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Statistics and Epidemiology

Annual Cases of Q Fever in the United States

Q fever was first recognized as a human disease in Australia in 1935 and in the United States in the early 1940’s. The "Q" stands for "query" and was applied at a time when the causative agent was unknown. Human Q fever is now known to be the result of infection with the obligate, intracellular bacterium, Coxiella burnetii. Cattle, sheep, and goats are commonly infected and may transmit infection to humans when they give birth. Coxiella burnetii can survive for long periods of time in the environment, and may be spread by wind and dust. The disease is global in distribution, with cases reported sporadically or occasionally as outbreaks. However, because Q fever may resemble other diseases, be mild, or even cause no symptoms in some people, cases of human Q fever are likely under recognized in the United States and elsewhere. Around 3% of the healthy adult U.S. population and 10-20% of persons in high-risk occupations (veterinarians, farmers, etc.) have antibodies to C. burnetii, suggesting past infection.

Q fever was made a notifiable disease condition in the United States in 1999 in order to better understand the epidemiology and magnitude of the disease. CDC compiles the number of cases reported by state and local health departments. The number of Q fever cases reported to CDC increased since the disease became reportable, from 17 cases with onset in 2000, to 167 cases with onset in 2007.  The incidence (the number of cases for every million persons) of Q fever increased similarly, from less than 0.1 case per million persons in 2000 to 0.6 cases per million persons in 2007.  The surveillance case definition for Q fever was modified in 2008 to revise laboratory criteria for diagnosis and to allow for the separate reporting of acute and chronic Q fever.  During 2008 –2010, the number of reported cases decreased slightly, relative to 2007, and the incidence rated has  decreased to 0.4 cases per million persons. One hundred thirty-one cases of Q fever were reported with onset in 2010; of these, 106 were acute Q fever and 25 were chronic Q fever.

Figure 1 - Number of U.S. Q Fever cases* reported to CDC, 1998 – 2010.

*Numbers presented here may differ from numbers presented in the MMWR Annual Summary of Notifiable Diseases. Data presented here are based on year of illness onset rather than reporting year.

	annual q fever cases 1998-2010

Number of annual Q fever cases 1998 – 2010:

The graph displays the number of human cases of Q fever cases reported to CDC annually from 1998 through 2010. Cases of Q fever increased steadily from 13 cases in 1999 when the disease became nationally notifiable, to 167 cases in both 2006 and 2007. Cases decreased significantly in 2008 and 2009 with total cases at their lowest in 2009 (n=113) with a slight increase in 2010. Beginning in 2008, cases were differentiated as acute or chronic. Acute cases generally make up 80-90% of cases reported.

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Hospitalization Rates

Illness among patients with recognized and reported Q fever may be severe with complications requiring hospitalization that may include endocarditis, encephalitis, pneumonia, hepatitis, and splenomegaly. Rates of hospitalization among Q fever cases for which information was available averaged over 50% during 2002 to 2008. However, it is likely that mild Q fever infections which do not require hospitalization may be more likely to be under-recognized and therefore under-represented in current national surveillance systems.

Figure 2 – Hospitalization rates, U.S. Q Fever cases reported to CDC, 2002 – 2010.

	q fever hospitalization rate 2002-2010

Q fever hospitalization rate, 2002 – 2010:

This figure shows the proportion of hospitalized cases among cases reported to CDC between 2002 and 2010. The lowest rate of hospitalization was in 2002 at 25%. Highest proportions of hospitalized cases were reported in 2007 and 2009, both of which reported hospitalization rates of roughly 70%.

Year Hospitalization Rate
2002 25%
2003 56%
2004 63%
2005 42%
2006 52%
2007 70%
2008 55%
2009 71%
2010 61%

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Cases of Q fever are most frequently reported from western and plains states where ranching and rearing of cattle are common. In other states, areas where sheep, goat, and cattle ranching are locally practiced may demonstrate increased incidence. Seven states (California, Colorado, Illinois, Kentucky, Missouri, Tennessee, and Texas) have accounted for more than half (52%) of all cases since human Q fever became notifiable. Cases of Q fever are reported less frequently in the eastern United States. Sporadic reports of cases may result from patients involved in animal research work and from patient travel to other states.

Figure 3 – Annual reported incidence (per million population) for Q Fever in the United States for 2010.

	geographical distribution of q fever incidence in 2010

Geographic distribution of Q fever incidence in 2010:

This figure shows the incidence of Q fever cases by state in 2010 per million persons. Q fever was not notifiable in Iowa, New Hampshire, and Vermont in 2010. The incidence rate was zero for Alabama, Alaska, Connecticut, Delaware, Hawaii, Idaho, Indiana, Kentucky, Louisiana, Maine, Massachusetts, Mississippi, Oklahoma, Rhode Island, South Carolina, Utah and West Virginia. Incidence ranged between 0.1 to 0.4 cases per million persons for Florida, Georgia, Maryland, Minnesota, North Carolina, Ohio, Pennsylvania, Tennessee and Virginia. Annual incidence ranged from 0.4 to 1.0 case per million persons in Arizona, California, Colorado, Illinois, Michigan, Missouri, New Jersey, New York, Texas, Washington, and Wisconsin. The highest incidence rates, ranging from 1.0 to 4.9 cases per million persons were found in Arkansas, the District of Columbia, Kansas, Montana, Nebraska, Nevada, New Mexico, North Dakota, Oregon, South Dakota and Wyoming.

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Although cases of Q fever can occur during any month of the year, most cases report onset of illness during the spring and early summer months, peaking in April and May. These increases coincide with increases in human outdoor activity, and with the birthing season for a number of domestic animal species.

Figure 4 – Percent of Q Fever cases reported to CDC by month of onset, 1998–2010.

	q fever cases reported each month 1998-2010

Percent of Q Fever cases reported each month of onset 1998 – 2010:

This figure shows the percent of cases reported from 1998 through 2010 by month of onset to give the seasonality of cases. There are cases reported in each month of the year, ranging from 3% reported in November, to 14% of cases reported in May. November is the only month in which less than 5% of cases are reported, and the majority (58%) of cases are reported between March and July.

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Persons at Risk

The frequency of reported cases of Q fever increases with age and is highest among males, which may reflect occupational risks for exposure among livestock handlers. A past history of heart valve defects, endocarditis, or valvular implants may increase the risk of chronic infection and severe disease outcome. Infection is also more common in patients with compromised immune systems (such as may occur through cancer treatments, advanced human immunodeficiency virus (HIV) infection, prior organ transplants, or some medications). Individuals who reside or spend time near ranches and livestock rearing facilities are at increased risk for infection.

Figure 5 – Average annual incidence of Q Fever by age group, 2000 – 2010

	q fever incidence by age group 2000-2010

Q Fever Incidence by Age Group, 2000 – 2010

This figure shows the average annual incidence of Q fever per million persons by age groups for 2000 through 2010. The graph shows that cases have been reported in every age group with increased incidence as age increases. The highest rate of incidence, more than 0.7 cases per million persons, is seen in persons ages 60 – 64 years.

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Q Fever Reporting and Surveillance

The 1999 Q fever surveillance case definition was used to classify cases with onset years 2000 – 2007 into the categories probable and confirmed.   The 2008 case definition became official as of January 1, 2008 and allows the categories probable and confirmed to be further categorized to acute and chronic manifestations of the disease. The 2008 case definition was again corrected in 2009 to clarify laboratory supportive serologic evidence. The 2009 case definition will remain current until a future revision is proposed and accepted. For more information on how to report cases of Q fever and to download a Q fever case report form (CRF) , visit the “In-Depth Information” section .

Further Reading

McQuiston JH, Holman RC, McCall CL, Childs JE, Swerdlow DL, Thompson HA. National Surveillance and the Epidemiology of Human Q Fever in the United States, 1978 – 2004. American Journal of Tropical Medicine and Hygiene 75(2006),36 – 40

Parker NR, Barralet JH, Bell AM. Q fever. Lancet 367(2006),679 – 88

McQuiston JH, Childs JE. Q Fever in Humans and Animals in the United States. Vector Borne and Zoonotic Diseases 2(2002),179 – 91
Raoult D, Tissot-Dupont H, Foucault C, Gouvernet J, Fournier PE, Bernit E, Stein A, Nesri M, Harle JR, Weiller PJ. Q Fever 1985 – 1998: Clinical and Epidemiologic Features of 1,383 Infections. Medicine 79(2000),109 – 23

Morbidity and Mortality Weekly Reports (MMWR), CDC. Q Fever --- California, Georgia, Pennsylvania, and Tennessee, 2000--2001. 51(2002),924-7