs as arbovirus maintenance hosts and their importance as amplifier hosts was studied. Blood samples from 464 pigs of all ages collected in 1962 and 1964 were tested against 10 arboviruses. Antibodies to Japanese encephalitis and Getah viruses were particularly prevalent and their calculated monthly infection rates were 19·5% and 13·3% respectively. In 1969, 447 pigs were bled monthly throughout the year and the infection rates for Japanese encephalitis virus were calculated in pigs during the first year of life. Infection rates were not uniform throughout the year; the rate increased as the pig grew older and there was a marked seasonal increase in the infection rate in the period from November to January. This coincided with the seasonal major population peak of Culex tritaeniorhynchus following intense breeding of this mosquito prior to rice planting. It is suggested that, in Sarawak, the pig acts as a maintenance host of Japanese encephalitis in a cycle involving C. gelidus mosquitoes and also acts as an important amplifier host towards the end of the year in a cycle involving C. tritaeniorhynchus. It is further suggested that Getah virus is maintained in a similar cycle between C. tritaeniorhynchus and pigs.