Vasculitis and infectious diseases.

Vasculitis usually presents without a well-known underline cause (idiopathic vasculitis), nevertheless, it is sometimes possible to find out one or more causative agents (secondary vasculitis). Nowadays, thanks to the increasing amount of precise diagnostic tools, a piece of idiopathic vasculitis is reclassified as associated with probable etiology, which can be set off by several factors, such as infections. Infections are considered to be the most common cause of secondary vasculitis. Virtually, every infectious agent can trigger a vasculitis by different mechanisms which can be divided in two main categories: direct and indirect. In the former, infectious agents destroy directly the vascular wall leading, eventually, to a subsequent inflammatory response. In the latter, indirect form, they stimulate an immune response against blood vessels. Different infectious agents are able to directly damage the vascular wall. Among these, it is possible to recognize Staphylococcus spp, Streptococcus spp, Salmonella spp, Treponema spp, Rickettsia spp, Cytomegalovirus, Herpes Simplex Virus 1 and 2, and many others which have a peculiar tropism for endothelial cells. Conversely, another group of microbial agents, such as Mycobacterium tuberculosis, Mycobacterium leprae, Hepatits B Virus, Human Immunodeficiency Virus and others, trigger vasculitis in the indirect way. This is due to the fact that they can share epitopes with the host or modify self-antigens, thus leading to a cross-self reaction of the immune system. These mechanism, in turn, leads to immunological responses classified as type I-IV by Gell-Coombs. Nevertheless, it is difficult to strictly separate the direct and indirect forms, because most infectious agents can cause vasculitis in both ways (mixed forms). This paper will analyze the link between infectious agents and vasculitis, focusing on direct and indirect secondary vasculitis, and on a group of probable infection-related idiopathic vasculitis, and finally on a group of idiopathic vasculitis with microbiological triggers. Furthermore, a diagnostic and therapeutic approach to vasculitis when an underline infection has been suspected is suggested.