Several recent studies conclude that an increase in the pathogenicity of SARS-CoV-2 cannot be ruled out. However, it should be noted that SARS-CoV-2 is a ‘direct’ respiratory virus – meaning it is usually spread by the respiratory route but does not routinely pass through the lymphatics like measles and smallpox. Providing its tropism does not change, it will be unique if its pathogenicity does not decrease until it becomes similar to common cold viruses. Ewald noted in the 1980s that respiratory viruses may evolve mildness because their spread benefits from the mobility of their hosts. This review examines factors that usually lower respiratory viruses’ severity, including heat sensitivity (which limits replication in the warmer lungs) and changes to the virus’s surface proteins. Other factors may, however, increase pathogenicity, such as replication in the lymphatic system and spreading via solid surfaces or faecal matter. Furthermore, human activities and political events could increase the harmfulness of SARS-CoV-2, including the following: large-scale testing, especially when the results are delayed; transmission in settings where people are close together and not free to move around; poor hygiene facilities; and social, political, or cultural influences that encourage sick individuals to remain active, including crises such as wars. If we can avoid these eventualities, SARS-CoV-2 is likely to evolve to be milder, although the timescale is uncertain. Observations of influenza-like pandemics suggest it may take around two decades for COVID-19 to become as mild as seasonal colds.

Entomopathogenic nematodes (EPNs) are closely associated with Popillia japonica and potentially used as their biological control agents, although field results proved inconsistent and evoked a continual pursuit of native EPNs more adapted to the environment. Therefore, we surveyed the Azorean Archipelago to isolate new strains of Heterorhabditis bacteriophora and to evaluate their virulence against the model organism Galleria mellonella under laboratory conditions. Six strains were obtained from pasture and coastal environments and both nematode and symbiont bacteria were molecularly identified. The bioassays revealed that Az172, Az186, and Az171 presented high virulence across the determination of a lethal dose (LD50) and short exposure time experiments with a comparable performance to Az29. After 72 hours, these virulent strains presented a mean determination of a lethal dose of 11 infective juveniles cm-2, a lethal time (LT50) of 34 hours, and achieved 40% mortality after an initial exposure time of only 60 minutes. Az170 exhibited an intermediate performance, whereas Az179 and Az180 were classified as low virulent strains. However, both strains presented the highest reproductive potential with means of 1700 infective juveniles/mg of larvae. The bioassays of the native EPNs obtained revealed that these strains hold the potential to be used in biological control initiatives targeting P. japonica because of their high virulence and locally adapted to environmental conditions.

Toxoplasma gondii has at least 318 genotypes distributed worldwide, and tropical regions usually have greater genetic diversity. Campeche is a state located in the southeastern region of México and has favourable climate conditions for the replication and dissemination of this protozoan, similar to those in South American countries where broad genetic diversity has been described. Thus, in this study, 4 T. gondii isolates were obtained from tissues of stray dogs and free-range chickens in Campeche, México, and were genotyped by Mn-PCR-RFLP with 10 typing markers (SAG1, altSAG2, SAG3, BTUB, GRA6, c22-8, c29-2, L358, PK1 and Apico) and 5 virulence markers (CS3, ROP16, ROP17, ROP18 and ROP5) to provide new information about the distribution and virulence prediction of T. gondii genotypes. Two isolates of T. gondii genotype #116 and 2 of genotype #38 were obtained from stray dogs and chickens, respectively. The parasite load found in these species was between <50 and more than 35 000 tachyzoites per mg of tissue. Virulence marker genotyping revealed a recombinant 1&3 ROP5 RFLP pattern in 2 ToxoDB #116 isolates with no prediction of virulence in a murine model, while in the 2 ToxoDB #38 isolates, the ROP18/ROP5 combination predicted high virulence. Considering all the typed markers, there is a predominance of type I and III alleles, as constantly reported for the isolates characterized in various regions of México. It is crucial to determine their phenotype to corroborate the genetic virulence profile of the T. gondii isolates obtained in this study.

Metastrongyle lungworms could be particularly detrimental for diving animals such as marine mammals; however, little is known of the drivers of pathogenic host–parasite relationships in this group. This systematic review analysed the diversity of metastrongyles in marine mammals and the host and parasite traits associated with virulence. There have been at least 40 species of metastrongyles described in 66 species of marine mammals. After penalization for study biases, Halocercus hyperoodoni, Otostrongylus circumlitus, Parafilaroides gymnurus, Halocercus brasiliensis and Stenurus minor were the metastrongyles with the widest host range. Most studies (80.12%, n = 133/166) reported that metastrongyles caused bronchopneumonia, while in the cardiovascular system metastrongyles caused vasculitis in nearly half of the studies (45.45%, n = 5/11) that assessed these tissues. Metastrongyles were associated with otitis in 23.08% (n = 6/26) of the studies. Metastrongyle infection was considered a potential contributory to mortality in 44.78% (n = 90/201) of the studies while 10.45% (n = 21/201) of these studies considered metastrongyles the main cause of death. Metastrongyle species with a wider host range were more likely to induce pathogenic effects. Metastrongyles can cause significant tissue damage and mortality in marine mammals although virulent host–parasite relationships are dominated by a few metastrongyle species with wider host ranges.

Wild rabbits in Australia developed genetic resistance to the myxoma virus, which was introduced as a biological control agent. However, little is known about the rate at which this evolutionary change occurred. We collated data from challenge trials that estimated rabbit resistance to myxomatosis in Australia and expressed resistance on a continuous scale, enabling trends in its development to be assessed over 45 years up to 1995. Resistance initially increased rapidly, followed by a plateau lasting ten years, before a second rapid increase occurred associated with the introduction of European rabbit fleas as myxoma virus vectors. By contrast, in the United Kingdom, where rabbit flea vectors were already present when the myxoma virus initially spread, resistance developed more slowly. No estimates of rabbit resistance to myxomatosis have been made for almost 30 years, despite other highly lethal rabbit pathogens becoming established worldwide. Continued testing of wild-caught rabbits in Australia to determine current levels of resistance to myxomatosis is recommended to assess its current effectiveness for managing pest rabbits. Given the economic and environmental significance of invasive rabbits, it would be remiss to manage such biological resources and ecosystem services poorly.

Streptococcus pneumoniae (S. pneumoniae) is one of the most common community-associated pathogens responsible for pneumonia in children. This retrospective study aimed to investigate the molecular characteristics of S. pneumoniae isolated from children with community-acquired pneumonia (CAP) under 5 years in Chengdu, China. Molecular characteristics of S. pneumoniae included serotype and virulence factor performed by using PCR method and sequence types (STs) determined by sequencing seven housekeeping genes. In addition, the potential relationships between molecular characteristics were depicted by minimum spanning tree and correspondence analysis. The prevailing serotypes were 19F (18.52%), 6B (17.59%), 19A (13.89%), 6A (6.48%) and 23F (5.56%) among 108 isolates. The overall coverage rates of 7-valent, 10-valent, 13-valent, 15-valent and 20-valent pneumococcal conjugate vaccines (PCVs) were 47.32, 48.1, 75, 75 and 78.7%, respectively. Meanwhile, the coverage rates of PCV13 among the isolates from CAP patients aged <1 year were high up to 84.2%. MLST analysis results showed that there were 56 different STs identified, of which the dominant STs were ST271 (22.22%) and ST320 (12.04%). Five international clones among STs were observed, including Spain23F‐1, Spain6B‐2, Taiwan19F‐14, Netherlands3‐31 and Denmark14‐32. Additionally, most of the isolates carried ply, psaA, nanA, pavA, piaA and CC271 isolates expressed more of nanA than non-CC271 isolates. Moreover, there were strong relevant relationships among STs, serotypes and virulence factors. Considering serotypes and virulence factors together can be used as the foundation for the formulation of vaccine strategy.

Transmission mode is a key factor that influences host–parasite coevolution. Vector-borne pathogens are among the most important disease agents for humans and wildlife due to their broad distribution, high diversity, prevalence and lethality. They comprise some of the most important and widespread human pathogens, such as yellow fever, leishmania and malaria. Vector-borne parasites (in this review, those transmitted by blood-feeding Diptera) follow unique transmission routes towards their vertebrate hosts. Consequently, each part of this tri-partite (i.e. parasite, vector and host) interaction can influence co- and counter-evolutionary pressures among antagonists. This mode of transmission may favour the evolution of greater virulence to the vertebrate host; however, pathogen–vector interactions can also have a broad spectrum of fitness costs to the insect vector. To complete their life cycle, vector-borne pathogens must overcome immune responses from 2 unrelated organisms, since they can activate responses in both vertebrate and invertebrate hosts, possibly creating a trade-off between investments against both types of immunity. Here, we assess how dipteran vector-borne transmission shapes the evolution of hosts, vectors and the pathogens themselves. Hosts, vectors and pathogens co-evolve together in a constant antagonistic arms race with each participant's primary goal being to maximize its performance and fitness.

With the increase of crewed space missions and the rise of space microbiology, the research of microbes grown under microgravity environment has been attracting more attention. The research scope in space microbiology has been extended beyond pathogens directly related to spaceflight. Y. pestis, the causative agent of plague, is also of interest to researchers. After being cultivated for 40 consecutive passages in either simulated microgravity (SMG) or normal gravity (NG) conditions, the Y. pestis strain 201 cultures were analysed regarding their phenotypic features. By using crystal violet staining assays, increased biofilm amount was detected in Y. pestis grown under SMG condition. Besides that, the damage degrees of Hela cell caused by SMG-grown Y. pestis were found diminished in comparison to those under NG condition. Consistent with this observation, the death course was delayed in mice infected with SMG-grown Y. pestis, suggesting that microgravity condition can contribute the attenuated virulence. RNA-seq-based transcriptomics analysis showed that a total of 218 genes were differentially regulated, of which 91 upregulated and 127 downregulated. We found that dozens of virulence-associated genes were downregulated, which partially explained the reduced virulence of Y. pestis under SMG condition. Our study demonstrated that long-term exposure to SMG influences the pathogenesis and biofilm formation ability of Y. pestis, which provides a novel avenue to study the mechanism of physiology and virulence of this pathogen. Microgravity enhanced the ability of biofilm formation and reduced the virulence and cytotoxicity of Y. pestis. Many virulence-associated genes of Y. pestis were differentially regulated in response to the stimulated microgravity. However, there is no molecular evidence to explain the enhanced biofilm formation ability, which requires further research. Taken together, the phenotype changes of Y. pestis under SMG conditions can provide us a new research direction of its potential pathogenesis.

The Mormon cricket (MC), Anabrus simplex Haldeman, 1852 (Orthoptera: Tettigoniidae), has a long and negative history with agriculture in Utah and other western states of the USA. Most A. simplex populations migrate in large groups, and their feeding can cause significant damage to forage plants and cultivated crops. Chemical pesticides are often applied, but some settings (e.g. habitats of threatened and endangered species) call for non-chemical control measures. Studies in Africa, South America, and Australia have assessed certain isolates of Metarhizium acridum as very promising pathogens for Orthoptera: Acrididae (locust) biocontrol. In the current study, two isolates of Metarhizium robertsii, one isolate of Metarhizium brunneum, one isolate of Metarhizium guizhouense, and three isolates of M. acridum were tested for infectivity to MC nymphs and adults of either sex. Based on the speed of mortality, M. robertsii (ARSEF 23 and ARSEF 2575) and M. brunneum (ARSEF 7711) were the most virulent to instars 2 to 5 MC nymphs. M. guizhouense (ARSEF 7847) from Arizona was intermediate and the M. acridum isolates (ARSEF 324, 3341, and 3609) were the slowest killers. ARSEF 2575 was also the most virulent to instar 6 and 7 nymphs and adults of MC. All of the isolates at the conidial concentration of 1 × 107 conidia ml−1 induced approximately 100% mortality by 6 days post application of fungal conidia. In conclusion, isolates ARSEF 23, ARSEF 2575, and ARSEF 7711 acted most rapidly to kill MC under laboratory conditions. The M. acridum isolates, however, have much higher tolerance to heat and UV-B radiation, which may be critical to their successful use in field application.

The outbreak of pneumonia-like respiratory disorder at China and its rapid transmission world-wide resulted in public health emergency, which brought lineage B betacoronaviridae SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2) into spotlight. The fairly high mutation rate, frequent recombination and interspecies transmission in betacoronaviridae are largely responsible for their temporal changes in infectivity and virulence. Investigation of global SARS-CoV-2 genotypes revealed considerable mutations in structural, non-structural, accessory proteins as well as untranslated regions. Among the various types of mutations, single-nucleotide substitutions are the predominant ones. In addition, insertion, deletion and frame-shift mutations are also reported, albeit at a lower frequency. Among the structural proteins, spike glycoprotein and nucleocapsid phosphoprotein accumulated a larger number of mutations whereas envelope and membrane proteins are mostly conserved. Spike protein and RNA-dependent RNA polymerase variants, D614G and P323L in combination became dominant world-wide. Divergent genetic variants created serious challenge towards the development of therapeutics and vaccines. This review will consolidate mutations in different SARS-CoV-2 proteins and their implications on viral fitness.

In most of the world Toxoplasma gondii is comprised of archetypal types (types I, II and III); however, South America displays several non-archetypal strains. This study used an experimental mouse model to characterize the immune response and parasite kinetics following infection with different parasite genotypes. An oral inoculation of 50 oocysts per mouse from T. gondii M4 type II (archetypal, avirulent), BrI or BrIII (non-archetypal, virulent and intermediate virulent, respectively) for groups (G)2, G3 and G4, respectively was used. The levels of mRNA expression of cytokines, immune compounds, cell surface markers and receptor adapters [interferon gamma (IFNγ), interleukin (IL)-12, CD8, CD4, CD25, CXCR3 and MyD88] were quantified by SYBR green reverse transcription-quantitative polymerase chain reaction. Lesions were characterized by histology and detection by immunohistochemistry established distribution of parasites. Infection in G2 mice was mild and characterized by an early MyD88-dependent pathway. In G3, there were high levels of expression of pro-inflammatory cytokines IFNγ and IL-12 in the mice showing severe clinical symptoms at 8–11 days post infection (dpi), combined with the upregulation of CD25, abundant tachyzoites and tissue lesions in livers, lungs and intestines. Significant longer expression of IFNγ and IL-12 genes, with other Th1-balanced immune responses, such as increased levels of CXCR3 and MyD88 in G4, resulted in survival of mice and chronic toxoplasmosis, with the occurrence of tissue cysts in brain and lungs, at 14 and 21 dpi. Different immune responses and kinetics of gene expression appear to be elicited by the different strains and non-archetypal parasites demonstrated higher virulence.

The potato tuber moth, Phthorimaea operculella (Zeller), is a serious pest of potato and other commercial crops belonging to the Solanaceae family. In recent years, it has become an emerging problem in potato-growing regions of the Nilgiri hills of southern India. It is responsible for the reduced quality and quantity of marketable potatoes. In this regard, the development of an eco-friendly control method for the management of the potato tuber moth is urgently required. Therefore, in the present study, the virulence of Steinernema cholashanense CPRSUS01 originally isolated from the potato rhizosphere was tested on fourth-instar larvae and pupae of P. operculella. Steinernema cholashanense caused the greatest mortality in the fourth-instar larval stage (100%) than the pupae (30%). In addition to this, penetration and reproduction of this nematode was also studied in fourth-instar larvae of P. operculella and this is the first report of penetration and reproduction of any entomopathogenic nematode species on potato tuber moth larvae. The reproduction capacity of S. cholashanense on P. operculella is higher (702 infective juveniles mg−1 body weight). Our results indicated that S. cholashanense has good potential as an alternative tool for the management of P. operculella. But before including S. cholashanense in the integrated pest management program of P. operculella, its efficacy should be tested under field conditions.

Coxiella burnetii, the causative agent of Q fever, is widely present in dairy products around the world. It has been isolated from unpasteurised milk and cheese and can survive for extended periods of time under typical storage conditions for these products. Although consumption of contaminated dairy products has been suggested as a potential route for transmission, it remains controversial. Given the high prevalence of C. burnetii in dairy products, we sought to examine the feasibility of transmitting the major sequence types (ST16, ST8 and ST20) of C. burnetii circulating in the United States. We delivered three strains of C. burnetii, comprising each sequence type, directly into the stomachs of immunocompetent BALB/c mice via oral gavage (OG) and assessed them for clinical symptoms, serological response and bacterial dissemination. We found that mice receiving C. burnetii by OG had notable splenomegaly only after infection with ST16. A robust immune response and persistence in the stomach and mesenteric lymph nodes were observed in mice receiving ST16 and ST20 by OG, and dissemination of C. burnetii to peripheral tissues was observed in all OG infected mice. These findings support the oral route as a mode of transmission for C. burnetii.

Four Beauveria bassiana and three Metarhizium brunneum isolates were evaluated, as soil drenches, against Spodoptera littoralis prepupae. Treatment efficacy was determined by assessing total mortality during development from prepupae through to pupae and adults; mortality and sub-lethal effects on reproduction were also quantified for adults emerging from surviving prepupae/pupae. All isolates were pathogenic but overall mortality varied between 31.7 and 83.3% (0% for control); average survival time was 7.5–10.5 days (14.0 days for control). From 1.7–15.0% of adults emerging from surviving prepupae/pupae were deformed (0% in control). Contact with fungal suspensions as prepupae/pupae caused a significant reduction in fecundity of emerging adult females (15–58.9%), and a significant reduction in egg viability (6.8–28.4%) compared with controls. Two isolates were selected for virulence evaluation against S. littoralis prepupae. The LC50s were 1.7 × 107 and 1.8 × 107 conidia ml−1 and the median survival times were 7 and 6 days for isolates EAMa 01/58-Su and EAMb 09/01-Su, respectively. Destruxin A was present in pupae developing from prepupae treated with isolates EAMa 01/58-Su (0.010 ± 0.002 µg pupae−1) and EAMb 09/01-Su (0.015 ± 0.003 µg pupae−1). The use of entomopathogenic fungi as soil drenches could be a key component of S. littoralis IPM strategies due to direct reductions in the number of soil-dwelling life stages and, also, the significant reduction in reproductive potential of surviving adults.

Why some parasites evolve and maintain extreme levels of virulence is a question that remains largely unanswered. A body of theory predicts that parasites that form long-lived spores able to persist in the environment evolve higher virulence, known as the sit and wait hypothesis. Such parasites can obliterate their local host population and wait in the environment for further hosts to arrive, reducing some of the costs of high virulence. On the other hand, some models predict the opposite to be true, that virulence and environmental persistence are both costly and traded off, the resource allocation hypothesis. I conducted a meta-analysis on published data on the relationship between environmental persistence and virulence collected to date. I first examined all data available to date and then conducted a smaller analysis focussing on just those studies testing the specific predictions of the sit and wait hypothesis. Empirical work supports both hypotheses; however, the direction of the effect is largely associated with parasite type. In both analyses, viruses tend to show evidence of resource allocation trade-offs, these traits are positively correlated in bacterial and fungal parasites.

Chagas disease is a complex tropical pathology caused by the kinetoplastid Trypanosoma cruzi. This parasite displays massive genetic diversity and has been classified by international consensus in at least six Discrete Typing Units (DTUs) that are broadly distributed in the American continent. The main clinical manifestation of the disease is the chronic chagasic cardiomyopathy (CCC) that is lethal in the infected individuals. However, one intriguing feature is that only 30–40% of the infected individuals will develop CCC. Some authors have suggested that the immune response, host genetic factors, virulence factors and even the massive genetic heterogeneity of T. cruzi are responsible of this clinical pattern. To date, no conclusive data support the reason why a few percentages of the infected individuals will develop CCC. Therefore, we decided to conduct a systematic review analysing the host genetic factors, immune response, cytokine production, virulence factors and the plausible association of the parasite DTUs and CCC. The epidemiological and clinical implications are herein discussed.

We performed a long-term natural experiment investigating the impact of the diphyllobotriidean cestode Schistocephalus solidus on the body condition and clutch size (CS) of threespine stickleback Gasterosteus aculeatus, its second intermediate host, and the growth of larval parasites in host fish. We tested the hypothesis that single S. solidus infections were more virulent than multiple infections. We also asked whether the metrics of mean and total parasite mass (proxies for individual and total volume, respectively) were consistent with predictions of the resource constraints or the life history strategy (LHS) hypothesis for the growth of, hence exploitation by, larval helminths in intermediate hosts. The samples were drawn from Walby Lake, Alaska in eight of 11 years. Host body condition and CS (egg number per spawning bout) decreased significantly with intensity after adjustments for host size and parasite index. Thus, infections have an increasingly negative impact on measures of host fitness with greater intensity, in contrast to the hypothesis that single infections are more harmful than multiple infections. We also found that mean parasite mass decreased with intensity while total parasite mass increased with intensity as predicted by the LHS hypothesis.