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Arenavirus Infections in the Southwest

By Amesh A. Adalja, MD, FACP, FACEP, June 20, 2014

A new report published in Emerging Infectious Diseases confirms earlier studies showing that unsuspected arenavirus infections are occurring in the US Southwest.1 The arenaviruses are a class that includes some highly lethal pathogens such as Lassa virus, Junin virus, and Muchapo virus. The CDC classifies these hemorrhagic fever-causing viruses in the highest bioterrorism risk category alongside the pathogens that cause smallpox, ebola, and anthrax.

These viruses, which mostly have rodent reservoirs, are scattered across the globe and are divided into New World and Old World varieties. The Old World arenaviruses include Lassa fever and lymphocytic choriomeningitis virus (LCMV); the New World varieties include Junin (Argentinian) hemorrhagic fever and Muchapo (Bolivian) hemorrhagic fever.2

In the US, only LCM virus is known to regularly infect humans; it is a well-known cause of meningitis in the fall. Rodent species in the US do harbor other arenaviruses, but human infections have been reported only rarely and documented in only a handful of studies.3,4

Mimics Hantavirus

The authors of the EID report studied people who were hospitalized in New Mexico or Arizona from 1993 to 2001 who had illnesses resembling the unrelated hantavirus pulmonary syndrome, with thrombocytopenia and fever. Those individuals were tested for IgG antibodies against 3 arenaviruses: Whitewater Arroyo virus (WWAV), LCMV, and Ampari virus (AMAV).1 Of the 173 patients studied, 8 (4.6%) were positive for IgG antibodies against WWAV. None were positive for LCMV or AMAV. The clinical manifestations in the patients with positive serology included headache, myalgia, and thrombocytopenia. The diagnoses given to those who tested positive included acute parvovirus infection and ARDS; 6 cases had no diagnosis.1

Understanding the Unexplained

Because it is often assumed that exotic and novel viral infections occur in the developing world, but not here, clinicians in the US do not typically consider these and other exotic diagnoses. However, as these arenavirus studies make clear, no locale is immune to such spillover events.

The fact that this study was focused exclusively on those hospitalized with illnesses mimicking hantavirus is important because it confirms that unexpected viruses can cause severe disease and that hospitalized patients who test negative for common pathogens should undergo more extensive testing.

Future studies should aim to understand the spread, clinical spectrum, and epidemiology of WWAV and other related arenaviruses.


  1. Milazzo ML, Iralu J, Fulhorst CF, et al. Antibody against arenaviruses in humans, southwestern United States. Emerg Infect Dis 2014. Accessed June 10, 2014.
  2. Peters CJ. Lymphocytic choriomeningitis virus, lassa virus, and the South American hemorrhagic viruses. In: Mandell GL, Bennett JE, Dolin R, eds. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. 7th ed. Philadelphia, PA: Churchill Livingstone; 2010.
  3. Milazzo ML, Campbell G, Fulhorst CF. Novel arenavirus infection in humans, United States. Emerg Infect Dis 2011;17:1417-20.
  4. Fulhorst CF, Milazzo ML, Armstrong LR, et al. Hantavirus and arenavirus antibodies in persons with occupational rodent exposure. Emerg Infect Dis 2007;13:532-8.