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The Spectrum of Illness and the Role of Comorbidities in Zoonosis


Anna Rovid Spickler

, DVM, PhD, Center for Food Security & Public Health, College of Veterinary Medicine, Iowa State University

Reviewed/Revised Feb 2023 | Modified Jun 2023

The spectrum of zoonotic illness varies from skin eruptions or mild, self-limiting infections (easily misdiagnosed as influenza in humans) to serious, life-threatening disease.

Some zoonoses can affect healthy humans; others occur primarily in humans with debilitating illnesses and other conditions that compromise immunity. Veterinarians should be aware of and should alert clients of the potential risk to immunocompromised persons after diagnosing a zoonosis.

Zoonoses that cause mild or no clinical signs and symptoms in healthy hosts can be serious illnesses or have unusual clinical manifestations in those who are immunocompromised. In some cases, a suppressed immune response may also slow diagnosis if common tests rely on serologic testing.

Primary immunodeficiencies, which are congenital defects, may affect humoral or cell-mediated immunity or both. Some primary immunodeficiencies increase susceptibility to a single category of pathogens, while others broadly suppress defenses. Sometimes these conditions can remain unnoticed except as an unusual susceptibility to certain illnesses. Others are obvious from infancy.

Secondary immunodeficiencies can be due to any acquired condition that compromises the immune system. Examples include splenectomy, diseases that affect metabolism (eg, diabetes), illnesses such as cancer that result in generalized debilitation, and certain infections such as malaria or HIV. Other illnesses, such as chronic lung disease, can increase susceptibility by affecting innate (nonspecific) defenses. Injuries and burns can compromise the skin defenses that prevent pathogens from entering the body, as can indwelling catheters and implanted medical devices. Drugs can suppress immunity as an intended effect (eg, drugs used to treat autoimmune diseases or prevent rejection in organ transplant patients) or as an adverse effect. Some drugs used in cancer chemotherapy are highly immunosuppressive.

Physiologic states can also affect immunity. The immune system is relatively immature in newborns and young children, and it declines in older adults. Pregnancy may result in risks to the mother, the fetus, or both. For example, in some geographic locations, the case fatality rate for hepatitis E is ~1% in the general population but may reach 20% among pregnant women. Other pathogens, such as Toxoplasma gondii, may severely affect the fetus while causing only mild disease in the mother.

Key Points

  • Zoonotic diseases range from skin diseases and mild flu-like illnesses to severe, life-threatening disease.

  • Immunosuppressive conditions, which may include various primary and secondary immunodeficiencies, increase the risk and severity of some zoonoses; in some cases, they may also make the condition more difficult to diagnose.

  • Factors that compromise innate barriers, such as skin defenses, also increase the risk of acquiring a zoonotic disease.

  • Very young, elderly, or pregnant humans are often more susceptible to serious illness from zoonotic diseases.

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