Every outbreak is unique, which means epidemiologists must remain vigilant and open-minded in order to rapidly recognize and contain anything that might be atypical. That is the case in Michigan, where health officials are responding to an outbreak of hepatitis A. Several counties in southeast Michigan have been impacted, with nearly 200 confirmed cases and 10 deaths—10 times the number of cases from the previous year—with nearly 90% requiring hospitalization. One of the most significant challenges since the outbreak began in August 2016 has been identifying the source, or sources, of the disease.

Identifying the source of outbreaks

Epidemiologists rely on patterns and trends in data to identify, characterize, and respond to disease events, and atypical characteristics can complicate epidemiologic investigations and incident responses, potentially delaying public health and medical interventions and placing additional lives at risk. For example, an outbreak may occur outside of the normal seasonal pattern, such as the recent late-season surge of influenza in Taiwan and Hong Kong or an outbreak of norovirus—which usually peaks in the winter—during an outdoor summer obstacle course race in Michigan in 2013.

Unusual modes of transmission may also obscure outbreak investigations, as was seen during a 1996 hantavirus outbreak in Argentina in which the disease, normally spread through inhalation of aerosolized rodent waste, was linked to human-to-human transmission. Other factors, including symptomology consistent with numerous etiologic origins (eg, a rash may be caused by chikungunya, dengue, or Zika) and unusual symptomology not commonly seen with a given disease—such as acute flaccid myelitis during a 2014 enterovirus outbreak—may slow epidemiologic investigations.

Where does hepatitis A come from?

Hepatitis A is a viral liver infection that is “usually transmitted by the fecal-oral route, either through person-to-person contact or consumption of contaminated food or water.” More often, consumption of contaminated food or drinks is the culprit.

Recent high-profile examples include incidents in Hawai’i involving contaminated scallops and tuna sold at multiple retailers and distributed to several other states, which resulted in product recalls in Hawai’i, California, Oklahoma, Nevada, New York, and Texas; Ashton Kutcher’s 30th birthday party in 2008, during which a bartender infected with hepatitis A served drinks; and an outbreak at a Pennsylvania Chi Chi’s restaurant, resulting in more than 600 cases and several deaths—and likely contributing to the restaurant chain folding less than a year later.

Atypical source in Michigan

The current outbreak in Michigan likely has a different origin. According to information provided by the Michigan Department of Health and Human Services, health officials have been unable to identify a source of contaminated food or water. Transmission appears to be occurring “through illicit drug use, sexual activity, and close contact among household members.”

While fecal-oral transmission is far and away the most common mode of transmission for hepatitis A, use of contaminated illicit drugs (injection or non-injection) or drug paraphernalia (eg, needles) is also a potential route of transmission. Investigations of hepatitis A outbreaks among illicit drug users in Australia, England, Norway, and the United States (among others) discuss this possibility, but it has been difficult to definitively determine whether transmission is occurring via the drug use itself or via other associated risky activities (eg, unprotected sexual activity, general poor hygiene). Potential sources of contamination include transporting drugs via the mouth or rectum or by “tasting the drug to assess quality.” Several studies indicate that hepatitis A’s short viremic phase makes it unlikely that bloodborne transmission via needle sharing could sustain an outbreak.

Responding to a hepatitis A outbreak

Identifying cases and implementing interventions for hepatitis A outbreaks can be complex under typical circumstances. With the potential for asymptomatic transmission and a long incubation period, which may be nearly 2 months, it is often difficult to identify cases in time to prevent further transmission. Several of the reports discussed above explicitly addressed drug users’ aversion to providing potentially incriminating information about their drug use or sexual activity, drug users’ reluctance or inability to seek medical care for their condition, and the inability to identify subclinical cases as factors likely resulting in underestimates in the number of cases in previous outbreaks among illicit drug users.

Because the outbreak in Michigan does not appear to have a clearly defined exposure—as would be the case for a foodborne outbreak of hepatitis A—health officials face a tremendous challenge in identifying new cases and the broader at-risk population.

While hepatitis A is a vaccine-preventable disease, it was not part of routine childhood immunizations until 2006, and many adults remain susceptible. Beyond childhood vaccination, the CDC recommends vaccination for a number of at-risk populations, including travelers to countries with high rates of hepatitis A, men who have sex with men, users of injection and non-injection drugs, and those with chronic hepatitis B or C infection. Nearly half of the identified cases in southeast Michigan have a history of substance abuse, and 20% are co-infected with hepatitis C. Furthermore, several of the identified cases have been recently incarcerated, increasing the risk of transmission due to prolonged close contact with others.

Additionally, the populations most vulnerable to this outbreak may not have the means to access care when they become ill, or they may not seek care due to concern that issues such as their substance abuse or sexual history (eg, prostitution, same-sex partners) may be discovered and/or reported, further complicating surveillance efforts.

The latest on Michigan’s response

Knowing that the exposed population cannot be wholly identified, Michigan health officials have recommended vaccination for broad cross-sections of the area population, including “any person who wishes to be immune to hepatitis A” and “people who live, work, or recreate in SE Michigan and are concerned about getting hepatitis A,” alongside traditional high-risk populations.

This outbreak of hepatitis A underscores the importance of thorough epidemiological investigations that consider all possible scenarios during an infectious disease outbreak. In this case, an uncommon transmission pattern occurring within a particularly vulnerable population is further complicating the challenges already inherent to hepatitis A outbreak investigations. The recognition of these atypical characteristics, however, has enabled the health department to adapt their response and hopefully curb transmission.