Transovarian and Transstadial Transmission

Transstadial transmission is the persistence of a

symbiont Symbiosis (Ancient Greek : living with, companionship < : together; and ''bíōsis'': living) is any type of a close and long-term biological interaction, between two organisms of different species. The two organisms, termed symbionts, can fo ...

pathogen In biology, a pathogen (, "suffering", "passion" and , "producer of"), in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a Germ theory of d ...

in an organism from one life stage ("stadium") to the next, such as

larva A larva (; : larvae ) is a distinct juvenile form many animals undergo before metamorphosis into their next life stage. Animals with indirect development such as insects, some arachnids, amphibians, or cnidarians typically have a larval phase ...

nymph A nymph (; ; sometimes spelled nymphe) is a minor female nature deity in ancient Greek folklore. Distinct from other Greek goddesses, nymphs are generally regarded as personifications of nature; they are typically tied to a specific place, land ...

to adult. This type of transmission is typically observed in insects. For example, the bacterium ''

Borrelia burgdorferi ''Borrelia burgdorferi'' is a bacterial species of the spirochete class in the genus '' Borrelia'', and is one of the causative agents of Lyme disease in humans. Along with a few similar genospecies, some of which also cause Lyme disease, it m ...

'', the causative agent for

Lyme disease Lyme disease, also known as Lyme borreliosis, is a tick-borne disease caused by species of ''Borrelia'' bacteria, Disease vector, transmitted by blood-feeding ticks in the genus ''Ixodes''. It is the most common disease spread by ticks in th ...

, infects the

tick Ticks are parasitic arachnids of the order Ixodida. They are part of the mite superorder Parasitiformes. Adult ticks are approximately 3 to 5 mm in length depending on age, sex, and species, but can become larger when engorged. Ticks a ...

vector Vector most often refers to: * Euclidean vector, a quantity with a magnitude and a direction * Disease vector, an agent that carries and transmits an infectious pathogen into another living organism Vector may also refer to: Mathematics a ...

as a larva, with the infection maintained as the tick molts to a

and later develops as an adult. Transstadial transmission is also seen with other microbes such as other

bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...

fungi A fungus (: fungi , , , or ; or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as one ...

, and

viruses A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Viruses are found in almo ...

in numerous insects. In addition to ticks,

mite Mites are small arachnids (eight-legged arthropods) of two large orders, the Acariformes and the Parasitiformes, which were historically grouped together in the subclass Acari. However, most recent genetic analyses do not recover the two as eac ...

s are another common vector. Transstadial transmission is especially relevant to

public health Public health is "the science and art of preventing disease, prolonging life and promoting health through the organized efforts and informed choices of society, organizations, public and private, communities and individuals". Analyzing the de ...

, as several threats to public health are maintained in insect populations by transstadial transmission. Some debate exists regarding the classification of transstadial transmission as vertical transmission versus horizontal transmission. Reasons for this stem from further debate regarding transovarial transmission, described as the passage of a symbiont or pathogen from an infected female to its progeny, especially in eggs. Transstadial blockage is the opposite of transstadial transmission in which the symbiont or pathogen cannot be carried over from one life stage to the next. This aspect serves to hinder transmission as the symbiont or pathogen is with the host for a shorter period, allowing for lower chances of transmission.

Epidemiology

Outbreaks of emerging infectious diseases have been increasing in frequency as human populations expand and encroach on previously undisturbed or uninhabited areas. Vector-borne diseases have presented significant challenges to maintaining public health over the past century. Furthermore, arthropod-borne diseases have been at the forefront of vector-borne diseases, bringing

epidemics An epidemic (from Ancient Greek, Greek ἐπί ''epi'' "upon or above" and δῆμος ''demos'' "people") is the rapid spread of disease to a large number of Host (biology), hosts in a given population within a short period of time. For example ...

of plague,

tularemia Tularemia, also known as rabbit fever, is an infectious disease caused by the bacterium '' Francisella tularensis''. Symptoms may include fever, skin ulcers, and enlarged lymph nodes. Occasionally, a form that results in pneumonia or a throat ...

, yellow fever,

malaria Malaria is a Mosquito-borne disease, mosquito-borne infectious disease that affects vertebrates and ''Anopheles'' mosquitoes. Human malaria causes Signs and symptoms, symptoms that typically include fever, Fatigue (medical), fatigue, vomitin ...

, Japanese encephalitis, Eastern equine encephalitis, West Nile Virus disease,

leishmaniasis Leishmaniasis is a wide array of clinical manifestations caused by protozoal parasites of the Trypanosomatida genus ''Leishmania''. It is generally spread through the bite of Phlebotominae, phlebotomine Sandfly, sandflies, ''Phlebotomus'' an ...

, and several others. Epidemiologically-relevant transstadial transmission is primarily observed in mites, ticks, and mosquitoes that serve as arthropod vectors of disease.

Transstadial transmission of pathogens in arthropod vectors

Lyme Disease (''Borrelia burgdorferi'')

Lyme disease, caused by the bacterium ''Borrelia burgdorferi'', is spread to humans via the bite of an infected blacklegged tick, also known as a deer tick (''

Ixodes scapularis ''Ixodes scapularis'' is a hard-bodied tick found in much of the eastern half of North America. It is commonly known as the deer tick, owing to its habit of parasitizing the white-tailed deer. It is also sometimes known as the black-legged tick ( ...

''). ''B. burgdorferi'' is considered enzootic, meaning that it is perpetuated in animals in the environment, outside of humans. In the cycle from ticks to animals, an uninfected tick larva feeds on an infected host, such as a

deer A deer (: deer) or true deer is a hoofed ruminant ungulate of the family Cervidae (informally the deer family). Cervidae is divided into subfamilies Cervinae (which includes, among others, muntjac, elk (wapiti), red deer, and fallow deer) ...

or a

mouse A mouse (: mice) is a small rodent. Characteristically, mice are known to have a pointed snout, small rounded ears, a body-length scaly tail, and a high breeding rate. The best known mouse species is the common house mouse (''Mus musculus'' ...

, leading to infection of the tick. The infection is transstadial in ticks because the bacterium will persist in the tick as it molts from a larva to a nymph. Humans are a dead-end host for ''B. burgdorferi'' and do not function in the natural cycling of the bacterium in the environment. ''B. burgdorferi'' contains a 1Mb linear

chromosome A chromosome is a package of DNA containing part or all of the genetic material of an organism. In most chromosomes, the very long thin DNA fibers are coated with nucleosome-forming packaging proteins; in eukaryotic cells, the most import ...

with multiple linear and circular plasmids with

genes In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...

encoding lipoproteins that become activated under various conditions. Additionally, ''B. burgdorferi'' is an

auxotroph Auxotrophy ( "to increase"; ''τροφή'' "nourishment") is the inability of an organism to synthesize a particular organic compound required for its growth (as defined by IUPAC). An auxotroph is an organism that displays this characteristic; ''a ...

for all

amino acids Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the Proteinogenic amino acid, 22 α-amino acids incorporated into p ...

nucleotides Nucleotides are Organic compound, organic molecules composed of a nitrogenous base, a pentose sugar and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both o ...

, and

fatty acids In chemistry, in particular in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, ...

, and does not contain genes encoding proteins for use in the tricarboxylic acid cycle or

oxidative phosphorylation Oxidative phosphorylation(UK , US : or electron transport-linked phosphorylation or terminal oxidation, is the metabolic pathway in which Cell (biology), cells use enzymes to Redox, oxidize nutrients, thereby releasing chemical energy in order ...

. When the tick engages in its nymphal bloodmeal, ''B. burgdorferi'' undergoes many changes in

gene expression Gene expression is the process (including its Regulation of gene expression, regulation) by which information from a gene is used in the synthesis of a functional gene product that enables it to produce end products, proteins or non-coding RNA, ...

, upregulating protein generation for use in attachment to the host and establishing infection. Virulence genes are also activated in concert with tick

saliva Saliva (commonly referred as spit or drool) is an extracellular fluid produced and secreted by salivary glands in the mouth. In humans, saliva is around 99% water, plus electrolytes, mucus, white blood cells, epithelial cells (from which ...

production, further advancing the ability for the bacterium to become established in the new host.

La Crosse Virus

La Crosse virus is primarily transmitted via the Eastern Treehole mosquito ('' Aedes triseriatus''). Disease caused by La Crosse virus can result in serious

neurological Neurology (from , "string, nerve" and the suffix -logia, "study of") is the branch of medicine dealing with the diagnosis and treatment of all categories of conditions and disease involving the nervous system, which comprises the brain, the s ...

complications, such as La Crosse Encephalitis, most often occurring in children and is endemic to the eastern United States. La Crosse virus undergoes transovarial transmission in mosquitoes, passing from one generation to the next. Additionally, after infection of the mosquito egg, the virus remains in the mosquito as it progresses through its life stages, completing transstadial transmission. Mosquitoes may become infected by feeding on the blood of reservoir hosts in the environment, allowing for the transmission of the virus in the genetic line of the mosquitoes in the environment.

''Anaplasma spp.''

'' Anaplasma'' is a bacterial genus causing costly economic losses worldwide, particularly in the food animal agricultural sector. Infection by ''Anaplasma'' results in

anaplasmosis Anaplasmosis is a tick-borne disease affecting ruminants, dogs, and horses, and is caused by ''Anaplasma'' bacteria. Anaplasmosis is an infectious but not contagious disease. Anaplasmosis can be transmitted through mechanical and biological vector ...

, a disease which can cause both serious and

subclinical Asymptomatic (or clinically silent) is an adjective categorising the medical conditions (i.e., injuries or diseases) that patients carry but without experiencing their symptoms, despite an explicit diagnosis (e.g., a positive medical test). P ...

disease. ''Anaplasma'' is another arthropod-vectored disease and is also spread through the bites of infected ticks, particularly Ixodidae ticks and potentially mosquitoes as well''.'' Numerous scientific reports present claims of transovarial and transstadial transmission of ''Anaplasma'' in ticks. Ruminants are the primary reservoir for ''Anaplasma'' spp., but the bacteria can also be found in wild animals. In ticks, ''Anaplasma'' spp. can be transmitted transstadially, and the ticks become infected by feeding off infected animals, allowing for transmission to un-infected animals. Debate exists among scientists regarding the ability for transovarial transmission of ''Anaplasma'' spp. in ticks. Recent research, however, has built evidence for transovarial transmission of at least some ''Anaplasma'' species in ticks.

Transstadial transmission and fungal communities in mosquitoes

Symbioses and microbiomes are one of the main drivers in the development of immunity, particularly with

commensal Commensalism is a long-term biological interaction (symbiosis) in which members of one species gain benefits while those of the other species neither benefit nor are harmed. This is in contrast with mutualism, in which both organisms benefit f ...

organisms. Microbiota-immune system interactions promote the development of host-microbe partnerships through organismal cross-talk and maintaining organismal boundaries to ensure niche fulfillment in each organism involved in the symbiosis. Transstadial transmission is an important factor in the persistence of microbes in their hosts, particularly within arthropods such as insects, which morph from one life stage to another. Adult mosquitoes have been a hallmark organism for the study of transstadial transmission of microbes from the larval stage into adulthood, largely due to their involvement in the transmission of pathogens relevant to public health and the study of pathogen environmental persistence. One study, published in 2020, presented evidence for the involvement of

fungus A fungus (: fungi , , , or ; or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as one ...

in the mosquito microbiome influencing the development of differential bacterial components in the mosquito microbiome. The authors reported finding less bacteria in freshly emerged adult mosquitoes compared to the prior larval stage, significantly influenced by fungal

colonization 475px, Map of the year each country achieved List of sovereign states by date of formation, independence. Colonization (British English: colonisation) is a process of establishing occupation of or control over foreign territories or peoples f ...

of the

hindgut The hindgut (or epigaster) is the posterior ( caudal) part of the alimentary canal. In mammals, it includes the distal one third of the transverse colon and the splenic flexure, the descending colon, sigmoid colon and up to the ano-rectal junct ...

. The authors hypothesized that

physiological Physiology (; ) is the science, scientific study of function (biology), functions and mechanism (biology), mechanisms in a life, living system. As a branches of science, subdiscipline of biology, physiology focuses on how organisms, organ syst ...

and environmental changes associated with fungal colonization, such as less available nutrients and altered pH contributed to the decrease in bacteria from larvae to adult. It is clear from this study that further research is needed to understand the interplay between fungal colonizers and bacteria which can successfully transmit transstadially in mosquitoes to better understand bacterial persistence in these important disease-vectoring arthropods. Another study evaluated the effect of ''

Aspergillus oryzae ''Aspergillus oryzae'', also known as , is a mold used in East Asia to saccharify rice, sweet potato, and barley in the making of alcoholic beverages such as '' sake'' and '' shōchū'', and also to ferment soybeans for making soy sauce and '' ...

'' fungus on the malaria parasite ('' Plasmodium berghei'') in ''

Anopheles stephensi ''Anopheles stephensi'' is a primary mosquito vector of malaria in urban India and is included in the same subgenus as ''Anopheles gambiae'', the primary malaria vector in Africa. ''A. gambiae'' consists of a complex of morphologically identical ...

'' mosquitoes. Ultimately, the researchers determined that incorporating a recombinant ''A. oryzae'' fungus strain into the mosquito larvae promoted the persistence of the fungus in the adult mosquito stage and prevented oocyst formation of ''P. berghei'' in the mosquito

midgut The midgut is the portion of the human embryo from which almost all of the small intestine and approximately half of the large intestine develop. After it bends around the superior mesenteric artery, it is called the "midgut loop". It comprises ...

. The researchers determined that the fungus is a candidate for environmental control of malaria parasite control in mosquitoes by incorporation of the fungus into mosquitoes as a paratransgenesis model carrying effector proteins to prevent parasite development in the developing mosquito.

References

{{reflist Parasitology Epidemiology