Indirect interaction between an endemic and an invading pathogen: A case study of Plasmodium and Usutu virus dynamics in a shared bird host population

Infectious disease agents can influence each other's dynamics in shared host populations. We consider such influence for two mosquito-borne infections where one pathogen is endemic at the time that a second pathogen invades. We regard a setting where the vector has a bias towards biting host individuals infected with the endemic pathogen and where there is a cost to co-infected hosts. As a motivating case study, we regard Plasmodium spp., that cause avian malaria, as the endemic pathogen, and Usutu virus (USUV) as the invading pathogen. Hosts with malaria attract more mosquitoes compared to susceptible hosts, a phenomenon named vector bias. The possible trade-off between the vector-bias effect and the co-infection mortality is studied using a compartmental epidemic model. We focus first on the basic reproduction number R0 for Usutu virus invading into a malaria-endemic population, and then explore the long-term dynamics of both pathogens once Usutu virus has become established. We find that the vector bias facilitates the introduction of malaria into a susceptible population, as well as the introduction of Usutu in a malaria-endemic population. In the long term, however, both a vector bias and co-infection mortality lead to a decrease in the number of individuals infected with either pathogen, suggesting that avian malaria is unlikely to be a promoter of Usutu invasion. This proposed approach is general and allows for new insights into other negative associations between endemic and invading vector-borne pathogens.

Microbiomes associated with avian malaria survival differ between susceptible Hawaiian honeycreepers and sympatric malaria-resistant introduced birds

Of the estimated 55 Hawaiian honeycreepers (subfamily Carduelinae) only 17 species remain, nine of which the International Union for Conservation of Nature considers endangered. Among the most pressing threats to honeycreeper survival is avian malaria, caused by the introduced blood parasite Plasmodium relictum, which is increasing in distribution in Hawai'i as a result of climate change. Preventing further honeycreeper decline will require innovative conservation strategies that confront malaria from multiple angles. Research on mammals has revealed strong connections between gut microbiome composition and malaria susceptibility, illuminating a potential novel approach to malaria control through the manipulation of gut microbiota. One honeycreeper species, Hawai'i 'amakihi (Chlorodrepanis virens), persists in areas of high malaria prevalence, indicating they have acquired some level of immunity. To investigate if avian host-specific microbes may be associated with malaria survival, we characterized cloacal microbiomes and malaria infection for 174 'amakihi and 172 malaria-resistant warbling white-eyes (Zosterops japonicus) from Hawai'i Island using 16S rRNA gene metabarcoding and quantitative polymerase chain reaction. Neither microbial alpha nor beta diversity covaried with infection, but 149 microbes showed positive associations with malaria survivors. Among these were Escherichia and Lactobacillus spp., which appear to mitigate malaria severity in mammalian hosts, revealing promising candidates for future probiotic research for augmenting malaria immunity in sensitive endangered species.

Purifying selection leads to low protein diversity of the mitochondrial cyt b gene in avian malaria parasites

BACKGROUND

Mitochondrial respiration plays a central role in the survival of many eukaryotes, including apicomplexan parasites. A 479-bp fragment from the mitochondrial cytochrome b gene is widely used as a barcode to identify genetic lineages of avian malaria parasites Plasmodium and related haemosporidians. Here we looked for evidence of selection in the avian Plasmodium cyt b gene, using tests of selection and protein structure modeling. We also tested for the association between cyt b polymorphism and the host specificity of these parasites.

RESULTS

Based on 1,089 lineages retrieved from the Malavi database, we found that the frequency of the most conserved amino acids in most sites was more than 90%, indicating that the protein diversity of the avian Plasmodium cyt b barcode was low. The exceptions were four amino acid sites that were highly polymorphic, though the substitutions had only slight functional impacts on the encoded proteins. The selection analyses revealed that avian Plasmodium cyt b was under strong purifying selection, and no positively selected sites were detected. Besides, lineages with a wide host range tend to share cyt b protein haplotypes.

CONCLUSIONS

Our research indicates that purifying selection is the dominant force in the evolution of the avian Plasmodium cyt b lineages and leads to its low diversity at the protein level. Host specificity may also play a role in shaping the low mitochondrial diversity in the evolution of avian malaria parasites. Our results highlight the importance of considering selection pressure on the cyt b barcode region and lay a foundation for further understanding the evolutionary pattern of mitochondrial genes in avian malaria.

Suspected Mefloquine Toxicity in a Colony of Humboldt Penguins (Spheniscus humboldti)

Avian malaria is an important cause of mortality in captive penguins housed in outdoor exhibits. Mefloquine was used as a prophylaxis to treat a colony of 19 Humboldt penguins (Spheniscus humboldti) for avian malaria. A target dose of 30 mg/kg was obtained from anecdotal literature for sphenisciforms that was not based on pharmacokinetic or toxicity studies. For this reason, preliminary plasma concentrations of mefloquine were acquired after the first dose in some penguins to ensure that plasma concentrations reached human malaria prophylactic concentrations. Afterward, each penguin in the entire colony received mefloquine (26-31 mg/kg [125 mg in toto] PO q7d). Regurgitation was frequently observed starting after the fourth weekly administration. Plasma concentrations of mefloquine after the seventh dose showed elevated concentrations, and the treatment was immediately terminated. Eight penguins died during and after the treatment period. The first fatality occurred after the fifth weekly administration, and 7 birds died within 7-52 days after the seventh weekly administration. Three penguins were found dead without previous symptoms. The other five presented with marked lethargy, dyspnea, poor appetite, and vomiting, and all died despite medical care. The remaining 11 penguins of the colony survived without any supportive care; 5 did not exhibit any clinical disease signs, while the other 6 showed a mild apathy and decreased appetite. Mefloquine toxicity was highly suspected on the basis of clinical signs, the elevated mefloquine plasma concentrations, and no other underlying pathologic disease conditions identified through postmortem examinations. Nonspecific lesions, including pulmonary congestion and edema and hepatic perivascular hematopoiesis, were noted in the birds that died. Additionally, 1 case presented with myocarditis, and mycobacteria were observed within granulomas in the respiratory tract of 2 penguins. Caution is advised, and further studies are encouraged before administering mefloquine to penguins.

Relationships between avian malaria resilience and corticosterone, testosterone and prolactin in a Hawaiian songbird

Glucocorticoids, androgens, and prolactin regulate metabolism and reproduction, but they also play critical roles in immunomodulation. Since the introduction of avian malaria to Hawaii a century ago, low elevation populations of the Hawaii Amakihi (Chlorodrepanis virens) that have experienced strong selection by avian malaria have evolved increased resilience (the ability to recover from infection), while high elevation populations that have undergone weak selection remain less resilient. We investigated how variation in malaria selection has affected corticosterone, testosterone, and prolactin hormone levels in Amakihi during the breeding season. We predicted that baseline corticosterone and testosterone (which have immunosuppressive functions) would be reduced in low elevation and malaria-infected birds, while stress-induced corticosterone and prolactin (which have immunostimulatory functions) would be greater in low elevation and malaria-infected birds. As predicted, prolactin was significantly higher in malaria-infected than uninfected females (although more robust sample sizes would help to confirm this relationship), while testosterone trended higher in malaria-infected than uninfected males and, surprisingly, neither baseline nor stress-induced CORT varied with malaria infection. Contrary to our predictions, stress-induced corticosterone was significantly lower in low than high elevation birds while testosterone in males and prolactin in females did not vary by elevation, suggesting that Amakihi hormone modulation across elevation is determined by variables other than disease selection (e.g., timing of breeding, energetic challenges). Our results shed new light on relationships between introduced disease and hormone modulation, and they raise new questions that could be explored in experimental settings.

Evaluating community-level response to management actions across a diverse Hawaiian forest bird community

Although species-specific approaches are necessary to understand the dynamics of individual species composing a community, they do not offer a framework for making optimal management decisions at the community level. Here, we present a simple framework for comparing the response of entire communities to multiple management scenarios. Our approach uses a weighted average of standardized species-specific responses to produce a single integrative measure of the community response and employs mixed-effect linear models to quantify the increase in the community response due to each management action, or combination of actions. We demonstrate our approach with a simulation study assessing the potential benefits of multiple management actions on the avian community of Hakalau Forest National Wildlife Refuge, Hawai'i, which is composed of eight native species with conservation status varying from endangered to least concern. Management actions considered included a reduction of avian malaria transmission risk, reducing rat predation, and increasing forest habitat. We used three different prioritization (weighting) schemes to assess whether the response to management actions differed along a conservation gradient (from endangered to least concern). We also investigated whether future changes in disease distribution as a result of climate change will alter the relationship between management practices and community response. Our community-level analysis produced three important insights, highlighting the need to consider the response from multiple species to changing threats and management actions. First, increasing the amount of habitat always had the greatest positive impact on the avian community, regardless of the weighting scheme. Second, the community response to management was different under current vs. future conditions, with increased benefit in the future when disease risks were higher. Third, the response to management varied along the conservation gradient. Reducing malaria transmission risk in 2100 had the greatest benefit to endangered species, while increasing forest habitat had the greatest benefit to "least concern" species. However, reducing rat predation appeared to benefit a subset of species based on ecological factors unrelated to rarity, such as nest accessibility. Our approach is widely applicable using experimental, observational, or simulation-based data, allowing managers to consider the response of all species while weighting their priority levels.

Historical Biogeography and Extinction in the Hawaiian Honeycreepers

Hawaiian honeycreepers, comprising an endemic radiation of passerine birds in the Hawaiian archipelago, have suffered losses of individual island populations and the extinction of many species as a result of colonization of the islands by Polynesians and, more recently, introduced avian pox virus and avian malaria. Here, I test the idea that populations have an intrinsic tendency toward extinction regardless of the cause. The distribution of each species before the arrival of humans in the archipelago was inferred from present distribution, historical records, and fossil remains. On the basis of these records, each species was placed in one of four stages of the taxon cycle: (1) expanding or recently expanded, (2) differentiating, (3) fragmenting, or (4) single-island endemic. Subsequent extinction of individual island populations was most frequent in stage 3 species, which had already suffered loss of individual island populations, suggesting commonality in vulnerability to extinction from anthropogenic and nonanthropogenic causes.

Fine-scale distribution modeling of avian malaria vectors in north-central Kansas

Infectious diseases increasingly play a role in the decline of wildlife populations. Vector-borne diseases, in particular, have been implicated in mass mortality events and localized population declines are threatening some species with extinction. Transmission patterns for vector-borne diseases are influenced by the spatial distribution of vectors and are therefore not uniform across the landscape. Avian malaria is a globally distributed vector-borne disease that has been shown to affect endemic bird populations of North America. We evaluated shared habitat use between avian malaria vectors, mosquitoes in the genus Culex and a native grassland bird, the Greater Prairie-Chicken (Tympanuchus cupido), by (1) modeling the distribution of Culex spp. occurrence across the Smoky Hills of north-central Kansas using detection data and habitat variables, (2) assessing the occurrence of these vectors at nests of female Greater Prairie-Chickens, and (3) evaluating if shared habitat use between vectors and hosts is correlated with malarial infection status of the Greater Prairie-Chicken. Our results indicate that Culex occurrence increased at nest locations compared to other available but unoccupied grassland habitats; however the shared habitat use between vectors and hosts did not result in an increased prevalence of malarial parasites in Greater Prairie-Chickens that occupied habitats with high vector occurrence. We developed a predictive map to illustrate the associations between Culex occurrence and infection status with malarial parasites in an obligate grassland bird that may be used to guide management decisions to limit the spread of vector-borne diseases.

Latent infections in avian malaria in relation to the production of drug-resistance

1. No resistance to paludrine or to sulphadiazine was obtained after treating latent infections ofPlasmodium gallinaceumin chickens with twice daily doses of 20 mg./20 g. of sulphadiazine over periods of 171, 178 and 190 days.

2. No resistance to paludrine was obtained after treating a latent infection ofP. relictumin a canary over a period of 1 year with doses of paludrine increasing from 0·05 mg./20 g. once daily to 1·0 mg./ 20 g. twice daily.

3. It is considered that if drug-resistance arises by mutation and selection, then resistance should arise more readily when a large number of rapidly multiplying parasites is treated with a drug than when the population treated is small, with a low reproduction rate, i.e. the failure to obtain resistant strains of malaria by prolonged treatment of latent infections with large amounts of drug, lends support to the theory of the origin of resistant strains of malaria by the selection of resistant mutants.

Direct and indirect immunosuppression by a malaria parasite in its mosquito vector

Malaria parasites develop as oocysts within the haemocoel of their mosquito vector during a period that is longer than the average lifespan of many of their vectors. How can they escape from the mosquito's immune responses during their long development? Whereas older oocysts might camouflage themselves by incorporating mosquito-derived proteins into their surface capsule, younger stages are susceptible to the mosquito's immune response and must rely on other methods of immune evasion. We show that the malaria parasite Plasmodium gallinaceum suppresses the encapsulation immune response of its mosquito vector, Aedes aegypti, and in particular that the parasite uses both an indirect and a direct strategy for immunosuppression. Thus, when we fed mosquitoes with the plasma of infected chickens, the efficacy of the mosquitoes to encapsulate negatively charged Sephadex beads was considerably reduced, whether the parasite was present in the blood meal or not. In addition, zygotes that were created ex vivo and added to the blood of uninfected chickens reduced the efficacy of the encapsulation response. As dead zygotes had no effect on encapsulation, this result demonstrates active suppression of the mosquito's immune response by malaria parasites.

Changes in Haemoproteus sex ratios: fertility insurance or differential sex lifespan?

There is little direct evidence of the fitness effects of changes in malaria gametocyte sex ratio. Gametocyte sex ratios in haemospororin parasites (phylum Apicomplexa) are usually female skewed. However, in some cases and especially in Haemoproteus parasites, less female-biased and even male-biased sex ratios are encountered. The 'fertility insurance hypothesis' tries to explain these biases as an evolutionary strategy to facilitate gamete encounter. Thus, the hypothesis predicts that, if there is a reduction in gametocyte density (intensity of infection) or other factors preventing gametes from meeting, a change to a higher proportion of male gametocytes may be favoured. By contrast, a change in sex ratio may be caused by other non-adaptive mechanisms, for example differential survival of the gametocytes of each sex. We study within-host changes in Haemoproteus majoris sex ratios following an experimental reduction in the density of the parasites in the blood in a breeding population of blue tits (Parus caeruleus). Medication with the antimalarial drug primaquine induced a significant reduction in Haemoproteus gametocyte infection intensity in two different breeding seasons and under two different doses of medication. Sex ratios became male skewed following the experimental treatment in agreement with the predictions of the 'fertility insurance' hypothesis. Also in support of the hypothesis, a significant change towards male-biased sex ratios emerged for non-medicated birds in one year, probably owing to the natural immune reduction of the density of the parasites in the blood. The alternative possibility that changes are caused by different lifespans of gametocytes is not supported by changes in sex ratios in control hosts, where new production and release of gametocytes occur.

Studies on the extracellular cultivation of an intracellular parasite (avian malaria). I. Development of the organisms in erythrocyte extracts, and the favoring effect of adenosinetriphosphate

The erythrocytic stages of Plasmodium lophurae were freed from their host red cells by specific hemolysis directly into a favorable medium containing an extract of duck erythrocytes. Extracellular survival and development of the parasite in vitro occurred in culture media consisting essentially of a very concentrated extract of duck red cells prepared in a special nutrient solution. Omission or dilution of the red cell extract resulted in rapid degeneration of the parasites. Their survival and development were favored by the presence in the erythrocyte extract of gelatin, yeast adenylic acid, and cozymase, and especially by the further addition of adenosinetriphosphate and sodium pyruvate. Under the best conditions yet tested, all the free parasites continued their development extracellularly during the first two days of cultivation. Merozoites formed by the extracellular segmentation of the free parasites originally present developed further into trophozoites. On the third day a majority of the free parasites were still of normal appearance, but by the fourth day more were degenerate, and very few normal parasites remained on the fifth day.

Chitinases of the avian malaria parasite Plasmodium gallinaceum, a class of enzymes necessary for parasite invasion of the mosquito midgut

The Plasmodium ookinete produces chitinolytic activity that allows the parasite to penetrate the chitin-containing peritrophic matrix surrounding the blood meal in the mosquito midgut. Since the peritrophic matrix is a physical barrier that the parasite must cross to invade the mosquito, and the presence of allosamidin, a chitinase inhibitor, in a blood meal prevents the parasite from invading the midgut epithelium, chitinases (3.2.1.14) are potential targets of malaria parasite transmission-blocking interventions. We have purified a chitinase of the avian malaria parasite Plasmodium gallinaceum and cloned the gene, PgCHT1, encoding it. PgCHT1 encodes catalytic and substrate-binding sites characteristic of family 18 glycohydrolases. Expressed in Escherichia coli strain AD494 (DE3), recombinant PgCHT1 was found to hydrolyze polymeric chitin, native chitin oligosaccharides, and 4-methylumbelliferone derivatives of chitin oligosaccharides. Allosamidin inhibited recombinant PgCHT1 with an IC(50) of 7 microM and differentially inhibited two chromatographically separable P. gallinaceum ookinete-produced chitinase activities with IC(50) values of 7 and 12 microM, respectively. These two chitinase activities also had different pH activity profiles. These data suggest that the P. gallinaceum ookinete uses products of more than one chitinase gene to initiate mosquito midgut invasion.

Molecular heterogeneity of lactic dehydrogenase in avian malaria (Plasmodium lophurae)

Lactic dehydrogenase activity increased in direct proportion to the degree of parasitization in synchronous infections of duck erythrocytes. Deviations from this linearity could be accounted for on the basis of the developmental stage of the parasite. Erythrocyte-free P. lophurae showed activities which averaged 3 times that of uninfected erythrocytes, whereas infected erythrocytes had intermediate values. In addition, a patent infection was generally reflected by an increase in the lactic dehydrogenase activity in the plasma, but no direct correlation with parasitemia was established. Molecular heterogeneity of the enzyme was determined on the basis of kinetic data and electrophoretic isolation on a starch block. The uninfected red blood cell showed a major anodal and a minor cathodal peak of lactic dehydrogenase activity, and was further characterized by a kinetic constant representing a high pH optimum with low concentrations of substrate. Isolated P. lophurae had a single, cathodal peak of activity dissimilar from that of the uninfected erythrocyte, and a kinetic constant describing a low pH optimum with a high concentration of substrate. Infected erythrocytes showed a combination of these electrophoretic entities and an intermediate range of kinetic constants. The data indicate that the avian malaria parasite P. lophurae contains a lactic dehydrogenase qualitatively dissimilar from that of its host cell, and the increased enzymatic activity of infected erythrocytes is a result of the enzyme content of the growing parasite added to that of the red blood cell. It is suggested that the LDH of the parasite has a physiological advantage under those conditions which prevail inside the red blood cell.

Blood parasites of some columbiform and passeriform birds from Chile

Ninety-one birds of 23 species from Chile were examined for haematozoa; 13 birds of seven species harbored species of Haemoproteus, Leucocytozoon, Plasmodium, Trypanosoma and microfilariae. Haemoproteids (representing four species) were the most common parasites and occurred in 10 of the 13 infected birds.

The fine structure of the exoerythrocytic stages of Plasmodium fallax

The fine structure of the exoerythrocytic cycle of an avian malarial parasite, Plasmodium fallax, has been analyzed using preparations grown in a tissue culture system derived from embryonic turkey brain cells which were fixed in glutaraldehyde-OsO(4). The mature merozoite, an elongated cell 3- to 4-micro long and 1- to 2-micro wide, is ensheathed in a complex double-layered pellicle. The anterior end consists of a conoid, from which emanate two lobed paired organelles and several closely associated dense bodies. A nucleus is situated in the mid portion of the cell, while a single mitochondrion wrapped around a spherical body is found in the posterior end. On the pellicle of the merozoite near the nucleus a cytostomal cavity, 80 to 100 mmicro in diameter, is located. Based on changes in fine structure, the subsequent sequence of development is divided into three phases: first, the dedifferentiation phase, in which the merozoite loses many complex structures, i.e. the conoid, paired organelles, dense bodies, spherical body, and the thick inner layers of the pellicle, and transforms into a trophozoite; second, the growth phase, which consists of many nuclear divisions as well as parallel increases in mitochondria, endoplasmic reticulum, and ribosomes; and third, the redifferentiation and cytoplasmic schizogony phase, in which the specialized organelles reappear as the new merozoites bud off from the mother schizont.

A study of resistance to cycloguanil in Plasmodium gallinaceum in chicks

A cycloguanil-resistant strain of Plasmodium gallinaceum was produced relatively rapidly by passage through chicks treated with low but effective doses of the drug, the dose being increased as resistance developed.

The strain was cross-resistant to proguanil but not to pyrimethamine or chloroquine.

A strain highly resistant to proguanil was resistant to cycloguanil but only slightly resistant to pyrimethamine.

A strain highly resistant to pyrimethamine was resistant to proguanil and cycloguanil.

Passage for 20 months through birds treated with doses of cycloguanil which suppressed infection for relatively long periods failed to change the sensitivity of the strain to this drug or to proguanil. Although the relatively large dose did not eradicate the infection in any of the birds, subinoculations demonstrated that parasites were absent from the blood for a period in some of the birds, though infections finally developed.

I am indebted to Parke Davis and Company for the supply of cycloguanil, to Imperial Chemical Industries Ltd. for the proguanil hydrochloride and chloroquine phosphate and to Burroughs Wellcome and Company for the pyrimethamine base.

The feeding mechanism of avian malarial parasites

Electron microscope studies of the erythrocytic forms, including gametocytes and asexual schizonts, of the protozoa Plasmodium fallax, P. lophurae, and P. cathemerium, have revealed a "cytostome," a specialized organelle of the pellicular membrane which is active in the ingestion of host cell cytoplasm. In material fixed in glutaraldehyde and postfixed in OsO(4), the cytostome appears in face view as a pore limited by two dense circular membranes and having an inside diameter of approximately 190 mmicro. In cross-section, the cytostome is a cavity bounded on each side by two dense segments corresponding to the two dense circles observed in face view; its base consists of a single unit membrane. In the process of feeding, the cytostome cavity enlarges by expansion of its membrane, permitting a large quantity of red cell cytoplasm to come into contact with the cytostome wall. Subsequent digestion of erythrocyte cytoplasm occurs exclusively in food vacuoles which emanate from the cytostome invagination. As digestion progresses, the food vacuoles initially stain more densely and there is a marked build-up of hemozoin granules. In the final stage of digestion, a single membrane surrounds a cluster of residual pigment particles and very little of the original host cell cytoplasm remains. The cytostome in exoerythrocytic stages of P. fallax has been observed only in merozoites and does not seem to play the same role in the feeding mechanism.