Bats have been identified as reservoirs for a number of zoonotic viruses, including Ebola virus, Henipah viruses, SARS-like coronavirus and Australian bat lyssavirus. Bats have the ability to transmit these viruses (a process known as spillover) to other susceptible species leading to severe illness and in some cases death. While these bat borne viruses may cause significant disease in their spillover host, bats remain asymptomatic and rarely show any clinical signs of disease. The molecular mechanism by which bats control viral infection is currently unknown and is therefore of significant value.

Mammalian breast milk is rich in nutrients, hormones, growth factors and immunoactive molecules, which influence the growth, development and immune status of a newborn infant. It is hypothesized that the nutritional function of milk evolved subsequent to its protective function; that is, the mammary gland evolved from the innate immune system.

The comprehensive proteomic analysis of Pteropid bat milk, presented in this study, revealed that it is very concentrated and rich in proteins, with the majority of typical milk proteins identified. The most distinctive and interesting feature of P.alecto milk was the over production of zinc-α-2-glycoprotein (ZAG). This protein was detectable, albeit in much lower level, in human milk but was not detected in milk of other mammals analysed, including an Australian microbat (Chalinolobus gouldii).

ZAG is a protein of interest due to its postulated multifunctional role, although it is still considered a protein with an unknown function. One role postulated is as a novel adipokine (lipid metabolism regulatory molecule), whereas another proposed role is, due to structural organization and fold similar to MHC class I antigen-presenting molecule, in the expression of the immune response.  Hence the overexpression of ZAG in bat milk may be related to the molecular mechanism by which bats control viral infection.