Bird flu may be a sexually transmitted infection – at least in ducks. That’s the suggestion of an analysis of flu prevalence and mating behaviour that also proposes an easy way to spot duck populations most at risk of harbouring avian flu.

Surprisingly, the more rampant the sex a particular duck species indulges in, the lower the chance of spreading the virus. It’s all to do with penis size and the complexity of the females’ vagina.

Lethal strains of avian flu virus can evolve from harmless versions and then jump to other species, so it is important to also monitor less dangerous strains in wild birds. Ducks are the main wild hosts of bird flu, but surveillance is difficult without easy markers of infection risk.

Now Gergely Hegyi at Eötvös Loránd University in Budapest, Hungary, and colleagues think they may have found just such a marker: the “covert” wing patches that ducks display during mating. This finding builds on the previous work by the team, in which they found that species with large penises tend to have smaller wing patches, which are also more likely to be a single colour.


Sexual arms race

Most ducks form monogamous pairs but the males also often force sex on females because, unlike most birds, they have a phallus. This has led to an evolutionary “arms race” between the sexes, with males evolving longer penises the better to inseminate the female, and females responding by evolving more complex vaginal anatomy to make insemination by unwelcome partners less likely.

When the researchers compared data on the prevalence of low-pathogenic bird flu strains in different duck species with what is known about the anatomy of duck reproductive parts and mating behaviour, they found that ducks with the smallest penises and tamest sex lives had the highest flu levels.

“This is intriguing and a bit counter-intuitive because a long phallus prolongs copulation, and forced copulations characteristic to species with a large phallus should further promote virus transfer,” says Hegyi.

The explanation may lie in the counter-adaptation of the female ducks. “Long and elaborate vaginas may hinder unwanted fertilisation but may also make it difficult for viruses acquired during copulation to reach the site of egg formation,” Hegyi says.

Infection peak

This idea might help to explain why infections often peak during the breeding season. The researchers think that mating may spread flu by pushing water contaminated with the virus onto developing eggs and so infect chicks before or during hatching – and that this will happen more readily in ducks with less complex vaginas.

Bird flu specialist John McCauley at the National Institute for Medical Research in London says that the idea of sexual flu transmission and its dependence on reproductive anatomy and copulation habits is interesting. “It’s quite feasible that they are right, but it will be important to make sure that the epidemiological evidence is robust and not biased by differences in sampling methods.”

Because duck species with small penises have larger and more colourful wing patches, Hegyi hopes that confirmation of the link between sex and flu might help to identify which wild populations are most likely to spread bird flu. Monitoring effort can then be concentrated on these populations.

In addition, recognition of sexual transmission “may require different biosafety measures in captive birds than those currently implemented,” he says.

Journal reference: Behavioral Ecology, DOI: 10.1093/beheco/arp133