Pathogenicity index

The pathogenicity index (PI) is a key figure in virology to compare the disease-causing properties ( virulence ) of different virus isolates with one another and to assess their danger to an animal population . The indices are only used to characterize Newcastle Disease Virus (NDV) as the causative agent of Newcastle disease and in the case of influenza virus subtypes of avian influenza . The higher the index, the more virulent the virus strain examined is.

The various pathogenicity indices are important in the monitoring of the two diseases in birds to avert the danger of an animal disease , but also in the approval of attenuated live vaccines and inactivated inactivated vaccines .
When determining the indices, virus preparations of a specified dilution are injected into the brain ( intracerebral pathogenicity index ICPI) of newly hatched chicks ( Gallus gallus domesticus ) or a vein ( intravenous pathogenicity index IVPI) of adult chickens. The occurrence of signs of illness or the death of the animals is observed in the following days and the index is calculated from these data. Due to animal health regulations, the implementation and evaluation of ICPI and IVPI testing has been standardized and has been uniformly regulated by law for the European Union since 1993.

Intracerebral Pathogenicity Index (ICPI)

Different isolates of the Newcastle Disease Virus differ considerably in terms of their virulence, so a single detection of the virus is not sufficient to assess its risk. Only virus strains of the NDV with an index value of the ICPI of more than 0.7 are classified as relevant under disease law; only then will control measures be initiated.

execution

To determine the ICPI, allantoic fluid is taken from infected and incubated chicken eggs and the virus titer therein is determined using a hemagglutination inhibition test. If the titer is at least 1:24, the liquid is diluted 1:10 with isotonic saline solution . 0.05 ml of the virus preparation produced in this way is injected into the brain tissue of ten day-old chicks (chicks which hatched 24-40 hours ago). The animals are then checked every 24 hours for eight days and examined for signs of disease. The evaluation is based on the daily schedule according to healthy (0 points), sick (1) and dead (2).

evaluation

The index is now calculated according to the daily schedule, the number of points awarded and the number of animals according to the following example table of a test:

Symptoms	Days after vaccination (number of animals)								total	x points	rating
	1	2	3	4th	5	6th	7th	8th
healthy	10	8th	3	0	0	0	0	0	21st	21 × 0	= 0
ill	0	2	7th	10	8th	1	0	0	28	28 × 1	= 28
dead	0	0	0	0	2	9	10	10	31	31 × 2	= 62

The assessment points are now added (0 + 28 + 62 = 90) and related to the number of animals (10) and test days (8). This gives the average value per animal and test, which is defined as the pathogenicity index: ICPI = 90 / (8 × 10) = 1.125

Intravenous Pathogenicity Index (IVPI)

The intravenous pathogenicity index (IVPI) is mainly used to examine influenza virus strains that can cause avian influenza. A virus strain with an IVPI of at least 1.2 is considered to cause the disease and is therefore relevant as a possible pathogen of an animal disease.

execution

Similar to the ICPI test, infected allantoic fluid is used to determine the IVPI. This is diluted 1:10 in isotonic saline solution and 0.1 ml of this is injected intravenously into ten chickens at least six weeks old. The animals are then observed for ten days and examined daily for symptoms of the disease. The score is healthy (0 points), sick (1), seriously ill (2) and dead (3).

evaluation

The IVPI is determined using a table similar to the ICPI as in the following example:

Symptoms	Days after vaccination (number of animals)										total	x points	rating
	1	2	3	4th	5	6th	7th	8th	9	10
healthy	10	10	8th	1	0	0	0	0	0	0	29	29 × 0	= 0
ill	0	0	2	8th	6th	0	0	0	0	0	16	16 × 1	= 16
seriously ill	0	0	0	1	3	8th	1	0	0	0	13	13 × 2	= 26
dead	0	0	0	0	1	2	9	10	10	10	42	42 × 3	= 126

The evaluation points are now added up again (0 + 16 + 26 + 126 = 168) and related to the number of animals (10) and test days (10). This gives the average value per animal and test: IVPI = 168 / (10 × 10) = 1.68

ICPI in vaccine production

When producing attenuated live vaccines against the Newcastle Disease virus, only those virus strains may be used whose original strain (original seed virus , master seed virus ) meets the following ICPI values:

• ICPI less than 0.4 (if each bird in the ICPI test received no less than an infectious dose of 10 ⁷ EID50), or
• ICPI less than 0.5 (if each bird received no less than 10 ⁸ EID50 in the ICPI test )

Inactivated inactivated vaccines against the NDV must be made from an original virus strain that has an ICPI below 0.7 at an infection dose of at least 10 ⁸ EID50.

swell

• 93/342 / EEC: Commission decision of 12 May 1993 on the criteria for classifying third countries with regard to avian influenza and Newcastle disease (pdf) (PDF)

Individual evidence

1. ↑ H. Khadzhiev: Isolation, identification and typing of Newcastle disease virus isolates in a chick embryo cell culture . Vet. Med. Nauki. (1984) 21 (10): pp. 19-27 (abstract only), PMID 6531856 .
2. ↑ DJ Alexander and G. Parsons: Avian paramyxovirus type 1 infections of racing pigeons: 2 pathogenicity experiments in pigeons and chickens . Vet. Rec. (1984) 114 (19): pp. 466-469, PMID 6464308 .