Juice of Citrullus lanatus var. citroides (wild watermelon) inhibits the entry and propagation of influenza viruses in vitro and in vivo.

Vaccines and various anti-influenza drugs are clinically used to prevent and treat influenza infections. However, with the antigenic mismatch of vaccines and the emergence of drug-resistant viral strains, new approaches for treating influenza are warranted. This study focused on natural foods as potential candidates for the development of new treatment options for influenza infections. The screening of plants from the Cucurbitaceae family revealed that the juice of Citrullus lanatus var. citroides (wild watermelon) had the strongest ability to inhibit the replication of influenza virus in Madin-Darby canine kidney cells. The results of a time-of-addition assay indicated that wild watermelon juice (WWMJ) inhibits the adsorption and late stages of viral replication, suggesting that WWMJ contains multiple constituents with effective anti-influenza activity. A viral adsorption analysis showed that WWMJ reduces the amount of viral RNA in the cells at 37°C but not at 4°C, confirming that WWMJ inhibits viral entry into the host cells at 37°C. These results suggest that a mechanism other than the inhibition of viral attachment is involved in the anti-influenza action of WWMJ, which is perhaps responsible for a reduction in internalization of the virus. Administration of WWMJ into the nasal mucosa of BALB/c mice infected with the A/PR/8/34 mouse-adapted influenza virus was seen to significantly improve the survival rate. The findings of this study, therefore, demonstrate the anti-influenza potential of WWMJ in vitro and in vivo, thereby suggesting the candidature of WWMJ as a functional food product that can be used to develop anti-influenza agents and drugs.