Abstract
Rift Valley fever virus (RVFV) is transmitted among susceptible animals by mosquito vectors. Although the virus can be isolated from nasal and oral swabs of infected animals and is known to be highly infectious when administered experimentally via oral or respiratory route, horizontal transmission of the virus is only sporadically reported in literature. We considered that immunosuppression resulting from stressful conditions in the field may increase the susceptibility to horizontally transmitted RVFV. Additionally, we reasoned that horizontal transmission may induce immune responses that could affect the susceptibility of contact-exposed animals to subsequent infection via mosquito vectors. To address these two hypotheses, viremic lambs were brought into contact with sentinel lambs. One group of sentinel lambs was treated with the immunosuppressive synthetic glucocorticosteroid dexamethasone and monitored for signs of disease and presence of virus in the blood and target organs. Another group of contact-exposed sentinel lambs remained untreated for three weeks and was subsequently challenged with RVFV. We found that none of the dexamethasone-treated contact-exposed lambs developed detectable viremia, antibody responses or significant increases in cytokine mRNA levels. Susceptibility of immunocompetent lambs to RVFV infection was not influenced by previous contact-exposure. Our results are discussed in light of previous findings.
Highlights
Rift Valley fever virus (RVFV) causes considerable morbidity and mortality among domesticated ruminants and occasionally humans
We investigated the epidemiological significance of horizontal transmission by co-housing infected lambs with dexamethasone-treated lambs and by evaluating the susceptibility of immunocompetent lambs to experimental RVFV infection following contact exposure
To study if exposure to viremic lambs results in disease in immunocompromised lambs, 4 lambs were experimentally infected with RVFV via the intravenous route (Figure 1A, red group) and brought into contact with eight dexamethasonetreated sentinel lambs (Figure 1A, orange group) the day after inoculation
Summary
Rift Valley fever virus (RVFV) causes considerable morbidity and mortality among domesticated ruminants and occasionally humans. RVFV has a very broad host range, but domesticated ruminants play a key role in the epidemiology of the disease by developing sufficiently high viremia to allow transmission to susceptible mosquitoes. Future incursions into previously unaffected areas could have dramatic consequences as mosquito species associated with transmission of the virus in endemic areas are globally prevalent (Bird et al, 2009; Pepin et al, 2010). The earliest and perhaps most elegant studies on RVFV transmission were performed by Daubney and Hudson, who first characterized and named the disease (Daubney et al, 1931). Subsequent studies by Findlay revealed that experimental infection can be achieved by intramuscular, intravenous, subcutaneous, intraperitoneal, intracerebral, intranasal, or conjunctival inoculation (Findlay, 1932). Inhalation of virus-containing aerosols was found to result in infection (Brown et al, 1981; Reed et al, 2013; Hartman et al, 2014)
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