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Although vertebrate blood contains proteins, sugars, and lipids, as well as various micronutrients and minerals, it lacks the complete range of substances most organisms require to sustain life and to reproduce. Consequently, many of the animals, which derive most or all their nutrition from feeding on blood (haematophagy), have symbiotic relationships with bacteria that provide the missing substances, such as the B group of vitamins. The need for supplementary nutrients is particularly acute in blood sucking lice (sub‐order Anoplura) because they have lost the ability to lyse (break up) red blood cells, and therefore many nutrients will remain locked within these cells. In many cases, the symbiotic bacteria are held within special cells called mycetocytes that are grouped together to form an organ called a mycetome. Although these terms appear to indicate the involvement of fungi, they originate from a time when scientists could not distinguish between the presence of yeasts and bacteria within cells. Many scientists continue to use the term ‘mycetocyte’ regardless of the nature of the symbiont, but others use the term ‘bacteriocyte’ where it is known that the cells harbour only bacteria.

In blood‐feeding leeches belonging to the order Rhynchobdellida (there is a popular misconception that all leeches feed on blood; many of them are predatory), mycetomes surround or connect to the oesophagus. Mycetomes do not form in all blood‐feeding leeches, and in the medicinal leech, Hirudo medicinalis (Figure 12.1), the symbiotic bacteria live within the lumen of the gut (Graf et al. 2006). The bacteria present in H. medicinalis are Aeromonas veronii; earlier work on leeches often refers to this bacterium as Aeromonas hydrophila. Aeromonas veronii also forms associations with other blood‐feeding invertebrates, as well as vampire bats, but it can also live independently as a free‐living organism. Interestingly, both H. medicinalis and A. veronii produce antimicrobial peptides that suppress the growth of other microbes in the leech’s gut (Tasiemski et al. 2015). This reduces the diversity of the gut microbial flora and emphasises the close relationship between the two organisms. Aeromonas veronii is not always beneficial: in humans, it causes wound infections, septicaemia, and gastroenteritis. Blood‐feeding leeches are useful in modern medicine, particularly to aid wound drainage following reconstructive surgery, but there is a risk of them facilitating an Aeromonas infection in the patient. The infections are often trivial, but they can become serious and lead to abscesses or cellulitis. This is a difficult problem to solve because the symbiotic bacteria are essential for the long‐term survival of the leech. One cannot develop a strain or culture of Aeromonas‐free leeches. However, treating the leeches 1–4 weeks before use with an antibiotic such as ciprofloxacin removes the bacteria without compromising the willingness of the leech to feed (Mumcuoglu et al. 2010). A leech is only used once in reconstructive surgery because of their potential to transmit diseases between patients. Consequently, the long‐term survival of antibiotic‐treated leeches is not a concern.

The nymphs and adults of the human body louse Pediculus humanus humanus (sub‐order Anoplura) (Figure 7.11) have a symbiotic relationship with the gamma (γ) proteobacterium Riesia pediculicola – also referred to as Candidatus Riesia pediculicola. The term ‘Candidatus’ is used in prokaryote taxonomy for an organism that may be well characterised from molecular and other studies but cannot be cultured in the laboratory. Riesia pediculicola are primary intracellular endosymbionts that provide their hosts with pantothenic acid (vitamin B5) and are essential to the survival of the lice. In nymphs and adult male lice, the symbionts live within a mycetome that some texts refer to as the ‘stomach disc’. This is an unfortunate term because lice, like other insects, do not have a stomach in the mammalian sense of the word. Anyway, the mycetome is located on the ventral side of the mid‐gut but unlike the leeches mentioned previously, there is no connection between the mycetome and the gut lumen (Perotti et al. 2008). In adult female lice, the bacteria re‐locate to the oviducts and the developing eggs. Interestingly, molecular phylogenetic analysis cannot distinguish between the symbiotic bacteria of human head lice (Pediculus humanus capitis) and human body lice (Pediculus humanus humanus). This adds support to phylogenetic analysis of the lice themselves that indicates that although head lice and body lice occupy different ecological niches and body lice tend to lay their eggs on clothing whilst head lice attach their eggs to hair shafts, they are two morphotypes of the same species rather than two separate species. One suggestion is that body lice evolved from head lice relatively recently in human evolution once we started wearing clothing. The association between Riesia and Pediculus is between 12.95 and 25 million years old – which makes it one of the youngest host: primary endosymbiont relationships so far recorded (Allen et al. 2009). In common with other primary endosymbionts, Riesia has undergone a reduction in genome complexity and lost genes (Moran and Bennett 2014): this is because it has come to rely on its host for the provision of many nutrients and protection from the environment etc. In addition, because its transmission is via the eggs of its host, each louse symbiont population is in reproductive isolation and unable to undergo recombination with other strains of Riesia in other lice. This has led to the suggestion that Riesia will lack the capacity to develop rapid resistance mechanisms to antibiotics – and because the Riesia is essential for the lice, killing the symbiont would result in host mortality.