Polyparasitism

Sirs—The recent Raso et al. paper¹ addresses a phenomenon that is important, familiar, but neglected. Explicit studies of polyparasitism should be common: polyparasitism is common in the world, though seldom acknowledged. Figure 1 summarizes seven reports published in 1991–96 that include data on polyparasitism.^2–8 At 7 of the 11 study sites, more than half the sample population harboured a polyparasitic infection.

Figure 1.

Prevalence of mono- and polyparasitic infections at 11 sites

Even these sparse results point to deeper questions, e.g. why those seven sites include three of the four at which three parasite species were investigated, but only one of the two at which eleven were investigated. Studies that simply compare the observed with the expected prevalence of a specific polyparasitic infection—or present data that allow others to do so—can provide insights to guide intervention, e.g. for geohelminth infections,^9–10 or malaria.^11–12 But such studies are still rare.

It may be that only a few parasite species interact, in epidemiological or clinical terms, and that in most circumstances it is wise to focus on one infection and ignore the others. Differential diagnosis has proven an enormously valuable principle, and the postulates Koch developed—after concluding that ‘work with impure cultures yields nothing but nonsense’¹³—remain the standard for demonstrating a causal relationship between a microbe and a disease. But it was impure cultures—thanks to Fleming’s sloppiness, luck, and sharp eye—that led to penicillin. Wagner-Jauregg’s observation that syphilis patients were sometimes ‘healed through intercurrent infectious diseases,’ led him to ‘imitate this experiment of nature’¹⁴—which led to a Nobel Prize, and the treatment of thousands in malariatherapy clinics.¹⁵ Certainly Jenner could not have known that he was taking advantage of microbial interactions when he used the virus of cowpox as ‘vaccinia’ to protect against the virus of smallpox. In recent times, analysis of the accelerating co-incidence of familiar STDs might have helped us recognize more promptly that HIV was emerging, facilitating, and exacerbating other infections.¹⁶

Polyparasitism is widespread. Its combinatorics and complexity are daunting, but, for parasite species interconnected through immunity or transmission, likely to confound routine approaches to research. Raso et al. remind us that ‘multiple species parasitic infections are the norm rather than the exception,’ and therefore that polyparasitism deserves much more attention.