Coxiellosis is a zoonotic bacterial infection associated primarily with parturient ruminants; however, domestic animals such as cats and a variety of wild animals have been identified as sources of infection in humans. The zoonotic infection in humans associated with Coxiella burnetii is widely known as Q fever. Coxiella is considered a potential agent of bioterrorism because of its low infectious dose, stability in the environment, and ability to be dispersed in aerosol form.
Etiology, Epidemiology, and Transmission of Coxiellosis in Animals
Coxiellosis is due to the gram-negative coccobacillus C burnetii. Although formerly considered a rickettsial agent, phylogenetic analyses suggest that C burnetii is more closely related to Legionella and Francisella than to the genus Rickettsia.
C burnetii resides and reproduces in the acidified phagolysosomes of host monocytes and macrophages. Two forms exist: the large-cell variant is a vegetative form found in infected cells; the small-cell variant is the extracellular infectious form shed in milk, urine, vaginal secretions, semen, and feces, and found in high concentration in placental tissue and amniotic fluid. The small-cell variant is resistant to heat, desiccation, and many common disinfectants, and it remains viable for weeks to years in the environment. After infecting a domestic ruminant, C burnetii can localize in mammary glands, supramammary lymph nodes, placenta, and uterus, from which it may be shed in subsequent parturitions and lactations. Both acute and chronic signs of disease are observed.
The epidemiology of coxiellosis is complex because there are two major patterns of transmission. In one, C burnetii circulates between wild animals and their ectoparasites, mainly ticks. The other form of transmission occurs in domestic ruminants, independent of the wild animal cycle. Ixodid and argasid ticks can act as reservoirs of the organism. Sexual transmission in humans has been reported; however, it appears to be of minimal importance in the epidemiology of coxiellosis in animals. C burnetii is found throughout the world (except New Zealand), and the host range includes various wild and domestic mammals, arthropods, and birds. Coxiellosis is enzootic in most areas where cattle, sheep, and goats are kept.
In the US, seroprevalence studies have shown substantial variability in the prevalence of antibodies against C burnetii, according to differences in the test population, test used, and time of year. Evaluation of bulk-tank milk sampling of US cattle dairies has demonstrated a prevalence of the organism at the farm level of 77% to >90%. In a comprehensive seroprevalence study conducted in Canada, 48.6% of sheep operations and 63.2% of goat operations had at least one positive animal. Not surprisingly, seroprevalence in veterinarians and in farmers who raise small ruminants is also high.
The greatest risk of transmission occurs at parturition, through inhalation, ingestion, or direct contact with birth fluids or placenta. The organism is also shed in milk, urine, and feces. High-temperature pasteurization effectively kills the organism. Ticks may transmit the disease among domestic ruminants; however, they are not thought to play an epidemiologically important role in the transmission of disease to humans.
Clinical Findings of Coxiellosis in Animals
Coxiellosis infection in ruminants is usually subclinical; however, it can result in anorexia and late abortion. Subclinically affected animals shed much lower bacterial loads of the organism in fetal fluids and vaginal secretions than are shed in cases of abortion. Reports have implicated C burnetii as a cause of infertility and sporadic abortion with a necrotizing placentitis in ruminants. New evidence has shown an association of C burnetii with subclinical mastitis among dairy cows; however, additional work is required to verify C burnetii as the causative agent. Experimental infection in cats causes transient fever, malaise, and anorexia lasting several days.
Diagnosis of Coxiellosis in Animals
Microscopic evaluation of tissue smears
Culture of tissue samples
In domestic ruminants, gross lesions of coxiellosis are nonspecific, and differential diagnosis should include infectious and noninfectious agents that cause abortion. Because seroprevalence is high in many animal populations, pathogen confirmation is important for definitive diagnosis; severe placentitis is grossly evident and the organism can be readily observed on smears of histopathologic samples of these tissues. Culture, immunohistochemical, and PCR tests may then be used to confirm the diagnosis of C burnetii. Studies conducted in veterinary diagnostic laboratories suggest that C burnetii is often found concurrently with other organisms isolated in cases of infectious abortions, so mixed infections may be important. Seasonal variability in shedding of the organism hinders the interpretation of the results of a single PCR assay. Shedding is highest in the periparturient period and may drop below detectable levels for a lengthy period during the year, despite persistent infection. Immunofluorescence tests on paired serum samples obtained ≥ 2 weeks apart can be used to detect recent infection; in up to 20% of infected animals, however, C burnetii may be shed in the absence of a measurable serum antibody titer.
Coxiellosis is a reportable disease in many animal health jurisdictions, and it is considered an OIE-notifiable disease in many species.
Treatment and Control of Coxiellosis in Animals
Vaccination where available
Q fever in humans is a notifiable disease in the US, primarily because of its status as a possible bioterrorism agent; reporting requirements for animals vary by state. Vaccines for humans and animals have been developed; however, they are not commercially available in the US. Vaccination has prevented infection when administered to uninfected calves and has improved fertility and decreased shedding in previously infected animals.
Antimicrobials (doxycycline or trimethoprim/sulfamethoxazole) are typically used to treat acute coxiellosis in humans. However, appreciable benefit from tetracycline treatment has not been demonstrated in controlled studies of abortion outbreaks in sheep in Europe. Despite the lack of evidence, some veterinarians still advocate the use of parenteral tetracyclines during abortion storms. In herds known to be infected, the periparturient period represents a substantial risk for transmission because of the large amount of environmental contamination associated with abortion. Standard abortion control measures, including prompt removal of aborted materials (using zoonotic precautions), segregation of animals by pregnancy status, and diagnostic evaluation of abortions, are all warranted. Environmental control measures include keeping parturient animals downwind of the rest of the herd or housed in a controlled ventilation area, and minimizing dusty conditions on the premises.
Zoonotic Risk of Coxiellosis in Animals
Q fever occurs more frequently in persons who have occupational contact with high-risk species. The clinical presentation in humans is highly variable, ranging from a self-limiting, influenza-like illness to pneumonia, hepatitis, and endocarditis. C burnetii is highly infectious, and a single organism can reportedly cause infection via the aerosol route in humans. Individuals with valvular heart disease, vascular grafts, or arterial aneurysm, as well as anyone appreciably immunocompromised, are at particular risk. C burnetii has been associated with spontaneous abortions in people as well as animals, and pregnant women should take precautions to prevent exposure.
Most Q fever outbreaks in humans have been associated with wind dispersion of dust contaminated with C burnetii, from sites where sheep, goats, or cattle are kept. Farmers and veterinarians are at risk while assisting parturition. Slaughterhouse workers are at risk from contact with infected carcasses, hair, and wool. The pathogen may also be transmitted through the consumption of unpasteurized milk. Handling of infected tissue poses a threat to laboratory personnel. Q fever has occurred in personnel and human patients in medical institutions where latently infected sheep were used for research. Medical facilities using ruminants in research should attempt to purchase animals from flocks free of coxiellosis or use male animals when possible. In addition, workers should use adequate personal protective equipment to protect against small droplet and aerosol exposure during high-risk medical procedures.
Coxiellosis is an important cause of abortions, particularly in small ruminants. Cats may also be affected.
The pathogen, Coxiella burnetii, is zoonotic and quite prevalent in domestic ruminants. It is highly infectious and readily aerosolized during abortions.
Most people with coxiellosis are asymptomatic or have only mild symptoms, but certain individuals can be severely affected.
If C burnetii is identified in a population of animals, caretakers should be educated on the symptoms of human disease and should have personal protective equipment available when working with the animals.
Treatment with antimicrobials is not warranted in animals; vaccination is an effective strategy and the lack of availability of a vaccine in many parts of the world leaves supportive care, environmental control -with awareness of the zoonotic risk - as the primary means of management.
For More Information
The 2018 American College of Veterinary Internal Medicine consensus statement of coxiellosis provides in-depth recommendations regarding diagnosis, treatment, management, and zoonotic risk mitigation for this disease. Plummer PJ, McClure JT, Menzies P, Morley PS, Van den Brom R, Van Metre DC. Management of Coxiella burnetii infection in livestock populations and the associated zoonotic risk: a consensus statement. J Vet Intern Med. 2018;32(5):1481-1494. doi:10.1111/jvim.15229
National Association of State Public Health Veterinarians, National Assembly of State Animal Health Officials: Prevention and Control of Coxiella burnetii Infection Among Humans and Animals: Guidance for a Coordinated Public Health and Animal Health Response, 2013