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No. 17 Winter 2004
 
Immunity Predicted by Serology
By Ephraim Keren, VMD 

An experimental study in vaccinated cats led investigators to conclude that serologic tests could be used to predict whether cats are likely to be resistant or susceptible to three common feline infectious diseases. An abstract of this study(1) is reprinted here for the interest of our readers.  

Objective:
To determine whether detection of virus-specific serum antibodies correlates with resistance to challenge with virulent feline herpesvirus 1 (FHV-1), feline calicivirus (FCV), and feline parvovirus infection (FPV) in cats and to determine percentages of client-owned cats with serum antibodies to FHV-1, FCV, and FPV.  

Animals: 
72 laboratory-reared cats and 276 client-owned cats.  

Procedures: 
Laboratory-reared cats were vaccinated against FHV-1, FCV, and FPV, using 1 of 3 commercial vaccines, or maintained as unvaccinated controls. Between 9 and 36 months after vaccination, cats were challenged with virulent virus. Recombinant-antigen ELISA for detection of FHV-1-, FCV-, and FPV-specific antibodies was developed, and results were compared with results of hemagglutination inhibition (FPV) and virus neutralization (FHV-1 and FCV) assays and with resistance to viral challenge.  

Results: 
For vaccinated laboratory-reared cats, predictive values of positive results were 100% for the FPV and FCV ELISA and 90% for the FHV-1 ELISA. Results of the FHV-1, FCV, and FPV ELISA were positive for 195 (70.7%), 255 (92.4%), and 189 (68.5%), respectively, of the 276 client-owned cats.  

Conclusions & Clinical Relevance: 
Results suggest that for cats that have been vaccinated, detection of FHV-1-, FCV-, and FPV-specific antibodies is predictive of whether cats are susceptible to disease, regardless of vaccine type or vaccination interval. Because most client-owned cats had detectable serum antibodies suggestive of resistance to infection, use of arbitrary booster vaccination intervals is likely to lead to unnecessary vaccination.    

Reference:
(1) Lappin, M. R., et. al. (2002). Use of Serologic Tests to Predict Resistance to FHV-1, FCV, and FPV in Cats. JAVMA,
220(1), 38-42.   

“What You Should Know about Feline Toxoplasmosis?” (Part I)  
By Ephraim Keren, VMD 

This is the first of a two-part article about feline Toxoplasmosis. This article introduces the subject with some background, a review of the parasite’s life cycle and prevalence. Clinical aspects, diagnosis of infection and prevention and control will be discussed in the next issue.  

Background            
Clinical cases of feline Toxoplasmosis are rarely seen in the average veterinary practice. However, it is important for veterinarians to be informed about this disease since they are often called upon to clarify the potential risks associated with cats regarding the transmission of Toxoplasma gondii infection to human beings. Whereas infection by T. gondii in cats (and people) is relatively common, clinical cases of feline Toxoplasmosis are rarely seen in the average veterinary practice. It is important for veterinarians to be informed about this disease, however, since they are often called upon to clarify the potential risks associated with cats regarding the transmission of T. gondii infection to human beings. Veterinarians should be able to reassure their clients that it is possible to keep a pet act in the household and still avoid this zoonotic (from animal to humans) infection(1).  

This article is geared to provide veterinarians with information on the clinical and epidemiologic aspects of feline Toxoplasmosis, including diagnosis of latent infections in cats.  

Life Cycle              
Toxoplasma gondii is a coccidian parasite that infects most warm-blooded animals. The cat is regarded as the definitive host or reservoir for T. gondii because only in cats can the parasite complete its entire cycle(2). Cats typically acquire infection by preying upon rodents or birds. Cysts, which are released in the cat’s intestine upon digestion of infected tissues, continue to produce large numbers of oocysts that are then passed in the feces. Some of these mature forms of the parasite (bradyzoites) penetrate the intestine, multiply (tachyzoite stage) and migrate to other body sites. Usually, the cat’s immune system arrests further development of the parasites. Oocysts, which are shed in the feces, become infectious several days after leaving the cat and can remain viable in the environment for over a year. Other animals and humans may become infected via direct or secondary exposure to contaminated soil.  

Prevalence              
Serologic testing in cats and people has shown that Toxoplasma infection occurs commonly throughout the world. The seroprevalence rates vary in different regions. Infection rates of T. gondii in both cats and humans vary in different countries according to reports from serologic surveys (See Tables 1 & 2). These studies have consistently found a low incidence of clinical disease even in regions with high seroprevalence.  
  
Table 1.  T. gondii Seroprevalence in Humans  
Country                       Rate                            Reference
Norway                       11%                            Jenun (1998)
S. Africa                      21%                            Joubert (1997)
Israel                           21%-56%                  Raz & Nishri (1993)
USA                            22%                            CDC (2001)
Spain                          43%                            del-Castillo (1998)
France                        85%                            CDC (2001)    

Table 2.  T. gondii Seroprevalence in Cats  
Country                       Rate                            Reference
Japan                         9%                              Maruyama (1998)
Israel                         11%-24%                Salant, et al. (2001)
Egypt                          20%                            Abu-zakham (1989)
USA                            30%                            Lappin (1996)
Check Rep.                  61%                            Svobodova (1998)
Turkey                          70%                            Poyraz (1998)    

References:  
1.  Centers for Disease Control and Prevention: www.dpd.cdc.gov.dpdx/frames/s-z/toxo              www.cdc.gov/ncidod/dpd/parasites/toxoplasmosis   
2. Cornell Feline Health Center, web. vet. cornell.edu/public/FHC/toxo.html    
3.  Lappin, M., et al. (1992). Serologic prevalence of selected infectious disease in cats with uveitis. JAVMA, 201(7),
pp. 1005-9.  

From the Editor:  Medical and Economic Considerations regarding Serologic Testing      
By Ephraim Keren, VMD

This edition of the ImmunoComb® News features two articles about the value of serologic testing; one in cats and the other in poultry.   

In the feline article, Dr. Lappin and co-workers suggest that cats with specific antibodies to FHV- 1, FCV, and FPV are likely to be resistant to infection from these agents. Dr. Lappin’s suggestion has long been adopted in the poultry industry, as noted in my reply to Dr. Macalalad regarding Newcastle Disease (ND).  

Why then, is serologic testing for evaluating immunization status in dogs and cats practically unheard of, while it is routinely practiced in poultry management? In my opinion, one reason is that the cost / benefit calculations for bringing healthy birds to market are quite different than keeping healthy pets at home.    

In poultry, when serologic tests show that a flock has a high level of antibodies vs. a potentially devastating disease such as ND, the owner will most likely choose not to revaccinate the birds. The owner saves the cost of revaccination and any potential losses associated with vaccination related side effects in the birds.  

In dogs, annual revaccinations for Distemper and Parvovirus are routinely practiced. Some pet owners have come to view the veterinarian primarily as the administrator of the vaccine without perceiving further professional input. Therefore, many clients have turned to less expensive vaccination clinics or over- the-counter means of purchasing these vaccinations. In these circumstances, the dog’s regular veterinarian misses a valuable opportunity to examine the animal and review other aspects of the pet’s health record, which are as important as giving the vaccination.  

The veterinarian provides important professional services to clients regarding the preventive health care and treatment of their pets. These services include educating clients and providing information about vaccinations. For the past few years, the subject of revaccinations has been under review within the veterinary profession. Guideline recently published by the American Animal Hospital Association state that revaccination every 3 years for Distemper and Parvovirus is considered protective(1). However, the AAHA still holds the veterinarian responsible for doing what s/he determines to be in the best interest of the patient.  

Serology provides actual data, which the veterinarian can use to determine if the dog has a protective level of antibodies (i.e. immune to Distemper and Parvovirus) or should be revaccinated. Whereas the economic considerations in pets are different than with poultry, the approach and methodology is the same. As such, I encourage veterinarians to consider using serology as part of their vaccination programs. 

References:
(1) Paul, M. A., et. al. (2003). JAAHA, 39, pp. 119-131.

Vet Forum: Serologic Testing in Poultry  

Dr. Keren,  
Please continue sending me the ImmunoComb® Newsletter, which I find both interesting and helpful. I would appreciate it if you would include a poultry-related article in the next issue and ask that you consider addressing the following question: How could a Newcastle Disease (ND) outbreak occur in a flock of chickens that has been previously vaccinated for this disease?  

Thank you.  
Marigold Macalalad, DVM
Glenwood Technologies International, Inc.
Philippines      

Dear Dr. Macalalad,  
Thank you for your positive feedback on the ImmunoComb® Newsletter. I am happy to reply to your poultry-related queries in the Vet Forum section of this issue. Your question addresses a critical point in animal husbandry, particularly regarding poultry and other farm animals, but also which pertains to pets. Namely, that faulty vaccination programs are often associated with an outbreak (or clinical case) of a particular disease.

Whereas a vaccine was administered to a particular flock, this procedure may not have conferred protective humoral immunity for a variety of reasons. For example, chicks that have a high level of maternal immunity at the time of vaccination and would need a second  ‘immunization’ after their maternal antibody levels have waned. (This is well recognized in puppies.) Secondly, the desired immune effect may be compromised if the route of vaccination (e.g. eye drop vs. drinking water) is wrong.

There are other possible explanations, so a complete history of the flock and the vaccination program is necessary in order to determine why an outbreak occurred. If the technical aspects seem to be in order, the vaccine itself may be at the root of the problem, possibly at the manufacturer level or more likely related to improper storage of the product.

While the veterinarian usually makes the final evaluation regarding the overall immune status of the flock, it is vital that the poultry owner and/or serviceman have an adequate working knowledge regarding the interpretation of serologic testing results.

I hope that my comments have been helpful. If you (or any of our other readers) would like further information or references on this subject, please don’t hesitate to let me know.

Sincerely,
Ephraim Keren, VMD
IC News Editor