Novel pathogens make inroads through human encroachment and global travel

By Gary Tufel

In recent years, populations in a number of regions throughout the world have found themselves at ground zero for an increasing variety of previously unknown infectious and zoonotic diseases. Such occurrences are becoming more and more widespread, and with the ease and speed of modern global travel has come even greater rapidity of disease transmission. In addition, some diseases long thought to have been eradicated in certain regions have begun to reappear—sometimes in antibiotic-resistant forms.

The number of such previously unknown diseases is growing, but it only takes one uncontrolled epidemic to create public health turmoil throughout the world. As a result of the recent ebola epidemic in West Africa, many healthcare professionals have a newfound understanding about how every nation’s health security increasingly depends on global health security. Yet, it can be extremely challenging to sustain public and private investment for the purpose of developing medical countermeasures (MCMs) before an emerging infectious disease becomes a public health emergency in the United States and worldwide.

A recent Institute of Medicine (IOM) workshop featured discussions on the development process for MCMs in the face of three types of recent infectious disease outbreaks, and explored the regulatory and operational challenges encountered when rapid response is needed for an already evolving worldwide emergency. Participants at the workshop also considered potential business models for future public-private partnerships for the creation of MCMs, and discussed what level of sustainable funding is needed to meet the demands of emerging threats across the globe.1

THE DISEASES

The National Center for Emerging and Zoonotic Infectious Diseases (NCEZID) at the US Centers for Disease Control and Prevention (CDC), defines an “emerging disease” as one for which infections have increased recently or are threatening to increase in the near future. Such infectious diseases may be:

  • Completely new, such as Bourbon virus (recently discovered in Kansas) or Middle East respiratory syndrome (MERS; linked to travel or residence in any of nine Middle Eastern nations).
  • Completely new to an area, as in the rise of chikungunya in Florida.
  • Reappearing in an area, as in the reappearance of dengue fever in South Florida, Texas, and most recently, Hawaii.
  • Caused by bacteria that have become resistant to antibiotics, such as Clostridium difficile, drug-resistant tuberculosis, or methicillin-resistant Staphylococcus aureus (MRSA).

NCEZID defines a “zoonotic disease” as an infectious animal disease that can be spread to humans by ticks, mosquitoes, fleas, or direct contact with animals. Examples include:

  • Lyme disease (spread by ticks).
  • Rabies (spread by mammals).
  • Salmonella (spread by poultry).

But those are by no means the only such diseases; they’re only the tip of the iceberg. The following sections describe other emerging and zoonotic diseases that have recenlty become problematic for humans.

Bubonic Plague. Prevalent in medieval times, but rare today, even bubonic plague is making something of a comeback. Plague is a disease that affects humans and other mammals. It is caused by the bacterium Yersinia pestis.

In the form most familiar to the modern world, this infectious bacterial disease is carried by squirrels, chipmunks, and other wild rodents—and their fleas. When an infected rodent becomes sick and dies, its fleas can carry the infection to other animals or humans through bites.2 Humans usually get plague after being bitten by a rodent flea that is carrying the plague bacterium or by handling an animal infected with plague. In this form, bubonic plague is infamous for having killed millions of people—roughly a third of Western Europe’s population—during the fourteenth century.

But recently, researchers from the University of Copenhagen have discovered that Y. pestis was not always transmitted in this way. The team sequenced Y. pestis DNA isolated from the teeth of human remains dated as being between 2,800 and 5,000 years old. The researchers determined that the ancient bacterial genomes were basal to all known Y. pestis lineages, and estimated the origin of the Y. pestis lineage to some 5,783 years ago—about 3,300 years earlier than previous estimates. The team’s analysis concluded that the ancient Y. pestis strains could only lead to pneumonic and septicemic plague—not to bubonic plague—and that Y. pestis didn’t become a flea-borne mammalian pathogen until the beginning of the first millennium BC.3

In recent decades, according to CDC, an average of seven human plague cases have been reported in the United States each year. Last October, a teen girl from eastern Oregon contracted bubonic plague. The Crook County girl was believed to have acquired the disease from a flea bite during a hunting trip. The trip started on October 16; she fell ill five days later, and was hospitalized three days afterward.

Today, modern antibiotics are effective in treating plague. But without prompt treatment, the disease can cause serious illness or death. Presently, human plague infections continue to occur in the western United States, but significantly more cases occur in parts of Africa and Asia.4

Cancer. Scientists at CDC recently discovered that cancer cells originating in a common tapeworm may take root in people with weakened immune systems, causing cancer-like tumors. The discovery arose from the case of a Colombian man whose lung tumors turned out to have an unusual cause. The rapidly growing masses weren’t actually made of human cells, but were from a tapeworm living inside him.

This first known case of a person becoming ill from cancer cells that developed in a parasite—in this case, Hymenolepis nana, the dwarf tapeworm—has raised concern that similar cases might be misdiagnosed as human cancer—especially in less-developed countries where the dwarf tapeworm and immune-system-suppressing illnesses such as HIV are widespread.5,6

Chagas Disease (American trypanosomiasis). An exotic and deadly bug-borne infection known by the name of Chagas disease is spreading in Texas. According to one report, one in every 6,500 blood donors in the state are infected with the disease, compared with one in every 27,500 donors across the country.7

Also known as American trypanosomiasis, Chagas disease is a parasitic disease that is spread mostly by insects known as triatomine bugs or “kissing bugs.” The disease is spread when insects carrying the parasite Trypanosoma cruzi are feeding on someone’s blood, and drop contaminated feces on the skin while feeding. Transmission occurs when the fecal material is rubbed into the bite wound or into a mucous membrane (such as the eye or mouth), and the parasite enters the body. The parasite can then enter the bloodstream, enabling it to affect the heart and gastrointestinal system.

Ebola. While public health authorities appear to have broken the West African epidemic of ebola virus that began in 2014, the high mortality rate of the disease makes it an ongoing subject of concern and planning. The wide-ranging impact of the disease at many levels of society—with social and economic consequences for nongovernmental organizations, public health authorities, diagnostics suppliers, healthcare professionals, as well as patients and their families—virtually ensures that ebola will remain an area of concern for many years to come. According to Thomas Theuringer, director of public relations for Qiagen, Germantown, Md, the company played a pivotal role during the West African outbreak, supplying extraction kits and enzymes, and commercializing Filovirus and Ebolavirus kits.

Norman Moore, PhD, Alere Inc.

Norman Moore, PhD, Alere Inc.

Influenza. Because influenza viruses can mutate rapidly, keeping up with the current strains in circulation remains a top concern of health professionals around the world. Public health authorities have long cautioned that lack of preparedness could result in a pandemic, potentially killing millions of people. “Influenza can mutate into unique strains, against which people have no immunity,” says Norman Moore, PhD, director of scientific affairs for infectious diseases at Alere, Waltham, Mass.

In the case of influenza, novel strains of the disease can be the result of people living in close proximity to animals, says Moore. “Influenza usually transmits well from birds to humans. However, pigs are genetically similar to people. Therefore, if a pig acquires a high-pathogen influenza virus from a chicken, and a highly infectious influenza virus from a person, they can recombine in the same cell and produce something that is both highly infectious and high mortality,” Moore explains.”

Valley Fever (Coccidioidomycosis). Digging in parched dirt or simply breathing in dry air is not without peril in certain parts of the Southwest, where a fungus called Coccidioides is known to infect many who inhale its spores. The resulting illness, called Valley Fever, has sickened an increasing number of people over the past decade and a half.

While often mild, Valley Fever can cause lingering flu-like symptoms and even lead to chronic illness when it spreads to other parts of the body, sometimes triggering such life-threatening complications as meningitis. “In cases where it gets into the brain and into the spinal cord, it can be very challenging to treat,” sometimes requiring lifelong therapy or even causing death, says Tom Chiller, MD, MPHTM, deputy chief of CDC’s mycotic diseases branch, which is dedicated to preventing death and disability from fungal diseases.

Fortunately, Valley Fever is rarely deadly, with fewer than 200 deaths attributed to it annually. But Chiller notes that it’s currently one of the most common infectious diseases in Arizona. It’s also common in California—including the San Joaquin Valley, from which the disease derives its name—as well as in Nevada, Texas, and lower Utah. In those regions where the Coccidiodes fungus grows, Valley Fever afflicts agricultural and other workers who disturb the soil as well as thousands of residents who come in contact with airborne spores.8

CDC recently published the report of a Valley Fever outbreak that affected at least 44 workers constructing two solar power farms in California’s San Luis Obispo County between 2011 and 2014. Those affected did work ranging from digging to electrical, which often put them in proximity to fungal spores that could be breathed in, causing infection. The workers reported symptoms ranging from fatigue and night sweats to fever, difficulty breathing, joint and muscle pain, and weight loss. Most needed to take time off from work and, for two, the disease spread elsewhere in their bodies—to the skin of one individual and the bones of another.9

Performing testing for West Nile virus using the Rotor-Gene Q real-time polymerase chain reaction cycler by Qiagen.

Performing testing for West Nile virus using the Rotor-Gene Q real-time polymerase chain reaction cycler by Qiagen.

West Nile Virus. The emergence and spread of West Nile virus in the United States has been rapid. The virus is transmitted to humans by mosquitos.10 Last November, California health officials reported 28 confirmed deaths so far in 2015 from West Nile virus. According to Qiagen’s Theuringer, the company offers a PCR kit for West Nile virus, which is sold predominantly in Canada and the Middle East.

RECENT EMERGENCE MAKES TESTING DIFFICULT

Kelly Wroblewski, MPH, MT(ASCP), Association of Public Health Laboratories.

Kelly Wroblewski, MPH, MT(ASCP), Association of Public Health Laboratories.

According to Kelly Wroblewski, MPH, MT(ASCP), director of infectious diseases at the Association of Public Health Laboratories (APHL), Silver Spring, Md, two zoonotic diseases of great concern worldwide are chikungunya and Middle East respiratory syndrome (MERS-CoV). As with other emerging diseases, US labs are finding it difficult to test for these diseases because of their recent emergence in the United States. Thus, Wroblewski says, labs need to be continually aware of the emergence of such diseases and vigilant in detecting them. (For more information, see the companion article, “One Health: Public Health Labs Crossing Spheres of Health.”)

Detecting new diseases is difficult, agrees Alere’s Moore. Quite often, labs simply don’t have the right tests. While many of the commercially available influenza tests should work on emerging strains, for instance, they won’t provide a result that indicates whether the detected strain is unique. “If we don’t have the appropriate tests for laboratories, it then becomes a question of noticing unusual trends and reporting them to the state laboratories that can then coordinate with CDC.”

The Alere i Influenza A&B assay is an automated, CLIA-waived, diagnostic test that detects influenza A and B viruses from nasal swab specimens within 15 minutes.

The Alere i Influenza A&B assay is an automated, CLIA-waived, diagnostic test that detects influenza A and B viruses from nasal swab specimens within 15 minutes.

In the case of influenza, says Moore, many tests for influenza do exist, including the Alere i Influenza A&B assay, an automated, CLIA-waived diagnostic test that detects influenza A and B viruses from nasal swab specimens within 15 minutes. “When there is an outbreak of something that can be deadly, the last thing we want is for those people to bring it to a hospital, to then spread it to other patients and the medical staff,” he adds. “Future pandemic outbreaks are going to have to be triaged much better by bringing the testing closer to the patient.

“We have seen the panic of ebola. We now know that we have a significant problem in training people on the appropriate way to take off gowns and gloves after dealing with potentially infectious patients,” says Moore. “For many years, microbiology was being centralized; now we see that having immediate answers closer to the patient is far better.”

Colorized transmission electron micrograph of avian influenza A H5N1 viruses (gold) grown in MDCK cells (green). Avian flu viruses are airborne and affect all birds. They do not usually infect humans, but several instances of infection and outbreaks have been reported. Source: Cynthia Goldsmith courtesy CDC.

Colorized transmission electron micrograph of avian influenza A H5N1 viruses (gold) grown in MDCK cells (green). Avian flu viruses are airborne and affect all birds. They do not usually infect humans, but several instances of infection and outbreaks have been reported. Source: Cynthia Goldsmith courtesy CDC.

Avian Influenza. Topping the list of newly emerging diseases receiving media attention is avian influenza (H5N1). The scope of the virus is wide, as it is airborne, affects all birds, and is easily transmitted to humans. Primary areas to be monitored for the transmission of avian influenza are the open-air markets of Asia, where birds are often slaughtered and sold. But according to Wroblewski, the disease has also become fairly widespread among commercial flocks in the American Midwest—most notably in Iowa and Minnesota—and in Washington State. (For more information, see the companion article, “Connecting Human + Animal + Environmental Health.”)

Where avian flu infection has been detected in the United States, the result has typically been significant culling of turkey and chicken flocks—and elevated concern that the disease will spread to humans. Avian-to-human transmission is not known to have occurred in North America, but even the possibility of transmission to humans is very concerning. “So labs need to be aware of the travel history of patients, their contact with birds, and with avian influenza,” says Wroblewski.

“The challenge is to be aware of what might be circulating when you are not able to identify a cause of illness,” says Wroblewski. Using respiratory panel tests, for instance, clinical labs may be able to detect avian influenza and some species of MERS, but they are typically not able to distinguish subtypes of these diseases.

“Because labs can’t offer tests for every disease,” says Wroblewski, “it’s critical for clinical labs to be aware of the activities of the Laboratory Response Network (LRN),” which tests for agents of biological and chemical terrorism, emerging infectious diseases, and other public health threats. The LRN was founded in 1999 by APHL, CDC, and the Federal Bureau of Investigation.

Chikungunya. The mosquito-borne chikungunya virus is passed to humans by two species of mosquito, Aedes albopictus and Aedes aegypti. The virus originated in Africa and began to appear in the Caribbean about 2 years ago. Symptoms are flu-like and include fever and achiness; although the disease is rarely fatal, it is very painful. “It is very debilitating,” says Wroblewski.

Travelers from the Caribbean region, Mexico, and South America began bringing the chikungunya virus from those areas to the United States in the summer of 2014. In addition, the disease has been transmitted to humans from native mosquito populations in South Florida. Animal reservoirs of the virus also include monkeys, birds, cattle, and rodents.

Although incidence of the disease is still relatively low worldwide, it is growing in Puerto Rico and the US Virgin Islands, which raises concern about chikungunya in the mainland United States. “The disease can be identified using molecular- or serology-based assays, but is not widely tested for yet,” says Wroblewski.

Lyme Disease. “When I think of established diseases in the United States that are spreading, I think of Lyme disease as well as other tick- and mosquito-borne diseases,” says Moore. “Unfortunately, one of the additional consequences of global warming is the spread of infectious agents. We now have disease-causing ticks in areas of the country where they simply weren’t decades ago.”

REEMERGING DISEASES

A “reemerging disease” is any disorder—usually an infectious disease such as cholera, malaria, or tuberculosis—that has been on the decline in the global population, but is now increasing due to changes in the health status of a susceptible population.

Some reemerging diseases, such as bubonic plague, were thought to have been eradicated. Others, such as tuberculosis, have decreased in the United States, but not globally. In either case, there is a danger that the reemerged strains of a disease may be more virulent or even resistant to current antibiotics. Some observers have even suggested that several emerging or reemerging infectious agents could be used as biowarfare agents.

“With increased travel and globalization, and with the appearance of extensively drug-resistant tuberculosis (XDR-TB), it’s vital that clinical labs be alert,” says Wroblewski.

Dengue Fever. According to Qiagen’s Theuringer, dengue fever is a major global unmet health challenge. “It is the most important vector-borne disease, with 3.6 billion people at risk of infection, 390 million cases in 2010 alone, 96 million symptomatic patients, 500,000 hospitalizations annually, and an annual fatality rate of 5% (25,000 patients),” he says.

Dengue originally came from Asia. But its reemergence on the world stage is much broader. According to a mid-year report, Brazil recorded 750,000 cases of dengue during the first half of 2015. The causes of dengue’s reemergence is also multi-factorial, including increasing population, increasing travel, climate change, deforestation, changes in pathogens, farming, and other factors.

Together with the Liverpool School of Hygiene and Tropical Medicine, Qiagen is currently developing a test kit for the dengue virus, says Theuringer.

Colorized scanning electron micrograph of Middle East respiratory syndrome coronavirus (MERS-CoV) particles (yellow) on the surface of a Vero E6 cell (blue). A zoonotic disease of great concern globally, MERS-CoV currently represents a low risk to the US public. Source: National Institute of Allergy and Infectious Diseases.

Colorized scanning electron micrograph of Middle East respiratory syndrome coronavirus (MERS-CoV) particles (yellow) on the surface of a Vero E6 cell (blue). A zoonotic disease of great concern globally, MERS-CoV currently represents a low risk to the US public. Source: National Institute of Allergy and Infectious Diseases.

Middle East Respiratory Syndrome Coronavirus (MERS-CoV). MERS-CoV first emerged in Saudi Arabia in 2012. It recently reemerged, with most of the incidents occurring in South Korea and the Middle East. During the spring and summer of 2015, MERS garnered more US press than chikungunya, says Wroblewski, even though the latter disease was and remains the more imminent threat to US residents.

The field of molecular diagnostics continues to evolve very rapidly, and is having an undeniable impact on the diagnosis of infectious diseases. Molecular tools have played a pivotal role in discovering and characterizing several emerging infectious agents, and have now become the gold standard for the diagnosis of infectious diseases caused by fastidious or uncultivable agents. FDA has already authorized a number of automated instruments capable of sample processing, multiplexed nucleic acid amplification, and postamplification analysis in the clinical setting.

Analysts expect to see continued exponential growth in the molecular techniques used for the initial diagnosis of infectious diseases. Molecular tools will also continue to have an impact on disease prognosis and response to therapeutic interventions. Automation, multiplexing, and miniaturization will continue to be driving forces in the development of new instruments. Nevertheless, multiple challenges still remain for the widespread use of cost-effective, validated, and commercially available molecular tools.11

ZOONOSIS AND THE FOOD SUPPLY

CDC defines a “tropical disease” as one belonging to a group of parasitic and bacterial diseases that causes illness in as many as 1 billion people worldwide.This group includes zoonotic diseases, which can be caused by viruses, bacteria, parasites, and fungi. Such diseases are very common. CDC scientists estimate that more than 6 out of every 10 infectious diseases in humans are spread via animals.

There are many ways such zoonotic diseases can spread. “When it comes to diseases like ebola,” says Moore, “transmission from animals can be the result of humans encroaching on areas normally inhabited by other primates.”

Peter Hotez, MD, PhD, founding dean of the National School of Tropical Medicine at Baylor University’s College of Medicine, estimates that as many as 12 million people in the United States are living with a neglected tropical disease. “We already have dengue (and chikungunya) in Texas and in the Gulf Coast, and I think we’ll start to see transmission of both diseases in the coming years,” says Hotez.12

Gilbert Greub, MD, PhD, director of the institute of microbiology at the University of Lausanne, points to such other zoonotic organisms as Coxiella burnetti, a zoonotic agent from sheep and goats that caused a small outbreak of Q fever in Lausanne; and Chlamydophila psitacci, a zoonotic agent from birds that often goes unrecognized unless PCR-based diagnostic tests are available.

Food-borne pathogens are often zoonotic. An example is shiga toxin-producing Escherichia coli (STEC), which originates in cows. Food-borne outbreaks can also be caused by a combination of animal and environmental conditions. In the case of an E. coli outbreak that occurred in 2006, for example, run-off from nearby cattle farms contaminated spinach crops, ultimately resulting in the removal of packaged spinach from the shelves of grocery stores across the United States.

Public health laboratories have long worked to protect people from food-borne illness through the PulseNet program and the Food Emergency Response Network (FERN). Teams from human, animal, and environmental backgrounds work together to solve food-borne outbreaks and prevent future occurrences.13

Today, FDA is funding public health laboratories to help with the accreditation of food testing and animal feed laboratories. Accreditation creates stronger labs with better data and a more robust public health laboratory system.

Alere’s Moore says the emerging infectious diseases and zoonotic diseases currently causing the most concern on a daily basis are the ones in our food supply.

“We can get Campylobacter, E. coli, and salmonella from contaminated or undercooked meat. We also run into problems where contaminated meat may come into contact with surfaces like cutting boards, where vegetables can then be contaminated,” says Moore. “For food-borne diseases, we are getting more of our produce from other countries that can be contaminated or, when there is a contamination in the United States, it can go into frozen food and spread over a significant distance.”

REDUCING ZOONOTIC INCIDENTS

Many people interact with animals in their daily lives, for food or as pets. Close human-animal contact might come at a county fair or petting zoo, or with wildlife when wooded land is cleared for new construction. Because of these interactions, it’s important to be aware of the different ways people can get zoonotic diseases. These can include:

  • Coming into contact with the saliva, blood, urine, or feces of an infected animal.
  • Being bitten by an infected tick or mosquito (often called a “vector”).
  • Eating or drinking something unsafe (such as unpasteurized milk, undercooked meat, or unwashed fruits and vegetables) that is contaminated with feces from an infected animal.

But according to CDC, there are also many ways that people can protect themselves from zoonotic diseases:

There are preventive measures, but one thing that doesn’t work is systematic surveillance in animals, which was never useful for preventing emerging infections, says Jean Paul Stahl, MD, professor of infectious diseases at the University Hospital, University of Grenoble, France. “I don’t think we have any example of this. In fact, we react when the disease emerges and then decide on a policy. It works in case the transmission remains animal-to-human exclusively.

“One of the best examples is brucellosis in Western Europe,” Stahl says. “In case of an evolution in human-to-human transmission, animals are no longer the problem, or only a little part of the problem. The most recent example is ebola, transmitted by bats to humans, and then it was no more a zoonosis. Flu or MERS-CoV infections are the same type of example.”

Stahl notes that complicating labs’ role in diagnosing such diseases is that most zoonosis is rare in developed countries. It is difficult to determine the diagnosis immediately, so adapted tests are used when the first case occurs. “In case of a known outbreak, the difficulty is to evoke other diagnoses; during a flu outbreak, pneumococcal pneumonia or endocarditis are still possible. So in case of fever, a panel of diagnostic tests is necessary,” he says.

“The final role is to collaborate with public health authorities in order to evaluate the evolution of the outbreak,” says Stahl. “In this case, all diseases are specific, so it is not possible to give a global scheme.”

Gary Tufel is a contributing writer for CLP. For further information, contact CLP chief editor Steve Halasey via [email protected].

REFERENCES

  1. National Academies of Sciences, Engineering, and Medicine. Rapid medical countermeasure response to infectious diseases: enabling sustainable capabilities through ongoing public- and private-sector partnerships: workshop summary. Washington, DC: The National Academies Press, 2015. Available at: http://iom.nationalacademies.org/Reports/2015/Rapid-Medical-Countermeasure-Response-to-Infectious-Diseases.aspx#sthash.n2QKRdha.6BdSv5ev.dpuf. Accessed December 6, 2015.
  2. Oregon public health officials: teen girl has bubonic plague. Associated Press, October 29, 2015. Available at: http://bigstory.ap.org/article/666ea2a6d5cd4d0db8944cb09d6b03d2/oregon-public-health-officials-teen-girl-has-bubonic-plague. Accessed December 6, 2015.
  1. Zimmer C. In ancient DNA, evidence of plague much earlier than previously known. New York Times, October 22, 2015. Available at: http://www.nytimes.com/2015/10/23/science/in-ancient-dna-evidence-of-plague-much-earlier-than-previously-known.html?_r=0. Accessed December 6, 2015.
  1. Plague [online]. Atlanta: Centers for Disease Control and Prevention, 2015. Available at: www.cdc.gov/plague. Accessed December 6, 2015.
  1. Muehlenbachs A, Bhatnagar J, Agudelo CA, et al. Malignant transformation of Hymenolepis nana in a human host. N Engl J Med. 2015;373:1845–1852; doi: 10.1056/nejmoa1505892.
  1. Rettner R. Tapeworm spreads deadly cancer to human. Scientific American, November 5, 2015. Available at: www.scientificamerican.com/article/tapeworm-spreads-deadly-cancer-to-human. Accessed December 6, 2015.
  1. Barrera J. Chagas disease spreading in the US: 10 things to know about deadly borne infection. Latin Times, November 17, 2015. Available at: www.latintimes.com/chagas-disease-spreading-texas-10-things-know-about-deadly-borne-infection-353746. Accessed December 6, 2015.
  1. Schroeder MO. The mysterious rise of valley fever. US News & World Report, November 12, 2015. Available at: http://health.usnews.com/health-news/patient-advice/articles/2015/11/12/the-mysterious-rise-of-valley-fever?int=a57b09. Accessed December 6, 2015.
  1. Wilken JA, Sondermeyer G, Shusterman D, et al. Coccidioidomycosis among workers constructing solar power farms, California, USA, 2011–2014. Emerg Infect Dis. 2015;21(11): 1997–2005; doi: 10.3201/eid2111.150129.
  1. Cardine S, Perry T. 28 deaths from West Nile virus confirmed in California so far in 2015. Los Angeles Times, November 2, 2015. Available at: www.latimes.com/local/lanow/la-me-ln-28-deaths-in-california-so-far-in-2015-from-west-nile-virus-20151101-story.html. Accessed December 6, 2015.
  1. Olano JP, Walker DH. Diagnosing emerging and reemerging infectious diseases: the pivotal role of the pathologist. Arch Pathol Lab Med. 2011;135(1):83–91; doi: 10.1043/2010-0260-rar.1.
  1. Johnson SR. Coming to America: neglected tropical diseases taking hold in US. Modern Healthcare, October 10, 2015. Available at: www.modernhealthcare.com/article/20151010/MAGAZINE/310109987. Accessed December 6, 2015.
  1. One World, One Health [online]. Silver Spring, Md: Association of Public Health Laboratories, 2015. Available at: www.aphl.org/aphlprograms/pages/one-world-one-health.aspx#sthash.JYOsB7GO.dpuf. Accessed December 7, 2015.