Introduction

The role that pets play in many people's lives and the close contact that is inherent with this relationship can be highly beneficial, but is also associated with some risk of pathogen transmission. Few would argue that pets are part of the family socially and emotionally. At the same time, it is becoming increasingly clear that pets are part of the family microbiologically, and that transmission of microorganisms between people and pets is probably a common event. Zoonotic diseases are an inherent risk of any contact with animals and are generally considered an acceptable risk by most pet owners, considering the low incidence of serious zoonoses and the positive social and emotional benefits of pet contact. However, there are certain situations where people may be at higher risk for zoonotic diseases or be more prone to serious outcomes. One such group is people with compromised immune systems. Advances in medicine have been able to sustain lives of such individuals but leave them in a more susceptible state for infectious diseases, including zoonotic diseases. Others who may be at increased risk include infants, pregnant women and the elderly. The aging population, in particular, increases the pool of individuals with waning immunity.

A ‘knee-jerk’ reaction sometimes occurs when issues regarding zoonotic diseases and immunocompromised individuals are discussed. The simplest way to reduce the risk of zoonotic disease exposure from pets is to avoid all contact with pets. However, this is generally not an acceptable course of action, nor is it commonly (if ever) required. The importance of the human-animal bond and its benefits are also documented in individuals who are immunocompromised, and this fact must not be lost amongst infectious disease concerns. It is clear that while there must be consideration of potential health risks associated with pet contact, the emphasis should be on determining the cost-benefit of pet ownership and implementation of methods that may be able to reduce (but never eliminate) the risk of zoonotic disease transmission.

What Is “Immunocompromised”?

While all people with some degree of compromise of their immune system are often lumped into the generic ‘immunocompromised’ category, it is critical to recognize that there are marked differences between different conditions and even between individuals with the same condition.

Acquired immunodeficiencies are the most common causes of immune dysfunction. These can include numerous biologic disorders, normal physiologic states (e.g., pregnancy), and drug-induced effects on the immune system. Different conditions have different effects on the immune system, both in terms of severity of compromise and the component of the immune system that is affected. Both of these are critical factors since certain components of the immune system are critical for preventing or eliminating certain infections. A person with a drug-associated immunosuppression may have markedly different risks compared to someone without a functional spleen, despite the fact that both are at increased risk of infection in broad terms.

Pathogens of Concern

There are various types of pathogens that are of concern in immunocompromised individuals. These include organisms that are also of concern in immunocompetent individuals, those that do not typically cause disease in immunocompetent individuals but can cause disease in immunocompromised individuals, and pathogens where disease is potentially much more severe in immunocompromised individuals. There are not always clear delineations between these groups.

Organisms that are primarily pathogens only in immunocompromised individuals or select populations

People are constantly exposed to various bacteria, viruses and fungi. Some are primary pathogens that typically cause disease when they are encountered. More are opportunistic pathogens that can cause disease under appropriate circumstances. Others are organisms that rarely cause disease in otherwise healthy people but can cause disease in individuals with compromised immune systems. For some organisms, diagnosis of infection essentially indicates that immunocompromise is present, even if it has not been previously diagnosed. Zoonotic pathogens in this category include Capnocytophaga spp, Rhodococcus equi and Toxoplasma gondii.

Organisms that are more likely to cause disease and/or to cause more severe disease in immunocompromised individuals

Some infectious agents may cause disease in immunocompetent people; however, disease tends to be mild and either self-limiting or readily treated. The same pathogens may cause severe or life-threatening disease in immunocompromised individuals. These include organisms such as Cryptosporidium spp, Salmonella and a host of other common enteric or oro-pharyngeal inhabitants.

Risk Reduction

Absolute elimination of risk is impossible. However, practical measures can be taken to reduce the risk. Some examples are outlined below.

Client Communication

This is perhaps the most important aspect and the one for which veterinarians are the least trained. A key component is simply raising the subject, as many high-risk owners may not appreciate potential zoonotic disease concerns or have any concept of basic infection prevention measures.

Animal Selection

Zoonotic risks wary between and within species. Reptiles are particularly high risk because of Salmonella and are generally not recommended to be present in high-risk households.

Young animals and animals from shelters or similar facilities are more likely to shed various infectious agents, so they are not the best choice of a pet. Further, younger animals are more likely to bite and scratch, and it is more difficult to assess their temperament. Older animals that are already in households are ideal. Species such as rabbits where there is moderate information about infectious disease risks and management are relatively low risk.

Birds are a somewhat controversial area. Rates of shedding of some potential pathogens can be high, including serious pathogens such as Chlamydophila psittaci. There are also concerns about shedding of organisms such as Cryptococcus spp, although the risk various geographically. There has been little objective evaluation of risks associated with pet bird ownership and some degree of risk must be considered, although it can probably be minimized by safe management practices, particularly those involving avoidance of fecal material.

Owners should be made aware of certain syndromes that might indicate an infectious process. These include diarrhea, vomiting, coughing and skin lesions. In households with immunocompromised owners, earlier or more comprehensive diagnostic testing may be indicated to allow for prompt identification of potentially zoonotic pathogens.

Training

Bites and scratches are probably the most common adverse events from animals and associated with the greatest morbidity and mortality. While good animal training and handling are important in any household, they are critical in households where the implications of those events can be higher.

Preventive Medicine

Optimizing animal health is required to reduce the risk of infections that could pose a zoonotic risk. Regular examination is critical to detect problems that could put the owner at risk. A thorough veterinary examination should be performed at least yearly, if not more frequently. A proper preventive medicine program is important.

Animal Disease Management

A more aggressive approach to diagnostic testing may be indicated in some cases, particularly when an infectious process is suspected. Identifying pathogens of higher concern (e.g., MRSA) in animals can help counsel owners on prevention measures. Further, pets owned by high-risk individuals may be at increased risk of infections caused by certain human-associated multidrug resistant bacteria.

Another aspect is controlling underlying disease processes. While this is important for all patients, it is of particular need in high-risk households as a means of preventing secondary infections that could be caused by zoonotic pathogens. For example, control of allergic skin disease is important to reduce the risk of pyoderma, which could be caused by a zoonotic pathogen.

Conclusion

A large percentage of households contain at least one higher risk individual, and pet ownership by immunocompromised people, including highly compromised individuals, is common. Veterinarians have an important role as part of the family health team. Rarely, if ever, are pets inappropriate for high-risk people. However, certain pets, certain management practices and certain activities may be contraindicated. Veterinarians need to engage higher-risk owners to maximize animal health and counsel owners on optimal practices to reduce their exposure to important zoonotic pathogens.