Enhancement of Virus Transmission by Tick Salivary Glands

Abstract

1895 SUMMARYPrevious studies have demonstrated that Thogoto (THO) virus is transmitted from infected to uninfected ticks cofeeding on an uninfected guinea-pig, although the guinea-pig does not develop a detectable viraemia. To investigate this mode of transmission, guinea-pigs were infested with uninfected Rhipicephalus appendiculatus nymphs prior to inoculation with either a mixture of THO virus and tick salivary gland extract, or with THO virus alone. The number of ticks that acquired the virus from feeding on animals inoculated with a mixture of virus and salivary gland extract was 10-fold greater than the number that became infected by feeding on animals inoculated with virus alone. The increase in the number of ticks that became infected was greatest when the salivary glands used in the inoculum were derived from uninfected ticks, which had partially fed for a period of 6 days. Viraemia was not detected in any of the guinea-pigs tested during the experiments. These results indicate that THO virus transmission is enhanced by factor(s) associated with the salivary glands of ticks, and that these factor(s) may facilitate 'non-viraemic' transmission between infected and uninfected ticks.In nature, infected and uninfected vectors of arthropod-transmitted viruses must frequently feed together on the same vertebrate host. In general, the host is considered to take part in the virus transmission cycle if it becomes infected and develops viraemia, providing a virus-laden blood-meal for the vector. On this premise, animals are considered important in the epidemiology of an arthropod-borne virus disease if they develop a viraemia that satisfies the threshold level considered necessary to infect the arthropod vector (Hardy et al., 1983). However, in the laboratory we have demonstrated that transmission of a tick-borne virus can occur without the vertebrate developing a detectable viraemia. Moreover, in these studies 'nonviraemic transmission' (NVT) was more efficient than classical 'viraemic transmission' (Jones et al., 1987). This study was undertaken to investigate the mechanism of NVT.Experiments were conducted with Thogoto (THO) virus, a virus of medical and veterinary significance that is structurally and morphogenetically similar to the influenza viruses (Haig et al., 1965;Clerx et al., 1983;Davies et al., 1986). The virus was originally isolated from ticks collected in Kenya (Haig et al., 1965) and has subsequently been detected throughout central Africa and in parts of the Middle East and southern Europe (Davies et al., 1986).THO virus is transmitted biologically by the African three-host ixodid tick species, Rhipicephalus appendiculatus (Davies et al., 1986). A laboratory colony of R, appendiculatus was maintained by feeding the ticks on guinea-pigs as previously described (Jones et al., 1988). The inter-feeding stages were maintained in perforated tubes held inside a desiccator at a temperature of 28 °C and at 809/oo relative humidity. The Sicilian SiAr isolate of THO virus (Albanese et al., 1972) was used t...

Viraemic transmission of Crimean-Congo haemorrhagic fever virus to ticks

“…Recently, however, Jones and colleagues [20] have shown transmission of Thogoto virus to uninfected Rhipicephalus appendiculatus which were co-fed with infected ticks on hosts in which viraemia was undetectable. This 'non-viraemic' transmission was enhanced by factors present in tick salivary glands [21]. Whether such transmission occurs with CCHF or the related Dugbe virus has not been conclusively established [7,22].…”

“…The possibility that such transmission may be augmented in the wild by other forms of transmission, such as non-viremic transmission mediated by salivary gland factors [21,22], cannot be discounted and requires further study.…”

Impact of an ectoparasite on the immunity and pathology of an invertebrate: Evidence for host immunosuppression and viral amplification

“…It is well documented that some ectoparasites, especially ticks (1-4), immunosuppress their vertebrate hosts, and that the immunocompromised vertebrate hosts have been shown to be more susceptible to infectious diseases (5)(6)(7)(8). Little is known, however, of how an ectoparasite affects the immunity and pathology of its invertebrate hosts.…”

Sandfly Maxadilan Exacerbates Infection with Leishmania major and Vaccinating Against It Protects Against L. major Infection

“…While enhanced blood flow insures the feeding success of the arthropod, inhibiting the immune response of the host may prevent the host from becoming sensitized to the bite of the arthropod. However, there is now mounting evidence that the saliva of an arthropod vector can also enhance the infectivity of pathogens that the arthropod transmits (5)(6)(7)(8)(9)(10). Therefore, injecting arthropod-borne pathogens by syringe does not mimic natural transmission.…”