Vector-borne pathogen transmission through tick saliva

The most important aspect of the tick significance for livestock and wildlife is that they transmit many infectious agents as vectors, in addition to the piercing the host’s skins.

Examples of some important clinical tick-borne infectious agents in livestock are Anaplasma marginale causing the disease bovine anaplasmosis, Babesia bigemina causing the disease bovine babesiosis or African redwater, Ehrlichia rumanntium causing heartwater and strains of Theileria parva causing East Cost Fever, Corridor Disease and January Disease. The carriers of these diseases are mostly the wildlife species, with an additional importance to cause the zoonotic diseases in humans.

Salivary secretions transmit most of the pathogens, although the argasid ticks can transmit vectored pathogens through coxal fluid which is used for irrigation of the feeding lesions. The efficacy of transmission of a diseased pathogen by ticks to the host can be determined by the amount of secreted saliva because salivary glands are the most important route of pathogen transmission.

After feeding on blood meal, the pathogens reside in the gut contents with ingested blood having different mechanisms to pass through the membranes of gut. Laboratory trials have indicated that certain pathogens may find their way to the gut cells through the expression of molecules that allow to recognize specific receptors in the cell membrane.

Some well-known tick species transmit pathogens to humans and they have been very well studied due their public health significance. In this case, the example of Ixodes (I.) ricinus can be given which transmits a bacterium Borrelia (B.) burgdorferi in many regions. Same is the case with Hyalomma genus of ticks because it transmits the most important disease Crimean Congo Hemorrhagic Fever (CCHF) to humans, thus researchers have devoted them for its investigation.

However, some tick species transmit pathogens in cryptic cycles without their direct involvement, and ultimately pathogen transmission becomes perpetuated in the ecosystem. The active foci of pathogen transmission can be sustained in the ecosystem by these cryptic cycles. Thus, the clinical cases will not be noticed in the human population because of the absence of vector linkage between reservoir hosts and humans.

Trans-ovarial and Trans-stadial Transmission:

Epidemiologically, ticks have two important events to maintain the transmitted pathogens. The ticks feed only once at each stage, the pathogen need to persist at every stage after molting, so that it may be passed to the next vertebrate cohort by an infective bite. This means of stage to stage transmission is called trans-stadial passage.

In this phenomenon, the tick feeds on a reservoir host and acquires a pathogen, thus the pathogen has to further persist in the newly molted stage of the tick. Vertical or trans-ovarial passage is another important feature involved in the transmission of pathogen by ticks.

The tick acquires a persisted pathogen, while engorged female ticks transfer it to the cohort of eggs. This phenomenon is considered to be responsible for the transfer of many pathogens to the unaffected host and the new generation of ticks. The rate of the transfer of pathogens to the next generation of ticks is a species specific mechanism and both phenomena of pathogen transfer are dependent upon this mechanism.

The mechanism of transmission and maintenance of pathogens by the ticks and their hosts cannot only be determined by the field studies because such studies only report small sequence of nucleic acids from big population of ticks. Complementary laboratory studies should also be adopted as an essential part to testify the hypothesis made in the field trials.

The ticks should possess following qualities to be a successful vector of pathogens, i.e., (a) feeding on infectious vertebrates (b) pathogen acquiring during feeding on blood meal (c) maintenance of pathogen during single or more trans-stadial molts and (d) transmission of pathogen to previously unaffected hosts. The status of carrier or non-carrier state of the host is determined by the feeding of ticks and transmission of pathogen to the host.

According to this definition, pathogen carrying hosts are called the carrier hosts, however, it is not necessary that the carrier hosts are infected by the ticks, thus infectivity status must be defined by the terms reservoir and non-reservoir. Non-reservoir hosts are those which are unable to transmit the pathogens to un-infected ticks.

It is suggested that if the tick species responsible for pathogen transmission to wild vertebrates are present in the ecosystem, the presence of pathogens will be un-noticed until the availability of ticks that will link it to humans. Furthermore, seasonality and abundance of hosts, regional weather and variations in the abundance of ticks can change the epidemiological landscape by their interaction.

Therefore, a specific mechanism of pathogen transmission in a community of ticks, hosts and pathogens cannot be implemented in another community because these are realized as the regional or local processes.

Mechanism of Pathogen Transmission through Salivary Glands:

A pathogen has to pass through various major barriers during its transmission from one host to the other. First barrier is the vector’s contents of gut lumen that could be potentially destructive. Then the pathogen has to penetrate the gut epithelium which is another physical barrier.

Innate immune system is comprised of haemolymph of vector containing various substances which present another potentially harmful barrier for pathogen.

Further steps for the pathogen to reach another host are to cross the salivary gland epithelium and the chemical contents contained in the vector’s saliva. However, it has been suggested that the transmission of pathogen is supported by saliva instead of being a barrier. Numerous substances are found in the tick saliva which regulate the blood flow towards feeding lesion.

Understanding of pathogen transmission has proved to be helpful by the development of new techniques related to pharmacology and physiology of tick salivary secretions. Behaviors of vectors are assumed to have been affected a little by pathogens and need to be measured according to defined conditions.

As stated in a hypothesis that the rate of secretion of fluid is affected by the infection of pathogen to salivary glands. There are two reasonable hypotheses that have been made regarding this situation: (i) cellular damage of salivary glands can cause inhibition of salivary secretion by pathogen infection or (ii) salivation might be enhanced by the pathogen infection which is advantageous for pathogen. The second view is of more significance because many parasites increase the likelihood of pathogen transmission through manipulation of their host’s or vector’s physiology.

The hemostasis phenomenon is triggered by vascular injury, accompanied by a triad of platelet aggregation, blood coagulation and vasoconstriction. The mechanism of hemostasis is started within few seconds of tissue injury. Scar formation is induced by the mechanism of tissue repair which starts with the injury and continues for several days.

Cellular and humoral immune responses are triggered by the immune system that cause modification at the site of tick feeding. Such type of immune responses are immediate as seen in antigen/antibody/complement reactions, or may take longer periods in their occurrence similar to cellular reactions where leukocyte infiltrates are formed like basophilic infiltrates.

As Argasid and Ixodid ticks take from few hours to several days during feeding, various feeding strategies are involved to influence the host immune response. However, the activation of host immune system depends upon many factors such as health status, genetic background of the host and species of the ticks and hosts involved.

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