Plasmodium infection

Dhondt, Andre ́ A., Dhondt, Keila V. & Nazeri, Sophie, 2017, Apparent effect of chronic Plasmodium infections on disease severity caused by experimental infections with Mycoplasma gallisepticum in house finches, International Journal for Parasitology: Parasites and Wildlife 6 (2), pp. 49-53 : 50-51

3.3. Horizontal transmission and Plasmodium infection

3.3.1. Time to fi rst infection

To determine the time to first infection through horizontal transmission of each naïve individual we determined what day post

CA2008 NC2008 CA2015

No Plasmodium (n = 2) 17.62 ± 1.17 19.26 ± 1.52 15.99 ± 2.05 Plasmodium (n = 2) 15.74 ± 9.80 21.65 ± 21.65 a 33.08 ± 5.04

a One of the index birds, although it seroconverted, did not develop disease and hence would not have transmitted M. gallisepticum . The summed M. gallisepticum - load of the 2nd bird in the same aviary was 43.29.

introduction (PI) of M. gallisepticum in the aviary each individual developed any sign of infection: eye lesions (eye score>0), M. gallisepticum -DNA present in the eye swab, or evidence of presence of antibodies if neither other sign of infection was observed.

Survival analysis showed that the three M. gallisepticum strains differed significantly in transmission rates (χ 2 = 13.32, df = 2, P = 0.0013). Transmission was significantly slower in the group with CA2008 than in both other groups (NC2008 versus CA2008: χ 2 = 8.39, df = 1, P = 0.004; CA2015 versus CA2008: χ 2 = 8.20, df = 1, P = 0.004), but there was no significant difference between CA2015 and NC2008 (χ 2 = 0.28, df = 1, P = 0.60). The lower transmission in the groups exposed to CA2008 was primarily caused by the fact that by day 59 PI, when the experiment was terminated, half of the naïve birds still showed no signs of having become infected, while in the other groups more birds showed evidence of having been infected by M. gallisepticum .

While the time to infection did differ between groups exposed to different M. gallisepticum isolates, there was no effect of Plasmodium : birds with and without Plasmodium did not differ in time to infection in any of the groups (all P> 0.13).

3.3.2. Severity of disease if infected

Some birds did not develop any signs of infection. Because we wanted to address the question how birds responded if infected by horizontal transmission we used only those birds that either developed eye lesions or in which M. gallisepticum -load in the conjunctiva was non-zero. This reduced the sample sizes to 10/20 for CA2008, 16/20 for NC2008 and 18/20 for CA2015.

In order to answer questions 2 (effect of Plasmodium ?) and 3 (effect of M. gallisepticum strain used?) we performed a two-way analysis of variance with interaction to determine if either M. gallisepticum -strain or the presence of Plasmodium impacted M. gallisepticum -load and/or eye lesions. Given that the interaction term was not significant for M. gallisepticum -load (P = 0.80) nor for eye lesions (P = 0.12) we only report the results without interaction ( Table 2) and illustrate the change of M. gallisepticum -load and eye lesions for one M. gallisepticum -strain over time in Fig.1 . The results show that the response to infection through horizontal transmission in a group varied significantly between M. gallisepticum - strains to which the birds were exposed (P <0.0001) but also that birds with a chronic Plasmodium infection developed more severe clinical disease (P <0.03).

Figures

Fig. 1. Mean severity of eye lesions ± SE (circles), and M. gallisepticum-load (log (qPCR+1)) (triangles) of birds infected through horizontal transmission with the CA2015 strain of M. gallisepticum. Birds in which Plasmodium was detected by PCR (grey symbols) developed more severe disease than birds in which Plasmodium was not found (white symbol). As there is no significant interaction between M. gallisepticum ‾strain and Plasmodium infection (Table 2) the results for the other M. gallisepticum strains are qualitatively similar and therefore not shown.