Generating super-shedders: co-infection increases bacterial load and egg production of a gastrointestinal helminth

Abstract

Co-infection by multiple parasites is common within individuals. Interactions between co-infecting parasites include resource competition, direct competition and immune-mediated interactions and each are likely to alter the dynamics of single parasites. We posit that co-infection is a driver of variation in parasite establishment and growth, ultimately altering the production of parasite transmission stages. To test this hypothesis, three different treatment groups of laboratory mice were infected with the gastrointestinal helminth Heligmosomoides polygyrus, the respiratory bacterial pathogen Bordetella bronchiseptica lux(+) or co-infected with both parasites. To follow co-infection simultaneously, self-bioluminescent bacteria were used to quantify infection in vivo and in real-time, while helminth egg production was monitored in real-time using faecal samples. Co-infection resulted in high bacterial loads early in the infection (within the first 5 days) that could cause host mortality. Co-infection also produced helminth 'super-shedders'; individuals that chronically shed the helminth eggs in larger than average numbers. Our study shows that co-infection may be one of the underlying mechanisms for the often-observed high variance in parasite load and shedding rates, and should thus be taken into consideration for disease management and control. Further, using self-bioluminescent bacterial reporters allowed quantification of the progression of infection within the whole animal of the same individuals at a fine temporal scale (daily) and significantly reduced the number of animals used (by 85%) compared with experiments that do not use in vivo techniques. Thus, we present bioluminescent imaging as a novel, non-invasive tool offering great potential to be taken forward into other applications of infectious disease ecology.

Figure 1.

Correlation between light output (measured in relative light units) and colony-forming units in hosts infected with self-bioluminescent Bordetella bronchiseptica lux⁺.

Figure 2.

Bacterial load over time of individual mice infected with a respiratory bacterium, as measured by photons per unit area (photons s⁻¹ cm⁻²) emitted by self-bioluminescent Bordetella bronchiseptica (B. bronchiseptica lux⁺) from a subsample (n = 3) of a larger treatment group of BALB/c mice (n = 10) over a 22-day period. The bacterial load is represented by the colours displayed on a rainbow scale, where violet is assigned to the lowest light output and red to the highest, allowing easy identification of bright light regions.

Figure 3.

Bacterial load of individual mice over time co-infected with a respiratory bacterium and a gastrointestinal helminth, as measured by photons per unit area (photons s⁻¹ cm⁻²) emitted by self-bioluminescent Bordetella bronchiseptica (B. bronchiseptica lux⁺) from a subsample (n = 3) of a larger treatment group of BALB/c mice (n = 10) over a 22-day period that were co-infected with the helminth, Heligmosomoides polygyrus. The bacterial load is represented by the colours displayed on a rainbow scale, where violet is assigned to the lowest light output and red to the highest, allowing easy identification of bright light regions. Note on day 4 the lux measurement of the animal in position 3 was 2 s.d.s higher than average and it met the criteria for removal from the experiment.

Figure 4.

Survivorship curves over 300 days of three different treatment groups (n = 10 per groups); infection with the respiratory bacterium Bordetella bronchiseptica (dotted line), infection with the gastrointestinal helminth Heligmosomoides polygyrus (dashed line) and co-infection with both B. bronchiseptica and H. polygyrus (solid line).

Figure 5.

Shedding of helminth eggs in mice infected with a helminth-only or co-infected with a bacterium and helminth. (a) Mean number of EPG host faeces ± s.e. Black symbols represent hosts infected with the gastrointestinal helminth Heligmosomoides polygyrus only, white symbols are hosts simultaneously infected with H. polygyrus and the respiratory bacterium Bordetella bronchiseptica. (b) Number of eggs shed for each host separately. Solid lines represent hosts infected with H. polygyrus only, dashed lines are hosts simultaneously infected with H. polygyrus and B. bronchiseptica.

Figure 6.

Helminth numbers and per capita number of eggs shed from mice that have been infected with either the gastrointestinal helminth Heligmosomoides polygyrus alone (black bars/symbols) or co-infected with the helminth and the respiratory bacterium Bordetella bronchiseptica (white bars/symbols). Groups of mice were euthanized and dissected 12, 24 or 48 days after infection. (a) Numbers of adult H. polygyrus in mice infected with the helminth-only or co-infected. (b) Number of eggs per adult helminth in single and co-infected mice.