The Complete Guide to Feline Infectious Diseases

Infectious diseases are among the leading causes of illness and death in domestic cats worldwide. From highly contagious viral infections that sweep through catteries in days, to chronic bacterial conditions that persist for months, the landscape of feline infectious disease is broad, clinically complex, and constantly evolving.

This guide is designed as a comprehensive reference for cat owners, breeders, and veterinary professionals. It covers the most clinically significant feline infectious diseases, explains how each one spreads, what signs to watch for, how diagnosis is made, and what can be done to treat and prevent each condition.

Why Feline Infectious Diseases Matter

Cats are social animals. In multi-cat households, catteries, shelters, and rescue facilities, a single infected individual can rapidly expose dozens of others. Many feline pathogens are extremely stable in the environment, survive on surfaces for extended periods, and are resistant to common disinfectants.

Furthermore, some of the most common feline pathogens — including feline calicivirus (FCV) and feline herpesvirus (FHV-1) — mutate rapidly, meaning that vaccination provides partial rather than complete protection. Understanding the biology of these organisms, and the principles that govern their transmission, is essential for anyone responsible for feline health.

The Major Feline Infectious Pathogens

Feline infectious diseases can be caused by viruses, bacteria, intracellular organisms, and fungi. The most clinically important are covered in detail in the articles linked throughout this guide. Below is an overview of each major pathogen category.

Viral Infections

Viruses are responsible for the most common and often most serious feline infectious diseases. They are transmitted primarily through direct contact with infected cats, aerosol droplets, and contaminated secretions.

Feline Calicivirus (FCV): One of the two primary causes of cat flu. Causes oral ulceration, respiratory disease, and in virulent strains, life-threatening systemic disease. Highly mutable; vaccination provides approximately 75% protection.
Feline Herpesvirus Type 1 (FHV-1): The other major cause of cat flu. Causes severe rhinotracheitis, conjunctivitis, and corneal ulceration. Establishes lifelong latency in trigeminal ganglia; stress-induced reactivation is common.
Feline Panleukopenia Virus (FPV): A parvovirus causing severe, potentially fatal gastroenteritis and bone marrow suppression. Highly contagious; the vaccine is one of the most effective in veterinary medicine.
Feline Leukemia Virus (FeLV): A retrovirus causing immunosuppression, anemia, and lymphoma. Spread by close contact with infected cats. Vaccination available; testing and isolation are key control strategies.
Feline Immunodeficiency Virus (FIV): A lentivirus causing progressive immune dysfunction. Spread primarily by bite wounds. No curative treatment; management focuses on supportive care and preventing secondary infections.

Bacterial and Intracellular Infections

Bacterial infections often arise as secondary complications of viral disease or primary infections in their own right. Several intracellular bacteria — organisms that live inside host cells — have emerged as clinically important pathogens in cats.

Mycoplasma spp.: Causes both respiratory infections and hemolytic anemia. Mycoplasma haemofelis is the primary cause of feline infectious anemia. Diagnosis requires PCR; treatment involves doxycycline.
Chlamydia felis: An obligate intracellular bacterium causing chronic conjunctivitis and mild respiratory signs. Spread by direct contact. Responds well to doxycycline and fluoroquinolone antibiotics.
Bordetella bronchiseptica: A gram-negative bacterium capable of causing respiratory disease, particularly in kittens and immunocompromised cats. Can be primary or secondary pathogen.

Transmission Principles

Understanding how feline pathogens spread is fundamental to their control. The major routes of transmission include:

Direct contact: Nose-to-nose contact, mutual grooming, and shared feeding areas account for the majority of transmission events for FCV, FHV-1, FIV, and FeLV.
Aerosol / respiratory droplets: FCV, FHV-1, Bordetella, and Chlamydia felis are all transmissible via respiratory secretions, with effective droplet spread over short distances.
Fomite transmission: Contaminated food bowls, litter trays, bedding, and human hands can carry pathogens between cats. FPV and FCV are particularly resistant to environmental inactivation.
Vector-borne: Mycoplasma haemofelis is transmitted by fleas and potentially other ectoparasites; rigorous parasite control is essential.

Key Clinical Syndromes and Their Causes

Many feline infectious diseases present with overlapping clinical signs. The table below summarizes the most common syndromes and their principal infectious causes.

Clinical Syndrome	Primary Causes	Key Distinguishing Features
Upper respiratory infection	FCV, FHV-1, Chlamydia felis, Bordetella	Sneezing, nasal discharge, conjunctivitis
Oral ulceration	FCV (primary)	Tongue, palate, nostril erosions
Conjunctivitis	FHV-1, Chlamydia felis (primary)	Chlamydia causes unilateral onset; FHV bilateral
Anemia / pale mucous membranes	Mycoplasma haemofelis, FeLV	Check PCV; PCR for Mycoplasma
Systemic illness / multi-organ	FCV-VSD, FPV, FeLV, FIV	Varies widely; requires full work-up
Chronic weight loss / immunosuppression	FIV, FeLV	Recurrent infections; lymphadenopathy

Diagnosis: A Systematic Approach

Accurate diagnosis of feline infectious disease requires combining clinical assessment with appropriate laboratory testing. Treating empirically without a diagnosis risks under-treating serious conditions, inducing antimicrobial resistance, and missing concurrent infections.

History and Physical Examination

A thorough history should capture vaccination status, exposure history, living situation (indoor only vs. outdoor access, multi-cat household, recent cattery stay), and any recent stressors. Physical examination should assess body condition, hydration, lymph node size, ocular and nasal discharge, oral cavity, skin integrity, and mucous membrane color.

Laboratory Diagnostics

PCR testing: The gold standard for most feline respiratory and vector-borne pathogens. RT-PCR is used for FCV and FHV-1; DNA PCR for Chlamydia felis and Mycoplasma spp. Commercially available worldwide.
ELISA and rapid antigen tests: Widely used in-clinic for FeLV antigen and FIV antibody detection. High sensitivity and specificity for screening.
Complete blood count (CBC): Essential for assessing anemia severity in suspected Mycoplasma haemofelis infection; reveals lymphopenia and neutropenia in viral infections.
Serology: Useful for FIV and FeLV confirmation; less useful for respiratory pathogens due to widespread vaccine-induced antibody titers.
Culture and sensitivity: Indicated for chronic or refractory bacterial infections where antimicrobial resistance is suspected.

Vaccination: The Cornerstone of Prevention

Vaccination remains the most cost-effective strategy for preventing feline infectious disease at the individual and population level. Core vaccines — those recommended for all cats regardless of lifestyle — provide protection against the most severe and widespread feline pathogens.

Core Vaccines

Feline Herpesvirus Type 1 (FHV-1): Core. Reduces severity of disease; does not prevent infection or latency. Included in all standard combination vaccines.
Feline Calicivirus (FCV): Core. Approximately 75% efficacy due to genetic diversity of circulating strains. Newer multivalent vaccines offer broader coverage.
Feline Panleukopenia Virus (FPV): Core. Highly efficacious; duration of immunity may exceed 7 years following the primary course.

Non-Core Vaccines

Feline Leukemia Virus (FeLV): Recommended for all kittens; ongoing need assessed by lifestyle risk.
Chlamydia felis: Recommended in multi-cat households and catteries with documented infection.
Bordetella bronchiseptica: Considered in high-risk environments such as shelters and catteries.

Infection Control in Multi-Cat Environments

Multi-cat households, catteries, and shelters present unique infection control challenges. The following principles underpin effective disease management:

Strict quarantine of new arrivals for a minimum of 2 weeks before introduction to resident cats
Testing all new cats for FeLV and FIV prior to introduction
Maintaining up-to-date vaccination records for all resident cats
Using dedicated equipment (food bowls, litter trays) for isolated or infected cats
Regular disinfection with agents effective against non-enveloped viruses (e.g., bleach dilutions for FPV and FCV)
Avoiding overcrowding, which amplifies stress-induced viral reactivation and increases transmission risk
Ectoparasite control to reduce vector-borne transmission of Mycoplasma haemofelis

Special Considerations for Kittens

Kittens are disproportionately vulnerable to feline infectious diseases for several reasons. Maternal antibody levels decline from approximately 6 to 16 weeks of age, creating a window of susceptibility before active immunity from vaccination develops. Kittens born to vaccinated queens may have reduced but not reliable protection, particularly against rapidly mutating pathogens like FCV.

The limping syndrome associated with certain FCV strains, neonatal FPV infection, and Mycoplasma haemofelis all carry particularly serious consequences in young kittens. Timely vaccination, colostrum intake in the first hours of life, and hygiene during the neonatal period are all critical protective measures.

Articles in This Series

References

1. August JR (2006). Consultations in Feline Internal Medicine, Vol. 5. Elsevier Saunders.

2. Sykes JE (2014). Canine and Feline Infectious Diseases. Elsevier Saunders.

3. Hartmann K et al. (2015). ABCD guidelines on feline infectious diseases. Journal of Feline Medicine and Surgery.

4. Day MJ et al. (2016). WSAVA guidelines for the vaccination of dogs and cats. J Small Anim Pract 57(1):E1–E45.

5. Lloret A et al. (2013). Chlamydiosis in cats: ABCD guidelines on prevention and management. J Feline Med Surg 15(7):622–32.

6. Tasker S (2010). Haemotropic mycoplasmas: What’s their real significance in cats? J Feline Med Surg 12(5):369–81.