Infected Lice Research Articles

Integrating core subtractive proteomics and reverse vaccinology for multi-epitope vaccine design against Rickettsia prowazekii endemic typhus.

Rickettsia prowazekii is an intracellular, obligate, gram-negative coccobacillus responsible for epidemic typhus. Usually, the infected body louse or its excrement when rubbed into the skin abrasions transmits the disease. The infection withR. prowazekiicauses the highest death rate (> 20% without antibiotic treatment and now 1-7%), followed by epidemic typhus, which often manifests in unsanitary conditions (up to 15-30%). Conventionally, vaccine design has required pathogen growth and both assays (in vivo and in vitro), which are costly and time-consuming. However, advancements in bioinformatics and computational biology have accelerated the development of effective vaccine designs, reducing the need for traditional, time-consuming laboratory experiments. Subtractive genomics and reverse vaccinology have become prominent computational methods for vaccine model construction. Therefore, the RefSeq sequence of Rickettsia prowazekii (strain Madrid E) (Proteome ID: UP000002480) was subjected to subtractive genomic analysis, including factors such as non-similarity to host proteome, essentiality, subcellular localization, antigenicity, non-allergenicity, and stability. Based on these parameters, the vaccine design process selected specific proteins such as outer membrane protein R (O05971_RICPR PETR; OmpR). Eventually, the OmpR was subjected to a reverse vaccinology approach that included molecular docking, immunological simulation, and the discovery of B-cell epitopes and MHC-I and MHC-II epitopes. Consequently, a chimeric or multi-epitope-based vaccine was proposed by selecting the V11 vaccine and its 3D structure modeling along with molecular docking against TLR and HLA protein, in silico simulation, and vector designing. The obtained results from this investigation resulted in a new perception of inhibitory ways against Rickettsia prowazekii by instigating novel immunogenic targets. To further assess the efficacy and protective ability of the newly designed V11 vaccine against Rickettsia prowazekii infections, additional evaluation such as in vitro or in vivo immunoassays is recommended.

Novel 3D in situ visualization of seal heartworm (Acanthocheilonema spirocauda) larvae in the seal louse (Echinophthirius horridus) by X-ray microCT

The seal heartworm Acanthocheilonema spirocauda (Nematoda: Onchocercidae) parasitizes the heart and pulmonary arteries of various phocid seals of the Northern Hemisphere. Over many decades, potential vectors of this parasite have been discussed, and to this date, the life cycle is not fully known. The seal louse Echinophthirius horridus (Anoplura: Echinophthiriidae) is an obligatory, permanent and haematophagous ectoparasite of phocids that has been hypothesized to function as obligate intermediate host for A. spirocauda. We examined 11 adult E. horridus specimens collected from stranded harbour seals (Phoca vitulina) in rehabilitation at the Sealcentre Pieterburen by X-ray microCT imaging, aiming to illustrate larval A. spirocauda infection sites in situ. In three of these specimens, thread-like larvae were detected in insect organs. Detailed imaging of the most infected louse revealed a total of 54 A. spirocauda larvae located either in fat bodies or the haemocoel. Histological analysis of the same specimen illustrated nematode cross-sections, confirming X-ray microCT data. The current data strongly suggest that E. horridus is a natural intermediate host for A. spirocauda. Moreover, we demonstrate the potential of X-ray microCT-based imaging as a non-destructive method to analyze host-parasite interactions, especially in the neglected field of marine mammal parasitology.

The contributions of James Carmichael Smyth, Archibald Menzies and Robert Jackson to the treatment of typhus in royal naval vessels in the late 18th century.

In late 18th century Britain, typhus fever plagued the mass mobilisation of soldiers and posed a significant challenge to physicians of the time. Epidemic typhus was spread through highly infectious faeces of infected lice and carried a high mortality in patients and healthcare staff alike. Physicians James Carmichael Smyth (1741-1821) and Archibald Menzies (1754-1842) theorized that typhus fever was caused by infection of human exhalation. They trialled the use of vapourised nitrous acid to fumigate patients, their clothes and their bedspace, with apparent success. Despite this, typhus fever continued to ravage deployments of soldiers into the early 19th century, stimulating the continuing evolution of the understanding of typhus and its treatment.

Effects of the vertically transmitted microsporidian Facilispora margolisi and the parasiticide emamectin benzoate on salmon lice (Lepeophtheirus salmonis)

BackgroundMicrosporidia are highly specialized, parasitic fungi that infect a wide range of eukaryotic hosts from all major taxa. Infections cause a variety of damaging effects on host physiology from increased stress to death. The microsporidian Facilispora margolisi infects the Pacific salmon louse (Lepeophtheirus salmonis oncorhynchi), an economically and ecologically important ectoparasitic copepod that can impact wild and cultured salmonids.ResultsVertical transmission of F. margolisi was demonstrated by using PCR and in situ hybridization to identify and localize microsporidia in female L. salmonis and their offspring. Spores and developmental structures of F. margolisi were identified in 77% of F1 generation copepods derived from infected females while offspring from uninfected females all tested negative for the microsporidia. The transcriptomic response of the salmon louse to F. margolisi was profiled at both the copepodid larval stage and the pre-adult stage using microarray technology. Infected copepodids differentially expressed 577 transcripts related to stress, ATP generation and structural components of muscle and cuticle. The infection also impacted the response of the copepodid to the parasiticide emamectin benzoate (EMB) at a low dose of 1.0 ppb for 24 h. A set of 48 transcripts putatively involved in feeding and host immunomodulation were up to 8-fold underexpressed in the F. margolisi infected copepodids treated with EMB compared with controls or either stressor alone. Additionally, these infected lice treated with EMB also overexpressed 101 transcripts involved in stress resistance and signalling compared to the other groups. In contrast, infected pre-adult lice did not display a stress response, suggesting a decrease in microsporidian virulence associated with lice maturity. Furthermore, copepodid infectivity and moulting was not affected by the microsporidian infection.ConclusionsThis study demonstrated that F. margolisi is transmitted vertically between salmon louse generations and that biological impacts of infection differ depending on the stage of the copepod host. The infection caused significant perturbations of larval transcriptomes and therefore must be considered in future studies in which impacts to host development and environmental factors are assessed. Fitness impacts are probably minor, although the interaction between pesticide exposure and microsporidian infection merits further study.

Fungal airsacculitis associated with serratospiculiasis in captive falcons of the United Arab Emirates

Serratospiculum is a filarial nematode responsible for the parasitic disease serratospiculiasis. There are nine species of Serratospiculum recognised with cosmopolitan distribution, but S seurati is the only species identified in...

Arthropod-Borne Diseases Associated with Political and Social Disorder

The living conditions and the crowded situations of the homeless, war refugees, or victims of a natural disaster provide ideal conditions for the spread of lice, fleas, ticks, flies and mites. The consequence of arthropod infestation in these situations is underestimated. Along with louse-borne infections such as typhus, trench fever, and relapsing fever, the relationship between Acinetobacter spp.-infected lice and bacteremia in the homeless is not clear. Murine typhus, tungiasis, and myiasis are likely underestimated, and there has been a reemergence of bed bugs. Attempted eradication of the body louse, despite specific measures, has been disappointing, and infections with Bartonella quintana continue to be reported. The efficacy of ivermectin in eradicating the human body louse, although the effect is not sustained, might provide new therapeutic approaches. Arthropod-borne diseases continue to emerge within the deprived population. Public health programs should be engaged rapidly to control these pests and reduce the incidence of these transmissible diseases.

Body Lice,Yersinia pestisOrientalis, and Black Death

To the Editor: Wild rodent fleas are the most common vectors of Yersinia pestis, the plague agent (1). The human body louse (Pediculus humanus) has been proposed as a probable additional vector during historical epidemics (2) because human cases of louse-borne plague have been suspected (3) and body louse–borne plague has been demonstrated experimentally with rabbits (4). Using rabbits, we tested the ability of the 3 Y. pestis main biovars to produce a successful rabbit-louse-rabbit-louse cycle of transmission (4). Two New Zealand White (Oryctolagus cuniculi) rabbits were inoculated intravenously with phosphate-buffered saline alone (negative controls) or phosphate-buffered saline containing 109 CFU of Y. pestis biotype Nairobi-Rattus Antiqua, biotype 14–47 Medievalis, or biotype 6/69M Orientalis. PCR ensured detection of the virulence factor–encoding plasmids. The rabbits inoculated with biotypes Antiqua, Medievalis, or Orientalis had septicemia of ≈2 × 103 CFU/mL of blood 14 hours postinoculation and died at 20–22 hours, 18–20 hours, or 16–18 hours postinoculation, respectively. In contrast, the negative control rabbits remained healthy for 3 weeks. Five minutes postinoculation, 150 uninfected lice fed for 1 hour on rabbits and took an equivalent blood meal as measured by weight, regardless of the rabbit used. Y. pestis was isolated from all 120 randomly tested lice and their feces. Five days postinfection, the death rate of Orientalis-fed lice (95.3%) was significantly higher than that of the control (4%), Antiqua-fed, (78.6%), and Medievalis-fed (74%) (p<0.0001) lice. One third of Orientalis-infected lice remained alive 3 days after the contaminating blood meal. Lice fed on septicemic rabbits further fed on 2 uninfected rabbits for 1 hour daily for up to 6 days. The rabbits bitten by Orientalis-infected lice had 2.7 × 102 CFU/mL of blood 4 days postinfection and died 1 day later. In contrast, the rabbits bitten by Antiqua-infected or Medievalis-infected lice looked healthy and lacked septicemia 3 weeks after challenge. New groups of 150 uninfected lice fed for 1 hour daily on Orientalis-infected rabbits started to die earlier than did lice fed on Antiqua, Medievalis (1 vs. 2–3 days after blood meal), and uninfected rabbits. Furthermore, 21 days after their first blood meal, lice fed on Orientalis-infected rabbits had a significantly higher death rate (90%) than did control (3%) (p<0.0001), Antiqua-infected (16%), and Medievalis-infected (10%) lice; the latter values were significantly higher than that of the negative controls (p = 0.046). Y. pestis could be cultured only from lice and their feces if the lice were fed on rabbits previously bitten by Orientalis-infected lice (Appendix Figure). Our observation that body lice effectively transmitted Y. pestis through a complete cycle of transmission confirms previous experimental (4) and field observations of experimental transmission that used body lice collected from plague patients from the same family in the absence of any other ectoparasite (3). Transmission of Orientalis but not Antiqua or Medievalis organisms did not result merely from experimental bias because negative controls remained negative, data were duplicated, rabbits exhibited equivalent bacteremia, and lice took equivalent blood meals regardless of biotype. Our observations shed new light on the Black Death, a medieval epidemic of plague (5). Historical records indicate that persons with the Black Death had bubonic plague, indicating an ectoparasite-borne transmission (1). Pulex irritans fleas were documented in a medieval setting in Viking Greenland (6). However, their poor competence (7) and the Black Death that swept Russia and Scandinavia are not fully compatible with flea-borne transmission alone. Ten infected lice are sufficient for plague transmission (4), and our observation that one third of infected lice remained alive 3 days after infection indicates that an index plague patient carrying as few as 30 body lice could be a source for plague up to 3 days after dying. This figure was highly plausible during the Black Death because body lice currently infest almost 85% of homeless persons, with a mean of 57 lice per person (8). Although the role of fleas as vectors of Y. pestis from rodents to humans is undisputed, this tabulation sustains the potential role of body lice as an additional vector of plague from human to human during the Black Death (2). Paleomicrobiology suggested that most historical cases in Europe resulted from Orientalis (5). This observation challenged the scenario that Antiqua, Medievalis, and Orientalis were responsible for ancient, medieval, and modern plague pandemics, respectively (9). The latter scenario had been hypothesized after the biotypes were observed to have a geographic repartition matching that of the hypothetical sources of the 3 historical pandemics (10) and was further propagated as dogma without further confirmation. Our data support an alternative scenario of the historical plague epidemics transmitted by body lice, with Orientalis being the only such louse-borne transmissible biotype. This point justifies studies during ongoing epidemics in cold countries, keeping in mind the need to understand and control re-emerging plague in modern populations exposed to body lice.

Human Complement Regulators C4b-Binding Protein and C1 Esterase Inhibitor Interact with a Novel Outer Surface Protein of Borrelia recurrentis

The spirochete Borrelia recurrentis is the causal agent of louse-borne relapsing fever and is transmitted to humans by the infected body louse Pediculus humanus. We have recently demonstrated that the B. recurrentis surface receptor, HcpA, specifically binds factor H, the regulator of the alternative pathway of complement activation, thereby inhibiting complement mediated bacteriolysis. Here, we show that B. recurrentis spirochetes express another potential outer membrane lipoprotein, termed CihC, and acquire C4b-binding protein (C4bp) and human C1 esterase inhibitor (C1-Inh), the major inhibitors of the classical and lectin pathway of complement activation. A highly homologous receptor for C4bp was also found in the African tick-borne relapsing fever spirochete B. duttonii. Upon its binding to B. recurrentis or recombinant CihC, C4bp retains its functional potential, i.e. facilitating the factor I-mediated degradation of C4b. The additional finding that ectopic expression of CihC in serum sensitive B. burgdorferi significantly increased spirochetal resistance against human complement suggests this receptor to substantially contribute, together with other known strategies, to immune evasion of B. recurrentis.

First Case Report of Louse – Borne Relapsing Fever in Saudi Arabia

Relapsing fever is endemic in some parts of the world such as Africa, South America, and Far East.1-3 Epidemics have occurred among refugees and war victims in some parts of Africa.4,5 There is very little information about this infection in Saudi Arabia. The prevalence or incidence in Saudi Arabia is unknown. Its apparent rarity may be due to under diagnosis and under reporting. A high index of suspicion in appropriate clinical situations will lead to its early recognition and treatment. Relapsing fever is an acute febrile illness caused by blood spirochetes belonging to genus borrelia. This spirochete can be acquired through the bite of an infected soft tick (Ornithodoros species) or contact with the hemolymph of an infected human body louse (Pediculus humanus) and thus constituting two types of relapsing fever (RF) depending upon the responsible vector i.e. Tick – borne relapsing fever (TBRF) and Louse – borne relapsing fever (LBRF). It is characterized by recurrent cycles of pyrexia which are separated by intervals of apparent recovery. During pyrexia, organisms can be found in blood and that is the only way of reliable diagnosis in many situations.6,7 While during afebrile period, organisms hide themselves in spleen, liver and even central nervous system.

Different genes govern Yersinia pestis pathogenicity in Caenorhabditis elegans and human lice

To assess the role of virulence factors identified in Caenorhabditis elegans in the transmission of plague by lice, we infected 100 lice by feeding them on rabbits and made them bacteremic; the rabbits had been intravenously inoculated with 10 9 CFU of six different mutant Yersinia pestis strains of lower pathogenicity for C. elegans, obtained from the KIM5 strain. This strain lacks genes used for biofilm formation. High mortality rates were observed in all lice, which excreted viable bacteria in their feces. Mutants killed rabbits when infected intravenously, but mutants were not transmitted to rabbits by infected lice. We conclude that the genes governing pathogenicity in C. elegans and louse are not identical.

Rickettsia prowazekiiand Real-time Polymerase Chain Reaction

Rickettsia prowazekii is the causative agent of epidemic typhus and a potential bioterrorism agent. Sensitive and specific rapid assays are needed to complement existing methods of detecting this organism. We developed a real-time quantitative polymerase chain reaction assay by using a species-specific probe targeting the gltA gene. This assay, which was rapid, specific for R. prowazekii only, and sensitive (cutoff detection of 1 to 5 copies per sample), detected and directly identified R. prowazekii in blood of 12 experimentally infected mice sampled at day 3 and 6 postinfection or in naturally or experimentally infected lice. Because our assay is highly standardized and easily adaptable, it could improve epidemic typhus surveillance in public health programs, especially for countries with underdiagnosed or unrecognized human cases.

Typhus Group Rickettsiae Antibodies in Rural Mexico

Typhus Group Rickettsiae Antibodies in Rural Mexico

An experimental model of human body louse infection with Rickettsia typhi.

Murine (endemic) typhus caused by Rickettsia typhi, one of the most widely distributed arthropod-borne diseases, is transmitted to humans mainly by the oriental rat flea. The human body louse, Pediculus humanus corporis, has been suspected to have a role in the transmission of R. typhi to humans. To evaluate the potential role of Pediculus humanus corporis as a vector of murine typhus, we used R. typhi in an experimental model of human body louse infection previously used with R. prowazekii. A rabbit was made bacteremic by inoculating it with 2 x 10(6) plaque-forming units of R. typhi; it remained bacteremic for at least 59 hours. Two hundred body lice infected by feeding on the bacteremic rabbit were compared to 200 uninfected control lice. Each louse population was fed once a day on the abdomen of a seronegative rabbit. On day 8 post-infection, as a result of disruption of the gut cells and leakage of the blood meal into the hemolymph, four infected lice became bright red and died within four hours. The life span of infected lice was 20 days less than that of the controls. Infected lice did not transmit R. typhi to their progeny (eggs and larvae) as demonstrated by PCR amplification and cell culture. With an immunofluorescence assay, R. typhi was detected in feces from day 7 post-infection, and the organism remained viable in feces for up to 80 days as demonstrated by cell culture. From the 21st day post-infection, the rabbit used to feed the R. typhi-infected lice developed an immunoglobulin response with a titer of 1:50 increasing to 1:200 at the 42nd day post-infection. It showed no clinical signs of infection. The rabbit that was used to feed uninfected lice remained seronegative. Although body lice are not clearly identified vectors of R. typhi, it seems that under certain circumstances they could transmit R. typhi.

How Charles Nicolle of the Pasteur Institute discovered that epidemic typhus is transmitted by lice: reminiscences from my years at the Pasteur Institute in Paris.

How Charles Nicolle of the Pasteur Institute discovered that epidemic typhus is transmitted by lice: reminiscences from my years at the Pasteur Institute in Paris.

Some observations on Pseudomenopon pilosum (Amblycera: Menoponidae), the louse vector of Pelecitus fulicaeatrae (Nematoda: Filarioidea) of coots, Fulica americana (Aves: Gruiformes)

Breeding populations of Pseudomenopon pilosum (Scopoli, 1763) became established on 10 laboratory-reared juvenile American Coots (Fulica americana Gmelin) initially infested with 5 adult male and 5 adult female lice. Eggs of P. pilosum hatched less than 10 days after deposition and the combined duration of the three nymphal instars was 10–20 days. Nymphs and adults occupied all regions of the body. Pseudomenopon pilosum might thus acquire microfilariae of Pelecitus fulicaeatrae (Diesing, 1861) by simply randomly moving to and feeding on the legs where, in infected coots, the skin-inhabiting microfilariae of P. fulicaeatrae are known mainly to occur. Pseudomenopon pilosum occurred on all of 13 adult coots and three 1-week-old coot chicks collected in June in western Canada where P. fulicaeatrae is enzootic. Third-stage larvae of P. fulicaeatrae were found in adult P. pilosum on two of four adult coots harbouring microfilariae, but prevalence in lice was low (5.5% of 18 lice on one coot and 1.1% of 90 on the second) and only one third-stage larva was present in each infected louse. Four other species of lice were present on adult coots but only one other on 1-week-old chicks. Experiments showed that P. pilosum could occur as a straggler on chickens (Gallus gallus (L.)) and Red-necked Grebes (Podiceps grisegena (Boddaert)) although it did not establish on either species.

Ultrastructural Studies of Rickettsia prowazeki From Louse Midgut Cells to Feces: Search For “Dormant” Forms

An electron microscope study of infected human louse gut cells and feces was made to determine whether a valid correlation exists between the increased resistance of Rickettsia prowazeki (in the louse feces) to adverse environmental influences and changes in the organism which might be reflected in its ultrastructure. Upon fine structural examination of this intracellular parasite as it passed from the louse midgut cell to the feces, it was apparent that no such morphological changes had occurred.

The development of Spirochaeta duttoni and S. recurrentis in Pediculus humanus

1. The development of S. duttoni and S. recurrentis was studied in lice.2. After penetrating the anterior part of the midgut spirochaetes were seen in the haemocoele, haemocytes, neural ganglia and muscles of lice. They were never seen elsewhere.3. The haemocytes of infected lice sometimes contained coarse granules believed to be the breakdown products of spirochaetes due to phagocytosis.

Immunization Against Epidemic Typhus

Discussion and SummaryThe evidence herein presented indicates that anti-typhus vaccines of the inactivated type are of well-established value. While most of the evidence relates to Cox-type vaccines of yolk-sac origin, the conclusions to be drawn probably are also applicable to potent vaccines derived from infected lice or animal lungs. It is clear that adequate primary immunization will greatly modify such disease as may occur and will virtually eliminate fatality. The degree of reduction in morbidity remains undetermined but is undoubtedly significant. Data as to duration of real immunity after primary immunization do not exist (our small challenge experiments failed to reveal solid immunity at 9 and 21 months post-vaccination). However, serologic evidence indicates that administration of single booster inoculum from 9 months to as long as 6 years later results in effective restimulation of immunity. While repeated booster inocula will undoubtedly serve to maintain a good level of immunity, an appropriate schedule of inoculations has not yet been worked out.Use of a living rickettsial vaccine offers obvious possible advantages in that a single inoculation should result in solid and long-lasting immunity. Nearly all previous efforts to use living rickettsiae involved, basically, the use of minute doses of agents of unmodified virulence, minute because of dilution or partial inactivation. The concensus is that such vaccines, when given to non-immune persons, either failed to infect and to immunize, or produced unmodified, inoculation typhus.The evidence available with respect to strain E indicates that it is indeed a strain of R. prowazeki of greatly reduced virulence for man. Its primary safety is attested by its use in upwards of 17,000 persons with no single known instance of truly serious illness resulting. Further, it is unlikely that it could be picked up and spread by lice because of its apparent inability to invade the blood stream. Admittedly, reactions do follow the use of strain E. However, there is a dose range (4.0 to 5.5 log E.I.D.) which induces both a tolerable level of reactions and a satisfactorily uniform immediate seroimmune response. Available evidence, with respect to both seroimmunity and ability to resist virulent challenge, indicates that, while immunity induced by strain E infection may not be fully comparable to that following natural infection, it is solidly effective for at least 3 years and probably considerably longer.There are, however, some problems remaining. That actual infection of man does occur is suggested on clinical grounds by the development of regional adenitis and of reactions of the delayed type. It also is suggested by the fact that the minimum immunizing dose of formol-inactivated rickettsiae is 2.5 logs greater than that of living strain E (Everritt et al., 1954). However, knowledge as to the fact and possible sites of persistence of the agent in man should be sought. Also, while strain E-induced immunity seems superior to that following primary immunization with Cox-type vaccine, a comparison also should be made with that in persons given in addition a booster inoculum of inactive vaccine. Again, while the considerable field use of the vaccine in Peru testifies to its practical applicability under field conditions, criteria as to minimum infectivity, recommended dosage and expiration dates of production lots of vaccine remain to be defined. Finally, although the results of the challenge experiments offer much promise as to effectiveness, there remains the crucial problem of obtaining definitive data as to the effectiveness of strain E vaccine in preventing the natural occurrence of disease.

Typhus fever in county tyrone

Summary o (1) The facts surrounding and the events which led to the diagnosis of two cases of typhus fever at Fintona, County Tyrone, have been described. (2) The steps which were taken to control the spread of the disease in the county have been enumerated. (3) The great assistance to be obtained from modern serological methods which allow of identifying the insect vector of typhus have been demonstrated. (4) The indiscriminate dusting with D.D.T. powder of all persons remotely suspected of harbouring infected lice was used as the principal measure for controlling the disease. How far this contributed to the fact that no further cases occurred is difficult to estimate. (5) In spite of exhaustive enquiries and investigations, it was not possible to demonstrate a proved source of the disease. The greatest weight of evidence pointed to itinerant gypsies or tinkers as being the source because :— o (a) An outbreak of typhus amongst them is known to have occurred in County Donegal in 1940. (b) The only close association with the outside world which the affected family had was with the tinkers (the boy of seven did not go to school). (c) This family is one of the very few house-dwellers which associates with itinerants. (d) The generally verminous state of the tinkers. (6) The ability of the recently formed health services to deal with an epidemic emergency has been demonstrated.

An epidemic of louse-borne relapsing fever in Kenya

Summary 1. An epidemic of relapsing fever involving the hinterland of Mombasa and the Kenya Coast is described. 2. There were nearly 2,000 cases, with a 40 per cent, mortality in untreated cases. 3. Control measures rapidly terminated the epidemic. About 100,000 people were disinfested with 5 per cent, DDT powder. Various administrative steps (such as abolition of funeral ceremonies, prevention of travel, etc.) were also important in the control of the spread of the disease. 4. Clinically, cases were characterized by the prominence of neurological symptoms, cardiac involvement, positive Weil-Felix reactions and negative Kahns. 5. The disease was introduced from Arabia and was of louse-borne origin. Infected lice were recovered from patients and the disease was transmitted from these lice to monkeys. There was no evidence incriminating other possible vectors and the spirochaete was found to be non-transmissible by the ticks O. moubata and savignyi . 6. The behaviour of the spirochaete in different animals was observed and compared with that of S. duttoni . Rabbits, guineapigs, bush-babies, white rats, white mice, gerbilles and monkeys were found capable of being infected, their relative susceptibility being in the order shown, with rabbits the least and mice and monkeys the most susceptible. The following observations are particularly to be noted:— (i) Repeated passage in mice was easily maintained. (ii) Neurotropism occurred in S. duttoni infections in mice, but scarcely ever in recurrentis . (iii) Relapses of S. recurrentis occurred in 69 per cent. of monkeys, the interval between the relapses being 10 to 12 days. Relapses tended to be more severe than the original attack. (iv) S. duttoni infections differed from recurrentis in being much more fatal in monkeys and much more persistent in rats and white mice. 7. The infections in lice showed a well-marked negative phase lasting till the 16th day; lice collected from patients and examined within 24 hours never showed spirochaetes. 8. No cross immunity exists in animals (except bush-babies) between S. recurrentis and S. duttoni . There was cross immunity in two out of three monkeys tested against a relapse strain. 9. Auto-agglutination of spirochaetes in old infections (primary attacks) occurs in monkeys but not in rats. 10. Some evidence was produced to show the immunizing effect of killed spirochaetes. 11. The pathogenicity of the Kenya spirochaete in man and animals resembles most closely that of S. carteri and differs markedly from the Chinese and Abyssinian strains. 12. It is suggested that the louse-borne spirochaetes may be divided into two groups. The first exhibits a long negative phase in the louse (European, North African and Kenya strains); the second never shows this phase and spirochaetes are present throughout the louse cycle (Abyssinian and Chinese strains). 13. The pathological changes resembled those previously described except that in three instances degeneration of ganglion cells was found in the cerebellum without meningovascular inflammation.