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The infectious agent that causes clinical signs in pigs most similar to Haemophilus parasuis is Streptococcus suis. Both of these organisms are known to be involved in similar disease processes in swine, particularly associated with systemic infections and conditions like polyserositis and meningitis, which manifest with similar clinical signs such as fever, joint swelling, and neurological symptoms.

Streptococcus suis is a significant pathogen in pigs, causing diseases that can resemble those caused by Haemophilus parasuis. In particular, they can both lead to outbreaks of diseases like Glässer's disease, characterized by serous inflammation of multiple body cavities, as well as neurological and joint issues.

While the other infectious agents listed—Fusobacterium necrophorum, Mycoplasma hyosynoviae, and Erysipelothrix rhusiopathiae—can also cause diseases in pigs, the clinical manifestations are distinct from those associated with Haemophilus parasuis. For instance, Fusobacterium necrophorum is primarily associated with necrotic laryngitis and liver abscesses, Mycoplasma hyosynoviae is linked to polyarthritis without the systemic signs typical of Streptococcus suis infections, and Erysipel