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By: Jeanine P. Wiener-Kronish, MD, Anesthetist-in-Chief, Massachusetts General Hospital, Boston, Massachusetts
For this particular example generic zoloft 50 mg with amex depression symptoms in child, it turns out that resource competition by itself typically reduces the potential for coexistence of antigenic variants compared with the case in which no competition occurs discount zoloft online american express mood disorder herbal remedy. If Iy drives y to extinction in the absence of competition purchase zoloft 25mg depression symptoms sleep, then additional competition for resources will usually not save y buy cheap lady era 100mg on-line. Rather order line toradol,thecom petition from x further decreases y’s chances for survival super cialis 80 mg on-line. Several studies suggest that resource competition between parasites may sometimes inﬂuence the within-host dynamics of infection. In per- sistent malaria infections, competition between Plasmodium for suscep- tible erythrocytes apparently playsanimportant role (Gravenor et al. These studies did not directly discuss antigenic variation, but they suggest that resource com- petition may be important. RESOURCE SPECIALIZATION Antigenic variants may specialize on diﬀerent host resources. For example, parasite surface molecules may inﬂuence tropism for host cell PARASITE ESCAPE WITHIN HOSTS 105 type or eﬃciency in uptake of diﬀerent host nutrients. To the extent that antigenic variants do diﬀer in their use of host resources, coexistence becomes easier to maintain by reducing the direct competition between the variants. Variation in tissue tropism appearstobeassociated with antigenically variable surface molecules in Neisseria gonorrhoeae (Gray-Owen et al. In Neisseria,variablecelltropism may be impor- tant in sequentially colonizing diﬀerent tissues as invasion and spread develop, with little direct competition between the antigenic variants. HIV provides a potential case of competition and resource specializa- tion between variants. The population of early viruses used a narrow range of coreceptors, whereas the late viruses were highly polymorphic for a diverse array of host coreceptors. As the population of viruses builds and depresses the abundance of commonly infected cell types, diversiﬁcation to diﬀerent cell tropisms reduces competition. SPATIAL SEGREGATION Variable resource concentration can also favor antigenic diversity. The ﬁrst has a sur- face antigen that provides superior entry into host cells, but this variant is cleared at a higher rate. The second variant has a lower rate of entry into host cells, but is cleared at a lower rate. The ﬁrst typeinterferes with the second whenbotharecommon. This infection-clearance trade-oﬀ can promote coexistence by spatial segregation. For example, host compartments with low resource lev- els cannot sustain the ﬁrst type—limitedhostcells reduce the produc- tion rate below the high clearance rate. By contrast, in compartments with high resource levels, the stronger type dominates by outcompeting the weaker type.
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Dioxolanthymidine (DOT) is a newer thymidine analog discount zoloft 25mg with amex depression worse at night. Dioxolane appeared to be relatively good in preclinical trials (Chung 2005 purchase zoloft online from canada anxiety groups, Liang 2006) buy zoloft overnight mood disorder with depressive features. Presently order viagra jelly 100 mg, prodrugs are being tested buy viagra super active 25mg cheap, however cheap 100 mg caverta with mastercard, clinical studies have yet to be conducted (Liang 2009). EFda or 4-ethynol-2-fluor-deoxyadenosine seems to be a very effective NRTI accord- ing to the results of monkey testing. The SIV viral load decreased after 7 days by 2–3 logs (Parniak 2009, Michailidis 2014). It is also being evaluated as a potential microbicide. There is an licensing agreement with MSD for the development of this novel drug with plans for full scale clinical development. Elvucitabine (ACH-126,443) is a cytidine analog developed by Achillion Pharma- ceuticals. It is an enantiomer of dexelvucitabine and is also effective against HBV. In vitro studies show potency even in the presence of numerous resistance mutations (Fabrycki 2003). It is also of interest as it seems to have an extremely long half-life of up to 150 hours – this may allow once-weekly dosing (Colucci 2005). A small double-blind study showed a reduction in viral load of between 0. However, this study had to be prematurely terminated, as 6/56 patients developed leucopenia or rash on a dose of 100 mg (Dunkle 2003). It seems that mitochondrial toxicity is lower than that of dexelvucitabine. On the other hand, this lower toxicity may also lower the efficiency of incorporation by drug-resistant versions of HIV-1 RT (Murakami 2004). In a smaller Phase II study in 77 therapy-naïve patients (with efavirenz and tenofovir), elvucitabine was comparable to 3TC at 96 weeks (DeJesus 2010). There appear to be problems with interactions with ritonavir, which may be due to ritonavir inhibiting an efflux gut transporter with activity present at various levels in subjects (Colucci 2009). Fosalvudine is an NRTI from Heidelberg Pharma, a prodrug of the fluorothymidine alovudine. The active part is released only after enzymatic cleavage in the tissue. It is hoped that the toxicity commonly seen with fluorothymidines can thus be reduced. In a Phase II trial with 43 ART-naïve HIV+ patients, fosalvudine was well- tolerated and after 2 weeks of monotherapy with 5–40 mg, viral load decreased by up to 1 log (Cahn 2007).
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Measles virus eﬀectively has only one antigenic type—a host’s ﬁrst infection and recovery provides lifelong protection order generic zoloft pills anxiety x blood and bone. The spatiotempo- ral landscape of measles spread follows the waxing and waning of the numbers of infected individuals cheap 100 mg zoloft with mastercard anxiety natural remedies, driven by immunological memory purchase zoloft amex depression symptoms names, re- cruitment of newborns purchase fildena uk, and migration between patches best buy for tadacip. Now imagine a parasite with distinct antigenic variants buy levitra professional 20 mg free shipping, for which memory to one variant does not provide any cross-protection against the other variants. The variants behave in eﬀect as completely distinct parasites. In a patch, the waxing andwaningofonevariant may be syn- chronized with or uncoupled from the dynamics of the other variants. If the variants change asynchronously within patches, then the spatio- temporal landscape is covered by multiple surfaces of rising and falling peaks, the surfaces moving independently of each other. Ihavediscussed infection landscapes in a rather abstract way. But there is nothing out of the ordinary about hosts spread over space and infected over time by diﬀerent antigenic variants of a parasite. The diﬃ- culty is to identify what general consequences arise from the interaction between antigenic variation and spatial processes. The landscape I have described so far has strains of antigenic variants that do not interact or interfere with each other. Thus, each strain changes independently of other strains, and no interaction occurs between space and antigenic variation. Now consider antigenic variants for which some pairs of variants cause cross-reactive memory. It is not so easy to imagine the spatio- temporal landscape because the spread of each variant has diﬀering quantitative eﬀects on the dynamics ofothervariants. One simple anal- ogy with age structure hints at thesortofprocesses that may occur. In inﬂuenza, it may be that children have immunodominant memory focused on only one or a few antigenic sites. In the simpliﬁed example Idiscussed above, at ﬁrst a virus strain with two sites, A/B,spreads. Some children develop immunodominant memory against A;otherchil- dren develop immunodominant memory against B. Mutantviruses can spread through the en- tire population in two steps. First, a mutant A /B can attack children with memory against A. The key is that diﬀerent classes of hosts provide a pathway of connectivity by which single mutations of the virus can eventually spread through the entire population. This spatial distribution of immunological memory creates astepwisepathway of connectivity for a parasite. For example, if a parasite A /B ﬁrst invades patch 1, then it can by a single mutation change into A/B and attack patch 2.