Transstadial transmission is the persistence of a symbiont or pathogen in an organism from one life stage ("stadium") to the next, such as larva to nymph to adult. [1] [2] [3] This type of transmission is typically observed in insects. For example, the bacterium Borrelia burgdorferi , the causative agent for Lyme disease, infects the tick vector as a larva, with the infection maintained as the tick molts to a nymph and later develops as an adult. [1] [2] Transstadial transmission is also seen with other microbes such as other bacteria, fungi, and viruses in numerous insects. [3] In addition to ticks, mites are another common vector. [3] [4] Transstadial transmission is especially relevant to public health, 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. [5] [6] [7] [8]
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. [4] 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.
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 of plague, tularemia, yellow fever, malaria, Japanese encephalitis, Eastern equine encephalitis, West Nile Virus disease, leishmaniasis, and several others. Epidemiologically-relevant transstadial transmission is primarily observed in mites, ticks, and mosquitoes that serve as arthropod vectors of disease.
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 ). 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 or a mouse, 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 with multiple linear and circular plasmids with genes encoding lipoproteins that become activated under various conditions. Additionally, B. burgdorferi is an auxotroph for all amino acids, nucleotides, and fatty acids, and does not contain genes encoding proteins for use in the tricarboxylic acid cycle or oxidative phosphorylation. When the tick engages in its nymphal bloodmeal, B. burgdorferi undergoes many changes in gene expression, upregulating protein generation for use in attachment to the host and establishing infection. Virulence genes are also activated in concert with tick saliva production, further advancing the ability for the bacterium to become established in the new host.
La Crosse virus is primarily transmitted via the Eastern Treehole mosquito ( Aedes triseriatus ). Disease caused by La Crosse virus can result in serious neurological complications, such as La Crosse Encephalitis, most often occurring in children and is endemic to the eastern United States. [9]
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. [10] 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 is a bacterial genus causing costly economic losses worldwide, particularly in the food animal agricultural sector. Infection by Anaplasma results in anaplasmosis, a disease which can cause both serious and subclinical disease. [11] Anaplasma is another arthropod-vectored disease and is also spread through the bites of infected ticks, particularly Ixodidae ticks and potentially mosquitoes as well. [12] 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. [13] 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. [13] Debate exists among scientists regarding the ability for transovarial transmission of Anaplasma spp. in ticks. [14] [15] [16] [17] [18] Recent research, however, has built evidence for transovarial transmission of at least some Anaplasma species in ticks. [19] [20] [21]
Symbioses and microbiomes are one of the main drivers in the development of immunity, particularly with commensal organisms. [22] 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. [22]
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 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 of the hindgut. The authors hypothesized that physiological 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. [23]
Another study evaluated the effect of Aspergillus oryzae fungus on the malaria parasite ( Plasmodium berghei ) in Anopheles stephensi 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 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. [24]
Arbovirus is an informal name for any virus that is transmitted by arthropod vectors. The term arbovirus is a portmanteau word. Tibovirus is sometimes used to more specifically describe viruses transmitted by ticks, a superorder within the arthropods. Arboviruses can affect both animals and plants. In humans, symptoms of arbovirus infection generally occur 3–15 days after exposure to the virus and last three or four days. The most common clinical features of infection are fever, headache, and malaise, but encephalitis and viral hemorrhagic fever may also occur.
Wolbachia is a genus of gram-negative bacteria infecting many species of arthropods and filarial nematodes. The symbiotic relationship ranges from parasitism to obligate mutualism. It is one of the most common parasitic microbes of arthropods, and is possibly the most widespread reproductive parasite bacterium in the biosphere. Its interactions with hosts are complex and highly diverse across different host species. Some host species cannot reproduce, or even survive, without Wolbachia colonisation. One study concluded that more than 16% of neotropical insect species carry bacteria of this genus, and as many as 25 to 70% of all insect species are estimated to be potential hosts.
La Crosse encephalitis is an encephalitis caused by an arbovirus which has a mosquito vector.
Rickettsia rickettsii is a Gram-negative, intracellular, cocco-bacillus bacterium that was first discovered in 1902. Having a reduced genome, the bacterium harvests nutrients from its host cell to carry out respiration, making it an organo-heterotroph. Maintenance of its genome is carried out through vertical gene transfer where specialization of the bacterium allows it to shuttle host sugars directly into its TCA cycle.
Tick-borne diseases, which afflict humans and other animals, are caused by infectious agents transmitted by tick bites. They are caused by infection with a variety of pathogens, including rickettsia and other types of bacteria, viruses, and protozoa. The economic impact of tick-borne diseases is considered to be substantial in humans, and tick-borne diseases are estimated to affect ~80 % of cattle worldwide. Most of these pathogens require passage through vertebrate hosts as part of their life cycle. Tick-borne infections in humans, farm animals, and companion animals are primarily associated with wildlife animal reservoirs. Many tick-borne infections in humans involve a complex cycle between wildlife animal reservoirs and tick vectors. The survival and transmission of these tick-borne viruses are closely linked to their interactions with tick vectors and host cells. These viruses are classified into different families, including Asfarviridae, Reoviridae, Rhabdoviridae, Orthomyxoviridae, Bunyaviridae, and Flaviviridae.
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 makes up the species complex of Borrelia burgdorferi sensu lato. The complex currently comprises 20 accepted and 3 proposed genospecies. B. burgdorferi sensu stricto exists in North America and Eurasia and until 2016 was the only known cause of Lyme disease in North America. B. burgdorferi are often mistakenly described as Gram negative because of their two external membranes, but they lack lipopolysaccharide and possess many surface lipoproteins, unlike true Gram-negative bacteria.
A canine vector-borne disease (CVBD) is one of "a group of globally distributed and rapidly spreading illnesses that are caused by a range of pathogens transmitted by arthropods including ticks, fleas, mosquitoes and phlebotomine sandflies." CVBDs are important in the fields of veterinary medicine, animal welfare, and public health. Some CVBDs are of zoonotic concern.
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 processes. Anaplasmosis can also be referred to as "yellow bag" or "yellow fever" because the infected animal can develop a jaundiced look. Other signs of infection include weight loss, diarrhea, paleness of the skin, aggressive behavior, and high fever.
The discipline of medical entomology, or public health entomology, and also veterinary entomology is focused upon insects and arthropods that impact human health. Veterinary entomology is included in this category, because many animal diseases can "jump species" and become a human health threat, for example, bovine encephalitis. Medical entomology also includes scientific research on the behavior, ecology, and epidemiology of arthropod disease vectors, and involves a tremendous outreach to the public, including local and state officials and other stake holders in the interest of public safety.
Transovarial or transovarian transmission occurs in certain arthropod vectors as they transmit pathogens from parent to offspring. This process, used by a wide variety of parasites, is also known as vertical transmission. For example, Rickettsia rickettsii, carried within ticks, is passed on from parent to offspring tick by transovarial transmission. This is in contrast to parasites such as Rickettsia prowazekii, which are not passed on by transovarian transmission due to killing the vector that carries it. Other examples of parasites that use this mechanism of transmission include the aedes mosquito vector of the yellow fever virus and in phlebotomine sandflies that transmit pappataci fever.
Ehrlichia chaffeensis is an obligate intracellular, Gram-negative species of Rickettsiales bacteria. It is a zoonotic pathogen transmitted to humans by the lone star tick. It is the causative agent of human monocytic ehrlichiosis.
Human granulocytic anaplasmosis (HGA) is a tick-borne, infectious disease caused by Anaplasma phagocytophilum, an obligate intracellular bacterium that is typically transmitted to humans by ticks of the Ixodes ricinus species complex, including Ixodes scapularis and Ixodes pacificus in North America. These ticks also transmit Lyme disease and other tick-borne diseases.
Mosquito-borne diseases or mosquito-borne illnesses are diseases caused by bacteria, viruses or parasites transmitted by mosquitoes. Nearly 700 million people contract mosquito-borne illnesses each year, resulting in more than a million deaths.
Ticks are insects known for attaching to and sucking blood from land-dwelling animals. Ticks fall under the category of 'arthropod', and while they are often thought of in the context of disease transmission, they are also known to cause direct harm to hosts through bites, toxin release, and infestation. Infestation can cause symptoms ranging from mild to severe and may even cause death. Hosts can include any number of vertebrates, though humans and livestock are more likely to be the interest of researchers.
Ehrlichia canis is an obligate intracellular bacterium that acts as the causative agent of ehrlichiosis, a disease most commonly affecting canine species. This pathogen is present throughout the United States, South America, Asia, Africa and recently in the Kimberley region of Australia. First defined in 1935, E. canis emerged in the United States in 1963 and its presence has since been found in all 48 contiguous United States. Reported primarily in dogs, E. canis has also been documented in felines and humans, where it is transferred most commonly via Rhipicephalus sanguineus, the brown dog tick.
Rickettsia felis is a species of bacterium, the pathogen that causes cat-flea typhus in humans, also known as flea-borne spotted fever. Rickettsia felis also is regarded as the causative organism of many cases of illnesses generally classed as fevers of unknown origin in humans in Africa.
Ticks of domestic animals directly cause poor health and loss of production to their hosts. Ticks also transmit numerous kinds of viruses, bacteria, and protozoa between domestic animals. These microbes cause diseases which can be severely debilitating or fatal to domestic animals, and may also affect humans. Ticks are especially important to domestic animals in tropical and subtropical countries, where the warm climate enables many species to flourish. Also, the large populations of wild animals in warm countries provide a reservoir of ticks and infective microbes that spread to domestic animals. Farmers of livestock animals use many methods to control ticks, and related treatments are used to reduce infestation of companion animals.
Anaplasma bovis is gram negative, obligate intracellular organism, which can be found in wild and domestic ruminants, and potentially a wide variety of other species. It is one of the last species of the Family Anaplasmaceae to be formally described. It preferentially infects host monocytes, and is often diagnosed via blood smears, PCR, and ELISA. A. bovis is not currently considered zoonotic, and does not frequently cause serious clinical disease in its host. This organism is transmitted by tick vectors, so tick bite prevention is the mainstay of A. bovis control, although clinical infections can be treated with tetracyclines. This organism has a global distribution, with infections noted in many areas, including Korea, Japan, Europe, Brazil, Africa, and North America.
Global climate change has increased the occurrence of some infectious diseases. Infectious diseases whose transmission is impacted by climate change include, for example, vector-borne diseases like dengue fever, malaria, tick-borne diseases, leishmaniasis, zika fever, chikungunya and Ebola. One mechanism contributing to increased disease transmission is that climate change is altering the geographic range and seasonality of the insects that can carry the diseases. Scientists stated a clear observation in 2022: "The occurrence of climate-related food-borne and waterborne diseases has increased ."
Elizabeth Ann McGraw is an American biologist who is a professor in entomology at Pennsylvania State University. She is the Director of the Center for Infectious Disease Dynamics and a Huck Scholar in Entomology. Her research investigates the bacterium Wolbachia as a strategy for biocontrol and to better understand the basis of its interactions with insects. She was elected a Fellow of the American Society for Microbiology.