Trench fever is caused by a specific type of bacteria called *Bartonella quintana*. This bacterium is transmitted to humans primarily through the human body louse, an insect that lives on clothing and feeds on human blood. The body louse acts as a vector, meaning it carries the bacteria from one person to another, spreading the infection. Humans are the only known reservoir for *Bartonella quintana*, so the disease circulates mainly in human populations where body lice infestations are common.
The transmission process begins when a body louse feeds on an infected person, ingesting the bacteria along with the blood meal. The bacteria then multiply within the louse’s gut. When the louse bites another person, it defecates near the bite site. The infected louse feces contain *Bartonella quintana*, and when the person scratches the itchy bite, the bacteria enter the skin through small breaks or abrasions, leading to infection. Unlike some other vector-borne diseases, the bacteria are not transmitted directly by the bite itself but through contamination with louse feces.
Trench fever got its name during World War I, when it was a common and debilitating illness among soldiers living in the unsanitary, crowded conditions of the trenches, where body lice thrived. The disease causes symptoms such as recurring fever, severe headache, muscle pain (especially in the legs and back), and sometimes a rash. It is called “five-day fever” because the fever often recurs in cycles every five days.
Poor hygiene, overcrowding, and lack of access to clean clothing and bathing facilities create ideal conditions for body lice infestations, which in turn increase the risk of trench fever outbreaks. Today, trench fever is rare in the general population but can still occur in homeless populations or in situations where people live in crowded, unhygienic conditions.
In addition to body lice, recent research has suggested that other modes of transmission might exist, but the body louse remains the primary and most well-established vector. The disease does not spread directly from person to person without the involvement of lice.
The bacterium *Bartonella quintana* is a gram-negative bacillus that can survive inside human red blood cells and endothelial cells, which helps it evade the immune system and cause persistent infection. This intracellular lifestyle contributes to the relapsing nature of the fever and symptoms.
In summary, trench fever is caused by the bacterium *Bartonella quintana*, which is transmitted to humans through the feces of infected body lice. The disease is closely linked to poor sanitary conditions that favor lice infestations, and it primarily affects humans living in crowded or unhygienic environments.
