Playing the field The sexual habits of ladybirds could shed light on the spread of sexually transmitted diseases in human populations.
Although sexually transmitted diseases (STDs) are very common among animals and humans, we actually know very little about how they spread in populations," says Dr Robert Knell at the School of Biological Sciences at Queen Mary and Westfield College in London. "There are plenty of theories, but they've never been tested in an experimental model."
Theories about the transmission spread of 'typical' infectious diseases, such as measles, assume that the rate at which they spread is directly influenced by the density of infectious and susceptible individuals in a population. "That means you can lower the number of susceptible hosts - either by slaughtering them, as people did for foot and mouth, or by vaccinating - to a certain threshold, below which the disease won't be able to transmit itself," says Dr Knell. "So if we vaccinate 95 per cent of the population with MMR, we don't have any measles at all, because there are so few susceptible individuals that the disease cannot sustain itself in the population."
Because of the way STDs are spread, however, models of STD transmission are based on a different model. Their spread is dependent on the number of sexual encounters involving an infected partner, which is thought to be dependent not on densities but on the number of animals that have the STD (i.e. on the proportion of infected individuals within the population). "If that model is correct, it means you won't be able to eliminate an STD simply by reducing the proportion of susceptible hosts, as you can with normal infectious diseases," says Dr Knell.
He also points out that this proportion-based model assumes that the rate of partner change stays the same, regardless of the number of possible partners available. "If you change sexual partners at a set rate, your likelihood of getting infected is related to the proportion of infected individuals in the population. But is that really what happens in populations?
We suspect that as the density of populations increased - in other words, as populations grow and there are more hosts per unit area - the mating rate increases too. And that will affect the transmission dynamics of STDs in the population."
This is the hypothesis that Dr Knell and colleagues now aim to test. A couple of years ago a colleague of Dr Knell's at University College London, Dr Greg Hirst, published a description of a ladybird system, which could be used in the laboratory as a model to test ideas about STD population biology in general. "Ladybirds have a very simple mating system where the males walk around and if they come across a female they try and mate with it," says Dr Knell. "The STD we're looking at is a parasite, a mite called Coccipolipus hippodamiae, which lives under the ladybird's wing cover and jumps from one ladybird to the next during mating. The mite punches a stylet through the insect's skin into its blood and feeds on the blood."
Interestingly, the sexual behaviour of ladybirds appears to differ across Europe. English ladybirds are less promiscuous than their continental counterparts, while Polish ladybirds are particularly sexually active.
The experimental part of the project is actually quite straightforward. "The basic experimental design is that we put laboratory cultures of infected and uninfected ladybirds together, at different densities, and at the end of the experiment we'll know how many of those susceptible individuals have become infected, so we'll be able to analyse the transmission rate."
As well as the impact of behaviour on transmission, his team also aims to look at how changes in physiology, such as the immune response, can affect transmission. Recent research suggests that animals might respond to a higher population density by up-regulating their immune response, which will have an impact on the transmission of the parasite. The team will measure the haemocyte count, the blood cells in insects that mediate the immune response, and the presence of phenoloxidase, an enzyme important in making melanin. Since increased amounts of melanin harden the insect's cuticle (outer skin), possibly affording some protection against the mite's stylet, it could indicate an up-regulation of the insect's immune response.
Data from the study will be fed into a mathematical model, enabling Dr Knell and his team to calculate the relationship between transmission, population density and infection.
The fundamental question the team will be asking is whether the transmission rate is related to the proportion of infected hosts, or to changes in the density of the host population: whether, in fact, opportunity proves too good to resist. In Dr Knell's view, the most likely scenario will fall somewhere between these two models, but the results of this very first experimental study of STD transmission will hopefully lead to a better understanding of STD population dynamics in both animals and people.
|Dr Rob Knell at the School of Biological Sciences, Queen Mary, University of London: Research interests|
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