CHAPTER 34 Coxiella burnetii infection

francisco ruiz-fons

Instituto de Investigation en Recursos Cinegeticos IREC (CSIC-UCLM-JCCM), Ciudad Real, Spain

Coxiella burnetii is the causal agent of Q fever, a disease affecting humans and animals.

AETIOLOGY

Coxiella burnetii, previously known as Rickettsia diaporica or R. burnetii, is a small, obligate, intracellular, Gram­negative bacterium (0.2—0.4 μm wide and 0.4—1 μm long) that belongs to the family Coxiellaceae and has a genome that ranges from 1.5 to 2.4 Mb, depending on the strain. Coxiella burnetii is found in two different morphological forms in persistently infected cells: the ‘small cell variant’ form and the ‘large cell variant’ form. These forms corre­spond to different intracellular development stages of C. burnetii. Coxiella burnetii also displays antigenic variations owing to mutations in the lipopolysaccharide (LPS). The smooth form of LPS corresponds to the phase I and the rough form of LPS to the phase II antigenic variant of C. burnetii. The phase I variant is found in naturally infected hosts and is highly infectious. The phase II form has low infectivity and only occurs after serial passages in cell cul­tures or embryonated egg cultures⁽¹⁾.

EPIDEMIOLOGY

GEOGRAPHICAL DISTRIBUTION AND HOSTS IN EUROPE

Coxiella burnetii is a ubiquitous infectious agent reported in almost worldwide distribution, including infection in livestock in many European countries. It has been shown to circulate among European wildlife in Cyprus (24 local and migratory bird species), the Czech Republic (red deer ( Cervus elaphus), roe deer ( Capreolus capreolus), fallow deer (Dama dama), mouflon ( Ovis aries) and Eurasian wild boar (Sus scrofa)), France (chamois (Rupicapra rupic- apra)), Italy (fallow deer and lion (Panthera leo) in a safari park), Poland (European bison (Bison bonasus)), Portugal (waterbuck (Cobus ellipsiprymnus) and sable antelope (Hippotragus niger niger) from a zoo), Slovakia (mouflon, red deer, fallow deer and yellow-necked mouse (Apodemus flavicollis)), Spain (red deer, roe deer, mouflon, Eurasian wild boar, European hare (L epus europaeus), wood mouse (Apodemussylvaticus), house mouse (Mus musculus), griffon vulture ( Gyps fulvus) and black kite (Milvus migrans)) and the UK (brown rat (Rattus norvegicus}).

Infectious Diseases of Wild Mammals and Birds in Europe, First Edition. Edited by Dolores Gavier-Widen, J. Paul Duff, and Anna Meredith. © 2012 Blackwell Publishing Ltd. Published 2012 by Blackwell Publishing Ltd.

HOST FACTORS

Virtually all life kingdoms, including mammals, birds and arthropods, are considered to be able to harbour C. bur­netii'¹'¹. It has been reported in domestic mammals, such as camel, horse, water buffalo, cattle, sheep, goat, swine, rabbit, guinea pig and mouse and domesticated birds such as chicken, pigeon, duck, goose or turkey⁽¹⁾. In wildlife, C. burnetii infection has been reported in ruminants, suids, carnivores (including marine carnivores) and small mammals, as well as in several migratory and resident bird species and reptiles, such as tortoises and snakes. The information available on C. burnetii in wildlife is mainly about exposure to the pathogen, and little is known about its ecological drivers. In humans, incidence of Q fever tends to be higher in men than women, and this is thought to be caused by an increased risk of occupational expo- sure⁽¹⁾. Risk of exposure to C. burnetii in sheep and goats seems to increase with age⁽²⁾, but management may be a significant driver of this risk. Based on known C. burnetii epidemiological patterns in livestock, host population factors such as density, social behaviour or the structure of the ecological community can be expected to influence the risk of exposure of humans to C. burnetii from wildlife sources.

ENVIRONMENTAL FACTORS

Coxiella burnetii has been found in more than 40 tick species, and ticks are presumed to play a relevant role in the wild cycle of C. burnetii. However, the epidemiologic role of ticks is not sufficiently understood and is currently controversial.

Human outbreaks of Q fever appear to be linked to season, with higher incidence in spring and summer⁽¹⁾, probably in association with the seasonality of livestock reproduction rather than to any specific climatic condi­tions.

Current knowledge is insufficient to define the epide­miological role of wildlife species in C. burnetii infection. Nonetheless, there are reports that suggest that wild mammals excrete C. burnetii in the same manner as domes­tic ruminants; these potentially infective tissues include products of abortion and stillbirth in wild ruminants⁽⁴⁾ and rabbits⁽¹⁾ as well as infective placenta in the case of a Pacific harbour seal (Phoca vitulina richardsi). Seroprevalence in black-tailed deer (Odocoileus hemionus columbianus) in North America was found to peak in January, then decrease until April and increase again after the calving season in May, suggesting a higher excretion of C. burnetii around the birth season⁽⁵⁾.

Transmission of C. burnetii from animal to animal and from animal to human occurs mainly by infective aerosols. Owing to the expected higher excretion associated with gestation and parturition, airborne transmission when animals abort or give birth could also be an epidemiologi­cal driver in wildlife.

PATHOGENESIS, PATHOLOGY AND IMMUNITY

Owing to the importance of airborne transmission of C. burnetii, the respiratory route of infection is considered the most important. Aerosol- transmitted infective forms of the organism have been found to infect humans with doses as low as just a single infective cell⁽¹⁾. The digestive, vertical and sexual routes of transmission are also possible. Coxiella burnetii is also excreted in milk.

Experimentally it has been shown that C. burnetii is engulfed by local macrophages after it enters the body. Bacteria are thereafter disseminated to the rest of the body, and localize particularly in tissues of the reticuloendothe­lial system, especially the spleen and liver.

In humans, where many cases of infection are asymp­tomatic, C. burnetii may cause atypical pneumonia, char­acterized by gross consolidation of the lungs, interstitial pneumonia and alveolar exudates, and granulomatous hepatitis⁽¹⁾. In domestic ruminants, infected placentae may show intercotyledonary fibrous thickening and discolored exudates; the myometrium and the stroma adjacent to the placentomal area may show a severe inflammatory response; trophoblast cells appear altered and foamy. Metritis may be observed in cattle. Similar pathological manifestations probably occur in wild animals. Chronic coxiellosis has not been reported in animals, although latent carriage of C. burnetii may occur in animals, as apparently happens in humans(¹). The cells, tissues and organs in which persist­ent C. burnetii infection occurs are currently not known.

Clearance of infection in pregnant goats observed after abortion was suggested to be caused by an effective immune response¹-⁶). After infection, the host produces anti-phase I and II antibodies to neutralize C. burnetii infection. Serologic patterns differ between species; whereas sheep are able to mount an early immune humoral response (within 2 weeks), anti-phase II antibodies reach their peak in cows some weeks later. Infection induces gamma inter­feron (FN-γ) production that activates monocytes and macrophages, producing nitrogen and oxygen intermedi­ates that lead to the intracellular killing of the pathogen. In vitro experiments have found that T-cell immunity is effective in the control of Q fever, although it cannot clear the infection and prevent chronic manifestations of the disease.

CLINICAL SIGNS

Clinical outcome of infection by C. burnetii is highly vari­able and depends primarily on the infection route, the infectious dose, the pathogen strain and host immune status(¹). In animals, disease caused by C. burnetii is rarely reported, and only reproductive manifestations of infection may be apparent. Experimental infection of chickens(⁷) and goats(⁶) showed no clinical outcome of infection, except for pregnant goats, in which late-term abortion occurred. Even though most cases are subclinical, both in animals and humans, acute onset of disease may be seen in humans (fever, atypical pneumonia, hepatitis, abortion) and animals (pneumonia, abortion, stillbirth, weak newborn animals, infertility, metritis, mastitis); and chronic disease (a chronic fatigue type syndrome, sometimes with endocarditis) has only been described in humans. The clinical outcome of infection by C. burnetii in wildlife consists of a few reports of abortion and stillbirth in zoo antelopes(⁴) and abortion in rabbits(¹), and recently Coxiella-Hike organisms have been found to cause fatal disease in psittacines and toucans in North America(⁸).

DIAGNOSIS

Diagnosis of acute Q fever in cases of abortion or stillbirth requires detection of C. burnetii. This can be done through staining tissue samples, immunohistochemistry, direct isolation in cell cultures, embryonated eggs or laboratory animals, capture enzyme-linked immunosorbent assay (ELISA) or by polymerase chain reaction (PCR); however, examinations must be done in biosecure facilities. There are several serologic tests employed for demonstrating the existence of anti- C. burnetii circulating antibodies¹-⁹). Detection of complement-fixing antibodies has been widely used to provide evidence of exposure to C. burnetii but indirect fluorescence assay (IFA) and ELISA have proven more sensitive and specific serologic methods. Dis­tinct detection of anti-phase I and II antibodies also helps to distinguish recent infections from historic infections.

Analysis of anti-C. burnetii antibodies (especially by IFA or ELISA) is the quickest and cheapest diagnostic method for epidemiological studies in wild animals. Some tests developed for domestic animals can be used in wildlife, but there is a need to develop serological tests for a wide range of wildlife species. For live animals and ticks, PCR is a very sensitive and quick method to detect C. burnetii presence or excretion by shedders, but many infected animals do not shed bacteria, reducing its diagnostic power. PCR undertaken on target organs could also be a good screening method alone or in combination with sero­logic tests for diagnostic purposes in wildlife.

MANAGEMENT, CONTROL AND REGULATIONS

Owing to its ubiquitous nature, high environmental resist­ance, wide host range and low infective dose, control and eradication of C. burnetii from endemic natural foci is difficult, if not impossible. Mass vaccination with phase I microorganisms/antigens has proven to be effective in reducing shedding and incidence of abortion in livestock when performed over a significant period of time and in conjunction with testing and culling¹-¹⁰). No vaccination control trials have been carried out in wildlife

PUBLIC HEALTH CONCERN

Q fever is considered an emergent or re-emergent disease. Recent massive outbreaks of Q fever in the Netherlands have indicated the need to control C. burnetii infection in livestock and to stimulate more intense epidemiological investigations. Most human outbreaks arise from infection from livestock, but little is known about the role of wild animal species. Increased human/wildlife exposure rates, increasing wild ungulate populations across Europe and the particular characteristics of C. burnetii, as men­tioned, make the need for research in wildlife of special relevance.

Significance and implications for

ANIMAL HEALTH

Coxiella burnetii has a significant impact in livestock because of the reproductive disorders it causes. As an example, 9% of ovine abortions in the Basque Country (Spain) were found to be caused by Q fever(¹¹). By contrast, recent surveys in the region have found low circulating C. burnetii antibodies in wild mammals and birds¹-¹²’¹³). This difference in seroprevalence may indicate that wildlife is not important in the epidemiology of livestock infection in this region, which is considered to be hyperendemic for Q fever(²). These results suggest that livestock are the more significant reservoir of C. burnetii rather than wildlife. The significance of wild species in the epidemiology of C. burnettii within and between human, domesticated and wild species is not known at present, and more research is required in this field.

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