Hilary Hurd

Apoptosis in the malaria protozoan, Plasmodium berghei: a possible mechanism for limiting intensity of infection in the mosquito

Death by apoptosis regulates cell numbers in metazoan tissues and it is mediated by activation of caspases and results in characteristic morphological and biochemical changes. We report here that the malaria protozoan, Plasmodium berghei, exhibits features typical of metazoan apoptotic cells including condensation of chromatin, fragmentation of the nuclear DNA and movement of phosphatidylserine from the inner to the outer lamellae of the cell membrane. In addition, proteins with caspase-like activity were identified in the cytoplasm of the ookinete suggesting that the cellular mechanism of cell death may be similar to that of multicellular eukaryotes. Our data show that more than 50% of the mosquito midgut stages of the parasite die naturally by apoptosis before gut invasion. Cell death was prevented by a caspase inhibitor, treatment resulting in a doubling of parasite intensity. All these features also occur in vitro. Cell suicide thus plays a major and hitherto unrecognised role in controlling parasite populations and could be a novel target for malaria control strategies.

Host fecundity reduction: a strategy for damage limitation?

Host fecundity reduction is a life-history trait that is commonly exhibited in parasitic associations. It is particularly prevalent in female invertebrate hosts that invest heavily in egg production during a relatively short life span. Here, Hilary Hurd uses examples of parasitized insects and trematode infections of snails to consider the evolutionary significance of this response to infection. Studies of host egg production and reports of the physiological mechanisms underlying reduction of host reproductive success are used to evaluate the hypotheses that fecundity reduction might be a by-product of infection, or an adaptive strategy on the part of parasite or host.

Physiological and Behavioural Interactions Between Parasites and Invertebrate Hosts

Publisher Summary This chapter focuses on the host–parasite interactions that provide new insights into the biochemistry, physiology, ethology, and ecology of the respective partners. This is, particularly, so in the case of parasitic infections of invertebrates for the following reasons: (i) the short generation time and large population size of many invertebrate hosts make them convenient models for study, and the need to understand the role of invertebrate vectors in the transmission cycles of parasitic disease is of particular relevance; and (ii) invertebrates are utilized by a variety of protozoan and helminth parasites as definitive or intermediate hosts in complex life cycles, some of which involve vertebrates. Using examples of parasitic protozoans, helminths, crustaceans, and insects in association with a wide range of invertebrate hosts, strategies, such as the synchronization of host–parasite life cycles, alteration of host growth rates, the impairment of host reproductive capacities, parasite establishment in hosts of different sexes, and alterations in host behavior are examined. A parasitic association succeeds only if the host survives long enough to enable the parasite to complete the particular phase of its life cycle and if the host population is able to persist in time. The mode and degree of integration of a variety of invertebrates and their parasites are also examined.

The effects of natural Plasmodium falciparum infection on the fecundity and mortality of Anopheles gambiae s. l. in north east Tanzania.

Rodent and avian malaria parasites have been reported to have an adverse affect upon the reproductive fitness of mosquitoes. In order to determine whether fecundity reduction occurs in Anopheles gambiae s. l. infected with human malaria a study of wild-caught mosquitoes was undertaken in the Muheza district of north east Tanzania. Fully engorged, indoor resting females were collected daily for 4 months and maintained for 5 days. A sporozoite rate of 11.5% was detected for the whole collection and of those females alive on day 6 an additional 17.5% were infected with oocysts alone. Oocyst, but not sporozoite, infection resulted in a 17.5% reduction in egg production. Fecundity reduction was not caused by a reduction in bloodmeal size in infected females and no size difference was detected between oocyst-infected and uninfected females although sporozoite-positive females were significantly larger. Comparisons in parity between uninfected and infected groups indicate that infection does not affect survival beyond the first gonotrophic cycle as no changes in survivorship occurred as a result of sporozoite infection.

The effect of Plasmodium yoelii nigeriensis infection on the feeding persistence of Anopheles stephensi Liston throughout the sporogonic cycle

Vector-borne parasites such as malaria have been shown to modify the feeding behaviour of their invertebrate hosts so as to increase the probability of transmission. However, evolutionary consideration of developmental changes in malaria within Anopheles mosquitoes suggests that the nature of altered feeding by mosquitoes should differ depending on the developmental stage of the parasite. We present laboratory evidence that the feeding persistence of female Anopheles stephensi towards a human host is decreased in the presence of Plasmodium yoelii nigeriensis oocysts (which cannot be transmitted), but increased when the malaria has developed into transmissible sporozoites in the salivary glands. In ten–minute trials, 33% of uninfected mosquitoes gave up their feeding attempt before the test period had ended, 53% of those harbouring oocysts had given up, but only 20% of those infected with sporozoites gave up by this time. We conclude that changes in feeding behaviour of mosquitoes mediated by parasite infection are sensitive to the developmental stage of the parasite and that these changes have important implications for malaria epidemiology.

EVALUATING THE COSTS OF MOSQUITO RESISTANCE TO MALARIA PARASITES

Abstract Costly resistance mechanisms have been cited as an explanation for the widespread occurrence of parasitic infections, yet few studies have examined these costs in detail. A malaria-mosquito model has been used to test this concept by making a comparison of the fitness of highly susceptible lines of mosquitoes with lines that are resistant to infection. Malaria infection is known to cause a decrease in fecundity and fertility of mosquitoes; resistant mosquitoes were thus predicted to be fitter than susceptible ones. Anopheles gambiae were selected for refractoriness/resistance or for increased susceptibility to infection by Plasmodium yoelii nigeriensis. Additional lines that acted as controls for inbreeding depression were raised in parallel but not exposed to selection pressure. Selections were made in triplicate so that founder effects could be detected. Resistance mechanisms that were selected included melanotic encapsulation of parasites within 24 h postinfection and the complete disappearance of parasites from the gut. Costs of immune surveillance were assessed after an uninfected feed, and costs of immune deployment were assessed after exposure to infection and to infection and additional stresses. Mosquito survivorship was unaffected by either resistance to infection or by an increased burden of infection when compared with low levels of infection. In most cases reproductive fitness was equally affected by refractoriness or by infection. Resistant mosquitoes did not gain a fitness advantage by eliminating the parasites. Costs were consistently associated with larval production and egg hatch rate but rarely attributed to changes in blood feeding and never to changes in mosquito size. No advantages appeared to be gained by the offspring of resistant mosquitoes. Furthermore, we were unable to select for refractoriness in groups of mosquitoes in which 100% or 50% of the population were exposed to infection every generation for 22 generations. Under these selection pressures, no population had become completely refractory and only one became more resistant. Variations in fitness relative to control lines in different groups were attributed to founder effects. Our conclusion from these findings is that refractoriness to malaria is as costly as tolerance of infection.

Malaria-induced reduction of fecundity during the first gonotrophic cycle of Anopheles Stephensi mosquitoes

Abstract. Anopheles stephensi mosquitoes which had fed upon mice infected with Plasmodium yoelii nigeriensis malaria parasites produced significantly fewer eggs than mosquitoes fed on an uninfected mouse. Fecundity reduction was more pronounced when the bloodmeal contained malaria gametocytes and the mosquitoes developed oocysts. Egg production and haematin excretion were correlated for uninfected bloodfed mosquitoes; the presence of P.y. nigeriensis in the blood affected this relationship. Reduced fecundity was associated with a significant reduction of bloodmeal size (measured by haematin excretion) in mosquitoes which ingested gametocytaemic blood. The bloodmeal size in mosquitoes fed on parasitaemic blood without gametocytes was not significantly reduced. The use of haematin assays for determination of bloodmeal size in mosquitoes is discussed.

Plasmodium yoelii nigeriensis : the effect of high and low intensity of infection upon the egg production and bloodmeal size of Anopheles stephensi during three gonotrophic cycles

Anopheles stephensi mosquitoes showed a reduction in fecundity over 3 successive gonotrophic cycles, after becoming infected with Plasmodium yoelii nigeriensis. This effect could be observed at high oocyst burdens (> 75) or at low oocyst burdens (mean of 4.36). Mean bloodmeal size of the infected mosquitoes was significantly reduced only when feeding upon a mouse with a high gametocytaemia and the conversion of the bloodmeal into eggs by the infected mosquitoes was disrupted. Patterns of infected mosquito mortality, over the 3 gonotrophic cycles, varied with severity of infection. Although in 1 case increased mortality and decreased bloodmeal size may have affected fecundity, this could not have accounted for all of the observed fecundity reduction. We propose that other, unknown parasite related factors, are involved.

Apoptotic markers in protozoan parasites

The execution of the apoptotic death program in metazoans is characterized by a sequence of morphological and biochemical changes that include cell shrinkage, presentation of phosphatidylserine at the cell surface, mitochondrial alterations, chromatin condensation, nuclear fragmentation, membrane blebbing and the formation of apoptotic bodies. Methodologies for measuring apoptosis are based on these markers. Except for membrane blebbing and formation of apoptotic bodies, all other events have been observed in most protozoan parasites undergoing cell death. However, while techniques exist to detect these markers, they are often optimised for metazoan cells and therefore may not pick up subtle differences between the events occurring in unicellular organisms and multi-cellular organisms.In this review we discuss the markers most frequently used to analyze cell death in protozoan parasites, paying special attention to changes in cell morphology, mitochondrial activity, chromatin structure and plasma membrane structure/permeability. Regarding classical regulators/executors of apoptosis, we have reviewed the present knowledge of caspase-like and nuclease activities.

Exploiting host compensatory responses: the ‘must’ of manipulation?

Parasite-induced alterations of the host phenotype have been reported in many systems. These changes are traditionally categorized into three kinds of phenomena: secondary outcomes of infection with no adaptive value, host adaptations that reduce the detrimental consequences of infection and parasitic adaptations that facilitate transmission. However, this categorization is a simple view, and host modifications should be considered as co-evolved traits, rather than a total takeover. Here, we present a novel scenario of manipulation, which has considerable potential to resolve issues that are specific to the evolution of behavioural alterations induced by parasites. It is proposed that certain parasites affect fitness-related traits in their hosts to trigger host compensatory responses because these responses can meet the transmission objectives of parasites.