Epidemics in competition

When two or more epidemic agents are simultaneously present in a population, they may interact to increase or decrease each other's effectiveness. One form of interaction is competition where each agent confers immunity to the others. Such competition occurs, for example, between different strains of myxomatosis in rabbit populations. We consider some consequences of introducing competition into mathematical epidemic models. Both deterministic and stochastic simple epidemic models are examined. In either case the conclusions are similar: the faster spreading epidemic has a considerable advantage.     

Double-antibody solid-phase enzyme immunoassay for the detection of staphylococcal enterotoxin A.

A simple double-antibody enzyme immunoassay that uses a microtechnique was developed for detecting staphylococcal enterotoxin A in food products. Sample preparation can be completed in less than 15 min. Assay sensitivity ranges from 0.4 ng (20-h test time) to 3.2 ng (1- to 3-h test time) of toxin per ml of prepared sample. Separation and detection of enterotoxin from spiked food products ranged between 72 and 98% of the amount added.     

Presence of the cyanogenic glucoside dhurrin in isolated vacuoles from sorghum

Large numbers of vacuoles (10⁶-10⁷) have been isolated from Sorghum bicolor protoplasts and analyzed for the cyanogenic glucoside dhurrin. Leaves from light-grown seedlings were incubated for 4 hours in 1.5% cellulysin and 0.5% macerase to yield mesophyll protoplasts which then were recovered by centrifugation, quantitated by a hemocytometer, and assayed for cyanogenic glucosides. Mature vacuoles, released from the protoplasts by osmotic shock, were purified on a discontinuous Ficoll gradient and monitored for intactness by their ability to maintain a slightly acid interior while suspended in an alkaline buffer as indicated by neutral red stain. Cyanide analysis of the protoplasts and the vacuoles obtained there from yielded equivalent values of 11 μmoles of cyanogenic glucoside dhurrin per 10⁷ protoplasts or 10⁷ vacuoles. This work supports an earlier study from this laboratory which demonstrated that the vacuole is the site of accumulation of the cyanogenic glucoside in Sorghum.     

Importance of markers of hepatitis B virus in alcoholic liver disease.

To determine the importance of the presence of serological markers of hepatitis B virus infection in patients with alcohol related liver disease we compared cumulative alcohol intake and clinical and histological features in patients with markers of hepatitis B virus infection and in those without. Hepatitis B surface antigen (HBsAg) was detected in five (2%) out of 285 patients studied and antibody to HBsAg (anti-HBs) in 41 (14%); one patient had antibody to hepatitis B core antigen alone. The combined prevalence of markers of hepatitis B virus infection was similar in patients with alcoholic cirrhosis (18%) and precirrhotic liver disease (13%). Two patients positive for HBsAg had histological features of both alcoholic liver disease and chronic active hepatitis, with stainable HBsAg. Patients with anti-HBs were, however, histologically indistinguishable from patients without markers, and the mean cumulative alcohol intake of patients with anti-HBs was similar to or even higher than that of patients with liver disease of comparable severity who had no evidence of previous infection. The presence of markers of hepatitis B virus infection was related to former residence in countries with a high prevalence of the infection and to previous parenteral treatment and blood transfusions. Infection with hepatitis B virus does not enhance the development of chronic liver disease in heavy drinkers, except in the small number who remain positive for HBsAg.     

Severity of alcohol dependence in patients with alcoholic liver disease.

To determine the severity of dependence on alcohol in patients with alcoholic liver disease the severity of alcohol dependence questionnaire was administered to 193 patients with various types of alcoholic liver disease established histologically, in whom a detailed history of lifetime alcohol consumption was also obtained. Only 34 patients (18%) were classified as being severely dependent compared with 56% of patients without overt liver disease who were attending a neighbouring alcohol treatment unit. There was a significant correlation between the severity of dependence and mean daily alcohol consumption (r = 0.45 and 0.39 for men and women, respectively) but not duration of drinking. Dependence scores tended to be lower in patients with cirrhosis than in those with precirrhotic liver disease, but this difference reached significance only in women. These findings confirm that patients who develop chronic alcoholic liver disease are usually only mildly dependent on alcohol and support the hypothesis that patients who escape florid symptoms of alcohol dependence are at greater risk of developing liver damage because they are able to sustain a continual consumption of alcohol over many years.     

23Na and flame photometric studies of the NMR visibility of sodium in rat muscle.

23Na nuclear magnetic resonance spectroscopy (NMR) is increasingly being used to study Na+ gradients and fluxes in biological tissues. However, the quantitative aspects of 23Na NMR applied to living systems remain controversial. This paper compares sodium concentrations determined by 23Na NMR in intact rat hindlimb (n = 8) and excised rat gastrocnemius muscle (n = 4) with those obtained by flame photometric methods. In both types of samples, 90% of the sodium measured by flame photometry was found to be NMR-visible. This is much higher than previously reported values. The NMR measurements for intact hindlimb correlated linearly with the flame photometric measurements, implying that one pool of sodium, predominantly extracellular, is 100% visible. From measurements on excised muscle, in which extracellular space is more clearly defined, the NMR visibility of intracellular Na+ was calculated to be 70%, assuming an extracellular space of 12% of the total tissue water volume and an extracellular NMR visibility of 100%. 23Na transverse relaxation measurements were carried out using a Hahn spin echo on both intact hindlimb (n = 1) and excised muscle (n = 2) samples. These showed relaxation curves that could each be described adequately using two relaxation times. The rapidly relaxing component showed a T2 value of 3-4 ms and the slowly relaxing component a T2 of 21-37 ms. A spin lattice relaxation (T1) measurement on intact hindlimb yielded a value of 51 ms. These relatively long relaxation times show that the quadrupolar relaxation effect of Na+ complexing to large macromolecules or being otherwise motionally restricted is relatively weak. This is consistent with the high NMR visibilities reported here.     

Regulation of GABAB receptors by histamine and neuronal activity in the isolated spinal cord of neonatal opossum in culture.

The aim of these experiments has been to analyse the properties of receptors for the transmitter gamma-aminobutyric acid (GABA) in developing mammalian nervous system. Changes in responses of GABAB receptors have been measured after alterations of the chemical environment and the level of electrical activity. We have previously shown that when the central nervous system (CNS) of the new-born opossum, Monodelphis domestica, is cultured for three to five days in the presence of histidine, inhibition by baclofen, a GABAB agonist, disappears (Stewart et al. 1991). We have now investigated whether histidine acts indirectly by way of conversion to histamine. As with histidine, culture with 150 microM histamine for five days virtually abolished the inhibition by baclofen. The effects of histidine, as well as histamine, were blocked by mepyramine, a histamine H1-receptor antagonist, and by ranitidine, an H2-antagonist. Tetrodotoxin (TTX), which blocks all electrical activity, protected preparations from the action of histidine but not histamine. Our results suggest that histidine is converted to histamine, which reduces the efficacy of GABAB agonists. We conclude that, in the developing mammalian CNS, transmitter levels and electrical activity can selectively influence the properties of receptors.     

Kinesin-related proteins required for structural integrity of the mitotic spindle.

For S. cerevisiae cells, the assembly of a bipolar mitotic spindle requires the action of either Cin8p or Kip1p, gene products related to the mechanochemical enzyme kinesin. In this paper we demonstrate that the activity of either one of these proteins is also required following spindle assembly. When their function was eliminated, preanaphase bipolar spindles rapidly collapsed, with previously separated poles being drawn together. In contrast, anaphase spindles were apparently resistant to collapse. Deletion of kinesin-related KAR3 partially suppressed the phenotypes associated with loss of Cin8p/Kip1p function. Our findings suggest that the structure of the preanaphase bipolar spindle is maintained by counteracting forces produced by kinesin-related proteins.