Lesser known rickettsial infections in animals and humans

Previous authors: K J SUMPTION AND G R SCOTT

Current authors:
A L FEDROW - Assistant Professor, MS, PhD, Shippensburg University, 1871 Old Main Drive, Shippensburg, Pennsylvania, 17257, United States of America
K E MULLINS - Associate Director of Chemistry, Point of Care, and Clinical Laboratory Support, PhD, 111 Michigan Avenue, N.W., Washington DC, 20010, United States of America
M L LEHMAN - Professor, PhD, Biology Department, Shippensburg University, 1871 Old Main Drive, Shippensburg, Pennsylvania, 17257, United States of America
R L STEWART - Shippensburg University, 1871 Old Main Drive, Shippensburg, Pennsylvania, 17257, United States of America

GENERAL INTRODUCTION: RICKETTSIAL, CHLAMYDIAL AND HAEMOTROPIC MYCOPLASMAL DISEASES

General introduction

In 2001, the order Rickettsiales was subjected to a taxonomic reorganization based upon genetic analyses of 16S rRNA, groESL, and surface protein genes⁸⁷ The major changes were in the families Rickettsiaceaeand Anaplasmataceae, including:

1. the family Rickettsiaeceaewas amended to include the genera Rickettsia and Orientia; and
2. the family Anaplasmataceaewas broadened to include all species in the genera Ehrlichia, Anaplasma, Cowdria, Wolbachia, and Neorickettsia.  As such, the genus Anaplasma was modified: 1) to include the organisms originally designated as Ehrlichia phagocytophila, Ehrlichia bovis, and Ehrlichia platys; 2) to include Ehrlichia equi and the Ehrlichia HGE agent as synonymous with Anaplasma phagocytophilum; and 3) to place Cowdria ruminantium into the genus Ehrlichia (i.e. Ehrlichia ruminantium).⁸⁷  Consequently, the re-classification resulted in nomenclature changes of certain animal pathogens that are discussed in this chapter (Table 1).

Table 1 Nomenclature changes associated with reorganization of the order Rickettsiales in 2001⁸⁷

Infections of domestic animals associated with members of the Rickettsiales were reported in Europe since approximately 1780, with historical reports describing febrile diseases (sometimes lethal) associated with tick infestations in farm animals and dogs.^{110, 111, 259}  Members of the Rickettsiaceaeand the Anaplasmataceae are transmitted to humans by ticks, mites, fleas, and lice, and can result in disease(s) ranging from subclinical, mild, to severe to deadly. These include, Rickettsia prowazekii (the causative agent of endemic typhus), Rickettsia conorii (the causative agent of Mediterranean spotted fever), Ehrlichia chaffeensis (the causative agent of human monocytic ehrlichiosis), and A. phagocytophilum (the causative agent of human granulocytic anaplasmosis) The members of Rickettsiales that cause disease in both domestic animals and humans are not always mutually exclusive, occur worldwide, are often transmitted by specific vectors (e.g. ticks, fleas) in certain geographic regions, and vaccines are most often not available for the disease-causing agents.  Despite the advent of more advanced molecular and serological diagnostic techniques, cross- reactivity among the different genera/species within the order Rickettsiales still pose a problem in making a definitive diagnosis, particularly in resource-restricted countries or laboratories.

In Africa, tick-borne pathogens are responsible for many important and serious diseases^{93, 226, 227, 232} resulting in livestock productivity and economic losses and may also pose public health risks.  The ecological richness of the continent of Africa provides a variety of ecosystems that are ideal for supporting an array of tick/ectoparasite species, and consequently the maintenance of pathogen diversity.  Moreover, close interfaces between humans and livestock/wildlife on the continent has a bearing on farming systems as well as ecotourism/tourism industries.  At these interfaces there is often increased transmission of pathogens between livestock and wildlife species, resulting in the emergence of unknown pathogens or new diseases or endemicity (see Infectious diseases of animals in sub-Saharan Africa: The wildlife/livestock interface). Consequently, the need for continued and new molecular and serological surveys throughout Africa cannot be overemphasised.  In this chapter, the lesser known members of Rickettsiales (Anaplasmataceae and Rickettsiaceae) are discussed, some of which have a history of causing disease amongst animals and humans in Africa.

Order Rickettsiales

Members of the order Rickettsiales are all Gram-negative, obligate intracellular bacteria vectored by either arthropods or trematodes; within their host cell they replicate in vacuoles in the cytoplasm and/or can be found within the host nucleus.^{224, 241} Bacteria in the order Rickettsiales that are responsible for diseases in humans and animals are associated with the genera Rickettsia (rickettsioses), Orientia (scrub typhus), Anaplasma (anaplasmoses) and Ehrlichia (ehrlichioses).

Family Anaplasmataceae (genera Anaplasma and Ehrlichia)

The Anaplasma and Ehrlichia are coccoid/ellipsoidal (can also be pleomorphic), multiply in membrane-bound, cytoplasmic vacuoles within their host cell forming morulae, and can be present in both reticulate and dense core forms.^{234, 279, 293}

Anaplasma species cause disease in animals (domestic ruminants, equines, canines, felines, deer, rodents) and humans. Moreover, there have been recent reports of human infections caused by species previously thought only to infect domestic animals.^{20, 51, 187}

Ehrlichia species infect domestic ruminants, canines, rodents, and humans.²⁴⁵ Ticks are the primary vectors of Anaplasma and Ehrlichia species. However, Anaplasma species can also be transmitted by biting flies, contaminated fomites, and/or via blood transfusions.^{22, 166, 277} The host target cells of Anaplasma and Ehrlichia are species-dependent, with a predominance of infection occuring in erythrocytes, monocytes, macrophages, granulocytes, endothelial cells, and/or platelets.^{279, 293} As observed throughout the order Rickettsiales, symptoms associated with Anaplasma and Ehrlichia infections are often flu-like, and can be mild or progress to severe/lethal outcomes; the course of disease in animals and humans is dependent upon the infecting strain.  The tetracyclines (doxycycline) are the antibiotic of choice for treatment of Anaplasma and Ehrlichia speciesinfections;^{120, 316} some fluoroquinolones are effective against some Anaplasma species,¹⁹⁶ and chloramphenicol has been described with both failure and success in the treatment of Ehrlichia infections.^{27, 95, 115, 186}

Some Anaplasma and Ehrlichia species that cause livestock disease are well known in Africa, particularly those species associated with bovine and ovine and caprine anaplasmosis (Anaplasma marginale, Anaplasma ovis, and Anaplasma centrale), and  heartwater/cowdriosis (Ehrlichia ruminantium).  Molecular and serological surveys have revealed the presence of Anaplasma and Ehrlichia species in animals, humans and/or ticks previously not known to harbour them, Moreover, newly identified/unclassified Anaplasma and Ehrlichia species, with proposed zoonotic potential, have been reported recently in Africa.  In this chapter, the lesser known species associated with infections in Africa are dealt with, including Anaplasma bovis, Anaplasma platys, Anaplasma phagocytophilum, Ehrlichia chaffeensis, Ehrlichia canis, Ehrlichia ewingii, and Ehrlichia muris.

Rickettsia species infections

The Rickettsia bacteria are short rods (paired in some instances), 0.3-0.5 x 0.8-2.0 µm, free in the cytoplasm of host cells, or in some instances intranuclear, and divide by binary fission.²⁹³  Within the genus Rickettsia, the species are divided into the spotted fever group rickettsiae, the typhus group rickettsiae, and the ancestral/transitional group.²⁰² Hard ticks (Ixodidae) are the primary vectors of the spotted fever group rickettsiae in hosts such as humans, livestock, canines, felines,  birds, and reptiles.  Some members of the spotted fever group rickettsiae maintainance vector hosts are fleas (R. felis⁴⁸ and R. asembonensis),¹⁸⁹ tsetse flies, mites, mosquitoes, ticks (R. felis-like organisms (RFLO)),²¹⁸ soft ticks in the Argasidae family (R. hoogstraalii⁸⁵and Candidatus R. lussitaniae) and mouse  mite (Liponyssoides sanguineus)¹²⁷ (R. akari). Fleas and lice are the vectors of the typhus group rickettsiae, which are transmitted to humans upon contamination of the vector bite site with the faeces from the infected vector.¹³

Following the bite of an infected host, rickettsiae disseminate via the bloodstream and replicate in the cytoplasm primarily of endothelial cells.²⁴⁴ Flu–like symptoms are usually associated with rickettsial infections in humans, and an eschar may be present at the site of the vector bite.  Disease can range from mild to lethal.²²⁴ Doxycycline is the drug of choice for treatment of diseases associated with species of the Rickettsia genus but other tetracyclines, chloramphenicol, rifampin, fluoroquinolones, and some macrolides have also been used.^{47, 239}

The potential for bacteria of the genus Rickettsia to cause disease in domestic animals in Africa has only recently become clearer, primarily due to the availability of advanced molecular techniques to identify and characterize these agents.  Most studies in Africa in livestock focused on the presence of the rickettsial agent(s) in ticks collected from the host and/or the immediate surroundings and serological evidence of infection in their hosts. Despite the paucity of information about the pathogenicity of Rickettsia species in livestock, it is important to consider their disease-causing and zoonotic potential in the diverse ecological or climatic niches at the human/animals interfaces in Africa.

Anaplasma bovis infections (Bovine anaplasmosis; Monocytic anaplasmosis)

Anaplasma bovis is one of the causative agents of bovine anaplasmosis or monocytic anaplasmosis (formerly bovine ehrlichiosis)^{83, 87, 245, 259} in Asia and Africa.^{245, 259} Early reports of the disease implicated A. phagocytophilum, an organism capable of infecting granulocytes, as the disease causative agent.^{118, 130, 245, 259}

Anaplasma bovis primarily infects circulating monocytes,^{39, 87, 174, 292} but may also infect tissue macrophages.³¹⁷ It has been reported in ruminant species (cattle, goats and wild deer),^{57, 174, 317, 319} small wild animals,¹⁹⁴ dogs,²⁶² cats,²⁷⁰ and rabbits.¹⁰⁸ It has been suggested that small mammals are reservoirs of A.bovis.^{108, 259}

Anaplasma bovis are small pleomorphic coccoid to ellipsoid cells, approximately 0.3 µm in diameter, with a similar morphology to other members of the genus.²⁹³ It has not yet been cultivated in vitro,^{87, 293} but can be  propagated by serial passage in cattle.²⁹³ The parasites in bovine monocytes stain dark blue with Romanowsky methods.^{87, 293} Although the host cell ligand is not known for this species, another member of the group, A. phagocytophilum, utilizes the platelet glycoprotein selectin ligand 1 (PGSL-1) to bind to host cells.^{87, 121} The organism is internalized and remains within vacuoles in the infected cell in vivo.²⁹³

Phylogenetic analysis demonstrated that A. bovis was more closely related to A. phagocytophilum than to either A. marginale or A. centrale.⁸⁷ A more recent study in 2014 showed that it consistently fell into the cluster that included A. phagocytophilum, A. platys and Anaplasma sp. Japan.^{34, 321} Studies of the genetic variability of A. bovis recovered from different geographic regions and different hosts demonstrated that similarity between strains exceeds 99 per cent.^{139, 141, 245, 271}

There are few studies on the prevalence of A. bovis infections. Anaplasma bovis has been reported in countries in Africa, North America, Japan and Brazil,^{108, 141, 267} although most reports of infection in ruminants  emanated  from African countries. In Tunisia and Algeria in North Africa²⁵⁹ A.bovis  has been reported in cattle, sheep and goats, but not in camels.^{29-32, 38, 255} Tunisian cattle had  a prevalence rate of 3.9 per cent for A.bovis,25.4 per cent  for A. marginale and 15.1 per cent for A. centrale.²⁹  Another study showed  prevalence rates of A. bovis in South Mediterranean sheep and goats of 42.7 and 23.8 per cent, respectively.³⁸ This study demonstrated that the strains of A. bovis isolated from goats and sheep clustered with other strain sequences previously published in GenBank and were similar to the results from sequence analysis conducted in Tunisian cattle.²⁹ It was suggested that sheep and goat populations could play a key role in the bovine anaplasmosis disease cycle in the Mediterranean region of Tunisia.³⁴ A more recent study in the north of Tunisia that utilized molecular techniques showed an overall prevalence rate of 4.9 per cent  for  A. bovis infection in cattle.³¹ A separate longitudinal field study in Mediterranean sheep and goats demonstrated the average prevalence of A. bovis was 7.4 per cent in sheep and 10.1 per cent in goats.³⁰ The infection rates in cattle and sheep varied seasonally.^{30, 31} The A. bovis prevalence rates in these studies were lower than those of an earlier Tunisian study (sheep 42.7 per cent and goats 23.8 per cent),³⁸ as well as rates in other ruminant populations from different geographic regions, including goats in China (49.6 per cent) and deer (23 per cent) and cattle (15-53.3 per cent) in Japan.^{135, 141, 174} These rates were higher than reported in cattle populations from Iran, India and Italy (2.7 per cent, 3.3 per cent and 4.2 per cent, respectively).^{57, 210, 215} Various factors were incriminated as an explanation for these different prevalence rates.^{30, 38}

In a study conducted in northeast Algeria, A. bovis infection was demonstrated to be 4.4 per cent in cattle,²⁵⁵ similar to the rate observed in a Tunisian study.³¹ The A. bovis variant detected in cattle in the recent Algerian study demonstrated high homology with other A. bovis variants isolated from cattle, goats and sheep in Tunisia,^{29, 31, 39} variants isolated from Haemophysalis longicornis ticks in South Korea and variants isolated from wild deer populations in Japan.²⁵⁵ The authors indicated that this was the first report demonstrating the presence of A. bovis (along with other Anaplasma spp.) in cattle in this region.  

Anaplasma bovis infections are usually subclinical but clinical signs may include fever, weight loss, weakness, anaemia, depression, enlarged lymph nodes, and pale mucous membranes.^{29, 39, 215, 267, 306} Abortion and death occur rarely.^{39, 259, 267} Treatment with tetracyclines can be used in severe cases of A. bovis.

Multiple tick vectors have been implicated in the transmission of A. bovis. In Africa Amblyomma variegatum and Rhipicephalus appendiculatus have been implicated but different vectors, such as a Hyalomma sp. in Iran and  Amblyomma cajennense in Brazil are also considered to be vectors.^{87, 245, 307} Anaplasma bovis or A. bovis-like organisms have been detected  in different Haemaphysalis spp. including H. longicornis in Korea and Japan,^{141, 151} Haemaphysalis concinna in  Russia,²⁵⁵ Haemaphysalis lagrangei in Thailand²⁶⁴ and Dermacentor andersoni in Canada.⁷⁸

Anaplasma platys infections (Infectious canine cyclic thrombocytopenia (ICCT))

Anaplasma platys (formerly known as Ehrlichia platys)is the causative agent of infectious cyclic thrombocytopenia (ICCT) in dogs, infecting platelets. It occurs worldwide.^{89, 259} The bacterium was first identified in dogs from Florida, United States of America(US).¹¹⁸ It is assumed to be vectored by Rhipicephalus sanguineus senso lato (brown dog tick), although the tick’s competence as a vector has not been definitively established.²⁸⁰ Anaplasma platys  has also been detected in Ixodes ricinus, Ixodes persculatus, Hyalomma detritum, Hyalomma excavatum, Haemaphysalis longicornis, Rhipicephalus camicasi, Rhipicephalus evertsi, Rhipicephalus pulchellus, and Rhipicephalus pravus,suggesting that the bacterium may be vectored by a variety of ticks.^{152, 195, 221, 269}

In dogs, ICCT is characterized by thrombocytopenia, fever, weakness, lethargy, anorexia, ocular discharges, splenomegaly, hyperkeratosis of the muzzle, and respiratory signs.²⁶⁰ The incubation period of A. platys is one to two weeks, which is followed by cyclical periods (every one totwo weeks) of thrombocytopenia and fever.¹⁰⁴ The severity of the clinical signs^{14, 18, 68, 288} is influenced by the breed, age and immune status of the dog: fatalities are uncommon  unless fatal haemorrhaging occurs following  accidents or surgery.²⁴⁵ While the disease is often  asymptomatic/subclinical in the US and Australia, it is clinically more severe in South America, Europe, Africa, and Asia.⁴⁹ This discrepancy is possibly due  to co-infections with other pathogens, but is more likely the result  of A. platys strains and/or A. platys-like strains with different pathogenicity.⁴⁹ Infections are commonly treated with  tetracyclines (particularly doxycycline) for 8-10 days or with enrofloxacin for 14-21 days.²⁶¹

Anaplasma platys has not yet been cultivated in vitro and no vaccine against A. platys is available.

In addition to infecting dogs, Anaplasma platys has a wide host range and may also infect sheep, goats, cattle, dromedary camels, buffalo, and deer,^{8, 32, 35, 40, 61, 67, 81, 163, 177, 178, 184} as well as humans^{20, 51, 187} and cats.^{172, 263} Anaplasma platys infections have been reported in dogs and/or ticks collected from dogs^{64, 112, 162, 191, 195, 265, 269, 275} throughout Africa. Apart from fever, most infections in the abovementioned animal species are subclinical.

Anaplasma phagocytophilum infections (Tick-borne fever or Pasture fever)

Anaplasma phagocytophilum is the causative agent of tick-borne fever (TBF) or pasture fever in domestic ruminants (primarily sheep and cattle), and human granulocytic anaplasmosis (HGA) and also infect dogs, cats, goats, donkeys, horses, and deer.  Cattle, sheep, wild ruminants (cervids), and rodents (white-footed mouse) are thought to serve as reservoirs of A. phagocytophilum, although definitive reservoir hosts have not yet been identified.²¹

In Europe, A. phagocytophilum has long been a well-known agent of disease in domestic ruminants, where it was first described as a “Rickettsia” in sheep during the early 1900s.^{110, 111} The bacterium was first associated with human infection in 1994 in the US, and was originally referred to as the “human granulocytic ehrlichia agent”.⁵⁹ Anaplasma phagocytophilum primarily infects neutrophils in cattle, sheep, goats, horses, deer, dogs, and humans^{99, 100, 299}   Morulae can be observed in infected cells.  In both animals and humans, disease can range from subclinical or mild to severe and can be fatal  The severity of infection is dependent upon a multitude of factors, including, age, immune status of the host, host species, and the variant of A. phagocytophilum that is causing the infection.  Infections in domestic ruminants are often characterized by high fever, anorexia, lethargy, weight loss, reduction in milk production, and abortion.^{102, 113, 259, 296, 299} Similar signs are observed in other susceptible animal species.   In addition, petechial haemorrhages in horses,⁹⁹ depression and lameness in dogs,⁸⁸ and hyperaesthesia, conjunctivitis, and incoordination in cats have been associated with the disease.^{45, 119} In humans, symptoms associated with HGA manifest one to two weeks after the bite of an infected tick and are generally flu-like, including fever, headache, myalgia, and lethargy.  Thrombocytopenia, leukopenia, and liver damage have also been reported.^{25, 310} In the US, the case fatality rate associated with A. phagocytophilum infections is less than 1 per cent,⁴¹ and hospitalization rates have been reported at 36 per cent, of which 7 per cent of the patients needed intensive care.⁸⁶ Human infections are much less frequently reported in Europe and Asia.  However, serosurveys revealed antibodies in humans throughout both continents.²⁴⁵ The drug of choice for treatment of human infections is doxycycline (rifampicin can be used as an alternative if necessary).²⁴ In domestic animals, prophylactic treatment with tetracycline has been used when moving animals into tick infested pastures.⁵² Long-acting oxytetracycline is effective in treating infections in domestic ruminants, horses, dogs, and cats.²¹

Anaplasma phagocytophilum has been detected by molecular methods in a variety of tick species, but the vector competence has only been demonstrated for Ixodes scapularis, Ixodes pacificus, Ixodes ricinus, and Ixodes spinipalpis.³¹⁵ In North America, A. phagocytophilum has been detected in I. pacificus, I. spinipalpis, Dermacentor variabilis, and Dermacentor occidentalis in the western parts of the USA;^{124, 161, 324} I. scapularis and Amblyomma americanum in the eastern, upper midwestern and southern USA^{63, 179, 256} and I. scapularis and Dermacentor albipictus in Canada.^{26, 158} Similarly, A. phagocytophilum has been detected in a wide range of tick species throughout Europe including I. ricinus (the main vector), Ixodes persulcatus, Dermacentor reticulatus, Haemaphysalis concinna,and Ixodes ventalloi.^{231, 266, 297, 303} In Africa, although fewer surveillance studies for A. phagocytophilum have been performed compared to other tick-borne diseases, the studies have nonetheless revealed the molecular presence of the bacterium in multiple species of ticks including Rhipicephalus sanguineus (Egypt);¹⁰⁵ Amblyomma variegatum, Amblyomma lepidum, Amblyomma cohaerens (Ethiopia);^{125, 301} Hyalomma marginatum, Hyalomma excavatum, Rhipicephalus bursa, I. ricinus, R. sanguineus, Hyalomma detritum (Algeria, Tunisia, Morocco);^{183, 269} Rhipicephalus evertsi evertsi, Rhipicephalus decoloratus, Amblyomma hebraeum (South Africa);²⁰⁷ Rhipicephalus pulchellus (Kenya);²²³ and Amblyomma flavomaculatum (collected from lizards imported from Ghana).²¹⁷ The abovementioned ticks were collected from animals (domestic or wild), and from vegetation of a wide range of habitats.

The occurrence of A. phagocytohilum (and its variants) in domestic and wild animals and humans has been reported in a limited number of studies in Africa including in humans in Morocco,¹⁴⁸ and dogs in Tunisia, Morocco, Algeria, South Africa, and Cape Verde.^{23, 147, 153, 157, 182} In Tunisia, immunofluorescence assays (IFA) and nested polymerase chain reaction  (PCR) assays and DNA sequencing were used to detect the presence of A. phagocytophilum in healthy horses.^{37, 183}

In Europe, A. phagocytophilum has been reported to cause persistent infections in sheep that may act as a reservoir.²⁹⁵ Anaplasma phagocytophilum has been identified in the blood of clinically ill sheep in Senegal using PCR and DNA sequencing.⁸¹ In Algeria, infected dairy cows  showed fever, decreased milk production, cough, and  oedema of the distal parts of the body.⁶⁷ The widespread occurrence of infection in Africain a variety of both ticks and mammals underscores the need for additional surveillance and transmission studies.

Variants based on the highly conserved 16S rRNA gene⁵⁶ of A. phagocytophilum have been detected in ticks and domestic/wild animals worldwide.  Among the 15 genetic variants that have been recognized based on comparison of the 5’ variable fragments of the 16S rRNA gene, variants 1 (accession number U02521, prototype strain, Ap-ha) and 2 (accession number AF093789, CAHU-HGE1)  are most often reported in surveillance studies.²⁴⁵ Variants 1 and 2 have been reported in I. scapularis, I. ricinus, I. persulcatus, and Ixodes ovatus ticks, as well as in sheep, cattle, horses, dogs, cats, deer, and bison in both the US and Europe.^{59, 193, 298, 309} Moreover, human cases of anaplasmosis associated with both variants have been reported in the US.^{58, 59} In Africa, closely related variant strains of A. phagocytophilum have been reported in sheep, goats, and cattle from Tunisia and Algeria and have also been detected in ticks and mammals in Japan and China.^{34, 36, 67, 135, 139, 220, 320, 323}

Ehrlichia chaffeensis infections (Canine ehrlichosis, Human monocytic ehrlichiosis)

Ehrlichia chaffeensis is a causative agent of both human monocytic ehrlichiosis and canine ehrlichiosis in North America, South America, Asia, and Africa. It is transmitted by ticks²⁴⁵ and infects monocytes and macrophages.  Amblyomma americanum was the first tick species identified as a vector for E. chaffeensis in the US, in 1993.^{12, 72, 74} Since then, E. chaffeensis has been detected in a variety of tick species including Ixodes pacificus and Dermacentor variabilis in the US, Haemaphysalis longicornis and Ixodes persulcatus in South Korea, Dermacentor silvarum, Amblyomma testudinarium and Haemaphysalis yeni in China, Amblyomma parvum in Argentina,^{55, 151, 165, 272, 304, 312} Hyalomma impeltatum in Nigeria and in Rhipicephalus sanguineus in Cameroon.^{212, 252}

Ehrlichia chaffeensis was first isolated in1991 from a patient with symptoms of human monocytic ehrlichiosis.^{10, 212, 245} Human monocytic ehrlichiosis was first described in 1987 and thought to be caused by E. canis, but it is possible that E. chaffeensis was the aetiologic agent in this case.^{12, 186, 212, 245} While, E. chaffeensis DNA has been detected in febrile patients in Cameroon, the organism has only been isolated from symptomatic humans in the US.^{245, 247} Various Ehrlichia species (E. canis, E. ewingii, E. chaffeensis ) may cause  canine ehrlichiosis.^{73, 245}

There is a close relationship between E. chaffeensis and E. canis. and both can infect dogs.^{50, 71, 73, 170, 173} Dogs infected with E. chaffeensis may show fever, lethargy and thrombocytopenia.^{50, 170, 173} Experimental infection of dogs  showed that E. chaffeensis can result in persistent infections lasting at least two to four months, thereby providing evidence that dogs are likely a reservoir for E. chaffeensis.³²⁸ North American white-tailed deer can become infected with E. chaffeensis and are believed to be reservoirs of the parasite.^{71, 72, 173, 245} Erlichia chaffeensis has been detected in many vertebrate hosts including goats in China and the US, Sika deer in Japan and South Korea, coyotes in the US, and wild rats in China.^{84, 156, 164, 312, 327}

In an attempt to better understand E. chaffeensis, red and grey foxes, white- tailed deer, and calves were experimentally infected with E. chaffeensis,^{69, 74, 77, 308}which resulted in persistent infections in white-tailed deer^{70, 74, 308} and red but not grey foxes,⁶⁹ Calves developed mild clinical signs such as fever, neutropenia, and thrombocytopenia to severe disease with lethargy, muscular weakness, and death.⁷⁷ Amblyomma americanum and Dermacentor variabilis ticks that fed on the infected calves  became infected with E. chaffeensis.⁷⁷

Ehrlichia canis infection (Canine monocytic ehrlichiosis)

Ehrlichia canis was first described in dogs from Algeria in 1935.⁸² It occurs worldwide causing canine monocytic ehrlichiosis and ehrlichiosis in humans, with increased infection rates in tropical and subtropical regions.^{82, 245, 281} The main vectors are Rhipicephalus sanguineus and Dermacentor variabilis^{82, 97, 114, 136, 213, 233, 245, 281}but E. canis has also been recovered from Haemaphysalis sulcata and Dermacentor marginatus in Italy and from Haemaphysalis longicornis and Ixodes turdus in South Korea,^{152, 272} and Rhipicephalus bursa in Sardinia. Few studies have been undertaken to determine if livestock are susceptible to infection with E. canis.¹⁹²

Dogs and wild canids such as silver backed jackals, coyotes, red and grey foxes, and wolves and wolf x dog hybrids, are reservoir hosts for E. canis.^{82, 97, 114, 236, 245, 299} Rhipicephalus sanguineus and Dermacentor variabilis ticks have been shown experimentally to acquire infections from dogs and were able to transmit E. canis to dogs.^{82, 97, 114, 136, 236}

Subclinical infections are common in dogs.  Canine monocytic ehrlichiosis can result in subclinical infection and acute and chronic disease.^{65, 245, 254} Canine ehrlichiosis caused by E. canis is much more severe than disease caused by other Ehrlichia species.^{65, 245, 254} Acute disease is characterized by an incubation period of 8-20 days, fever, depression, dyspnoea, anorexia, splenomegaly, thrombocytopenia, leukopenia, anaemia, and hypergammaglogulinaemia and the presence of parasites in monocytes.^{65, 117, 245, 281} Subclinical infections can last for years in otherwise healthy dogs.  Chronic canine monocytic ehrichiosis is usually the most severe form of the disease, leading to lethargy, anorexia, weight loss and death.^{117, 281}

A novel Ehrlichia most closely related to E. canis has been detected in North American and Brazilian cattle, and a calf experimentally infected with it exhibited clinical signs of ehrlichiosis including fever, depression, lethargy, and thrombocytopenia.^{4, 101}

Ehrlichia ewingii infections (Canine granulocytic ehrlichiosis)

Ehrlichia ewingii, unlike other Ehrlichia species, infects granulocytes, and was first identified as a new strain of E. canis causing canine granulocytic ehrlichiosis in 1971, but was later realized to be a completely different species of Ehrlichia, and subsequently named E. ewingii in 1992.^{10, 245, 318} Ehrlichia. ewingii occurs in the US, Africa, and Asia. It is the cause of human ewingii ehrlichiosis.^{11, 54, 245} Amblyomma americanum and Dermacentor variabilis in the US, Rhipicephalus sanguineus in the US and Cameroon, and Haemaphysalis longicornis in Korea have been shown to harbour E. ewingii.^{19, 152, 209, 213, 294} Amblyomma. americanum has been shown experimentally to transmit E. ewingii to dogs.¹⁹

Ehrlichia ewingii causes a much milder disease in dogs than E.canis^{65, 109, 170, 245, 318}and clinical signs include fever, lameness, neutrophilic polyarthritis, and thrombocytopenia.^{11, 19, 65, 109, 245} Most E. ewingii infections in dogs are subclinical, leading to the belief that dogs are a reservoir host for E. ewingii.^{1, 2, 13, 50} Like E. chaffeensis, white-tailed deer in the US seem to be a reservoir host for E. ewingii.^{245, 318}

Recently, E. ewingii has been suggested as a possible cause of a febrile illness in goats in the US, after E. ewingii-infected A. americanum ticks were fed on them.¹⁷⁵ These goats showed fever, nasal discharge, lethargy, and neutropenia and became chronically infected and were able to transmit E. ewingii to A. americanum ticks.¹⁷⁵

Ehrlichia muris infections (Murine splenomegaly)

Like E. canis and E. chaffeensis, E. muris infects monocytes and macrophages.  Its distribution is currently limited to Eurasia.²⁴⁵ Vectors for E. muris are Haemaphysalis species and Ixodes species including Haemaphysalis flava in Japan, Ixodes ricinus in Europe, and Ixodes persulcatus in Russia and Japan.^{7, 92, 128, 140, 159, 186, 245, 246, 248, 278, 289}  Small rodents are the reservoir host for E. muris, as it has been detected in voles, field mice, shrews, and Siberian chipmunks in Russia, yellow-necked mice in Slovakia, and wild mice in Japan.^{140, 245, 246, 289} Mice have been shown to exhibit moderate splenomegaly due to infection with E. muris.³¹³ Sika deer in Japan have been found to harbour E. muris.³⁰⁰ Antibodies to E.muris have been detected in dogs, mice, monkeys, bears, and deer in Japan.¹⁴⁰

Genus Rickettsia

Diseases associated with the genus Rickettsia have a long, extensive history of plaguing mankind – both military and civilian populations;¹⁴² currently several rickettsial pathogens are considered to be emerging infectious agents and are noted as public health issues.²²⁷ Rickettsial pathogens can be found throughout the world, primarily as a result of the cosmopolitan nature of their associated vectors (ticks, fleas, lice, and mites).  Currently, there are more than 25 recognized rickettsial species in the spotted fever group and two species in the typhus group.⁶⁰ As observed with the organisms of the Anaplasmataceae, rickettsial pathogens and their associated vectors occur frequently at human/animal interfaces, underscoring the need for continued surveillance these often-emerging infectious agents.  Here, we discuss those Rickettsia that have been detected in surveillance studies, in Africa, many of which have also been detected in several tick species and countries all over the world, (see Table 2).^{5, 44, 75, 98, 106, 116, 149, 154, 169, 203, 222, 227, 228, 235, 237, 238, 243, 274, 290, 302, 326}

Rickettsia felis infections

Rickettsia felis was first identified in Ctenocephalides felis (commonly known as the cat flea) in the US in 1990.³ It is recognized as the causative agent of flea-borne spotted fever (aka cat-flea typhus, R. felis rickettsiosis).  Fleas, ticks, mites, lice, and mosquitoes are potential vectors of R. felis.^{1, 80, 250, 286}  Despite the detection of R. felis in a many potential vectors, the cat flea (Ctenocephalides felis) is currently the only recognized vector of R. felis.

Rickettsia felis infections in humans have been reported in countries in North America, South America, Europe, Africa, Asia, and New Zealand and Australia.^{171, 251, 314} Infection and disease associated with R. felis is widespread in Africa.⁸⁹ In Senegal, Kenya, Mali, Tunisia, and Algeria, 1-5 per cent of human febrile diseases were shown to be linked to infection with R. felis.^{201, 253, 285} However, other studies have reported R. felis infections associated with febrile diseases to be as high as 10-15 per cent in Senegal and Gabon.^{201, 206} Rickettsia felis has been detected in the blood and/or in fleas collected from domestic and wild animals, including dogs, cats, rodents, hedgehogs, and monkeys on several continents.^{6, 122, 123, 167, 257}

Recent studies detected R. felis-like organisms (RFLOs) in in a variety of ectoparasites.  Among the RFLOs  are Rickettsia asembonensis (fleas from Kenya^{180, 189}), Rickettsia species F144 and F143 (fleas from Thailand⁹⁶), Rickettsia species SE313 (fleas and mites from rats in Egypt¹⁷⁶), Rickettsia species RF2125 (fleas from the Thai-Myanmar border,²²⁹ US,^{214, 249} and Hungary,¹²⁶), Rickettsia species clone ric_Ag_101731 (mosquitoes from the Ivory Coast²⁸⁶), and Rickettsia species SG10 (tsetse flies in Senegal¹⁹⁸).  Full characterization studies including isolation in pure culture of these RFLOs are needed in order to better define their importance and pathogenicity in mammalian hosts.

Rickettsia typhi infections

Rickettsia typhi is the causative agent of flea-borne (endemic) typhus, and is vectored by the Oriental rat flea (Xenopsylla cheopis), with rodents (primarily Rattus rattus and Rattus norvegicus) serving as the reservoir hosts of the bacterium.  Besides the rat flea, R. typhi has been detected in other flea species including Xenopsylla brasiliensis (Tanzania),¹⁶⁸ Ctenocephalides felis (Spain),²¹⁶ Leptopsylla segnis (Cyprus),⁶² Ctenophthalmus congeneroides and Rhadinopsylla insolita (Korea).¹⁵⁵ Moreover, R. typhi has also been detected in ticks, lice, and mites,^{107, 110} indicating the possibility of a wide variety of vectors in the transmission of R. typhi. Rickettsia typhi has been detected in various rat and mouse species, shrews, opossums, and cats,^{46, 160, 216} as well as in domestic ruminants in Egypt.²⁷³Murine typhus was recognized as a different disease from epidemic typhus (Rickettsia prowazekii) in the 1920s.¹⁹⁷ 199 It has a worldwide distribution in a variety of climates, and is often found in port or coastal cities where rodents can be commonly found.¹⁴² In Africa, there are few studies on the prevalence of R. typhi.  It has been detected in fleas collected in Algeria, Benin, and Tanzania.^{43, 168} Human disease associated with murine typhus in Africa has been reported from Algeria, Tunisia, Morocco, Ivory Coast, Central African Republic, Madagascar, Reunion, Chad, Tanzania, and Senegal.^{15, 17, 103, 204, 311, 329} It has been suggested that the disease in Africa is possibly under reported considering the wide distribution of its vectors and reservoirhosts.

Rickettsia africae infections

Rickettsia africae is the causative agent of African tick bite fever (ATBF) and is known to be the most prevalent tick-borne rickettsiosis in sub-Saharan Africa.190 It is often found to be the cause of illness in tourists visiting countries in sub-Saharan Africa.^{9, 131, 133, 242} The ticks that are recognized as the principle reservoir hosts or vectors of R. africae are Amblyomma hebraeum  and Amblyomma variegatum¹³² (see Tick vectors: an African perspective). It has been demonstrated that these ticks can maintain infection with R. africae both transovarially and transtadially, allowing for transmission of the bacteria to animals and humans during all of these ticks’ life cycle stages.^{150, 283} Infection rates of these tick species tends to be very high, with rates of almost 100 per cent  in endemic areas.^{211, 225} In addition to A. hebraeum^{200, 264} and A. variegatum, R. africae has also been detected in a variety of ticks throughout Africa, although their role in the transmission of R. africae has not been determined yet. These include Amblyomma lepidum,²⁰⁸ Amblyomma compressum,²⁰⁰ Hyalomma marginatum rufipes,⁹⁰ Hyalomma impeltatum,²¹⁹ Hyalomma impressum,⁹⁰ Haemaphysalis paraleachi,²⁰⁰ Rhipicephalus annulatus,^{199, 200, 252} Rhipicephalus evertsi evertsi,^{199, 252} Rhipicephalus decoloratus,^{200, 219} Rhipicephalus geigyi,²⁰⁰ Rhipicephalus sanguineus,²¹⁹ and Rhipicephalus microplus.⁹⁰ Rickettsia africae variants have been identified in various tick species throughout Africa (i.e. Kenya, Niger, Mali, Sudan, Burundi, Mauritania);^{91, 181, 225} however, it has not been determined whether or not the variants are associated with ATBF.

Rickettsia africae is closely associated with animals in Africa including cattle, sheep, goats, and wild ruminants due to the main vectors’ (A. hebraeum, A. variegatum) preference for these hosts.¹⁸⁸ In particular, 80-100 per cent of cattle in endemic areas have been shown to have high seroreactivity to SFG rickettsiae, leading to the assumption that cattle may be involved in maintenance of the pathogen.^{143, 144, 230} Similarly, high seroprevalences have also been observed in goats and sheep.^{143, 188, 230} Despite seroprevalence among livestock, the levels have not been shown to be exclusively linked to exposure of the animals to ticks infected with R. africae,¹⁸⁸ emphasizing the necessity for additional studies to better understand the relationship between the vectors of R. africae, their livestock hosts, and associated human infections.

Rickettsia aeschlimannii was isolated in 1992 from Hyalomma marginatum marginatum ticks in Morocco²²⁶ and the first case of spotted fever due to infection with R. aeschlimannii was in a patient who had travelled to Morocco.²⁴⁰ More recently, cases have been reported in Algeria and South Africa.^{205, 226} Ticks of the genus Hyalomma are suspected to be the main vectors,⁴² and the bacterium has been detected in H. marginatum marginatum (Algeria, Morocco, Egypt),^{2, 44, 86} Amblyomma variegatum (Nigeria),²⁵² Hyalomma impeltatum (Nigeria, Senegal),^{252, 264} Hyalomma marginatum rufipes (Mali, Niger, Senegal, Nigeria, Ivory Coast)^{90, 137, 225, 264}Hyalomma truncatum (Senegal, (Ivory Coast),^{90, 199} Rhipicephalus evertsi evertsi (Nigeria, Senegal)^{252, 264} and Rhipicephalus annulatus (Nigeria).²⁵² The ticks in which R. aeschlimannii was detected were collected from domestic animals such as cattle, sheep, goats, donkeys, horses, and camels; however, no link between infections in animals and humans has yet been demonstrated.

Rickettsia conorii infections

Rickettsia conorii subsp. conorii is the causative agent of Mediterranean spotted fever (MSF) in humans and is vectored by Rhipicephalus sanguineus ticks.  The disease was first reported in humans in 1910 in Tunisia.⁶⁶ More recent cases of MSF have been reported in Algeria, Tunisia and Morocco,^{146, 258} as well as among travellers returning from Senegal and other countries in sub-Saharan Africa.^{33, 53, 199, 322} The animal reservoir associated with R. conorii subsp. conorii has not been explicitly defined; however, dogs and hedgehogs have been suggested, based on seroprevalence studies and human proximity to dogs as risk factors for contracting MSF.^{44, 232, 287} Additionally, R. conorii has been detected in ticks (Rhipicephalus mushamae, Haemaphysalis leachi, Rhipicephalus simus, Rhipicephalus evertsi evertsi, Haemaphysalis punctaleach) removed from domestic animals (cattle, dogs, and/or horses) in the Central African Republic, Zimbabwe, Senegal, and Uganda.^{199, 226, 284} The role of these ticks as vectors of R. conorii has not yet been determined.

Rickettsia conorii subsp. israelensis is also vectored and maintained transstadially³²⁵ by Rhipicephalus sanguineus. It is the causative agent of Israeli spotted fever (ISF) in humans, and dogs have been implicated as reservoir of the bacterium.^{28, 305}  The cases of ISF reported in Tunisia were in patients who did not report tick bites, but were in close association with dogs and/or worked in the livestock industry.³²¹ The bacterium has been detected in R. sanguineus ticks and in the blood of a dog in Nigeria.¹³⁸

Rickettsia massiliae infections

Rickettsia massiliae was first isolated from Rhipicephalus sanguineus ticks in 1992 near Marseille, France.²²⁷ It has been reported as a human pathogen in Europe and South America; in Africa reports of human infection are scant.  One study in Tunisia reported the presence of R. massiliae DNA in a patient.¹⁵⁰ The bacterium has been detected in many species of Rhipicephalus ticks in countries in North and West Africa (Algeria, Morocco, Senegal, Guinea, Nigeria, Ivory Coast).^{89, 227} In particular Rhipicephalus ticks collected from donkeys, cattle, and dogs showed high rates of infection, along with ticks collected from the vegetation,^{199, 200, 252} suggesting several possible reservoirs, as well as its subsistence in nature. Rickettsia massiliae was detected recently in the blood of cattle in Nigeria.¹⁷⁸

Rickettsia sibirica infections

Rickettsia sibirica subsp. mongolitimonae is the causative agent of lymphangitis-associated rickettsiosis (LAR), and was first isolated in 1991 from Hyalomma asiaticum ticks collected in Mongolia.²²⁶ Cases of LAR have been reported in visitors to Algeria, Egypt, and South Africa.^{98, 226, 282} In Senegal, infected Hyalomma truncatum ticks have been collected from cattle, donkeys, sheep, goats, and horses.¹⁹⁹

Rickettsia slovaca and Rickettsia raoultii infections

Both R. slovaca and R. raoultii have been linked to the syndrome scalp eschars and neck lymphadenopathy (SENLAT) following tick bites in 2010.¹⁶ The syndrome has also been termed tick-borne lymphadenopathy (TIBOLA) and/or DEBONEL (Dermacentor-borne necrotic erythema and lymphadenopathy).²²⁷ The bacteria have been detected in both Dermacentor marginatus and Dermacentor reticulatus ticks^{94, 134, 190} and serologically and molecularly in the blood of sheep, goats, and cattle in Europe.^{222, 226, 227}  Dermacentor marginatus ticks collected in Morocco and Algeria have been shown to harbour the agents of SENLAT.^{145, 268}

Rickettsia monacensis infections

Rickettsia monacensis was first identified as a human pathogen in 2005 in Spain and Italy,^{129, 185} and Ixodes ricinus ticks are the presumed vector.²²⁷ The bacterium has been detected in I. ricinus ticks collected in Morocco, Tunisia, and Algeria.^{79, 145, 268, 276}

Rickettsia helvetica infections

Rickettsia helvetica is vectored by Ixodes ricinus, the natural reservoir of the bacterium.²²⁷ In Europe, R. helvetica has been isolated from blood and/or tissue samples of hedgehogs, lizards, mice, deer, and wild boar.^{76, 291} The bacterium has been found in I. ricinus ticks in Morocco, Tunisia, and Algeria.^{145, 268, 276}

Table 2 Lesser known Rickettsia and their potential vectors, reservoirs, and geographical distribution

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