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I was studying the life cycle of Plasmodium, and I read that the infectious form (sporozoites) is what causes malaria in humans. I also read that this form resides in the salivary glands of the female Anopheles(carrirer of Plasmodium). My question is that if the sporozoites reside in the salivary glands, do they affect the mosquito itself? Or, in general, do the pathogens attack, or affect the disease vectors in any way?

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tl;dr yes, Plasmodium does appear to interact with its mosquito vectors — many species of mosquito have reduced fitness when infected with plasmodia due to the cost of mounting an immune response (this implies that they would lose even more fitness if they didn't mount an immune response ...). However, mosquito species that are frequently infected in the wild appear to have evolved tolerance, so they incur less fitness cost (there is even a positive correlation between infection intensity and fitness in some cases (for not-fully-understood reasons).

From Shaw et al 2022:

Numerous studies using rodent model species have detected a fitness cost of infection to mosquito fecundity [26–29], fertility [27,28], and survival [30], most likely due to the considerable investment by the infected female in immune responses (Box 1). When Anopheles stephensi or An. gambiae were infected with Plasmodium yoelii nigeriensis, there was a reduction in fecundity of 21–41% [26–28], but with no significant correlation between egg numbers and oocyst loads [26,28]. The reduction in fecundity was due to apoptosis of follicular epithelium cells, promoting resorption of ovarian follicles in infected females [31,32]. The density independence between egg and oocyst numbers suggests an all-or-nothing cost to activation of the mosquito immune system preventing significant egg investment [33,34]. An. stephensi also showed reduced survival when infected with a number of rodent malaria parasite species [26,35–37]. Compared with uninfected controls, mortality rates were significantly higher when mosquitoes were infected with P. berghei or with P. yoelii nigeriensis, while infections with P. chabaudi reduced median survival (with parasite strain- and experiment-dependent effects).

On the other hand, a meta-analysis on 11 published studies found that in infections involving P. falciparum and its natural mosquito vectors – An. gambiae, Anopheles funestus, and An. stephensi – no costs to survival were detected [38]. Similarly, in recent studies, An. gambiae and An. stephensi females infected with P. falciparum developed the same number of eggs as their uninfected sisters [39]. In An. gambiae, an unexpected positive correlation existed between paired egg and oocyst numbers from individual infected mosquitoes, with females that invest more in reproduction supporting higher infection intensities [40]. ...

Shaw, W. Robert, Perrine Marcenac, and Flaminia Catteruccia. “Plasmodium Development in Anopheles: A Tale of Shared Resources.” Trends in Parasitology 38, no. 2 (February 2022): 124–35. https://doi.org/10.1016/j.pt.2021.08.009.

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It does not because it causes no structural damage to mosquito in either stage i.e while penetrating into endolymph via gut and also while penetrating the salivary gland since there is complimentary protein ligand (on surface of gut and on surface of salivary gland) which facilitates its entry without causing any harm, on the other hand some research even goes to the extent that it's mutually beneficial since it enhances the smelling capacity of mosquito thus their relationship is advantageous hence sustained in nature over the course of evolution.

https://phys.org/news/2021-11-mosquitoes-mutual-symbiotic-relationship-malaria-causing.html

https://www.pnas.org/doi/10.1073/pnas.241491198

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